This year, Comet Lovejoy C/2014 Q2 will be passing “close” to Earth from December through January, just in time for the holidays. It should be bright enough to be seen by the naked eye on a dark evening between sunset and moon rise. For a sky chart showing where the comet will be in the northern sky, see the bottom of the page here.
Lets talk about a few details about this comet, and a bit of history. First when I say “close”, I’m talking about 0.47 AU, or 77 million km. For reference 1 AU is the average distance between the Earth and the Sun. So still a pretty long distance. This is not the closest comet or asteroid to Earth by far. NASA JPL keeps a running list of all large objects detected by telescopes that may have an trajectory that intersects with Earth. In other words they keep watch for Earth impactors. The most recent and closest approach was by an object classified as 2014 YN, which got within 0.027 AU of Earth on Dec 18, 2014. 2014 YN was only 15-30 meters in diameter. A much larger object 2005 XV77 (200-450 meters) passed by two days earlier at 0.13 AU. That one would have ended life if it hit Earth. So while comets and asteroids are pretty, they can also be very dangerous on a scale we cannot imagine.
The difference between comets and asteroids is primarily one of composition. Comets tend to be mostly ice with some rock, while asteroids are mostly rock and metal. The trail of a comet is due to the sun heating up the ice and cause it to release gases. So you’ll rarely see an asteroid blazing through the night sky as they’re mostly water-less. Asteroid originate from the appropriately names asteroid belt between Mars and Jupiters. The belt is hypothesizes to be the leftover pieces from the formation of the solar system. The belt is actually located in a prime location for a planet if we had another. So some think the asteroid belt is the remains of a planet that didn’t form. Interesting huh.
Comets originate mainly from the Kuiper belt, a large ring of frozen ice and dirty balls past the orbit of Neptune. The other source of comets may be the Oort Cloud, a huge spherical cloud of icy rocks about 50,000 AU from the Sun. The Oort Cloud is not a proven thing yet, but it has been proposed by astronomers and so far seems reasonable. The reason we can’t prove or disprove its existence is that fact we can’t see the Oort Cloud objects from Earth. While the cloud may be a dense sphere of icy rocks, dense is a relative term. There are likely billions of comets in the cloud, but the space between them may be millions of kilometers. They are also very small on the cosmic scale. With telescopes we can detect stars (which are very bright), and sometimes planets by their way they interact with their stars. But the Oort Cloud comets are much much smaller than planets and do not shine like starts. So we can’t see them with even the most advanced telescopes, and we have not yet traveled that far with probes.
The green glow from Comet Lovejoy is a nice coincidence for Christmas. The color is due to the release of of cynonitrogen (CN) a poisonous gas, and diatomic carbon (C2) from the comet. When these two gases are hit by specific wavelengths of light from the Sun, they emit a green light, a process called resonant florescence. This same process is used in research on Earth to observe various molecules in gases.
Finally a bit of history of the Christmas comet. Probably the first possible Christmas comet is the Star of Bethlehem. I hedge a lot here because we truly don’t know if it was a comet, or a really bright planet, or possible a star going nova somewhere in the galaxy. Halley’s comet was know to have passed by Earth in 11 BC. It was recorded by the Romans and used to make the death of a General Agrippa. However the timing seems to be off compared to the birth of Christ. There are four leading theories of what the star was.
Halley’s comet did make a second appearance as a Christmas Comet, and on Christmas day no less. Sir Edmund Halley, whom the comet was named after, was a great astronomer and mathematician, and a friend of Issac Newton. In 1705, Halley used Newton’s law of gravitation and found three comets seen in the years 1531, 1607, and 1682 were indeed the same comet. He then predicted it would return to Earth in 1758. Quite a boast one would think. Luckily it came true. On Christmas Day in 1758, the Halley’s comet returned. Unfortunately Halley was not around to see it for he died in 1742.
So Christmas Comets have been around a long time, and each time they are a cause of wonder and joy. I hope everyone will go out an take the chance to see what the sights the universe has provided for us. Merry Christmas and Happy Holidays.