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A satellite tracker’s analysis of a meteor seen over the United Kingdom on September 21 suggests that it was an Aten asteroid, not space debris as originally believed.

Aten asteroids orbit the Sun at an average distance less than 1 AU (in other words, inside Earth’s orbit). Because their orbits are highly eccentric, most Aten asteroids cross the Earth’s orbit when approaching aphelion. This makes Aten asteroids potentially hazardous objects.

The asteroid 99942 Apophis is an Aten asteroid. Apophis, which was discovered in 2004, made news because it is large enough to be dangerous and was calculated to have a significant probability (2.7%) of hitting the Earth in 2029. Later estimates reduced that probability greatly but there is still sufficient uncertainty that some observers would like additional data.The Russian space agency Roscosmos recently announced plans for an unmanned mission to place a radio beacon on Apophis.

Aten asteroids are not only dangerous to the Earth, they are also difficult to detect and observe. Because Atens spend most of their time inside the Earth’s orbit, their sky position, as seen from the Earth, is close to the Sun. The brightness of the daytime sky interferes with observations by ground-based telescopes. Spaceborne telescopes don’t have the bright sky to contend with, but they have insrument-safety considerations that limit their ability to observe objects close to the Sun. If a space telescope like Hubble was accidentally pointed at the Sun, it would burn out a camera that costs hundreds of millions of dollars (and can no longer be replaced, since the Shuttle is retired). So, operators impose strict limits on telescope pointing to prevent it from getting too near the Sun.

One solution to this problem is the Atsa Suborbital Observatory, being developed by astronomers at the Planetary Science Institute and The Citadel. The Atsa Suborbital Observatory is a fairly small telescope, based on a 14-inch Celestron reflector, which will fly aboard the XCOR Lynx Mark III. Because the Atsa telescope is easily serviceable, it can be pointed very close to the Sun with little danger. Accidentally burning out a $500 CCD chip that can replaced the next day is not a catastrophe like damaging the Hubble.

In other news, a meteor the size of a car landed in the Martinez Hills in the San Francisco Bay Area. There are no reports of casualties or damage, but this is another reminder of the fact that our Earth is constantly being hit by objects from space. Sooner or later, we will be hit by something much larger and more dangerous.