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Comment by adyus | Hacker News Reader

Let's hope it has enough velocity to escape Earth's field and carry on. I'm sure it does, but I'll keep my fingers crossed, just in case.

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Give science a little more credit :) The n-body problem is hard, but it's more like "we don't know if this same asteroid will hit the Earth 20 years from now" hard than "this asteroid could shift course at any moment!" hard.

From an earlier press release on this asteroid, from a 2010 pass:

[The radar imaging] reveals 2005 YU55 as a spherical object about 400 meters (1,300 feet) in size.

Not only can the radar provide data on an asteroid's dimensions, but also on its exact location in space. Using Arecibo's high-precision radar astrometry capability, scientists were able to reduce orbit uncertainties for YU55 by 50 percent.

"At one time we had classified 2005 YU55 as a potential threat," said Steve Chesley, a scientist at JPL's Near-Earth Object Program Office. "Prior to the Arecibo radar passes on April 19 thru 21 [2010], we had eliminated almost all upcoming Earth flybys as possibilities of impact. But there were a few that had a low remaining probability of impact. After incorporating the data from Arecibo, we were able to rule impacts out entirely for the next 100 years."

So, they do know this one will be well-behaved for 100 years.

The n-body problem is hard to solve analytically, but trivially easy to solve to arbitrarily high precision numerically.

Not entirely trivial. Because it is chaotic, it's not terribly well behaved. People who try to integrate the solar system forward in time have to put some effort into accuracy, and ultimately accuracy is limited by our limited knowledge of the initial state of the system.

I work at NASA, supporting the person who wrote the codes that is now used for planetary dynamics simulation, like this. It is an extraordinarily complex problem, and a specialized academic discipline in its own right.

n-body dynamics is a complicated, chaotic system. Getting any precision at all over any reasonable period of time is a feat of itself.

from NASA's gif, it doesn't even bend its trajectory.. and also it's gravity will not affect earth's tides and tectonic plates :)

EDIT: although from the gif it appears that it was a close call on the moon. But i close my eyes and i see a world where i get out and see a small piece of rock hit our moon with a decent explosion when the moon is full. Man if it does't affect out planet in any way, i would really like to see that happen. Gives me goosebumps every time i visualize it! amazing..

There's a thought. What would happen, hypothetically speaking, if an asteroid large enough were to hit the moon and spin it out of its orbit? What would happen to life here on Earth? Obviously tides would be affected, and I am assuming that this in itself would probably affect a lot of animals, fish, birds etc?

well moon moving out of its trajectory means alot more than tides (which in itself is pretty darn serious) IMO. The nights would be absolutely dark, also earth's rotation speed (AFAIK) is also affected by moon's gravity and bulges in both earth and moon's softer crust. Plus maybe we can adapt to such changes but our culture will change and it will be alot more difficult to live through that, imagine explaining to your kids/grandchildren what moon is (or rather was) and how it looked when it was closer/farther.

PS if moon gets a major hit, its highly unlikely earth stays 'physically' unaffected. The aftermath of the collision would leave numerous chunks of huge rocks in earth's near space and it would be almost certain that some of then then hit earth. And the effect of decent sized rock hitting the planet is better known to our long lost friends, the dinosaurs :)

Thanks for that. Quite a thought. Hopefully this will never happen - am assuming that the Earth has more chance of being hit than the moon (the moon being smaller and faster)?

But would the nights be totally dark? I've experienced a lot of nights out on a boat on a moonless night and the stars provide a (relatively) large amount of light on their own.

hmm well they do give some light but it would not be nearly as bright as our beloved moon :)

The Moon isn't faster. The velocities you care about for Solar System impacts are sun-relative. The Earth and Moon have essentially the same velocity relative to the sun, since they orbit it together. The Moon moves a bit slower at new phase and faster at full phase because its Earth-relative velocity subtracts from or adds to its Sun-relative velocity, but the former is 1 km/sec and the latter is 30 km/sec, small potatoes.

As for who gets hit; Earth's mass is 81 times that of the Moon so it attracts objects 81 times as strongly. Really, the Moon would only get hit if Earth already pulled an object into a collision course or near-miss and the Moon just happened to get in the way at the right time. Earth has 4x the diameter so presents 16x the cross-sectional target that the Moon does.

An asteroid that could move the moon would have to be moving insanely fast, or be close to the size of the moon (due to conservation of momentum).

I'm pretty sure that said hypothetical impact can't exist. The object either has far too little energy to significantly change its orbit, or alternatively has enough energy but there is nothing moon-like left after that collision.

The moon has a mass of 7.349×10²² kg. Its mean orbital velocity is 1023m/s. So, with a perfectly-aligned impact, and perfect efficiency, to add velocity, you'd need to add energy equal to difference in kinetic energy. For 1m/s, that'd be 10²⁵J (assuming the change in mass is negligible). That's an incredible amount of energy, about 20 times greater than the Chicxulub impact (believed to have triggered the mass extinctions at the K-T boundary).

That's 1 m/s. You need far more than that. Earth escape velocity is 11200m/s. So, that would seem to need 10³⁰ J. And an impact would deliver that in well under a minute. Which would be a problem, since that's an order of magnitude greater than the gravitational binding energy of the moon.

Conclusion: impact required would fully obliterate moon.

(Moon mass and velocity, and escape velocity from http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html. Gravitational binding energy and estimate of Chicxulub from Wolfram Alpha)

Don't forget that's a two-dimensional picture of 3D space, so it's probably not nearly as close to the moon as it appears. While most of the objects in the solar system are in kinda-sorta the same plane, they're not quite.

hmm well yes.. i almost forgot about the third dimension but that was a hypothetical situation after all.. :)

...except that the solar system is two dimensional, aligned on a plane?

See where I said "kinda sorta"?

If all the objects in the solar system were in exactly the same plane we'd have a solar eclipse every month. The moon's orbit, for instance, is inclined at five degrees to the ecliptic.

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