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We have satellites and telescopes looking outward and inward to the earth. Would any of these detect an extraterrestrial spacecraft before it landed, or in other words, would we know a spacecraft was approaching before it did (assuming its going at some reasonably detectable speed in our local area)?

Let's assume it is Saturn V like wrt detectability.

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    $\begingroup$ It's a good question and I think it is on-topic here because it's about what telescopes would see, not about the spacecraft itself. But I will mention that 99% of a Saturn V is used to get things launched into space, and only a tiny bit at the top really goes where it's supposed to, the rest is just one big fuel tank. $\endgroup$
    – uhoh
    Commented Dec 9, 2020 at 14:56
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    $\begingroup$ If it goes into a lower earth orbit before it lands, I think NORAD may be more likely to detect it than astronomers. $\endgroup$
    – Connor Garcia ♦
    Commented Dec 9, 2020 at 16:28
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    $\begingroup$ So far, we failed each and every time ... $\endgroup$
    – Hagen von Eitzen
    Commented Dec 10, 2020 at 13:21
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    $\begingroup$ @uhoh Maybe, if we restrict it to spacecraft using standard physics, not science fictional stuff like FTL or inertialess drives. Even then, I won't protest if mods or others deem it to be off-topic. $\endgroup$
    – PM 2Ring
    Commented Dec 10, 2020 at 21:09
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    $\begingroup$ @PM2Ring I'm inclined to agree with uhoh. The OP does seem to be limiting this to conventional physics. That said, I won't overturn it if it ends up getting closed. $\endgroup$
    – called2voyage ♦
    Commented Dec 11, 2020 at 14:37

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So NASA recently re-detected the upper stage of a old centaur rocket from Surveyor 2 identified as NEO 2020 SOO. Now, there was some advantages with this object as people had noticed it before so spent some time working out its orbit and where it would be. But it suggests we can find and observe objects smaller than a Saturn V a long time before they reach the atmosphere. Wikipedia says its closest approach was 50,000 km so that gives a lower limit for finding a spacecraft before it lands on Earth, but the actual distance when we first saw it was proper alot further.

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    $\begingroup$ +1 but there's more: Pan-STARRS detected it, NASA's Infrared Telescope Facility later confirmed that it was a rocket body, and yet other researchers confirmed that the object's orbit is consistent with it having originated from Earth at the time of Surveyor 2's launch. 1, 2 $\endgroup$
    – uhoh
    Commented Dec 13, 2020 at 23:56
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Some thoughts about Earth-based detection systems:

  1. We are looking at asteroids using radar: For instance the former Arecibo telescope could actively send out radio waves and calculate object position and size. However, we might be able to detect an incoming spaceship using this technology, assuming that the object approaches Earth within the radio beam, if we we are currently studying another object. In other words: Detecting an incoming object would be purely chance.

  2. An all sky camera network for meteorite detection is another outward-looking system worth mentioning, but that would be very very briefly before landing of a hypothetical extraterrestrial space ship. I am sure you are not after this option since we would be seeing the object already with naked eyes if it would not be incoming at a very remote location. However, this option is to my knowledge working 365/24/7 and covers large portions of the sky, I believe.

  3. Edit suggested by uhoh: There are also initiatives like NASA's Space Fence or ESA's Space Surveillance and Tracking (SST) segment monitoring space debris within the geospace which could possibly detect other objects as well, see also https://space.stackexchange.com/q/45144/12102

Concerning satellite-based observations: If the objects emits significant radiation e.g. in parts of the spectrum of communication satellites, we might detect anomalities or malfunctions in one or more satellire and by chance figure out the extraterrestrial source of the distortion. This would of course only hold for slow approaching objects.

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  • $\begingroup$ @uhoh Again, another edit to include your other comment. I am not sure what you exactly mean with "optimized for agility and numbers" though. $\endgroup$
    – B--rian
    Commented Dec 14, 2020 at 21:22
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    $\begingroup$ Looks great, I wish I could up vote twice! I just meant that it needs to be able to detect and measure a very large number of objects that range from huge to tiny in reflected signal strength so instead of aiming at one object at a time it has to "eat" a large swath of space at once. I'm no expert in any way, but for the "agility" part I'm imagining that the signal processing has to work hard to pick out tiny reflections that may be in close proximity in space to very large reflections. $\endgroup$
    – uhoh
    Commented Dec 14, 2020 at 23:55

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