How many comets are in the Solar System? SOHO's found 1500 of them.

A few days ago SOHO (Solar and Heliospheric Observatory) announced that it had discovered a total of 1500 comets so far, which isn't bad considering it wasn't designed to discover comets, but rather to just watch the sun. Here's what the article on spaceref.com says:

The ESA/NASA SOHO spacecraft has just discovered its 1500th comet, making it more successful than all other comet discoverers throughout history put together. Not bad for a spacecraft that was designed as a solar physics mission.

SOHO's record-breaking discovery was made on 25 June. The small and faint Kreutz-group comet was discovered by US-based veteran comet hunter and amateur astronomer Rob Matson.

Kreutz-group comets, or sungrazing comets have been observed for many hundreds of years. They travel very close to the Sun (if they were to hit it, they would become 'sunstrikers'), with perihelion distance less than 0.01 Astronomical Units (the mean distance between the Earth and the Sun), or some 1460000 km.

The reason why it's been able to discover so many comets is not because the SOHO team is expressly looking for them, but because of the work of amateur astronomers that go over the images to find them:

Of course, LASCO itself does not make the detections; that task falls to an open group of highly-skilled volunteers who scan the data as soon as it is downloaded to Earth. Once SOHO transmits to Earth, the data can be on the Internet and ready for analysis within 15 minutes.

Enthusiasts from all over the world look at each individual image for a tiny moving speck that could be a comet. When someone believes they have found one, they submit their results to Karl Battams at the Naval Research Laboratory, Washington DC, who checks all of SOHO's findings before submitting them to the Minor Planet Center, where the comet is catalogued and its orbit calculated.

All right then, so we've found 1500 so far, plus all the other ones discovered through other means. How many are left to discover? The ESA estimates the total number at about perhaps one trillion. Most of these are in the Oort Cloud though and will never get close enough to the sun to spot them with a craft like SOHO.

Anyone interested can find them too (it's remarkably easy to get started):

Anyone can help to search for SOHO's comets by visiting the Sungrazing comets page.

By the way, the Rosetta mission, launched to study a comet up-close, is going to fly by an asteroid in just two months, so keep an eye on that:

• September 5, 2008 - flyby of asteroid 2867 Šteins