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If an enormous asteroid, approximately the size of our moon (~2000-mile diameter), was passing close to Earth, how early might we detect something that large? Or alternatively, how close could something that large get without detection?

[edited to change term from meteor to asteroid]

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You want the time. Simply put, for that the minimum requirement is position (and hence the distance) and velocity. To know the position you need to detect it. Once you detect it, you can calculate the trajectory and thus the time you have to settle your issues (assuming it is on a collision course).

I got this from CNN:

The B612 Foundation is building the Sentinel Space Telescope, the world's most powerful asteroid detection and tracking system, to see the millions of asteroids we can't see today and could pose threats to our planet.

Also, NEOSSat, the Near Earth Object Surveillance Satellite, is a micro-satellite launched in February 2013 by the Canadian Space Agency (CSA) that will hunt for NEOs in space.

Tracking systems are recording asteroids even as large as 140 meters. Any asteroid with a radius more than 300 meters means an assured global catastrophe. Check this out. The size that you are asking about is so big that it will create noticeable gravitational effects (like perturbation in orbit) and so we will know about it.

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Since Pluto is about the size of our moon, clearly we can detect an object of that size at least at the distance of Pluto. We can probably spot something like that some distance into the Kuiper belt with current technology.

Of course the ability to detect something and actually detecting it are two different things. We have to be looking in the right place at the right time under the right conditions to detect it.

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To give an indication, comet ISON was seen first by two amateur astronomers when it was at magnitude 18.8, so a large body approaching from a distance might be found at about the same magnitude. Depending on albido, a moon size object could reach that magnitude at about twice the distance of pluto. The actual distance would also depend on how fast it was moving because it needs to move against background stars to be spotted by comet hunters.

A large professional telescope could see it at a much larger distance but that would require incredible luck.

If luck was against us it could come a little closer before being spotted. If it came from behind the Sun it might be missed until the Earth moved round but it is hard to imagine that it could come within the orbits of the gas giants before being seen unless it was coming in very fast.

Of course this is a highly unlikely event. we have never seen an object anything like that size come into the solar system and if it did then the chances of it coming close to Earth are miniscule. The real danger is from much smaller bodies.

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