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A journey that began four decades before Columbus sailed for the New World finally ended when the Kepler space telescope snared a few errant photons as they shot past Earth's orbit en route to infinity. The light had sped through space for 560 years, traveling more than three quadrillion miles from a star much like our sun. Captured by Kepler's digital sensors, transformed into bytes of data, and downloaded to computers at NASA's Ames Research Center near San Francisco, the processed starlight slowly revealed a remarkable story: A planet not much bigger than Earth was whipping around its native star at a blistering pace, completing an orbit—its version of a "year"—in just over 20 hours.
Aside from its size, the planet bears little resemblance to Earth. It circles so close to its star that its surface temperature probably exceeds 2,500 degrees Fahrenheit, hot enough to melt iron. Nevertheless, the planet's detection was a technical and intellectual coup, a rite of passage for Kepler. The planet, dubbed Kepler-10 b when NASA announced its existence this past January, was the smallest world yet found beyond our solar system. Its discovery proved that the Kepler spacecraft, which was launched in March 2009, could indeed do what its designers had boldly promised: find small, Earth-size planets around distant stars, a task that once seemed so difficult as to border on the absurd.
Kepler-10 b was merely a preview. A month after the January announcement, NASA released its first full data set from the Kepler mission, and the results left astronomers straining for superlatives. "Frankly, we're overwhelmed," says Geoff Marcy, an astronomer at the University of California, Berkeley, and a member of the Kepler team. "What NASA is doing is akin to the transoceanic voyages of the 15th century—the voyages that opened up the whole world. With the Kepler telescope, we're learning about the properties of planets across the cosmic ocean. This is history. It's Armstrong stepping off the bottom rung."
Kepler monitors 156,000 stars, less than 0.0001 percent of the galaxy's population. Like some cosmic wildcat drilling operation, Kepler has struck a gusher, discovering 1,235 possible new worlds in its first four months of operation. That number doubles the previous total of just over 500, painstakingly gathered over the last 16 years. Prior to 1992, keeping track of all the known planets around other stars like the sun was easy—the tally stood at an even zero.
The bulk of Kepler's data have not yet been studied, and the mission will keep going for at least two and a half more years. But it is already shredding the textbooks, showing that our galaxy (at least the fraction of it seen by the spacecraft) contains a far more exotic assortment of planets than astronomers expected to find. "We're learning about a diversity of worlds in our universe that we had no clue about beforehand," Marcy says. "Rocky planets, yeah, we thought there might be some of those. By the way, we're finding some rocky planets that are even denser than Earth. But we're also finding these mini-Neptunes, a class of planet for which we have no examples in our solar system. They're like small Neptunes but with huge amounts of liquid water around a rocky core."
Also on the list are 67 planets roughly the size of Earth, give or take a thousand miles or so in radius; 288 "super-Earths" up to twice Earth's diameter; 662 Neptune-size planets; and 184 giants rivaling or exceeding Jupiter in size. The simple statistics from Kepler say that Earth-size planets are widespread. But as William Borucki, the head of NASA's Kepler team, points out, Earth-size does not mean Earth-like. Many of these are inhospitable, roasting-hot worlds.
The most intriguing discovery from Kepler is that 53 of those 1,200-odd planets dwell in the life-friendly "Goldilocks" zones of their stars, regions where temperatures would be just right—not too cold and not too hot—for liquid water. Where there's water, scientists reason, there could be life. Four of these Goldilocks planets are super-Earth size or smaller, meaning that they could actually resemble our own home. Even the giant worlds may offer hope. "Some of the Jupiter-size planets could conceivably have Earth-size moons, and those moons would of course also be in the habitable zone," Borucki says.
Just two years into its mission, Kepler is well on its way toward determining whether planets like Earth are rare or common. But that is just the first domino as scientists try to topple the much bigger questions, the kinds that make the hairs stand up on the back of your neck. Does life exist on other planets? Are planets with life common? Are any other intelligent beings out there?
For the first time, we have a handle on the odds, and the numbers beaming in from Kepler are not only encouraging but staggering. "Our galaxy contains 200 billion stars," Marcy says. "I would guess that at least 30 percent of them have an Earth-size planet. So 30 percent of 200 billion, that's at least 60 billion Earth-size planets just in our galaxy alone."
When astronomers started finding planets around other stars in the 1990s, they fully expected to see the general structure of our own solar system repeated throughout the cosmos. Small, rocky planets like Mercury, Venus, Earth, and Mars would orbit close to the star. Giant, gaseous planets would occupy more distant orbits, just as Jupiter and Saturn do in our solar system.
The reasoning seemed self-evident. The heat from a parent star would tend to blast away lightweight elements like helium and hydrogen. Only rocky, sturdy planets could form nearby; giant planets would form farther out, where ices and cool gases could gather together. Astronomers had no way to check their assumptions, though, because for most of the 20th century there were no telescopes capable of detecting planets around other stars. The few astronomers who even attempted to look for them languished in obscurity, spending years in fruitless searching. Such was the state of astronomy when Borucki began his career at NASA.
Borucki, 72, is a mild-mannered man, patient with bureaucrats and unruffled by skeptics, traits that served him well in his three-decade quest to find those alien worlds. Without him there would be no Kepler spacecraft, and this year's bumper harvest of planets would not have happened.
Borucki joined NASA's Ames Research Center in 1962, straight out of the University of Wisconsin with a master's degree in physics. He couldn't imagine a better job. As a boy growing up in rural Wisconsin, he had launched homemade rockets. His first assignment at NASA—researching heat shields for the Apollo moon missions—suited him perfectly. Like everyone else who worked at the space agency in those heady days, Borucki dreamed of exploring other worlds. NASA Ames was also the headquarters for SETI, the Search for Extraterrestrial Intelligence, and Borucki became friendly with one of SETI's founders, the visionary astronomer Carl Sagan.
After the Apollo program ended, Borucki grew fascinated with the idea that Earth-like planets might be orbiting other stars, unseen. He was particularly intrigued by a presentation on transit photometry, a theoretical technique that would make it possible to bring those planets into view. The concept was to equip telescopes with extremely sensitive electronic light detectors that could record the slight dip in brightness that occurs when a planet passes in front of a star. It was a brilliant idea, but it was a case of theory outpacing technology; existing detectors were not precise enough to measure such small variations in starlight consistently.
Borucki began studying the problem in his spare time. In 1984 he published a paper sketching a plan to launch a telescope into space to hunt for planets outside our solar system. In the clear vacuum of space, it could avoid the distorting effects of our own planet's atmosphere.
That same year Borucki even managed to get a bit of funding from NASA to host a conference dedicated to transit photometry. During the conference, some scientists from the National Bureau of Standards suggested that silicon diode detectors might provide the kind of precision that Borucki needed for transit photometry. When struck by a particle of light, the detectors emit a single electron. And converting starlight efficiently into a detectable electronic signal was crucial for transit photometry.
The problem was that each detector could track just one star, and Borucki knew that stringing thousands of detectors together to track enough stars to make the statistics meaningful was wildly impractical. "Every year I'd go to my branch chief for a performance evaluation. Every year he would tell me, 'Bill, this is never going to work.' And I never got promoted. There wasn't much support. On the other hand, no one ever tried to stop me. NASA Ames has a director's discretionary fund, and anyone who has a new idea can apply for it. That supported me for quite a while."
By 1992 Borucki's efforts had advanced to the point where he had assembled a team that began making formal proposals for a fully funded mission. He had determined that chips called charge-coupled devices, or CCDs (the kind used in many digital cameras), could provide the precision needed to accurately measure extremely small changes in starlight. Equally important, they could be packed into arrays capable of monitoring thousands of stars simultaneously. Borucki's team submitted a plan for a mission it called FRESIP, short for Frequency of Earth-Sized Inner Planets. NASA rejected it, citing lack of proof that the CCD technology would work.
Borucki persisted. When NASA announced that it was accepting proposals for the Discovery program, a new class of low-cost, three-year space missions, he immediately applied. NASA's reviewers estimated that his proposed mission would be too expensive. Borucki disagreed, of course, and in 1996 he tried yet again.
This time his project no longer seemed so implausible. The year before, a Swiss team had found 51 Pegasi b, a remarkable planet beyond our own solar system—the first ever discovered around another sunlike star. It is a gas giant like Jupiter, but it orbits just 5 million miles from its sun, racing around the star in only four days. (For comparison, Jupiter orbits at a distance of 480 million miles and takes nearly 12 years to complete one circuit.) The finding completely overturned the old assumption that all planetary systems would look like ours and suggested that astronomers had been overlooking a whole class of planet. "All of a sudden, it went from 'Anybody who is doing it is crazy' to 'Anybody who's doing it is at the forefront of astronomy,' " Borucki recalls.
The Swiss team made its discovery using a ground-based technique pioneered by Geoff Marcy and Canadian astronomer Bruce Campbell in the 1980s. Known as the Doppler method, it measures the gravitational tug exerted on a star by a planet—a planet that could not be seen directly because it would be lost in the glare of its star. Astronomers could use Doppler measurements to find large, Jupiter-size planets, but to find smaller worlds they would need something more precise, something like the mission Borucki had in mind. "The technique has a limited ability," Marcy says. "It's not going to take us all the way to Earths that orbit as far from their stars as our own Earth orbits the sun."
Borucki, meanwhile, had also attracted some heavyweights to his cause. Carl Sagan had joined the team, and shortly before his death in 1996 he supported a suggestion to change the mission's name to something catchier than FRESIP. David Koch, a physicist at Ames who had been working on the mission with Borucki since 1991, suggested Kepler, after the great 17th-century astronomer who discovered the laws governing the orbits of planets around the sun. Kepler it was.
Borucki's mission plan now called for a spacecraft that would trail behind Earth as it orbited the sun. Being so far from Earth would give the telescope an unobstructed view of its target stars. It would be equipped with a 56-inch telescope to focus faint starlight on an array of 42 CCDs. By comparison, the cutting-edge Hubble Space Telescope, launched six years earlier, had just four detectors. Borucki felt confident. But NASA rejected the proposal yet again, questioning whether the hardware and software could simultaneously track thousands of stars.
Borucki's team went back to the lab again, this time building a miniature observatory: a 2-foot-long, 8-inch-wide telescope with a CCD array. Coupled with software to reduce assorted stellar background noise, it could measure light changes down to 20 parts per million, making it more than sensitive enough to detect an Earth-size planet around a sunlike star in an orbit as large as Earth's. With that, another proposal followed in 1998, but it, too, was rejected. By now, though, NASA's objections were becoming pickier: Could the telescope withstand the rigors of launch and perform reliably once in orbit?
To satisfy all the remaining objections to the project, the team organized a demonstration that proved Kepler could control its vibrations and temperature enough to withstand the rigors of space. Borucki submitted yet another proposal in 2000. At long last, NASA accepted it.
"After struggling for all those years," Borucki says, "it was exhilarating to finally hear a NASA review panel say not only that it could be done, but that it should be done."
The next year, NASA provided the Kepler team with a budget of $299 million and aimed for a launch date sometime in 2006. The job of transforming the vision of Borucki's team into a functioning spacecraft fell to Ball Aerospace in Boulder, Colorado. John Troeltzsch, the Kepler program manager at Ball, says the task was far from straightforward. "When a science team proposes to NASA to build a mission like this, the team crafts a story about how few technical advances are required," he says. "They'll say 'We've got the detectors; we've got the optics.' They make it sound easy. Well, then you've got to really build it, and all the things you thought would be easy aren't. One example is the CCD detectors. People have been flying CCDs in space now since the mid-90s. So the idea of taking a whole bunch of CCDs and putting them together—how hard could that be?"
It turned out to be hard enough to keep 2,000 engineers and technicians at Ball working more than a million hours over five years. The end result was a 2,320-pound spacecraft equipped with an outsize digital camera—made of 22,000 parts—and a 55-inch telescope, the largest NASA has ever launched beyond Earth orbit. Kepler ended up costing $600 million. The CCD array measures 11 inches by 11 inches, and the whole camera sits in a 1.5-cubic-foot box suspended in a graphite-epoxy frame in the middle of the telescope. "The detector is basically a 95-megapixel camera," Troeltzsch says. Besides being hardened against radiation, the CCDs must be kept very cold, at –120ºF, to detect very low levels of light. Meanwhile, the electronics controlling the camera, positioned just inches from the CCDs, run about 225 degrees warmer. Aluminum pipes passively conduct heat from the electronics to a radiator mounted on Kepler's surface.
After some funding delays, NASA finally launched Kepler on March 6, 2009, 17 years after Borucki's first formal proposal and 400 years after Johannes Kepler published his first two laws of planetary motion in his book Astronomia Nova. A Delta II rocket sent Kepler into a sun-centered orbit similar to the one that Earth follows, in perfect clockwork accord with Kepler's laws.
The launch, Troeltzsch says, was nerve-racking. "You build this huge, high-fidelity instrument, and then you put it on a rocket that is 20 stories high and you explode it into space. It takes about two days to settle down and say OK, we're good, and about 30 days before you really start to relax."
"Kepler is the most boring mission you can imagine," Jon Jenkins tells me, smiling. "It simply stares at one place in the sky." That simplicity has turned out to be a significant factor in Kepler's success; it greatly reduces the number of things that can go wrong. "We never change Kepler's attitude except to rotate it once every 90 days to keep its solar arrays pointed at the sun, and the sun shades in place to keep the sunlight out of the barrel of the telescope. We're just staring and waiting for the stars to wink at us. You never know when that's going to happen—when a planet is going to cross in front of a star."
Jenkins, an electrical engineer by training, has been on the Kepler team since 1995; his office is just down the hall from Borucki's. The Kepler mission, his own assertion notwithstanding, has never bored him. Jenkins spent years designing and refining the sophisticated software that sifts the raw data downloaded from Kepler, looking for the telltale changes in stellar brightness that might reveal the existence of a new world. "Turning pixels into planets" is how he describes his work.
For its entire mission, Kepler's view will remain fixed on one of the spiral arms of our galaxy, on a field of stars in the constellations Cygnus and Lyra. Most of the stars it sees lie between 600 and 3,000 light-years from Earth, but some are as close 30 light-years. The cyclic dimming of more distant stars would be too faint for Kepler to measure. Although Kepler cannot peer nearly as far into the cosmos as the Hubble telescope, its view is far more panoramic, as wide as 27 full moons across the sky. Hubble focuses on a dot of sky no larger than what you would see by looking through a coffee stirrer.
The challenge for Kepler—or more specifically, for Jenkins's software—is to tease out brightness changes caused by the passage of a planet and to distinguish them from all the normal stellar variations, such as flares and star spots (the stellar equivalent of sunspots) or even nearby eclipsing stars. As with many things in life, timing is crucial. Planets give themselves away by the length of time it takes them to pass across the face of a star—typically a few hours. Star spots, which are embedded in a star's surface, typically rotate on a scale of days or weeks. So Jenkins's software searches for dips in brightness lasting up to half a day. If a planet is indeed the cause of the change in brightness, the exact same change should recur days, months, or years later, depending on how long the planet takes to orbit its star. Ideally, the Kepler team waits until the spacecraft has recorded three identical dips in brightness separated by equal intervals before concluding that they have probably found a planet.
Kepler's onboard computers can store a bit more than two months of data; the data are highly compressed for efficient storage and transmission to Earth, where NASA's servers can hold a total of about 60 terabytes. Without the compression scheme, the Kepler mission never would have flown. "It would have taken us five times as long to downlink the data and five times as much hardware to store the data on board," Jenkins says. "Compression was absolutely crucial to the mission." As it is, the spacecraft has already beamed down more than a terabyte of brightness measurements. Kepler astronomers have enough data to keep them busy for decades.
NASA is planning to release results from Kepler regularly through November 2013, when the mission is scheduled to end. The most sought-after results still lie ahead. After Jenkins and his colleagues have weeded out sunspots and other planet poseurs from the data, Marcy and other astronomers use the Doppler wobble method with terrestrial telescopes to verify that the remaining planet candidates, or "objects of interest," are indeed planets. The process will demand at least three years to find a completely Earth-like planet: one that is in a yearlong, Earth-like orbit around a star just like the sun.
The grand prize for Kepler would be to find a world just like ours—no bigger, no smaller—orbiting a sunlike star in an orbit the same size as Earth's. That challenge lies at the very edge of Kepler's remarkable capabilities. And it is definitely beyond the reach of even the most powerful Earth-bound telescopes.
Doppler measurements have been used to confirm the existence of Earth-size planets discovered by Kepler in habitable zones around stars smaller and dimmer than the sun. Kepler-10 b is one such example. But Doppler measurements won't be able to confirm a true twin of Earth: A planet's gravitational effect on its star depends on their relative mass and the distance between them. An Earth-size world close to a lightweight, dim star is much easier to find than a planet more like our own. Marcy laments, "We won't ever definitely verify or measure the density of true Earth clones that orbit as far as our own Earth does from the sun."
There is a chance, though, that Kepler will get lucky. If the spacecraft finds an Earth-like planet circling a star orbited by at least one other planet, Kepler's data alone could be used to determine the masses of the planets without measuring the star's wobble. The gravitational interactions of the two planets would affect the timing of their orbits, which is something Kepler could measure. And the gravitational interactions, in turn, would reveal the planets' masses. The Kepler team has already pulled off that feat for two planets orbiting a star called Kepler-9, about 2,000 light-years from Earth. The planets orbiting Kepler-9, astronomers determined, are such low density they would float if placed in a giant bathtub filled with water. Nobody understands the nature of these bloated worlds. "It's a beautiful, beautiful example of what Kepler can do," says Natalie Batalha, the mission's deputy science team leader.
Kepler may well find planets that closely resemble Earth. Or it may continue to upend our notions of what is out there. "All planetary systems were supposed to look like ours," Borucki reminds me. "The planets we're finding are in the wrong places! And their orbits are unlike anything anyone predicted. Now, that's a warning. That tells you we don't know how to predict what's out there."
Kepler has proved that planets are the rule in the galaxy, not the exception, but it won't tell us if any of those planets actually harbor life. That's a question for future missions, and NASA has plans for an ambitious one. Called the Terrestrial Planet Finder, it would study the atmospheres of exoplanets and search for the chemical signatures of life: oxygen, water vapor, carbon dioxide. But congressional budget cuts have left the mission in limbo for now. It's a project sorely in need of its own Bill Borucki.
"If there is no life in the entire galaxy, that would still be pretty profound for mankind," Borucki says as we sit in his office. "So I think this is the most important problem we can attack. And I'm going to have to leave it to my grandkids."
Despite Kepler's jaw-dropping success, Borucki still frets about the mission, and one of my last questions touches a nerve. "By the end of Kepler's mission, three years from now . . ." I begin to ask, but Borucki doesn't let me finish.
"No!" he says, leaning forward over his desk, his soft voice rising in volume. "It's going to go on at least six years. Don't let people tell you that just because it's funded for three and a half years it's going to quit. It is not. We're going to get the money and continue for six years. The program managers don't believe that, but I believe it."
Then, pausing after each word and rapping his knuckles on his desk for emphasis, he says, "You—can't—shut—the mission—down. It's working, and as we look longer and longer, we'll find more and more small planets. Do you want to throw away an opportunity to find a huge number of planets in the galaxy because you didn't come up with the money? It's the only mission in the foreseeable future that can do this job and you're gonna turn it off? Nobody's gonna turn it off."
Kepler-10 b
About three-quarters of the planets discovered beyond our solar system are giant, gassy worlds. Several reside at the right distance from their stars to support liquid water, but balmy temperatures do not guarantee pleasurable conditions. Layer upon layer of gases can create pressure strong enough to crush a Brink's truck like a paper cup, and the lack of terra firma leaves little place for life to take hold. In January Kepler astronomers announced the discovery of the first definitively rocky planet outside our solar system, Kepler-10 b. At 1.4 times the diameter of Earth, it also happens to be the smallest exoplanet yet measured. Unfortunately, rocky does not ensure habitable either. Daytime temperatures on Kepler-10 b approach 3000 degrees Fahrenheit. The good news is that Kepler's latest results include 117 candidates at or below the size of Kepler-10 b and 23 smaller than Earth, strongly suggesting that the planet-hunting probe should soon find small, rocky exoplanets in kinder climates.
NN Ser (ab) c & d
About half the stars in the universe are gravitationally bound to a companion star. Coexisting with the heat and gravity of two suns is difficult for planets—especially for the pair discovered last year orbiting the binary star system
NN Serpentis, also called NN Ser (ab). Astronomers believe that the exoplanets, called NN Ser (ab) c and d, may have survived a cataclysmic event several million years ago, when one of their host stars swelled to 200 times the diameter of the sun, temporarily enveloping the planets.
Or perhaps the planets actually formed from material cast off during that expansion, a theory that would overturn our conventional understanding that planets and stars form together
at the same time.
By 2010 astronomers had discovered 54 stars hosting multiple planets, yet none of these planetary systems much resembled our own. In February researchers announced the discovery of a planetary system that looks slightly more familiar, Kepler-11. Six planets orbit a star roughly the size of the sun, and like our solar system, the outer
planets are gas giants while the inner ones seem to be denser. But there's a surprising twist: Five of the six planets are packed into orbits smaller than that of Mercury, their paths almost perfectly aligned in the same plane. Astronomers are at loss to explain how all the planets managed this configuration without crashing into each other.
Of the 1,235 planet candidates
announced in February, KOI 326.01 truly stood out. Seemingly Earth-size and just a little hotter (140°F), it appeared to be the most Earth-like planet ever discovered—until a month later, when scientists made a less exciting discovery: an error in their data. Punching in the correct brightness of the planet's star revealed a planet bigger and hotter than Earth. As disappointing as the news was, it was par for the course; picking out the tiny signatures of planets within trillions of bytes of data is notoriously difficult. In fact, about 10 percent of all "Kepler Objects of Interest" (KOIs) will end up as false positives, mission scientists say, while others will suffer demotions like that of KOI 326.01 based on follow-up observations. On the upside, Kepler researchers have discovered four other potential Earth-size planets with palatable temperatures.
GJ 1214 b
GJ 1214 b is nearly three times as wide and some 300ºF hotter than Earth, but it may nonetheless have an abundance of life's most prized commodity: liquid water. Telescopic observations of the planet as it passed in front of its star indicated that GJ 1214 b may be covered with vast, hot oceans, which would remain liquid because of the extreme pressure exerted by the overlying atmosphere. GJ 1214 b also holds the title for orbiting closest to its star—a mere 1.3 million miles away, 1/70 the distance between Earth and the sun. Even though the star GJ 1214 is a puny red dwarf, it would still look 17 times larger from GJ 1214 b than the sun does in our sky.
HD 80606 b
In our solar system, every planet except Mercury revolves around the sun in a nearly perfect circular orbit. In fact, theorists long believed that planetary orbits had to be circular. Nature apparently believes otherwise. Take the unusual planet HD 80606 b, a gas giant in the constellation Ursa Major discovered in 2001. Its elongated path around its parent star is more reminiscent of the orbit of a comet than that of a planet the size of Jupiter. HD 80606 b swings within 3 million miles of its star at its closest approach and reaches its most distant point, 81 million miles away, just 56 Earth-days later. In 2009 astronomers reported that temperatures on the planet jumped from 1000°F to 2200°F in six hours.
—Andrew Grant
Theoretical physicist Richard Obousy can easily imagine how the discovery might unfold: A successor mission to Kepler locates a planet that resembles our own in size and temperature. Further study shows intriguing chemical similarities as well—oxygen, water vapor, and methane, possible signatures of habitability, are all present. The next impulse would be to send a probe there to take a look. Does this Earth twin have oceans and continents? Signs of life? Signs of civilization?
Unfortunately, sending a spacecraft to even the closest star (Alpha Centauri, 4.3 light-years away) would take 70,000 years at the speed of today's chemical rockets. Crossing the vast gulf of interstellar space within a human lifespan will require a whole new wave of research. Obousy hopes to start that wave with Project Icarus, a collaboration between several dozen scattered scientists who are brainstorming concepts for the spacecraft of the future.
One possibility is a rocket powered by nuclear fusion, essentially a sequence of hydrogen bombs. The ship would be constructed in Earth orbit and then sent to Uranus to mine helium-3 fuel from the planet's atmosphere. "That's really pushing technology to the edge," says Marc Millis, head of the Tau Zero Foundation. He should know: Millis headed NASA's Breakthrough Propulsion Physics project, which folded in 2002 without achieving the kind
of radical advances they aspired to.
"If you look at the things within known physics that could actually take us to the stars, there are few options," says Les Johnson, deputy manager of NASA's Advanced Concepts Office. There are ultrathin solar sails pushed by the sun; Japan and the United States have already launched simple versions of this technology. The Ikaros sail, which took flight in May of last year, is only 14 square meters, however. Interstellar journeys would require a sail as big as Texas and orbiting lasers to provide a light boost; even then, the trip would take centuries, Johnson says.
There are also schemes to power electrical thrusters with nuclear fission—which, unlike fusion, is something we know how to do today. "Those designs are in NASA's hands," yellowing in a drawer somewhere, Millis says. The
neglect stems not only from technical challenges, he explains, but also from political reluctance to put nuclear reactors in space.
And then there is the best kind of rocket physicists can imagine: engines that smash together matter and antimatter to produce pure energy. "The energy density is one to two orders of magnitude better than nuclear," Millis says. But antimatter is no free lunch: You have to put in as much energy to make it in the first place as you get back when you annihilate it. Just synthesizing the requisite amount of antimatter (never mind figuring out how to use it in a rocket engine) lies far beyond humankind's current capabilities.
No matter what the approach, going fast and far will require heaps of cash. Right now the money is not there; Tau Zero and Icarus are
essentially self-funded projects run by hard-core enthusiasts. To make real progress on interstellar travel, NASA and the other space agencies will need a galvanizing spark. Like finding that second Earth.—Andrew Moseman
Discover Magazine


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Umbilical hernia repair is surgery to repair an umbilical hernia. An umbilical hernia is a sac (pouch) formed from the inner lining of your belly (abdominal cavity) that pushes through a hole in the abdominal wall at the belly button.
You will probably receive general anesthesia (asleep and pain-free) for this surgery. If your hernia is small, you may receive spinal or epidural block anesthesia and medicine to relax you. You will be awake but pain-free.
Your surgeon will make a surgical cut under your belly button.
Umbilical hernia can also be repaired using a laparoscope, a thin, lighted tube that lets the doctor see inside your belly. The laparoscope will be inserted through one of the cuts and instruments will be inserted through the other cuts.
If your child is having this surgery, the surgeon will discuss the type of anesthesia your child will receive. The surgeon will also describe how the surgery will be done.
Umbilical hernias are fairly common. A hernia at birth will push the belly button out. It shows more when a baby cries because the pressure from crying makes the hernia bulge out more.
In infants, the problem is not usually treated with surgery. Most of the time, the umbilical hernia shrinks and closes on its own by the time a child is 3 or 4 years old.
Umbilical hernia repair may be needed in children for these reasons:
Umbilical hernias are fairly common in adults. They are seen more in overweight people and in women, especially after pregnancy. They tend to get bigger over time.
Smaller hernias with no symptoms sometimes can be watched. Surgery may pose greater risks for patients with serious medical problems.
Without surgery, there is a risk that some fat or part of the intestine will get stuck (incarcerated) in the hernia and become impossible to push back in. This is usually painful. If the blood supply to this area is cut off (strangulation), urgent surgery is needed. You may experience nausea or vomiting, and the bulging area may turn blue or a darker color.
To avoid this problem, surgeons often recommend repairing the umbilical hernia in adults. Surgery is also used for hernias that are getting larger or are painful. Surgery secures the weakened abdominal wall tissue (fascia) and closes any holes.
Get medical care right away if you have a hernia that does not get smaller when you are lying down or that you cannot push back in.
The risks of surgery for umbilical hernia are usually very low, unless the patient also has other serious medical problems.
Risks for any anesthesia are:
Risks for any surgery are:
A specific risk of umbilical hernia surgery is injury to the bowel (large intestine). This is rare.
Your surgeon or anesthesia doctor will see you and give you instructions for you or your child.
An anesthesiologist will discuss your (or your child's) medical history to determine the right amount and type of anesthesia to use. You or your child may be asked to stop eating and drinking 6 hours before surgery. Make sure you tell your doctor or nurse about any medications, allergies, or history of bleeding problems.
Several days before surgery, you may be asked to stop taking:
Most umbilical hernia repairs are done on an outpatient basis, which means that you will likely go home on the same day. Some repairs may require a short hospital stay if the hernia is very large.
After surgery, your doctor and nurse will monitor your vital signs (pulse, blood pressure, and breathing). You will stay in the recovery area until you are stable. Your doctor will prescribe pain medicine if you need it.
Your doctor or nurse will show you how to care for your or your child's incision at home. You or your child should be able to do all of your normal activities in 2 - 4 weeks.
There is always a chance that the hernia can come back. However, for healthy patients, the risk of it coming back is very low.
Malangoni MA, Rosen MJ. Hernias. In: Townsend CM, Beauchamp RD, Evers BM, Mattox KL, eds. Sabiston Textbook of Surgery. 19th ed. Philadelphia, PA: Elsevier Saunders; 2012:chap 46.

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Phosphodiesterase-5 inhibitors (PDE-5 inhibitors) are
taken before you plan to be sexually active. PDE-5 inhibitors
will result in an erection only if you are sexually stimulated.
During an erection, blood fills tissue
in the penis, causing it to enlarge and become stiff. The PDE-5 inhibitors
relax smooth muscle, allowing the penis to fill with blood.
PDE-5 inhibitors can be used when an
erection problem (erectile dysfunction) is caused
PDE-5 inhibitors are also used in men who have an enlarged prostate, or benign prostatic hyperplasia (BPH), to help with symptoms.
All PDE-5 inhibitors are very
effective in treating erection problems, no matter what is causing the erection
How well the PDE-5 inhibitors work depends on the
person taking it and how much he takes. But these medicines seem to work in
about 7 out of 10 men who use them for erection problems.2
The medicine needs to be taken before you plan to have sex:
PDE-5 inhibitors, such
as tadalafil, may help reduce BPH symptoms in men (whether or not they have erection problems). Men have about a 5-point reduction in their symptom scores on the American Urological Association symptom index.3
All medicines have side effects. But many people don't feel the side effects, or they are able to deal with them. Ask your pharmacist about the side effects of each medicine you take. Side effects are also listed in the information that comes with your medicine.
Here are some important things to think about:
Call 911 or other emergency services right away if you have:
Call your doctor right away if you have:
Common side effects of this medicine include:
See Drug Reference for a full list of side effects.
(Drug Reference is not available in all systems.)
Do not take PDE-5
inhibitors if you are taking any medicine that contains
nitrates, such as nitroglycerin. This combination can
lower blood pressure dangerously, possibly leading to a
stroke, a
heart attack, or death. If you are taking a medicine
called an alpha-blocker—used to treat high blood pressure or an enlarged
prostate—check with your doctor before you take a PDE-5
It is important that you tell all of your health care providers that you take this medicine. If you need emergency medical care for a heart problem, your health care provider will need to know when you last took this medicine..
These medicines may be linked to a rare type of vision loss. If you already have eye disease or vision problems (such as
glaucoma, or
retinopathy), be sure to talk with
your eye doctor (ophthalmologist) before you start taking any PDE-5
Do not drink alcohol to excess (for example, 5 glasses of wine
or 5 shots of whiskey) when you are taking tadalafil. Large amounts of alcohol can increase your chances of getting a headache or feeling dizzy, increasing your heart rate, or lowering
your blood pressure.
Talk with your doctor about whether medicines for erection problems are safe for you if you:
Sexual activity is exercise. If you have a heart condition
and have not been sexually active for a while, talk with your doctor to make sure you can safely engage in sexual activity.
If you eat a high-fat meal
around the time you take sildenafil
or vardenafil, the medicine does not work as well as it otherwise would. So many doctors recommend you take this medicine on an empty
stomach. Tadalafil works without regard to what you eat.
If you are thinking about taking PDE-5 inhibitors for erection problems, it is important
to include your partner in your discussions.
PDE-5 inhibitors can be prescribed by any doctor. You do
not have to see a doctor who specializes in sexual health. PDE-5 inhibitors are
also available on the Internet. If you obtain the medicine through this route,
you should discuss its use with your doctor.
PDE-5 inhibitors may
affect fertility. If you are trying to have a child, you may want to ask your
doctor about how this medicine can affect your chances of conceiving.
Medicine is one of the many tools your doctor has to treat a health problem. Taking medicine as your doctor suggests will improve your health and may prevent future problems. If you don't take your medicines properly, you may be putting your health (and perhaps your life) at risk.
There are many reasons why people have trouble taking their medicine. But in most cases, there is something you can do. For suggestions on how to work around common problems, see the topic Taking Medicines as Prescribed.
Follow-up care is a key part of your treatment and safety. Be sure to make and go to all appointments, and call your doctor if you are having problems. It's also a good idea to know your test results and keep a list of the medicines you take.
Complete the new medication information form (PDF)(What is a PDF document?) to help you understand this medication.
CitationsAgency for Healthcare Research and Quality (2009). Diagnosis and Treatment of Erectile Dysfunction. Evidence Report/Technology Assessment No. 171 [AHRQ Publication No. 08(09)-E016]. Rockville, MD: Agency for Healthcare Research and Quality. Also available online: http://www.ahrq.gov/downloads/pub/evidence/pdf/erectiledys/erecdys.pdf.Burnett AL (2012). Evaluation and management of erectile dysfunction. In AJ Wein et al., eds., Campbell-Walsh Urology, 10th ed., vol. 1, pp. 721–748. Philadelphia: Saunders.Liu L, et al. (2011). Phosphodiesterase-5 inhibitors for lower urinary tract symptoms secondary to benign prostatic hyperplasia: A systematic review and meta-analysis. Urology, 77(1): 123–130.
Last Revised:
May 14, 2012
Healthwise Staff
Medical Review:
E. Gregory Thompson, MD - Internal Medicine & E. Gregory Thompson, MD - Internal Medicine & Christopher G. Wood, MD, FACS - Urology, Oncology & Christopher G. Wood, MD, FACS - Urology, Oncology
This information does not replace the advice of a doctor. Healthwise, Incorporated disclaims any warranty or liability for your use of this information. Your use of this information means that you agree to the Terms of Use. How this information was developed to help you make better health decisions.
To learn more visit Healthwise.org
© 1995-2012 Healthwise, Incorporated. Healthwise, Healthwise for every health decision, and the Healthwise logo are trademarks of Healthwise, Incorporated.


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Why Join The Google+ Real Estate Communities?
Many of you have as part of your daily ritual to log into Active Rain and read your favorite blog authors. There is certainly nothing wrong with that but if you are a real estate agent, mortgage broker, home inspector, stager, lawyer or anything else connected to the real estate industry you are missing out on a lot of great content if that is all you do everyday!
There are many great places online where you can participate in a real estate forum just like Active Rain. Many of you know what a huge avocate I am for using Google plus for your real estate business. What you may not know is that Google Plus has some exceptional real estate communities where some fantastic articles are posted daily.
I happen to own one of the three largest real estate communities on Google Plus. At the present time there are over 10,500 members. While that pails in comparison to the member numbers here on Active Rain the group is highly engaged. The difference is that you will get to see articles from outside the Active Rain forum that often times are truly exceptional.
You will see consumer related content like buying and selling tips, as well as agent centric content. Often times there are great articles covering important social media or SEO practices. There are writers who post information on the latest tool or technology to use in your business. If you click on the link above this will take you right to the group where you can opt in if you like.
If you want to get more eyeballs on your content the Google+ Real Estate community is a great place to increase your online visibility.
Examples of Great Real Estate Content
Do you want to see some examples of what you will see in the Google Plus Real Estate Community? I have hand picked some of the articles that you will find in the group. One thing I will make clear is the group does not support content that the majority of the members would find interesting.
For example we do not allow real estate listings, market reports or localized articles. We want the majority of the members to enjoy coming to the group daily and not be bogged down by content that doesn't appeal to the masses.
The group is highly moderated and daily we remove content that does not follow the community guidelines. Those that spam the group are given a warning and if they continue to break the rules they are banned. This ensures the community remains a positive and productive environment.
If you are not a member of the Google Plus Real Estate community here are the kind of articles you will discover.
Are Zillow Real Estate Values (Zestimates) Accurate?
Zillow is the bane of all real estate agents! While Zillow offers many excellent features on their real estate site their estimate of value feature is not one of them. On a daily basis countless Realtors have to explain to buyer and seller clients that Zillow Estimates are not accurate. They cause confusion with the general public and force real estate agents to constantly explain why they are not correct. In the article you will see an in-depth discussion covering the reasons why the "zestimate of value" is a bogus means of property valuation. For full disclosure this article on Zillows inacuracy was written by yours truly. If you have ever had a difficult client that thinks the Zillow value is a good measuring stick for market value send them this article. It should clear up quite a bit of confusion as to why Zillow does not hit the mark most of the time.
Home Improvement Projects That Increase a Homes Value
If you are A Realtor I am quite sure you have ran into the seller that believes every single improvement they make to their homes amounts to a comparative increase in value. As real estate agents we know this line of thinking is wrong. There are some improvements that are really worth making and others that offer very little return especially when you are selling. Kyle Hiscock and agent in New Your writes an excellent article about excellent improvements when selling a home. It only makes sense to spend your money wisely when you are about to sell real estate. Kyle does a great job explaining some of the best home improvements.
The Final Real Estate Walk Through
One of the standard procedures in a real estate sale is the buyer having the opportunity to walk through the property one last time before they head to the closing table. The purpose of course is to make sure the condition of the property is the same as when they wrote the purchase contract. Bad suprises are never fun so doing a final walkthrough is vital. Tammy Emineth a favorite hear at Active Rain does a great job summarizing what to look for at the final walk through. Tammy provides an excellent check list of things that are often overlooked by home buyers and their agents.
How a Realtor Determines Market Value?
One of the biggest weaknesses of many real estate agents is their ability to price a home correctly. Pricing a home is both a skill and an art. If you don't follow some basic principles in market valuation it is easy to make mistakes. There is a reason why you see countless properties reduced everyday. The vast majority of Realtors don't know how to nail the right listing price. Joe Manuasa does a fantastic job explaining real estate market value. In fact I was forunate enough to have Joe ask me to come by and offer some opinions for his article. Joe and I had a very neat question and answer session. If you are a new real estate agent this one would be a great read for you.
How to Sell a Luxury Home?
While selling a luxury home has similarities to selling other types of properties there are some differences as well. Debbie Drummond a Realtor from Las Vegas drops by to offer her insight on how to market and sell luxury properties. The article is filled with great tips and insight into marketing to those at the upper end of the market.
Should I Price My Home With Negotiating Room?
One of the age old real estate questions is how much room you should have in your asking price vs what the actual market value of the home happens to be. I am sure if you asked ten Realtors you would probably get a variety of answers. Without a doubt you would have numerous home owners who have pre concieved notions on how much negotiating room there should be. As is often the case the seller would be wrong. Lynn Pineda cuts through all the nonsense and gives us some straight answers on how a home should be priced. Lynn gets right to the point that you probably are putting too much of a gap between the list price and the actual value of your property and why this is a mistake.
These are just a few of the examples of some of the valueable information you will find in the Google Plus Real Estate community. We are always looking for new members and would love to see some of you here at Active Rain come by and take a look. Whether you have a Wordpress Blog, Blogger Blog or an Active Rain blog your content is welcome as long as you follow the guidelines of the community!
Why Use Google+?
If you have been under a rock for the last few years and don't understand why you should be using Google Plus as a major part of your social media strategy, I would encourage you to read this brilliant article by Mark Traphagen on why Real Estate agents should use Google+. Mark clearly explains the benefits how search plus your world can benefit your real estate business. With so many people logging into Google daily and asking questions it only makes sense that you should want to be in front of those people!
When you use Google Plus and create a large following of people all of those see your content in search when they are logged into Google. Most people stay logged in so the you are increasing your odds of being found in search when using Google Plus as a business tool. Mark delves into this topic like no other. If you don't do anything else take the time to read his article. I gaurantee you will be thinking to yourself I should be using Google Plus more than I am.
Don't forget to come by and connect at the Google Plus Real Estate community - you will be glad you did!


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Every compressed-air system begins with a compressor - the source of air flow for all the downstream equipment and processes. The main parameters of any air compressor are capacity, pressure, horsepower, and duty cycle. It is important to remember that capacity does the work; pressure affects the rate at which work is done. Adjusting an air compressor's discharge pressure does not change the compressor's capacity - even though many people seem to believe it will.
There are a number of basic air compressor designs - and variations of them - on the market today. They all fall into two general categories: positive displacement and dynamic. Although the operating specifications for two different types of air compressors may be very similar on the surface, other installation and performance factors can make one design superior to the other in a real-world application. Let's review some of the basic designs and terminology.
Reciprocating compressors
Reciprocating compressors are positive-displacement units that trap a charge of air and then physically reduce the space that confines it, causing its pressure to increase. Reciprocating units, commonly called piston compressors, use a piston, cylinder, and valve arrangement. Their operation is very similar to the familiar internal-combustion engine, but they simply trap and compress the air without adding fuel to explode it. Note that whenever air is compressed, heat is generated. Proper cooling of the internal parts of any air compressor is a critical part of its design.
There are three basic selection decisions that must be made about reciprocating compressors:
- single- or double-acting operation,
- single- or multi-stage configuration, and
- air or water cooling.
In a single-acting piston compressor, the piston only compresses air in one direction of its stroke. In a double-acting model, the piston compresses air with both directions of its stroke. Obviously, because both strokes perform work, a double-acting compressor is more efficient (in moving a volume of air per input hp) than a comparable-size single-acting unit.
A single-stage unit compresses air from inlet to discharge pressure in one operation. A multi-stage unit compresses from inlet to discharge pressure in two or more operations - generally passing the air through an intercooler to remove some of the heat of compression between each stage. This saves power and keeps the compressor's internal operating temperatures lower.
In air-cooled compressors, ambient air circulates around the compressor cylinders and finned heads to provide cooling. Heat transfers through the metal to the air. Air-cooled units are generally designed for 50% to 75% duty cycles, depending on the particular units and their application. In water-cooled compressors, integral water jackets surround the cylinders and heads. Heat transfers through the metal to the water - more effectively than through metal to air. Thus, water-cooled reciprocating units reduce internal temperatures more efficiently than comparable air-cooled units.
Most air-compressor manufacturers promote the two-stage compressor as the optimum machine for producing 100-psi class air - the base pressure level in most industrial plants - providing the best efficiency per dollar cost with adequate reliability of internal working parts. For a reciprocating compressor to be categorized as continuous duty, it is generally agreed that it must be double acting and water cooled. Double-acting, water-cooled reciprocating compressors are offered in a variety of styles that combine efficient air compression with durability and reliability. However, they also are heavy and bulky, making them relatively expensive to install. They generally have more-significant unbalanced forces, which combines with their size to require a special foundation and support.
When they meet selection criteria such as capacity, weight, size, and price, single- and two-stage single-acting reciprocating units are a good choice - particularly in the 50- to 150-psig pressure ranges. (Three-stage reciprocating units are offered, but generally are used for pressures above 250 psig.)
Oil-cooled rotary-screw compressors
The rotary-screw compressor is another positive-displacement machine. In an analogy with the reciprocating compressor, Figure 1, the male rotor is like a piston, pushing air along the female rotor, which is like the cylinder. The sealing strips are like piston rings, and air is compressed against the stationary end plate, which is like the bottom of the cylinder. This design has been around for about 50 years. However, until the mid 1970s, it was considered suitable only for engine-driven portables and small-horsepower electric-motor units because of low efficiency (the ratio of compressed-air delivery to power cost).
In the 1970s, development began on two-stage rotary screw compressors for pressures up to 250 psi. Rotor-profile development during the 1970s, 1980s, and early 1990s has led the oil-cooled rotary-screw design to become the significant choice in electric-motor-driven, lubricated, industrial air compressors, particularly in sizes from 20 to 300 hp.
Then, a significant breakthrough in air-end design occurred. The introduction of the unsymmetrical profile resulted in an efficiency improvement of approximately 15%. This improvement was significant enough to make the oil-cooled rotary-screw compressor competitive in the larger-horsepower sizes for continuous duty. It has almost the same efficiency as the single-stage double-acting units and smaller centrifugal compressors.
Two-stage rotary-screw compressors can approach and sometimes equal the full-load performance of two-stage reciprocating units in 100-psig class service. Today, two-stage oil-cooled rotary-screw compressors are frequently used in the 150- to 400-psia pressure range. They also are used for 100-psi service with significant power savings. Two stages offer advantages associated with lower compression ratio per stage. Reduced pressure differential across the rotors minimizes blow-by and significantly reduces thrust-bearing loads. (Obviously two-stage units require two air ends, which increase the initial cost.)
The unique characteristic of this compressor is that it is cooled by oil. Oil injected into the air stream absorbs the heat of compression while it is being generated. The heated oil then is taken to an air- or water-cooled heat exchanger for cooling. Because the cooling takes place right inside the compressor, the working parts are never subjected to extreme operating temperatures. The cooling oil never is cracked nor burnt. No matter what the load on the compressor is, there are no hot spots inside the airend. The resulting absence of wear produces trouble-free service and high efficiency. In other words, oil-cooled rotary-screw compressors can run at full load and full pressure -twenty-four hours a day, seven days a week. This compressor's useful life in operating hours and its maintenance cost per hour will be the same as under any other load condition.
Continuous duty
The availability of continuous-duty air-cooled compressors (particularly in large sizes) offers a great deal of flexibility for installing them. Such compressors can be mounted on any surface that will support their static weight. In many facilities, great savings also are available in piping cost, compared to other types of systems. These compressors lend themselves to either the central- or departmental-compressor system concept. Units are available with electric motor and engine drives - on bases, on skids, on wheels, etc.
Compared to other types of continuous-duty air compressors, oil-cooled rotary-screw compressors offer a number of advantages:
- Oil cooling holds internal temperatures to an optimum level. As a result, discharge air is relatively cool -no more than about 180° F higher than ambient.
- Discharge air is clean - free from burned oil or carbon.
- The rotary design lends itself to higher speeds, particularly in the larger sizes. Consequently, larger flow capacity is available from compressors with physically smaller envelopes - providing significant savings on floor space and foundation requirements.
- Because of their compact size and inherent quiet-running characteristics, it is relatively easy to suppress noise. Electric-motor-driven models are commercially available rated from 75 to 85 dB at one meter per the CAGI Pneurop Test Code.
- Most models have fewer moving parts, and those parts run under more ideal conditions - resulting in lower temperatures and less vibration.
- Fewer parts make it easier to stock them for the rotary designs, and the machines are easier to work on.
In summary, oil-cooled rotary-screw compressors offer users a continuous-duty source of compressed air in a neat, compact package that has low initial cost, maximum flexibility of installation, and easy maintenance.
Non-lubricated rotary screw and lobe
In addition to the non-lubricated reciprocating compressors that have become so common over the years, there are several versions of non-lubricated positive-displacement lobe or screw rotary compressors. These units are referred to as clearance-type compressors because the internal parts do not contact each other, so they require no lubrication in the compression chamber. Cooling is accomplished through the cylinder walls via water jackets.
The lobes or screws do not drive one another either; they are driven by some type of gear arrangement instead. This drive system also acts as a timing gear to maintain the rotor or lobe profile relationship accurately. Lubricant for the drive train must be confined to the bearing and gear area - and not allowed to get into the compression chamber.
In this basic design, there is a constant leakage rate for any fixed set of conditions. The critical internal clearances are between end covers and the rotor, between the rotor lobes, and between the rotor OD and the cylinder ID. These gaps, combined with no injected oil to help with sealing, are the main reasons why two stages are required for these units to produce acceptable efficiencies in 100-psi class applications.
Because these are rotary units, they enjoy all the advantages of rotaries over similar-sized non-lubricated reciprocating units:
- compact size,
- smooth delivery of cool air,
- ease of installation, and
- simple (but critical) maintenance
They also have some disadvantages, depending on the specific type of compressor and its duty cycle:
- more sensitive to dirty inlet air,
- lower efficiency - resulting in higher power cost, and
- any repair work is more sophisticated and requires specialized training, which the user may not have nor want to have. This means repair work will probably have to be performed by the distributor or the manufacturer.
Sliding-vane rotary compressors
Oil-cooled sliding-vane compressors, Figure 2, operate as other positive-displacement compressors do by trapping a charge of intake air - in this case, between the vanes. As the eccentric rotor turns, the vanes are forced into the rotor slots, shrinking the size of the cell holding the trapped air. The air is compressed to full discharge pressure when it reaches the outlet port. The heat of compression is removed by cooling oil sprayed right into the air while it is being compressed. The same oil helps with sealing the vane tips.
For decades, oil-cooled, sliding-vane rotary compressors have been popular for continuous-duty applications. Their design has a number of unique characteristics:
- light weight - yet continuous rating,
- integrated and compact configuration,
- efficient production of compressed air at relatively low rotary speeds,
- smooth operation with little vibration,
- extremely quiet operation,
- coolest possible discharge air, and
- few wearing parts, making the machine easy and economical to repair.
However, the oil-cooled rotary-vane design in its single-stage configuration is limited in capacity. Bending stress applied to the vanes is the problem. The speed, size, and weight of the vanes must be limited for the machine to be durable. Because of this, oil-cooled rotary-vane compressors generally are applied only in a size range between 2 and 100 hp.
Lubricated or lube-free?
Two fundamental groups of compressor types are lubricated and lube-free. Lubricated compressors use oil to reduce friction between moving parts. As a result, some oil is entrained in the air being compressed. The entrained oil must be removed from or tolerated by the downstream system.
Lube-free compressors use no oil in the airend, and thus add no oil to the compressed air they produce.
Power and efficiency
Brake horsepower is the input power required at the compressor input shaft for a specific speed, capacity, and pressure condition.
Motor or engine horsepower is the nominal rating of the prime mover.
The service factor is the additional power built into an electric motor above its nominal rating - expressed as percent. Within the service factor, the brake horsepower driving an air compressor can be higher than the motor's nominal horsepower.
The power efficiency of a compressor is the ratio of the air delivered by the compressor and its input electrical requirements. Efficiency usually is expressed as brake horsepower per 100 cfm of delivered air.
Water-cooled rotary screws
Another version of oil-free rotary-screw compressors is a single-stage design that uses water injection to cool and seal the rotors during compression. The bearings and drive gears are lubricated with oil and sealed from the compression chamber. These units serve a selected market and are a special design. In some applications, care must be taken to avoid the build-up of bacteria in the water.
Dynamic air compressors
Dynamic, or centrifugal compressors, Figure 3, are dissimilar to the positive-displacement machines already discussed because they raise the pressure of air by converting the energy of its velocity into pressure. First, rapidly rotating impellers (similar to fans) accelerate the air. Then, the fast flowing air passes through a diffuser section that converts its velocity head into pressure by directing it into a volute.
Because the centrifugal is a mass flow compressor, it has a limited stable operating range. This has a large effect on economic operation or bhp/100 cfm delivered at part load. Minimum turn-down capacities for centrifugals may vary from 20% to 30% of full load, depending on impeller design, number of stages, etc.
There are limits to the pressure rise that can be achieved in a single stage by a centrifugal compressor - due to both physical and economical restraints - so two- to four-stage units are built that incorporate one to three water-cooled intercoolers. Cooling the air between stages reduces the power required to compress the air further, resulting in more efficient operation. Intercooling actually may permit the desired compression to be accomplished in fewer stages.
The centrifugal compressor is definitely a continuous-duty unit because its service life is unaffected by full-load operation. However, it is also a relatively sensitive machine because it operates at high speeds - often as high as 50,000 rpm. Ambient factors which affect flow are altitude, inlet air temperature, and the relative humidity of inlet air. The operating life of this type of unit is primarily determined by the amount of entrained liquids and solids carried into the unit at the inlet - and the quality of the cooling water. As in all machinery, correct installation and maintenance is critical to the efficient production of compressed air and reaching a satisfactory operating life.
When a facility requires a continuous-duty, high-volume (2,000 to 25,000 cfm) supply of non-lubricated air, the centrifugal compressor is one of the best choices. In fact, it is the only choice in sizes above 1,000 hp. Whether or not it fits the installation best is another question to be answered after analyzing the job conditions. In any event, when correctly applied, installed, and maintained, a centrifugal compressor offers a reliable, continuous source of compressed air.
Advantages and disadvantages
After reviewing the comments on air compressors in this article, one conclusion is fairly obvious: each design has advantages and disadvantages which must be matched to a specific application. The table on this page summarizes a number of selection factors for the most common basic designs. Other factors, such as air quality and installation requirements, are difficult to quantify. The unavoidable cost factor - initial, operating, and maintenance - is noted with them in the following text.
Double-acting reciprocating - Advantages: highest efficiency, longest service life, field serviceability. Disadvantages: highest initial cost, high installation cost, high maintenance cost.
Oil-flooded, single-stage rotary screw - Advantages: low initial cost, low maintenance cost, packaged design. Disadvantage: low efficiency.
Oil-flooded, two-stage rotary screw - Advantages: higher efficiency, simple packaged design, same low maintenance cost. Disadvantage: higher initial cost.
Oil-free rotary screw - Advantages: high-quality air, moderate efficiency, simple packaged design. Disadvantage: higher initial cost.
Centrifugal - Advantages: the only type available above 600 hp, high-quality air, moderate efficiency, longer service life than other rotaries. Disadvantages: higher initial cost, must be water cooled, air flow is sensitive to changes in ambient conditions.
Importance of capacity controls
Many compressed-air conservation program on the demand side target such issues as:
- identifying and repairing air leaks,
- eliminating open blowing,
- fixing malfunctioning condensate drains, and
- managing all potential inappropriate uses.
When these programs are completed successfully, often it is found that the facility consumes less compressed air for production, but electrical energy consumption does not go down proportionally. The reason: without appropriate capacity controls operating correctly on compressors, it is impossible to effectively translate lower air use into lower electrical energy input.
When working effectively, compressor-unloading controls should:
- match air supply to demand when needed,
- eliminate or minimize system overpressure,
- maintain the necessary minimum acceptable operating system pressure,
- reduce the input power cost to the optimum point proportional to the air flow demand, and
- turn off unneeded air compressors and bring them back on when required.
Regardless of the type of air compressor, the operating principles of capacity controls can be grouped into several basic categories. (Note that some will only perform on certain types of compressors.) Here are descriptions of these categories with some of the pros and cons of each.
Automatic start-stop control - This control simply starts and stops the electric motor or driver automatically. It can operate any type of compressor. A pressure switch usually accomplishes this function, shutting off the motor at the upper pressure limit, restarting it at the minimum system pressure.
Pro: the air compressor runs at its two most efficient modes, fully loaded and off.
Con: most AC electric motors can survive only a finite number of starts over a given time frame, primarily due to heat build up. This limits the application of automatic start-stop controls - particularly for motors larger than 10 to 25 hp.
Con: the compressor must run above minimum system pressure to hold that pressure.
Con: the system must have adequate air-storage capacity to perform satisfactorily.
Continuous-run controls (step type) - With these controls, the driver or electric motor runs continuously while the air compressor is unloaded in some manner to match supply to demand. System pressure usually commands the unloading arrangement. Continuous-run controls can be categorized as step or modulating type.
The most common is the two-step control which holds the compressor inlet either fully open or fully shut. Over the complete operational band, the compressor runs fully loaded (or at full flow) from the preset minimum pressure (or load point) to the preset maximum pressure (or no-load point). At the latter, the control shuts off air flow completely. The unit then runs at no flow and full idle until system pressure falls back to the load point. The control then goes immediately to full-flow capacity. A pressure switch typically actuates the two-step control, which can be either the primary control or part of a dual-control system on virtually every type of air compressor. (Some reciprocating compressors can be fitted with 3- and 5-step controls.)
Pro: the compressor runs at its two most efficient modes - full load and full idle - which results in the lowest possible input power cost. Full idle at lowest input power is accomplished almost immediately, except in the case of lubricated or lubricant-cooled rotary-screw compressors.
Con: both correct piping and adequate air storage are necessary to allow enough idle time over the operational pressure band to generate any significant energy savings.
Con: when two-step controls are misapplied, not only is there little or no power cost savings, but short cycling (i.e.: 20 sec. on/ 20 sec. off) can damage the equipment and shorten the life of normal wearing parts.
Con: too much backpressure in the interconnecting system can cause short cycling or ineffective unloading.
Con: at 85% to 95% loads, step controls consume some extra power because they have to compress at full capacity to a higher pressure just to hold a lower design system pressures.
Continuous-run controls (modulating) - These controls match supply to demand very accurately all along the operating band pressure range. Most incorporate some type of regulator, which in effect converts the operating pressure control band into a proportional band. If system pressure fluctuates as little as 1 psi, the modulating control immediately decreases or increases flow proportionally, depending on the signal. (This control generally is installed only on lubricant-cooled rotary-screw and centrifugal compressors.)
Pro: the minimum set system pressure draws the most power. As system demand falls, pressure rises, flow cuts back, and power usage also falls. This results in a savings at higher demand (and is the opposite of 2-step unloading where the power draw actually increases as system demand falls).
Pro: more efficient at high loads.
Pro: holds a relatively steady pressure when demand is stable, and responds responds quickly to any change.
Pro: does not depend on storage capacity to operate effectively.
Con: is generally more inefficient at lower loads.
Con: too much backpressure in the interconnecting piping can force multiple units into running on part load, when one or more could be shut off.
Controls for rotary screws
Industry's most commonly used air compressor in sizes above 30 hp today is the lubricant-cooled rotary-screw compressor. A significant number (80% to 85%) of these compressors use some form of modulating control as the primary unloading control or as the upper-range portion of a dual control. Two types of these controls for oil-injected rotary-screw compressors are throttled inlet and variable displacement.
In a throttled inlet control, the compressor's inlet valve is opened or closed to match supply to demand as sensed by a pressure regulator. The inlet valve modulates continuously and responds immediately in to any change in the sensed system pressure. In effect, flow capacity is controlled by restricting air intake. The control holds a constant system pressure with minimal valve movement at any given steady system demand.
Pro: smooth, non-cycling control of system pressure is easier on the power train and most other components.
Pro: is relatively efficient at loads from 60% to 100%.
Pro: will not short cycle, regardless of storage capacity and or piping.
Pro: simple to operate and maintain.
Pro: usually results in lower lubricant carryover in lubricated units.
Con: relatively inefficient at loads below 60%.
Con: backpressure must be overcome in order to reach full capacity.
Con: instant response may make the machine back down and unload, even when flow is needed for the base load.
Con: sensitivity and rapid reaction make correct piping and backpressure control necessary for optimum operation. (Note: this is true for all types of unloading controls.)
Variable-displacement controls
These controls for rotary-screw compressors all match output to demand by modifying or controlling the effective length of the rotor compression volume. The inlet pressure remains the same throughout the turn down, and the compression ratio stays relatively stable. This method of reducing flow without increasing compression ratios has a power advantage over modulating and/or 2-step controls in the operating range from 50% to full load.
The two most common of these unloading controls are the spiral-cut high lead valve and the poppet valve. Both methods open or close selected ports in the compressor cylinder, thus changing the seal-off points. These ports are located at the start of the compression cycle where pressure is very low. Opening them even a small amount prevents compression from occurring until the rotor tip passes the cylinder bore casing that separates the ports. This effectively reduces the trapped volume of air to be compressed and consequently the horsepower needed to compress it.
Pro: very efficient part-load performance from 50% to 100%.
Pro: maintains set pressure at minimum system pressure. Pro: very responsive.
Con: at higher loads, some units lose efficiency due to increased leakage.
Con: the mechanism is complex.
Con: still must run 2-step or modulation in lower operating range.
Variable-speed drives
Variable-speed drives (VSDs) control the speed of the prime mover. In theory, the performance unloading curve for compressors powered by VSDs is very attractive. Depending on the type of compressor, model, conditions, etc., unloading can be almost optimal in the range from 50% or 60% to 90% of load - i.e.: 75% power could produce close to 75% flow. Variable-speed turbines and engines have proved effective for years on all types of compressors. These drives maintain system pressure at the minimum set point and will modulate back as soon as the sensed system pressure raises.
In the world of electric motors, the most commonly applied VSD has been the variable-frequency driver (VFD) - usually as a retrofit or part of a special package. VFDs convert 60-Hz alternating current to direct current, and then reconvert it to AC at the frequency required to turn the motor at the desired speed. This conversion usually consumes about 2% to 4% more energy, and therefore VFDs are less efficient at full load than other types of controls.
Many VFDs have been installed successfully on lubricant-cooled rotary-screw compressor packages over the years, but there are some areas of concern that have limited their economies relative to cost and overall performance - particularly in retrofits. First, the design of some rotary-screw compressors causes efficiency to drop at less than full-load speed. Second, changing speeds can produce harmonic amplification problems that were not considered at the original design speed. Third, the motor itself may have efficiency problems at the low end of the speed range, possibly because of inadequate heat rejection and cooling capacity. Compressors with air ends designed specifically for VFDs will eliminate or minimize many of these potential problems.
Switched-reluctance VSDs
Another type of VSD being offered is the switched-reluctance system. This electrical control converts standard 3-phase AC power into 2-phase DC. The rectified AC voltage is passed to a bank of capacitors where it is increased to 600-V DC and stored. The bank then supplies the power required by each phase of a brushless motor, eliminating surge currents in the main power supply. The brushless motor has the inherent ability to survive an unlimited number of starts and stops per hour because the absence of inrush current surges keeps its operating temperature low.
The true application for any compressor with a VSD should be as a trim machine, not as the plant air system's base-load unit.
Where to put it
Industrial air compressors are rugged machines that will perform under adverse conditions, but it always is advisable to provide proper operating conditions to maximize reliability at minimum operating cost. Traditionally, compressors have been located in separate rooms to isolate their noise. Such locations are almost mandatory today to meet OSHA requirements. However, it still is important that the compressor room have an adequate foundation (particularly for reciprocating machines) as well as ample space so that the machine is easily accessible for inspection and maintenance. Stairways and catwalks can assist these procedures on larger compressors.
The compressor room ideally should be clean and dry. Auxiliary equipment, piping, and wiring should be arranged so that it does not interfere with routine inspections. Instruments should be located within easy view of operators.
|Partial summary of air compressor selection factors - 100 psig service
|Capacity in scfm
|Cooling medium
|<1 to 3,018
|<1 to 600
|<100 hp - Air
>75 hp - Water
|For some models
lubricated rotary
|14 to 3,000
|5 to 700
|Air or water
lubricated rotary
|560 to 3,100
|100 to 600
|Air or water
|Dry rotary
|75 to 4,200
|40 to 900
|Air or water
|400 to 25,000
|125 to 6,000
|Water only
|Applicability of air compressor unloading controls
|Type of
rotary screw
rotary screw
|Automatic start-stop
|Two step
Three and five step
|Yes (dual)
|Throttled inlet
Variable displacement
|Variable speed
This information was contributed by Hank van Ormer, president of Air Power USA, Pickerington, Ohio.
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Actress Vera Farmiga's directorial debut "Higher Ground" is an adaptation of "This Dark World: A story of Faith Found and Lost," Carolyn Briggs memoir focusing on her time in the '70s as a member of a group of fundamentalist Christian hippies who blended vegetarianism and born-again religiosity. It's not hard to imagine the project's appeal to Farmiga, who herself grew up as part of a rather strict Ukrainian Catholic community in New Jersey, where she didn't learn English until she went to kindergarten.
"I grew up in a community, a very distinct community, as we all do—it's important stuff, communities," Farmiga told PopcornBiz. "And the search for self within those communities, you know? I grew up in a Ukrainian-Catholic Christian home, and so to me it felt very comfortable setting the story there, rather than Islam, as far as detail-gathering in my experiences and perceptions and the wisdom that I had retained, and who I am."
In discussing the film following its World Premiere at the Sundance Film Festival, however, Farmiga distanced the film from herself somewhat, saying that religion was only that setting that it could've been about golf or any other place where people find spirituality, and she remains steadfast in her determination to make clear that "High Ground" is not her own life.
"It's not a story about me, it's very much a story about Carolyn Briggs' experience and her search as a thoughtful seeker. I am able to bring my experience within the church, every church, or house of worship I've ever sat in on. People who have an immediate sense of what God means to them, I marvel at those people, I admire them. But as far as community goes… I don't know how much to say about how it directly influenced the film…"
Listening to her trail off, start over, pause and stammer, it's clear she's trying to fend off admitting that this is a deeply personal film for her. When she finally cracks, she confesses, not unreasonably, that a relationship with God is not the sort of thing one should have with a stranger. In a roundabout way, she declares that is what her art is for.
"Yeah, on a molecular level, I struggle with my concept of god. And the yearning to understand that is even greater than… I'm not even sure; it's such an intensely private thing to talk about in interviews with people I hardly know. The power of the film is that people experience it in their own very receptive resonant ways—that's the power of the film. I can't tell you anything about the film, as far as me and projections of my youth… I can tell you [that] within my upbringing, this community, there's a certain sense of identity and inherent in my identity is certain traits and characteristics that I develop as a storyteller, this particular story has been infused with those character traits. And I really do strive for compassion, I strive for openness, suppleness, vulnerability, kindness—it's really difficult to say how things have affected me or how it correlates to my upbringing.
But soon she's back to protesting too much.
"It's all very me, but the story's not me at all, but there's resonates with me on a deep level. And I think, above my approach was going to be the most important thing as a director. I often talk about choosing roles and [I stand] in defense of a character, defending that person, that woman and why she is the way she is, no matter what your preconceptions are of her case—I'm an advocate, like a court-appointed lawyer, you know? I'm rambling now…." She says before trailing off into laughter.
"Higher Ground" opens Friday, Aug. 26


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For Immediate Release: August 25, 2009
Contacts: Michelle Jeffers (415) 557-4282 email@example.com
New Ingleside Branch Library Opens Sept. 12
New Ocean Avenue branch features community program room, dedicated teen room, inviting children's area and public art sculpture
The Ingleside neighborhood will come together to celebrate the new Ingleside Branch Library on Sept. 12. Replacing a leased facility on Ocean Avenue and Faxon Street, the brand-new library at 1298 Ocean Ave. at Plymouth Street, will feature an expanded collection of material, a teen room, a children's area with window seats and skylight, plus a program room accessible after hours for community meetings.
A grand opening celebration will kick off with lion dancers and musicians in front of the new branch. Festivities begin at 1 p.m. Library services will start at 2 p.m. Plymouth Street will be closed at Ocean Avenue to through traffic during the event. City Librarian Luis Herrera, Mayor Gavin Newsom, Supervisors John Avalos and Sean Elsbernd and Acting State Librarian/Deputy State Librarian Stacey Aldrich are among the city and state officials planning to attend the opening celebration.
"After being in temporary or leased facilities since 1917, we are thrilled to provide the surrounding Ingleside and Westwood Park neighborhoods with a brand new library that will serve as a hub of this community well into the future," said City Librarian Luis Herrera. "The branch's location at the entrance to the Ocean Avenue commercial corridor and its nearness to neighborhood elementary, middle and high schools as well as City College will make it a grand and welcoming resource for everyone."
"The Ingleside Branch Library is an inclusive partnership between our community, our non-profit partner and the City. This collaboration allowed Public Works to transform what was once an auto mechanic shop into a dynamic library for every child and adult to enjoy. We are excited to be at the heart of this celebration by helping to deliver a state of the art facility that is accessible for our growing and diverse community," said Ed Reiskin, Director of Public Works.
Designed by Fougeron Architecture/Group 4, the new 6,100-square-foot branch will provide a prominent civic presence on Ocean Avenue, adding to the revitalization of this commercial strip at the nexus of a diverse community. The new branch library facilitates a central gathering space and enhances access to important resources for the neighborhood. The robust, light-filled, and sustainable architectural design highlights the virtues and aspirations of this community: valuing the accessibility of knowledge and education to everyone.
The most striking element of the front façade of the new branch library, positioned prominently at the corner, is an egg-shaped children's reading room with a large, bench-seat window that encourages use of the library by younger patrons. The children's room is capped by a high canopy roof, extending over the entry and the lower community room along Ocean Avenue. This roof strengthens the civic presence of the one-story structure.
The Library will be the neighborhood's first city-owned branch and will be seismically safe and ADA accessible. The new branch will offer more public computers, wireless access, self-checkout machines and also have a flexible design to accommodate future technologies.
Inside, the spaces are designed to fuse historic interpretations of libraries as "temples of knowledge" with more contemporary uses. The sloped ceiling of the space is capped with giant skylights coaxing sunlight deep into the room. Facing the courtyard, mahogany-clad carrels offer quite, intimate spaces to read and relax. Benches are built into the glass edges creating a simple and elegant relationship between courtyard and interior spaces. A number of sustainable features are incorporated in this new branch including an energy efficient heating system, operable glazing for ventilation and the use of low VOC and sustainable finishes.
Artist Eric Powell was selected to create a 30-foot by 4-foot low relief metal structure that is mounted above book shelves in the reading room of the new branch. The sculpture is composed of metal tubing fashioned by hand into an organic, curving design representing a nearby underground creek.
The construction of the Ingleside Branch was funded by a bond measure passed by San Francisco voters in 2000 with additional funds coming from state Proposition 14 for library construction. The highly competitive state matching grant from the California Public Library Construction and Renovation Board awarded $3.75 million for the construction, ensuring the new building will serve as an anchor for the community. Ingleside is the 11th library project to be completed under the Branch Library Improvement Program. The Department of Public Works is responsible for designing, renovating, and managing the construction of public libraries under the direction of the library's program. Project costs for Ingleside totaled about $7 million including property acquisition.
The Ingleside Branch will include new furniture, fixtures and equipment. Friends of the San Francisco Public Library is raising funds from private donations to pay for these items. To get involved in the Ingleside neighborhood campaign, contact email@example.com.
For more information about the Ingleside Branch Library's opening celebration, please call (415) 557-4277, or visit sfpl.org.