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Comment by phkahler | Hacker News Reader

Notice that this was spotted by an amateur only 2 weeks before closest approach. Why was it not found by the larger installations?

One problem I like to repeat at times like this, is that these hunts for near earth asteroids look for bright spots that move against the background stars. That's a nice way to find things nearby. But any object on a collision course with earth will not be moving against the background in the days and weeks leading up to the collision when they are also the brightest and most observable. I believe this is why the ones that have exploded in the atmosphere in recent years went completely undetected. It's not just that they're small, but that they don't have the relative motion against the sky to make them stand out.

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Congress mandated that NASA construct a surveillance capability for relatively-large NEOs (larger than the ones in OP), so the detection problem has been well-studied. A little googling will turn them up, but here's what I remember:

- Detection in optical by ground-based telescopes is a good technique. You need several exposures and good spot-tracking. Because of the relatively short time-base, you also need good follow-up or else the NEO diverges from your estimated orbit and is lost again!

- The parallax issue you mention comes up more regarding orbit determination than detection per se. (I.e., you need more than just detection to do anything useful.) One link going deep on the relationship of parallax to good orbits: [1]

- There are a bunch of such optical surveys, e.g. Pan-STARRs [3].

- But ground-based telescopes rely on reflected light, and so NEOs coming from certain regions (e.g., within Earth's orbit, but it's worse than that) don't have much reflected sunlight so are hard to detect optically.

- So, IR telescopes can be used, but you have to go above the atmosphere. See, NEOSurveyor [2]. IR is based on the infrared "glow" of a NEO against the background, and so it does not rely on reflected light. So IR telescopes get around many of the geometric limitations of optical.

- What about radar? It's very useful for follow-up orbit determination, but not so good for surveys, because the radar energy diffuses so much. Crudely, if the NEO is R away, you get a 1/R^2 on the way out, and a 1/R^2 on the way back. It's more complicated than that, but this is the reason that radar has limitations for this problem.

[1] https://iopscience.iop.org/article/10.1088/1538-3873/ac43ca/...

[2] https://www.jpl.nasa.gov/missions/near-earth-object-surveyor

[3] http://legacy.ifa.hawaii.edu/research/Pan-STARRS.shtml

None of those links addresses the problem. Finding NEOs is important because they pass by repeatedly and can be tracked. My point is that one on a collision course is not detectable with that method of looking for motion against the background.

The second point highlighted by a "-" is trying to get at that. It's not so much a pure detection issue, it's a tracking issue.

My understanding is that the robotic scanning telescopes can detect these objects, but that unless they get a high-quality track, the telescope has already moved on with its scan and the NEO will be lost. That is, detection is hard, but tracking is harder, and you need tracking.

And I guess my comments are also directed at the "systems" aspect of the NEO protection problem - detection, tracking, followup.

> Dan : [the President asks about the size of the asteroid] it’s the size of Texas, Mr President.

> President : Dan, we didn't see this thing coming?

> Dan : Well, our object collison budget's a million dollars, that allows us to track about 3% of the sky, and beg'n your pardon sir, but it's a big-ass sky.

But with computers and sky tracking charts shouldn't it be trivial to find new spots in the sky without using parallax? Anything new should be pretty note worthy.

Not all asteroids are “bright” enough to be imaged from earth, the problem is the low albedo asteroids you cant see, thats why we need this: https://solarsystem.nasa.gov/missions/neo-surveyor/in-depth/

Right. If you can't see them, you can't see them. It's not a parallax problem.

And even that won't see one on a collision course because there is NO Motion to detect when an object is on collision course.

how far away would you need them to be to detect/be able to respond

you could put LIGO-type "stationary" satellites around Earth to detect heavy things moving

in boxed grids then estimate trajectory over the affected cubes

Is there no parallax as the Earth moves around the Sun? Are these asteroids in some crazy hyperbolic orbit coming in straight from outside the solar system?

No, it looks like this asteroid had a similar trajectory as Earth around the sun, but after the encounter its path will be altered significantly.

"Before encountering Earth, the asteroid’s orbit around the Sun was roughly circular, approximating Earth’s orbit, taking 359 days to complete its orbit about the Sun. After its encounter, the asteroid’s orbit will be more elongated, moving it out to about halfway between Earth’s and Mars’ orbits at its farthest point from the Sun. The asteroid will then complete one orbit every 425 days."

How sure are they that this isn't just some spent rocket from 40 years ago?

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