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Posted 11/28/2005 4:29 PM     Updated 11/28/2005 5:27 PM
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A slow boat to Pluto, Ants with hundreds of tiny eyes
Q: How long does it take for sunlight to reach Pluto? (Samantha, Mudgee, New South Wales, Australia)

A. A beam of light blasts off the Sun at midnight, Earth time (00:00:00 Coordinated Universal Time, UTC) and soars outward. Passing each planet in turn, it zips by Earth in about 8 minutes at 00:08:19 UTC and hurtles on at 186 thousand miles per second (300,000 km/s) for another 320 minutes until it reaches cold and distant Pluto. About 5:30 in the morning (05:28:06 UTC), it arrives.

That's an average travel time. Pluto's distance from the Sun averages about 39 times the distance of Earth from the Sun (a distance unit that is called an Astronomical Unit, or AU).

Pluto's orbit — unlike any of our other planets — is far from circular. So Pluto's distance from the Sun varies from about 30 AU to almost 50 AU. Consequently, the sunbeam's travel time to Pluto can likewise vary — from about 4 hours to almost 7 hours.

By the way, Pluto is the only planet that a spacecraft from Earth has not yet visited. We hope to launch New Horizons (the current name for the Pluto-Kuiper Express mission) in 2006. It will take our half-ton (463-kg) spacecraft nine years to reach Pluto.

The trip, however, will be worth it. "The combination of Pluto's extreme distance from the Sun and tiny size means we have no good maps of its surface," emails astronomer Robert Massey of the Royal Observatory Greenwich in London. "From Earth, it appears 15,000 times smaller than the Moon so even our most powerful telescopes struggle to reveal any detail."

Also, we need to get there as soon as possible — while Pluto still has an atmosphere to investigate. From 1979 to 1999, Pluto was close to the Sun. (Then, temporarily, Neptune became the most distant major planet.) "At its closest to the Sun, Pluto developed a thin atmosphere as some of the ices on its surface turned to gas," says Massey. This will probably freeze solid again soon, which is why New Horizons needs to hurry. "Otherwise we'll have to wait until the 23rd century before Pluto looks the same again."

Further Reading:

Greenwich Royal Observatory: Pluto

Wikipedia: Pluto

Q: How many eyes does an ant have? (Linny, Norris Point, Newfoundland and Labrador)

A: Ants (except for the army ant) have two compound eyes (aqua in the figure), which are made up of many smaller eyelets (called ommatidia). Different species have different numbers of ommatidia forming their compound eye. For example, the wood ant has 460 eyelets and the black garden ant 120.

" ...each ommatidium is pointed at just a single area in space and contributes information about only one small area in the field of view," says biologist John W. Kimball, professor emeritus of Harvard University. Each eyelet tells the optical nerve that it sees light or dark.

The composite eye produces a "mosaic" of the light and dark spots from the individual unit responses and thus an image like a half-tone newspaper picture. The more ommatidia, the better the picture. The exceptionally keen-sighted red wood ant can see moving objects up to four inches (10 cm) away.

Army ants do not have compound eyes — instead two single eyes — but they do not function. The ants are blind and use their antennae to smell and touch.

Indeed, most ant species have poor eyesight. All ants rely on their antennae as their number-one sense organ. Antennae are jointed so the ant can probe and sniff forward and whip them back over her head when danger threatens.

Further Reading

Biology by John W. Kimball

The Linus Holmberg Ant site

Wikipedia: Ants

(Answered Nov. 29, 2005)




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