Is somebody following us?

Do you feel like you're being followed lately? Well, you are. Don't worry, it's no one or thing dangerous. Asteroid 2009BD, discovered in early January, has been following us in an orbit almost identical to Earth's.

Just 30-feet across, 2009BD is no threat to Earth. It does orbit, however, close enough to qualify as a Near-Earth Object -- NEO). It made its closest approach to Earth Jan. 25, when it was just 400,000 miles away, or about 1.8 times the distance to the moon (about 250,000). You can do the math.

For the next several months, perhaps even years, this space rock will stay close to Earth, an average of 0.1 astronomical units away. An astronomical unit is the distance from Earth to the sun which is about 93 million miles and you can do the math on that one too.

Astronomers are calling it a co-orbital asteroid. One researcher quipped, "Its almost like it is stalking us."

Co-orbital asteroids exist elsewhere in the Solar System. Two groups, collectively known as Trojans, share Jupiter's orbit about the sun; the Trojans lie about 60-degrees in front of and behind Jupiter in its orbit. Each group forms one corner of an equilateral triangle, with Jupiter and the sun forming the other corners. Approximately 3,000 Trojans larger than one kilometer in size are known to exist, and millions of smaller ones likely await discovery.

A co-orbital asteroid is one located at one of five "Lagrangian points," where the asteroid's motion is stationary relative to two objects, like the planet and the sun. These points were discovered in 1772 by a French mathematician, Joseph-Lewis Legrange; he was trying to solve a problem involving the relationship of gravity and three orbiting objects. In the process, he discovered there are five points around any planet where the forces of gravity exerted by two massive objects (like the sun and a planet) balance out and any small object placed there would stay there without any additional outside force.

Yeah, I missed that day in physics class too.

At present there is at least one satellite that I know of using a Legrangian point to maintain an orbit. It is SOHO, the Solar-Heliospheric Observatory which is traveling in front of Earth in an orbit that keeps it focused on the sun 24/7/365 to see what the sun's surface is doing. You can find photographs and information about SOHO at http://soho.nascom.nasa.gov/ especially if you are looking for sunspots.

Anyway, 2009BD is too small to see, but it is interesting to know it is out there, traveling as our Earth's companion in its journey around the Sun -- at least for the time being. It is always nice to have a friend traveling with you.

SKY WATCH: Full moon and penumbral eclipse of the moon on Monday, Feb. 9. Within the penumbra -- the outer part of Earth's shadow -- the moon barely dims at all and it won't be really worth looking for. If you feel you must look, then take a peek at about midnight. Venus is still making a fine showing in the early southwest evening sky. Saturn is up in the eastern sky these evenings. If you are looking at it with a telescope be warned, the ring plane is almost flat and nonexistent for viewing from our perspective. Look about 9:30 p.m. for Saturn well below and left of a just-past-full-moon. By the way, the star to the left of the moon on the 9th is Regulus, the brightest star in Leo, the Lion. On Tuesday, the 10th, the moon is much closer to the golden planet which is still lying to the lower left.

Next time: more astronomical blathering.