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The really interesting prospect raised in that Reddit thread is that while the velocity is likely wrong for an interstellar spaceship, it could be just right fo

The really interesting prospect raised in that Reddit thread is that while the velocity is likely wrong for an interstellar spaceship, it could be just right for the spent and discarded deceleration stage of an interstellar spaceship. In which case the spaceship would be arriving some time later...

cjsuk 9y ago
I like this explanation best, as long as they're friendly.
api 9y ago
Watch some rocket separations. Usually they end up tumbling.
valuearb 9y ago
It's already passed pedigree with the sun. It is traveling slowly enough that if it's a discarded stage the spaceship should already be here.
- semi-extrinsic 9y ago
  How do you know that, without knowing the non-booster-assisted deceleration rate of the spaceship? (Presumably the hypothetical ship separated from the booster when that was out of fuel, and then the ship continued decelerating on its own to get to a velocity congruent with a stable orbit in our system).
  - valuearb 9y ago
    The object is traveling at 26Km/sec, earth is orbiting at 30 Km/sec. To generate a "stable orbit" is a matter of maneuvering at that velocity. So it's really unlikely that the parent ship is traveling much slower than the discarded booster, and so it's arrival is unlikely to be much later.
- xelxebar 9y ago
  I'm assuming autocorrect helpfully fixed "perigee" to "pedigree" for you. It's a nice little malapropism though. Anyway, minor point but "perigee" usually means closest approach to Earth; "-gee" is from the Greek for Earth (as is "geo-"). In the case of the sun, we have the word "perihelion", and for generic orbits around an object, you generally hear "periapsis". And just for completeness, I'll mention that the counterpoint to periapsis is apoapsis, with apogee and aphelion being the obvious specializations. There are more words in this pattern for various different objects; Wikipædia has all the gory details: https://en.m.wikipedia.org/wiki/Apsis https://en.m.wikipedia.org/wiki/Apsis
  - valuearb 9y ago
    Autocorrect figured out I meant that we need to make sure aliens have sufficient pedigree before we deign to talk to them.

cjsuk 9y ago

I like this explanation best, as long as they're friendly.

api 9y ago

Watch some rocket separations. Usually they end up tumbling.

valuearb 9y ago

It's already passed pedigree with the sun. It is traveling slowly enough that if it's a discarded stage the spaceship should already be here.

semi-extrinsic 9y ago
How do you know that, without knowing the non-booster-assisted deceleration rate of the spaceship? (Presumably the hypothetical ship separated from the booster when that was out of fuel, and then the ship continued decelerating on its own to get to a velocity congruent with a stable orbit in our system).
- valuearb 9y ago
  The object is traveling at 26Km/sec, earth is orbiting at 30 Km/sec. To generate a "stable orbit" is a matter of maneuvering at that velocity. So it's really unlikely that the parent ship is traveling much slower than the discarded booster, and so it's arrival is unlikely to be much later.
xelxebar 9y ago
I'm assuming autocorrect helpfully fixed "perigee" to "pedigree" for you. It's a nice little malapropism though. Anyway, minor point but "perigee" usually means closest approach to Earth; "-gee" is from the Greek for Earth (as is "geo-"). In the case of the sun, we have the word "perihelion", and for generic orbits around an object, you generally hear "periapsis". And just for completeness, I'll mention that the counterpoint to periapsis is apoapsis, with apogee and aphelion being the obvious specializations. There are more words in this pattern for various different objects; Wikipædia has all the gory details: https://en.m.wikipedia.org/wiki/Apsis https://en.m.wikipedia.org/wiki/Apsis
- valuearb 9y ago
  Autocorrect figured out I meant that we need to make sure aliens have sufficient pedigree before we deign to talk to them.