I was watching an old Stargate episode with big alien ships in orbit, and I thought 'in real life that would be spotted pretty quickly... um, wouldn't it?'

I'm not saying I'm on the search for alien ships (if they do exist I don't think they have any trouble staying a step ahead of us anyhow). It just made me wonder about how much of what is in the sky, even pretty close by, we are really aware of.

Maybe there should be some parameters on this. Suppose a standard Federation star ship were to yet again accidentally go back in time to our era, due to their continuing disregard for basic tachyon safety procedures. The Enterprise is supposed to measure about 600 x 400 x 200 meters, weighs 4.5 million metric tons, and seems to have a metallic hull. How long would it take us to spot that in low Earth orbit? How about if it was in a high orbit?

  • $\begingroup$ Out of this body, David Vincent! $\endgroup$
    – mins
    Mar 7 '15 at 12:49
  • $\begingroup$ i had to look that up... $\endgroup$
    – kim holder
    Mar 7 '15 at 14:59
  • $\begingroup$ Wow bouncing from stargate to star trek. I wonder how different the answers would be if it were a death star, in a galaxy far, far away... $\endgroup$
    – ThePlanMan
    Mar 13 '15 at 2:03
  • $\begingroup$ GENERAL GREY: A meteor? / SECOND OFFICER: No Sir. Definitely not. / GENERAL GREY: How do you know? / SECOND OFFICER: Well, er... it's slowing down. / GENERAL GREY: It's doing what? / SECOND OFFICER: It's... slowing down, Sir. $\endgroup$
    – Mark Adler
    Apr 2 '15 at 3:57
  • $\begingroup$ Considering how easy it is to spot the ISS with the naked eye, I'd imagine spotting a bright silver ship that's 30 times larger would probably be noticeable in short order by just about any amateur astronomer that the ship flies over. $\endgroup$
    – Ellesedil
    Apr 2 '15 at 14:50

An object of that size would be detected almost instantly on first pass by the United States Strategic Command, which sweeps Earth's Sphere Of Influence to track and catalog objects that are in Earth orbit. Radar picks up most objects which are close and reasonably large. Anything greater than a metre in diameter can be tracked with relative ease, in fact, objects down to 1cm in size can be tracked too.

All operational satellites in Earth orbit are in known, cataloged orbits. Its fair to say an object larger than that of the ISS would be very observable (from radar) from any orbit it would be placed in.

  • $\begingroup$ So it's all radar? Up to what height will that detect things? Does it effectively track even up to something like a geosynchronous orbit? $\endgroup$
    – kim holder
    Mar 7 '15 at 3:44
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    $\begingroup$ @briligg - radar and telescopes, mostly, but also lidars and space-based multi-spectral detectors,,, $\endgroup$ Mar 7 '15 at 4:05
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    $\begingroup$ I know this wasn't in the question, but I think it'd be a good idea to consider the effects of radar stealth technology. Depending on the angle, massive planes can look like golf balls, and I think an orbiting ship could be designed to have a very low radar return when viewed from below. $\endgroup$ Mar 7 '15 at 6:06
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    $\begingroup$ @raptortech97 - your ship still radiates heat. $\endgroup$ Mar 7 '15 at 7:54
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    $\begingroup$ @osgx, true, but then again, the question is asking about an object far larger than the ISS. I was merely mentioning that such objects are trackable, not that they are always trackable. $\endgroup$ Mar 15 '15 at 1:27

The United States Space Surveillance Network has 20 sites around the world, and satellites as well, dedicated to detecting and monitoring objects near Earth.

map of space surveillance network sites worldwide

The European Space Agency also operates installations that monitor near-Earth space, full-time or part-time, in Germany, Norway, and Spain. I couldn't find public information on Russia's network. China could have installations too.

The majority of the ground-based installations are radar, but there are some optical telescopes, notably ESA's installation in Spain. Surveillance satellites are all optical.

Objects as small as a centimeter are detectable in low Earth orbit. In geosynchronous orbit objects of a meter are detectable by radar, the Spanish telescope can see things as small as 15 cm. So the Enterprise would really stick out.

While parts of this system are designed to track small objects and have a small field of view, other parts are designed to watch large sections of the sky for anything passing by. When not actively tracking an object, the tracking units scan back and forth so that they also cover large parts of the sky. Both sorts of units are shown in the photo below, they are parts of ESA's network.

EISCAT radar station, Sweden

Either kind of unit would be capable of seeing something as large as a ship. As the networks span the globe and operate continuously, even in a very high orbit a ship would be seen in a few minutes or less.

The most inconspicuous place for a ship to loiter near Earth would be in the Earth-Moon L2 Lagrange point, always out of sight behind the Moon.


A spaceship needs to keep it's occupants warm, and that means keeping it at a temperature easily detectable by IR telescopes (unless it's behind the moon). Firing the engines would be a dead giveaway.

  • $\begingroup$ It might take a while before one was pointed in the right direction to see you though. That is why at first i thought maybe a ship could go unnoticed for a long while. (I thought radar wasn't very effective beyond a pretty low orbit. Now i know better :)) $\endgroup$
    – kim holder
    Mar 13 '15 at 1:12
  • $\begingroup$ The interior temperature and the exterior temperature are not the same: If the ship is well insulated it can be very cold on the outside $\endgroup$
    – gosnold
    Mar 13 '15 at 12:41
  • $\begingroup$ The ship always will have waste heat, also, lots of insulation would be extremely heavy. It would also require enough insulation to keep the outer surface near absolute zero. A reactor of any sort would release LOTS of waste heat. $\endgroup$
    – ntno
    Mar 17 '15 at 20:58
  • $\begingroup$ Firing the engines: that assumes a certain technology. A high speed ion beam in space as a thruster could have almost no heat signature. Radiation from hot gas arises from collisions - and resulting charge acceleration hence Larmor effect - between the gas molecules and these don't happen much with a unidirectional beam expanding freely in space. Look up hot dark matter and hot interstellar medium - hot gasses in space can be very hard to detect. $\endgroup$ Mar 22 '15 at 9:50
  • $\begingroup$ If you look at tests of ion engines, there is a "flame". The density of the exhaust is high enough that collisions do happen. If the exhaust were sparse enough for there to be a minimal heat signature, the exhaust velocity would have to be HUGE to provide non-negligible thrust. $\endgroup$
    – ntno
    Jul 25 '15 at 20:20

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