Comets have been known to man since ancient times (at least since Aristotle, around 350BC) and we are currently aware of the existence of some three and a half thousand of them, yet they remain the most elusive and least understood bodies in our solar system.
Almost half of the comets we know about today are what we call Kreutz Sungrazers
believed to be fragments of one large comet that broke up at least 2000 years ago. As of Friday last week, (March 12th, 2010) there was one less as the NASA spacecraft SOHO watched a newly-discovered sungrazer get just a little too close to the Sun. You can see the final moments of this ill-fated comet at the SOHO movie theater
- I recommend selecting image type "LASCO C3" with the start date 2010-03-12 ... you can see the comet move from the bottom left of the image, getting increasingly brighter and more elongated as it approaches the Sun for probably the last time.
As far as I can tell, though I confess I haven't done much research on the subject, the comet didn't impact the Sun it might at first appear. There were certainly no Hollywood-style explosions on impact. As the comet got too close, the energy from the Sun will have vapourized it. According to spaceweather.com "several of these fragments pass by the sun and disintegrate every day. Most are too small to see but occasionally a big fragment--like this one--attracts attention."
This year seems to turning out to be a very good one for astronomers interested in comets.
In January this year, an object (named P/2010 A2) was discovered which was initially believed to be a rare asteroid-comet hybrid known as a main belt comet
- and by rare, I mean super-rare. There are some 3,600 known comets, and there are an estimated 1.2 million asteroids 1km across or larger in the main belt, but there are just 4 known main belt comets. But as astroengine
explained nicely it could also have been something even more exciting - the first ever sighting of a collision between two main belt asteroids.
The main asteroid belt is not what you might expect after watching SciFi like Star Trek, where the crew have to navigate between densely packed asteroids regularly bumping off one another. Despite the missions of asteroids in the belt, the average distance between them is still immense, and collisions between main belt bodies with a mean radius of 10 km are expected to occur just once every 10 million years.
When impacts do occur, the relative speeds of the asteroids can be quite significantly different. In the case of P/2010 A2 it's possible that a hypervelocity
impact occured - that is to say one in which the relative speeds are different by many km per second, which leads to some very interesting effects on the structures involved.
After photographs were taken with Hubble (see right), this assessment was revised and is indeed now considered to be the first sighting of a collision between two main belt asteroids.
Hubble's lead scientist David Jewitt said "The thing that we want to understand is how the asteroids smash into each other and destroy each other. It might help us understand even how to destroy an asteroid and prevent one from hitting us."
And some time today, the ESA Rosetta spacecraft will be taking photographs of P/2010 A2 with its OSIRIS cameras. While OSIRIS is not as powerful as Hubble, Rosetta is currently on its way towards the main belt where it will perform a flyby of the asteroid Lutetia on July 10th later this year, before continuing further out and ultimately rendezvousing with the comet 67P Churyumov-Gerasimenko in 2014. Rosetta has just passed the orbit of Mars, and is about 150 million km away from P/2010 A2 right now - a very long distance, the same distance as the Sun is from the Earth, but also much closer than the Earth (and Hubble) - 80 million km closer, to be precise. In fact, this distance is some 6 million km less than the closest distance the Earth and P/2010 A2 will ever be. Combining this relative closeness with the benefits of being able to take images from a different angle, should hopefully make these images extremely useful to scientists in putting together the pieces of this fascinating event (sorry, bad pun).