Well, obviously billions of years if it is the moon. Or not very long if it is only 100km above the Earth, due to atmospheric drag. Also, how long it stays up would depend on density ie how much energy it has to dissipate in order to lose speed to drag due to mass/cross sectional area.  However, somewhere in the middle we have satellites that will stay up for... how long? How long will geo-sync satellites remain in orbit? What forces are exerted on them that will affect their stay eg magnetic field effects, solar wind etc? If we wanted to put a satellite in orbit and keep it there (say) 10,000 years how close could we get to Earth before this became unrealistic?
The orbit of satellites around Earth eventually decays, or so I read. This is typically caused either by atmospheric drag, or by tides. I would assume most satellites have a limited service life in orbit. Hence the question - What is the typical orbital life of a satellite? How long before it's successor must be launched into orbit ?  EDIT: For instance, a weather satellite
What Does it Really Mean to Have Different Kinds of Infinities? Can someone explain to me how there can be different kinds of infinities?  I was reading "" by  and came across the concept of countable and uncountable infinities, but they're only words to me.  Any help would be appreciated. How can I replace a relay with a MOSFET Transistor? I have this working         &ndash; Schematic created using   An Arduino commute both relays and a power adapter charge each battery at a time.  I would like to replace each relay by a MOSFET. How can I do that?  I think the solution bellow will not work because DRAIN is always connected to SOURCE and this will short circuit batteries. MOSFETS only isolate from SOURCE to DRAIN.  When the D9 is HIGH, D10 will be LOW so the power adapter will never be charging both batteries simultaneously.          Sorry for the messy schematic.  Thank you very much. I have a Python program that uses the "threading" module. Once every second, my program starts a new thread that fetches some data from the web, and stores this data to my hard drive. I would like to use sqlite3 to store these results, but I can't get it to work. The issue seems to be about the following line:  conn = sqlite3.connect("mydatabase.db")    If I put this line of code inside each thread, I get an OperationalError telling me that the database file is locked. I guess this means that another thread has mydatabase.db open through a sqlite3 connection and has locked it. If I put this line of code in the main program and pass the connection object (conn) to each thread, I get a ProgrammingError, saying that SQLite objects created in a thread can only be used in that same thread.   Previously I was storing all my results in CSV files, and did not have any of these file-locking issues. Hopefully this will be possible with sqlite. Any ideas? Evaluating logistic regression and interpretation of Hosmer-Lemeshow Goodness of Fit Often I see code blocks pasted into questions with no formatting, or terrible formatting.  It's sometimes quite a chore to fix these: you have to either manually space out each line, or paste the code into an IDE, format it there, and paste it back.  Could we have a button/function where you could highlight a code block, and it would turn this:  foreach (var i in stuff) { if (i==1) { Whatever(); } }   into this:  foreach (var i in stuff) {   if (i==1) {     Whatever();   } }   I realise the function would have to work differently based on the language in the code block. Perhaps  it could use the same rules as the prettify code.  Also, it wouldn't be a big deal if it gets the language wrong and breaks the code, since you can always just ctrl+z. Rest mass of the photon Why is the rest mass of the photon is taken to be zero?  If rest mass of the photon is non zero, then it leads to infinite energy of the photon (by $E=mc^2$ equation) and there is a violation of conservation of energy. Is this a reason?  Also, is there any thing called the relativistic mass of the photon? How can swapoff be that slow? Somehow I happened to swap out 14 GB of memory. After having killed the culprit, I have tons of free memory again, so I thought I could  again. So with 5 GB out of 32 GB used and 14 GB of swap space used, I ran swapoff -a.... and 4 hours later about half of the work was finished.  This means less that 1 MB/s, while I can easily copy 200 MB/s. My swap is encrypted but so are all normal partitions and with  it leads to no noticeable CPU load (and filling the swap space took only a few minutes). I see that there's no special reason to optimize swapoff, however I wonder how it could get that slow?    Just adding some more data: My main memory is 32 GB and I have 32 GB swap space on each of 4 harddisks (surely an overkill, but who cares?). The whole swap space can be  (decrypted and) read in less than 5 minutes:  time -p sudo sh -c 'for i in /dev/mapper/cryptswap?; do md5sum $i &amp; done; wait' 014a2b7ef300e11094134785e1d882af  /dev/mapper/cryptswap1 a6d8ef09203c1d8d459109ff93b6627c  /dev/mapper/cryptswap4 05aff81f8d276ddf07cf26619726a405  /dev/mapper/cryptswap3 e7f606449327b9a016e88d46049c0c9a  /dev/mapper/cryptswap2 real 264.27   Reading a part of a partition can't be slower than reading it all. Yet reading about 1/10th of it takes about 100 times longer.  I observed that during swapoff both the CPU was mostly idle (maybe 10% of one core) and so were the disks ("measured" by the LEDs). I also saw that the swap spaces were turned off one after the other. What's the main difference between Intel processor generations? What are the main differences between each processor generation of Intel processors/chipsets? Is there a performance difference?  I've already seen processors from first to sixth generation, however I don't know/understand the exact differences. Is it possible to scale up an image to increase the the pixel density? I have taken a picture whose resolution is less. I can resize the image easily (i.e I can scale to higher resolution) but the image quality goes for a toss, which is true because the image might not have enough pixels to scale properly.   Is resizing a means of increasing the resolution of the image ? Is it possible to increase the pixel density of the image and then resize, so that quality of the image is not affected much?   I mainly use GIMP and Picasa. An operator has closed range if and only if the image of some closed subspace of finite codimension is closed. When does an expired domain become available for others to register? This is tricky. I would like to install Ubuntu on a machine that is intended to work as server, and for some reason I do not have an option to connect a screen to it.  I expect this may be similar to remote installation. However, there is currently no OS on that machine, so I will have to set up everything from scratch.  On the other hand, I do have a physical access to that machine. Therefore I can get it to boot from a CD/USB. But that's not enough.  Is there a way to do such installation remotely? Is there a version of Ubuntu image that comes with a SSH server preinstalled, and starts it once booted, so that I can boot the machine with such image, connect to it via SSH and process the installation from a different computer?  If not, what other options do I have? How do I check if an object has a specific property in JavaScript?  Consider:  x = {'key': 1}; if ( x.hasOwnProperty('key') ) {     //Do this }   Is that the best way to do it? Panasonic Lumix G 25mm f/1.7 ASPH vs Olympus M. Zuiko 25mm f/1.8 Looking for a function $f$ that is $n$-differentiable, but $f^{(n)}$ is not continuous I am looking for a real valued function of real variable that is $n$-differentiable, but whose $n$th derivative is not continuous.  This is my function:  $f_n(x) = x^{n+1} \cdot \sin{\frac{1}{x}}$, if $x \neq 0$ and 0, if $x=0$. $n\in \{0,1,2,\ldots\}$ For example, if $n=0$, $f_0$ it's continuous and non diferentiable in $0$. I was looking for a tree or graph data structure in C# but I guess there isn't one provided.  explains a bit about why.  Is there a convenient library which is commonly used to provide this functionality?  Perhaps through a strategy pattern to solve the issues presented in the article.  I feel a bit silly implementing my own tree, just as I would implementing my own ArrayList.     I just want a generic tree which can be unbalanced.  Think of a directory tree.  C5 looks nifty, but their tree structures seem to be implemented as balanced red-black trees better suited to search than representing a hierarchy of nodes. Captured variable in a loop in C# I met an interesting issue about C#. I have code like below.  List&lt;Func&lt;int&gt;&gt; actions = new List&lt;Func&lt;int&gt;&gt;();  int variable = 0; while (variable &lt; 5) {     actions.Add(() =&gt; variable * 2);     ++ variable; }  foreach (var act in actions) {     Console.WriteLine(act.Invoke()); }   I expect it to output 0, 2, 4, 6, 8. However, it actually outputs five 10s.  It seems that it is due to all actions referring to one captured variable. As a result, when they get invoked, they all have same output.  Is there a way to work round this limit to have each action instance have its own captured variable? Show that $f(z)=0$. Suppose that $f:\mathbb{C}\rightarrow\mathbb{C}$ is analytic on the open unit disc and continuous on the closed unit disc. Assume that $f(z)=0$ on an arc of the circle $\{z\in\mathbb{C}:|z|=1\}$. Show that $f(z)=0$. How to mount electronics on a vehicle chassis safely I want to implement a control system I designed for a light electric motorbike. I would like to mount it as professionally as possible on the chassis. It consists of a power system pcb and a SAMC21 development board which I'll merge later once this all works properly. I have tried screwing it directly on the aluminium chassis but have found that shocks and vibrations from the horn disturbed the system or even destroyed it upon large shocks like a few hammer hits. Is there a specific way car and motorcycle manufacturers mount their electronics to make their system as robust as possible? Thank you for your help! I'm looking to get data such as Size/Capacity, Serial No, Model No, Heads Sectors, Manufacturer and possibly SMART data.