The aliens could've reasoned that the moon's parent planet had been destroyed, which sent the moon on its journey.
But natural process can't hurl a planet or moon out of a solar system at relativistic speeds. If the moon is traveling fast enough for time dilation as several posters mention, or to pass a new solar system once a month without time dilation, it will be travelling many times faster than any natural interstellar planet sized object and thus be very interesting.
Note that the Moon would still rotate once every 27.321 days, which would indicate to aliens that it had probably been tidally locked into synchronous rotation and orbited its primary every 27.321 days.
The Earth-Moon system has an average orbital speed around the sun of 29.78 kilometers per second and the Moon has an average orbital speed of 1.022 kilometers per second. So if Earth instantly disappeared the Moon would add or subtract (depending on where it was in its orbit of Earth) up to 1.022 kilometers per second to or from the orbital speed of the former Earth-Moon system. Thus the speed of the Moon relative to the sun would be between 28.758 and 30.802 kilometers per second.
But at the Earth-Moon system's distance from the Sun the escape velocity from the sun (or third cosmic velocity) is 42.1 kilometers per second. So the Moon would not leave the Solar system if the Earth was destroyed.
The escape velocity from the Sun is 34.1 kilometers per second at the orbit of Mars, 18.5 at the orbit of Jupiter, 13.6 at the orbit of Saturn, 9.6 at the orbit of Uranus, and 7.7 at the orbit of Neptune.
A moon that orbited Jupiter with a period of 27.321 days would orbit between IV Callisto (16.698 days) and XVIII Themisto (129.87 days) and would have an orbital velocity between 8.204 and 4.098 Km/sec. Adding up to 8 km/sec to Jupiter's orbital velocity of 13.07 km/sec could give such a moon a velocity of up to 21.07 km/sec allowing it to escape from the solar system.
If the Moon rotated every 1.7691 days like Io it could have orbited a planet like Jupiter at a distance of 421,000 kilometers and an orbital speed of 17.334 kilometers per second. thus it could have escaped from the solar system at a significantly higher speed if its primary was destroyed.
But any moon that escaped from the Solar System after its primary was destroyed would be travelling at only a few kilometers per second and would take hundreds of thousands of years to reach and pass by another star system.
Of course a moon-sized object could be ejected from a solar system with a much greater velocity if the solar system contained two or more massive neutron stars or black holes orbiting very close to each other at great speeds. Of course such solar systems are very rare and so are objects ejected from them at great speeds.
So the aliens would consider the speed of the moon to be highly unusual.
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Fletcherj119
8 years ago
Well, the show would've been less interesting if every episode had been a "bottle episode" with no planet arrivals.
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magolding-94772
8 years ago
Well, the show would've been less interesting if every episode had been a "bottle episode" with no planet arrivals.
My point in that post wasn't about how unrealistic the premise was, but simply to point out that the speed of the moon was so many times the speed that any natural celestial body should have. Thus any aliens who detected the Moon should have found its speed highly unusual instead of thinking that it was some kind of normal rogue planet.
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Fletcherj119
8 years ago
Supposedly the moon kept passing through space warps and other phenomena, as opposed to having a faster-than-light cruising speed.
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