A Friend in the Darkness

~Planet Earth has a new friend in the vast emptiness of space: Comet Lovejoy. This newly-discovered comet was first detected on March 15th by Terry Lovejoy, an amateur astronomer in Brisbane, Australia, who located the comet not with a telescope but with a digital consumer camera! The numerical designation of the comet is C/2007 E2.

^ Comet Lovejoy currently has a magnitude of 9.5 (too dim to be visible with the naked eye), and is an interesting greenish-blue color. It is expected to become brighter, up to perhaps magnitude 7 during its closest approach to Earth in late April; that's still not visible to the naked eye, but easily visible through amateur telescopes. The above picture was taken with a more powerful telescope in New Zealand. Because of the comet's trajectory, it can only be seen from the southern hemisphere.

^ This computer simulation from NASA shows Comet Lovejoy's orbital path. Although it appears to go straight through Earth, bear in mind that the distances you're looking at in this image are massive. We're in no danger of being hit by the comet, I assure you.

Judging from its orbit, Comet Lovejoy is a "long-period" comet, with a highly elliptical orbit nearly perpendicular to the plane of the solar system's planets. Long-period comets originate in the Oort Cloud, a sphere of icy debris at the extreme outer edges of the sun's gravitational pull, perhaps up to a full light-year away. The recently-spectacular Comet McNaught is also a long-period comet, which none of us will be seeing again. Ever.

At the other end of the scale are "short-period" comets such as Halley's Comet, which swing by at small, regular intervals of less than 200 years. Halley's Comet, for example, has a period of about 76 years (which fluctuates a bit each time due to gravitational interactions with the planets it passes by). Since comets melt a bit every time they get close to the sun (that's what makes their gaseous tails), in astronomical terms short-period comets don't last very long before they melt completely or disintegrate. They usually (but not always) come from the Kuiper Belt, a rocky ring of comets and debris beyond the orbit of Neptune...which is a long way from Earth, though not nearly as far as the Oort Cloud.

^ Actually, I wasn't able to locate a period for Comet Lovejoy on the NASA site, but it's semi-major axis (the distance from the middle of the ellipse to the longest edge) is listed as "9.9999e+99 AU" (i.e., it's so far away that the number is not yet calculable). Not exactly a walk to the convenience store! The orbit is so stretched out that it isn't likely to swing by Earth again for millions of years.

An analogy springs to mind: when I was younger, I learned the difference between a "little-while friend" and a "long-while friend". A "little-while friend" is somebody you meet and are friends with, but only for a short time. Perhaps it's somebody you meet on a journey, or a family you chat with on a vacation. Ultimately, you have to leave them; you might promise to keep in touch, but in most cases both parties know it's more of a social nicety than a serious commitment.

In the case of Comet Lovejoy and other long-period comets, I don't really know how to classify their relationship with Earth. Are they little-while friends, because they'll only see each other for a brief instant before the comet sails off into the infinite abyss of space? Or are they long-while friends, since, although it may take unfathomable stretches of time, they will see each other again? What do you think? ~Oyasumi!