2009 BD: Curious cosmic clump co-orbits Earth

By William Atkins
Monday, 26 January 2009 18:59

Science - Space

A curious asteroid flew past Earth on Sunday, January 25, 2009. What is strange and curious about this asteroid called 2009 BD is that its orbit is almost identical with the orbit of Earth.


The asteroid 2009 BD, which was discovered by the Mt. Lemmon Survey, is about ten meters (33 feet) across. It is passing by Earth from a (closest) distance of about 644,000 kilometers (400,000 miles).

The Mt. Lemmon Survey is part of a larger astronomical project to survey near-earth objects (NEOs).

Such a distance away is of very little concern to people on Earth. This distance is further than the orbit of the Moon around the Earth, which has an apogee (furthest point in its orbit) of just over 400,000 kilometers (250,000 miles).

As SpaceWeather.com states, it “poses no significant threat, but it merits attention anyway.”

This special attention is due to the nature of its orbit and the Earth’s orbit. Being, a rare nearly “co-orbital asteroid,” 2009 BD is orbiting the Sun in “near-tandem” with our home base, the planet Earth.

“Co-orbital” means that it has the same orbit as Earth. Being “nearly co-orbital” means, in this case, that 2009 BD has an orbit that is slightly larger than the orbit of Earth.

Its semi-major axis is 1.007 astronomical units (AU), as compared to Earth’s semi-major axis, which is 1.000 AU. The definition of AU is the average distance between the Earth and the Sun.

Page two talks more about these two orbits.