Detecting things in the IR

There has been a fair bit of discussion about the IR and detecting things on
the list so I thought I would toss in my two cents (US).

It seems quite reasonable that in the near future IR telescopes will be a
really good way to find spaceships (from space). The temperature every talks
about is around ~300 Kelvin (or ~27 C). At those sorts of temperatures, space
ships will be radiating at a peak wavelenght of about 10 microns (this has
ramifications I will discuss later). For a comparison, the sun's peak
wavelenth is around 0.5 microns, which is of course where the human eyeball is
the most sensitive.

I did a rough back of the envelope calculation (assuming perfectly spherical
spaceships and all of that). It works out that at reasonable ranges, a
spaceship will be quite bright. Even at around a light second the flux from a
moderate sized ship (say a 100 meters in radius) would be about that of a
bright star. Beyond that, say 10 light seconds, and now you have to worry
about the ship dissapearing into the background of nearby stars. At 10 light
seconds, you will not be able to resolve ship with any reasonable sized
telescope (a few meters or so) and the number of stars at the same brightness
as the ship appear is pretty high (a few per square degree, or, say, one per
full moon sized area).

Nonetheless, it is still pretty easy to find things. Why? No star has a
temperature of 300 Kelvin. So you can use the color to determine if something
is a ship or not (ie. if something looks really really red
in the Infra-red, odds are it is a ship).

The other advantage you will have is that ships will be moving rather
differently then anything else in a solar system. They will appear to move
much faster than background objects (distant stars) unless they are

moving on an orbit that will send them straight at the observer.

So, how do you hide? The temperature of a ship I assume is the same as the
earth, so a low earth orbit will make you hard to find. Interestingly, at 10
microns the earth's atmospher glows. So a ship in space will be very hard to
detect from the ground.

The other trick to hide yourself is to reduce your surface temperature. The
luminosity of a blackbody (a blackbody is a technical term, the common
household oven, however, makes a pretty good black body, an incandescent
lightbulb also makes a good black body) depends on the temperature to the
fourth power. So, if you reduce the hull temperature by a factor of two, your
brightness to an observer is reduce by a factor of 16. Now, you still have to
get rid of the waste heat your ship generates but I can imagine a "stealth
mode" where ships deployed radiators (big hunks of superconductor or
something) that spread out the area over which the waste heat is dumped and
thus decreases the effective temperature of the ship. Of course this puts a
gap in your armor.

In terms of other objects confusing detectors, I don't think that is an issue
in most places like the solar system. There are not many rocks at the sizes of
a ship, and almost none of them have temperatures of 300 Kelvin. It possible
in other solar systems such places exists (in fact I can think of one place
where this is definitely true) but they are unusual places.

Someone mentioned that optical instruments have certain ranges in which
something is in focus. This is true when the object you are taking a picture
of is close, where close means the distance between you and the object is a
few times the focal length of the camera. The focal length of a telescope is
usually a few meters, so this will not be a problem for ships in space.

Finally, in terms of the data rate problem (can one ship process all the data
covering the whole sky?) Well, there a couple of projects to do all sky
monitoring now. Heck, there is one that is to be done in
semi-real time and is done by only amatures (well, kinda), check out:

http://www.tass-survey.org/tass/tass.shtml

Another example is ROTSE, which is not all sky yet (though I know
people are building up an all-sky follow up as I write this)

http://www.umich.edu/~rotse/

I hope what I wrote above is not too muddled.

cheers