I am trying to establish whether humanity could detect and track a fast-moving alien ship arriving in the Solar System and whether we would get some sort of "advance" warning that it is coming. Let us consider the following scenario:

The path

  • The alien ship exits FTL at 51 AU from the Sun. That event releases large amounts of heat, light and radiation - could potentially be comparable to a weak solar flare.

  • The ship will move towards Mars, traveling at 0.7c (70% of lightspeed)

  • At Mars it will decelarate and enter orbit for approximately fifteen minutes (deceleration and acceleration to/from 0.7c to oribtal speed will take about 2 minutes - yes, that's a lot of Gs. Also, there will be a minor but measurable increase in the ships signature - more heat, more gamma - but nowhere near as strong as the FTL flare)

  • It will then head for Earth at 0.7c

  • The aliens are making absolutely no attempts to hide or conceal their approach

The FTL exit point, Mars and Earth are all on the same side of the Sun. You can assume FTL exit-Mars distance of about 50AU and Mars-Earth distance of about 1.5 AU. According to my calculations it should take the ship approximately ten hours and twenty minutes since entering Sol to reach Earth. The first signs of its presence (the FTL flare) could be detectable from Earth as soon as six hours forty five minutes since its arrival.

The ship

  • The entire time in cruise the ship is outputting heat and electromagnetic radiation (notably radar and gamma) and leaving a trail of plasma
  • Emmisions will decrease but not disappear when in Mars orbit
  • There is no stealth tech involved
  • The ship's size should be irrelevant given the vastness of space - about 600m long for completeness
  • The ship does reflect light from the Sun

Human technology

It is the 21st century - any facilities, technolgies, sattelites and probes which are in service today or are planned to enter service within the next 5-or-so years can be considered.

The problem

This is where I might be completely wrong. The main problem is the vastness of space - while we have good knowledge of objects in Sol, we have issues at times detecting asteroids passing near Earth. On the other hand, these asteroids don't have an active signature like the ship. The other obvious problem is the ship's speed. There is also something to be said about limited FOV of human sensors as we're not watching every stretch of the sky, but apart from some probes which could maybe pick it up, there's Mars, which we are watching and where the ship will probably cross the FOV of at least one probe or telescope.

With all that: Could humans detect and track the alien ship before it reaches Earth?

  • $\begingroup$ Not enough information. The relative locations of Sun, Earth, Mars and the FTL exit point of the ship are extremely important. Eg if Mars and Earth are on opposite sides of the Sun the answer will be totally different than if they are on the same side. $\endgroup$
    – Tonny
    Commented Apr 4, 2022 at 10:24
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    $\begingroup$ @Tonny Added info to the OP. $\endgroup$
    – JANXOL
    Commented Apr 4, 2022 at 10:30
  • $\begingroup$ Big unknown here is the ship's propulsion mechanism. Quick acceleration/deceleration (0.7c - 0 - 0.7c) could be either as quiet as nothing, or as energetic as a gamma-ray burst. $\endgroup$
    – Alexander
    Commented Apr 4, 2022 at 19:18
  • $\begingroup$ @Alexander Good point. In this case the power spike would be present but minor, so we can assume there is a visible impact of deceleration or acceleration on signature, but nowhere near like the FTL flare. And as mentioned in the question, the signature when in orbit will be lower than when moving fast. $\endgroup$
    – JANXOL
    Commented Apr 4, 2022 at 20:17

2 Answers 2


It's very likely: the sky is being watched by dedicated programs tracking gamma ray burst, and something like what you describe, emitting gamma rays, won't go unnoticed.

Based on what I have read, once a gamma flash is detected, other telescopes are pointed to the likely source, imaging it in other portions of the EM spectrum. This would quickly make clear that something has happened within our solar system and that whatever is causing it is moving really fast. This would happen within hours.

Of course once the detection is communicated it will start the most time consuming part of the job: politicians and authorities deciding what to do. By the time the discussion is ongoing, the ship will be already close to Earth, solving at least the problem of "should we tell the people?"

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    $\begingroup$ Sad but probably true. By the time any scientist can get through to the higher echelons of government the aliens are already having a picnic on the lawn of the White House. $\endgroup$
    – Tonny
    Commented Apr 4, 2022 at 11:01

This looks like 2I/Borisov which was eventually noticed and then tracked all the way.

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    $\begingroup$ The issue here would be that the ship in question is moving about 6521.7 times faster than 2I/Borisov. Also Borisov was noticed 3AU from the Sun, so I wouldn't say it was tracked all the way, though it is being tracked now as it's leaving. $\endgroup$
    – JANXOL
    Commented Apr 4, 2022 at 20:40
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    $\begingroup$ It would improve your answer if you could expand on it, maybe relay information from the linked page that is of help to the OP. $\endgroup$
    – Joachim
    Commented Apr 7, 2022 at 6:30

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