Mostly what it says on the tin. If a spaceship, let's say about 200 meters long and 100 meters wide (made of modern materials) was approaching Earth and slowing down (say from 100 km/s to 3km/s), how far out would it be noticed? How clear of a picture could something take of it? Is is possible it wouldn't be seen until it reached the atmosphere?

The ship in question is not announcing its presence, but it also isn't trying to hide (and has no means of stealth), so it can probably be treated similarly to an asteroid.

  • $\begingroup$ I'll throw it out there that the answer does depend a LITTLE bit on what you're thinking of by 'Earth Orbit'. Technically your ship could approach from the far side of the moon and slot into the L2 Lagrange point on the far side and still be in 'Earth Orbit' even though it's half a million miles away with the moon in between it and us. We've got satellites there too though, so we'd almost certainly still notice it. $\endgroup$ Sep 13, 2019 at 16:10
  • $\begingroup$ The long version doesn't match the Title question. E.g. does it enter Earth orbit, or does it enter the atmosphere? E.g. is it seen while in orbit, or while slowing down? $\endgroup$ Sep 13, 2019 at 16:22
  • $\begingroup$ While this analysis of spacecraft detection analyses a stealth spacecraft, the hydrogen steamer, it has a lot of interesting explanations and useful equations. $\endgroup$ Sep 13, 2019 at 19:44

4 Answers 4


Without any kind of stealth technology, it is 100% likely to be seen long before it gets anywhere close to Earth Orbit. This might give you some perspective:

A comet coming in unannounced from intersteller space, from outside of the ecliptic, was detected at 3au (three times as far from the sun as the earth is), by more than 20 different telescopes.


There are literally hundreds of organizations and thousands of amateur observers constantly looking for things like that.

An even better example might be Asteroid 2019_OK, which is just about the same size as what you're describing. It was moving about 25 times as fast as your example, and almost a worst case scenario in terms of where it was coming from in terms of being able to see it, and it was still detected about a million and a half km out, three times the distance to the moon.

EDIT: For some additional perspective, here's an example of a quite small object that would serve as a pretty good analogue for a large spacecraft that DID have stealth technology. It took us three years to notice C26FED2 was in orbit, but it's only about the size of a very small car. A large spacecraft with good stealth tech would probably have a return about this size.

Oh, and it looks like this is a duplicate too. So, my bad. =)

  • $\begingroup$ Yet there are very large asteroids that we only ever learn about AFTER they've gone past the periapsis of their flyby. $\endgroup$ Sep 13, 2019 at 14:55
  • $\begingroup$ @Renan those are moving much, much faster than the OPs example, and generally don't get nearly as close as 'earth orbit'. See en.wikipedia.org/wiki/2019_OK. It was almost a worst-case scenario in terms of difficulty of detection because of both speed and solar opposition, and it was still caught more than a million miles out. $\endgroup$ Sep 13, 2019 at 14:58
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    $\begingroup$ the sky and space still seems like a pretty vast expanse to claim 100% chance to spot it $\endgroup$
    – BKlassen
    Sep 13, 2019 at 15:05
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    $\begingroup$ @BKlassen sky and space generally, yes. The bit specifically between the earth and the Moon's orbit is really, really intensely observed. We track lost WRENCHES in earth orbit. $\endgroup$ Sep 13, 2019 at 15:08
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    $\begingroup$ @BKlassen: The ship is big, and it's coming close to Earth. We are really, really interested in biggish objects coming close to Earth. $\endgroup$
    – AlexP
    Sep 13, 2019 at 15:54

More important than the size of the object is its heat signature. Slowing down from 200 km/s to 3 km/s, without having the ship be 99.9% propellant, requires a LOT of energy, and that energy will show up as a bright heat signature even if the deceleration burn starts months in advance. It's my understanding that heat signatures in space are very, very easy to detect, to the point that a detector small enough to be put on a modern-day satellite could detect the exhaust plume of a space shuttle... from the orbit of Pluto, and that such a detector could scan the entire sky about once an hour. Granted, that last part is hearsay and I have no numbers to back it up; but if we have a heat detection system even 1% that effective, we'll spot this ship coming in from it's exhaust plume long, long before it gets into range of any radar or optical telescopes.

Assuming, of course, that it's not using a reactionless drive, but that's a whole other can of worms.

  • $\begingroup$ +1 Jup. This thing will light up like a christmas tree in any IR survey telescope. $\endgroup$
    – Karl
    Feb 26, 2020 at 21:30

If the ship's orbit is perfectly circular and with a speed of 3km/s, it will be orbiting at an altitude of approximately 37,917 kilometers above sea level. That is just a very little bit above geosynchronous orbit.

This may be interesting for you: geosync altitude is kinda the sweet spot for communication satellites, so slots in it are in high demand. If you check the interactive graphic in this page, you will lots and lots of satellites there.

Then we have this article from Sky and Telescope:

Many geosynchronous satellites shine between magnitudes 10–12, so you can spot them in telescopes as small as 4 inches.

On top of that, being close to geosync altitude means that for an observer in the ground, the ship will be moving very slowly - so it will be easy to track once found.

How visible it will be, though, depends on its albedo. If it is completely dark, it will only be noticed if it passes in front of something - another planet, or the Moon. If however it is as shiny as a solar panel, lots of people stargazing will see it as a shiny dot. It might be visible by day, with the right angles and equipment.

If it has the albedo of the regular asteroid, it may probably be found before it settles into orbit. In 2018, 41.9% of the asteroids approaching Earth with a closest distance smaller than Lunar Distance were detected before closest approach.

We'll probably never know what hits us

The huge majority of those were in the 1 to 20 meters size range, so a much large ship might be easier to spot oncoming.

2019 is not over yet, but we seem to be doing slightly better at detecting things before they arrive. I'd give your ship's odds of coming in unexpected at around 50%. If it arrives without being detected, then once it settles in orbit it might be a matter of hours to days until it is seen.

P.s.: at a distance from the sun close to the Earth, the solar system's escape speed is around 42 km/s. The flight path of a 100km/s ship prior to decelleration would be a very broad, open curve entering the system at one side and exiting it again at the other side.

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    $\begingroup$ Your graph tends to obscure the fact that the fast majority of those 'After closest approach' objects are quite small. The only things on that graph that got within 100,000km without being detected were less than 10m across. You're also not taking into account the fact that anything trying to insert into earth orbit would come into the range of the hundreds of radar stations that track earth's orbital space. Those systems are able resolve objects of 1m in Geosynchronous orbit, so I don't see how anything 200m across is going to arrive without being detected. $\endgroup$ Sep 13, 2019 at 15:36

On top of the other answers, if your ship somehow made it to Earth's orbit without being detected, the Space Fence and other international systems would certainly detect it. The US Air Force S-Band Space Fence radar detects objects as small as 10 cm from LEO. And there are many others looking skyward -- both for Early Warning of missile attack and tracking of orbital satellites and space debris.

The radar net can track objects as small as 1 cm from LEO to high orbits. These systems are digital and may have discriminating filters to prevent them from alerting to small meteoroids traveling too fast and too high be of concern to orbital systems. Your ship's initial speed might cause it to be filtered out of reporting data, but as it slows its is sure to be reported by multiple agencies managing information about the space above their respective countries.


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