This year is going to be a very busy year for comets. Everyone is eagerly awaiting comet ISON later this year, but that is going to be our third visit by a comet this year alone!
Our first comet, which our Southern Hemisphere readers have already been enjoying for several weeks, is comet Pan-STARRS. This comet is just beginning to peek above the horizon directly after sunset for us in the Northern Hemisphere, peaking this month in altitude and luminosity.
On 7 June 2011, an observatory at the summit of Haleakala, on the island of Maui, one of the islands in the state of Hawaii, turned its 1.8 meter (5’11”) telescope - the Panoramic Survey Telescope and Rapid Response System (PanSTARRS) - to the constellation Scorpio. After taking several photos, and spotting one dot of light moving in a way it shouldn’t... we had the birth of a comet! The comet was officially named C/Comet 2011 C4 PanSTARRS, but that’s a mouthful, so we just shorten it to the name of the telescope that discovered it, PanSTARRS.
When this comet was discovered, it was 7.9 Astronomical Units from the Sun. An Astronomical Unit, or AU, is the average distance of the Earth from the Sun. 7.9 AU’s puts this comets at the distance from the Sun equal to somewhere between the orbits of Jupiter and Saturn. The comet's orbit is very deeply inclined at 84.2° compared to the ecliptic; the plane of the Solar System. That means the comet almost comes in from directly below the Earth, and leaves directly above.
PanSTARRS orbit is highly eccentric as well. Its orbit is estimated to be at 110,000 years. That is an insanely long orbit (in case you didn't know). Astronomers believe the last time this comet flew into the inner Solar System was…never. This is thought to be this comet’s first and ONLY trip into the Solar System since the Sun ignited into a star 4.5 billion years ago.
Now I know some of you may be confused. "If we’ve been spinning around for 4.5 billion years, how is it that this comet, on an 110,000 year orbit, is making its first trip into the planets?" Well, let’s take a trip through our Solar System. (and here is a great piece about orbital comets and asteroids)
We have the Sun in the center, followed by the terrestrial planets. Next we have the Asteroid Belt, then the gas giants. After the gas giants, we have the famous Kuiper Belt, home to everyone’s favorite dwarf planet, Pluto. Most people think that’s where the Solar System ends, but it actually continues...
Extending one light year out from the sun in every direction lies the Oort Cloud, a sphere of dusty-icy debris that was launched to the very edge of the Sun’s influence. These ice balls are as cold as you can get. They are one light year away from the sun, and the Voyager probes haven’t gotten anywhere near it yet. At the time of writing this article, Voyager I is currently a little over 34 light hours away.
Now, in this cloud, the ice rocks are lightly bumping into one another as they ever…EVER so slowly orbit in their sphere. Every once in a while, an ice rock is bumped in towards the Sun, and we have the birth of a brand new comet. That is what has happened with PanSTARRS: 110,000 years ago it began its fall into the Solar System on its first journey to the planets since the Solar System’s birth.
The next time this comet will be visiting us will be in 110,000 years, after it flies back to the Oort Cloud and back down again. To put that into some perspective, let’s say you live to be 100 years old, and all of your descendants will live to be 100 years old as well. It will be one thousand generations of descendants before this comet is seen again, cementing this event as once in a lifetime. When I was originally planning out this article, I was going to write “great” for every generation until this comet came around again, such as your “great, great grandchild” but I’d be writing “great” one thousand times. It’s take me a few days to write this, and the comet would be gone by then.
This comet is a rare comet as well. It will have a very bright coma, or head, telling us that it’s more icy than dusty, and will have two tails. People throughout history have considered comets to be the harbingers of death and destruction, but when a two tailed comet comes about, people just lose their minds. But actually, all comets have two tails; both cannot be seen all of the time, nor at the same time. A comet's first tail comes from dust particles and vaporized ice. These are affected by the radiation from the Sun, which pushes these particles back away from the comet along its trajectory. The second tail comes from ions which first come from the nucleus as neutral gaseous particles, and are swept into the second comet tail by interaction with the Sun's magnetic field. This field sweeps past the comet nucleus and carries the ions with it to form the tail. Because of the special interaction with the magnetic field, this tail always points directly away from the Sun. This comet will give us a view of both tails.