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Comment by darkerside | Hacker News Reader

Perhaps a dumb question but...

"The mysterious object flew within about 15 million miles of our planet on October 14.

But it wasn't until four days later that humanity finally spotted it in telescope data."

How do we know any of this if we didn't see it until later? Or are we just reverse extrapolating it's orbital path from what we saw later?

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In general, most laws of physics are reversible, so extrapolation backwards is as accurate as extrapolation forwards. In the case where it's just mostly motion and gravity, we can do it rather accurately.

I know that, but that makes the assumption that the flight path was unaffected by external forces. How is this distinguishable from, say, an alien spacecraft seeing it was spotted, boomeranging around the Sun, and exiting the solar system?

*Wow, sorry. Guess I'm a conspiracy nutjob for skiing an innocent question.

Theoretically we can't distinguish them. But then again, if CCTV shows a hooded guy exiting your shop's front door at 4 am, it would be safe to assume that the same guy entered the shop some time before that, even though theoretically it could be an alien lifeform teleported into your shop out of thin air.

It's never aliens. (Until we actually find them, of course.)

Seems more akin to saying, since we see this runner on the track, he must have been running around the track for an undeterminable period of the past time. It's just as possible that the runner just started running when you saw him.

All I wanted to know is if we observed the past path or inferred it. Seems a valid question to me, but whatever.

I think what they're saying is that we saw the flyby but it was in data from a few days ago.

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