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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.

peterh
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hisairnessag3
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    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. – uhoh Dec 09 '20 at 14:56
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    If it goes into a lower earth orbit before it lands, I think NORAD may be more likely to detect it than astronomers. – Connor Garcia Dec 09 '20 at 16:28
  • For those for whom NORAD does not ring a bell: https://en.wikipedia.org/wiki/North_American_Aerospace_Defense_Command – B--rian Dec 09 '20 at 18:15
  • What size is the craft ? What is it made of ? Is it a stealth design (by accident or intentionally doesn't matter) ? Does it emit any signal for any reason - e.g. active radar or something like that ? Far too many variables here to given a good answer in my opinion. – StephenG - Help Ukraine Dec 10 '20 at 08:31
  • the task of a Saturn V is to leave the Earth. After it is in orbit, it falls back (part of it might remain on orbit as space waste). Only a small cabin goes to Moon, in size roughly like a school bus. Probably we would not detect it until it nears the atmosphere. – peterh Dec 10 '20 at 12:28
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    So far, we failed each and every time ... – Hagen von Eitzen Dec 10 '20 at 13:21
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    @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. – PM 2Ring Dec 10 '20 at 21:09
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    @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. – called2voyage Dec 11 '20 at 14:37
  • Ok. Here are some numbers that may be helpful in framing answers. Presumably, we want to spot a ship some time after it crosses the orbit of Neptune, which has a mean orbital radius of 30.1 AU. A body freefalling from 30.1 AU takes ~24.63 years to reach 1 AU (Earth's mean orbital radius), and has a speed of ~42.122 km/s (relative to the Sun) when it gets here. That's $\sqrt2$ times Earth's orbital speed. A Hohmann trajectory takes ~30.66 years, and has a speed of 41.438 km/s at 1 AU, but that trajectory is tangential to Earth's orbit. – PM 2Ring Dec 15 '20 at 02:26
  • (In that freefall trajectory, the body is falling from "infinity" into the Solar System, and has a speed of 7.677 km/s at 30.1 AU). In contrast, a craft decelerating at a constant 1 g, aiming to lose most of its speed by the time it gets to 1 AU, takes around 11 or 12 days to get here from 30.1 AU. Its speed when it crosses Neptune's orbit is ~0.03 c, so it has to shed a lot of kinetic energy in that time frame. If it uses any kind of rocket engine, that means it will be blasting a lot of hot matter in our direction. – PM 2Ring Dec 15 '20 at 02:49

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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.

B--rian
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  • @uhoh Again, another edit to include your other comment. I am not sure what you exactly mean with "optimized for agility and numbers" though. – B--rian Dec 14 '20 at 21:22
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    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. – uhoh Dec 14 '20 at 23:55
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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.

Rob
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