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Near misses for Earth today

Asteroid Itokawa

This is asteroid Itokawa, which was visited in recent years by a Japanese spacecraft. The two asteroids due to pass by Earth today are much smaller than Itokawa.

Two small asteroids are due to pass close to Earth within the next 24 hours. So close, in fact, that they’ll be close to us than the Moon is.

Discovered on Sunday by astronomer Andrea Boatini at the Mount Lemmon Observatory in Arizona, they are known only by their catalogue names: 2010 RX30 and 2010 RF12.

2010 RX30 is thought to be 32 to 65 feet (10 to 20 metres) in width. It’ll pass within 0.6 of the distance between the Moon and the Earth (about 154,000 miles, or 248,000 kilometres) at 5:51am US EDT on Wednesday…that’s 7:51pm Sydney time on Wednesday).

The second object, 2010 RF12, estimated to be 20-46 feet (6 to 14 metres) in size, will pass within 0.2 of the Moon-Earth distance (about 49,088 miles or 79,000 kilometres) at 5:12pm US EDT on Wednesday…that’s 7:12am Sydney time on Thursday.

Asteroid orbit diagrams

Orbits of the inner planets (white circles) and the asteroids (light blue lines). Arrows show where the orbit of Earth and the asteroids' trajectories almost intersect.

Experienced amateur astronomers will have a chance of seeing them, depending upon where the observer is located on the Earth (eg. whether it is daylight or night-time at the observing location). Details are available on the Fawkes Telescope web site.

But the two small space rocks won’t be visible to the unaided eye—they’ll be hundreds of times fainter than the naked eye can see.

Neither of the objects have any chance of hitting us during these close approaches, nor indeed at any reasonable time into the future.

So why weren’t they discovered earlier?

Asteroids don’t give off any light of their own…all they do is reflect sunlight. And being made of rock, they aren’t very reflective. Add this dullness to the fact that small asteroids don’t reflect as much light as big ones do (because of their smaller surface area), and you can see that small ones are dim. And this makes them harder to spot.

They have to be almost upon us before they can be seen. Indeed, some have been spotted only once they’ve gone past!

Small asteroids such as these don’t pose much of a risk to anyone on Earth. If they hit, they would explode in the upper atmosphere—due to the tremendous deceleration forces experienced during atmospheric entry—with some small fragments probably surviving to fall to the ground as meteorites.

This is what happened in October 2008, when a small asteroid that had been spotted only a few days before, entered the atmosphere over Sudan in Africa and exploded. Searchers went out and found some fragments a few days later.

And we’re probably being buzzed by them all the time. NASA says that a “10-metre-sized near-Earth asteroid from the undiscovered population of about 50 million would be expected to pass almost daily within a lunar distance, and one might strike Earth’s atmosphere about every 10 years on average.”

Adapted from information issued by NASA / JAXA.

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