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Actually, comets are considered building blocks of the solar system. That's why there's a large push from NASA and the ESA to send spacecraft to comets and land on them and/or gather samples from them. Here are a few links:

Stardust

Contour (failed)

Rosetta (to be launched)

Here's the latest press release from the CONTOUR people, which can be found here:

CONTOUR: Latest News
August 16, 2002 -- 1 p.m. (EDT)
Search for CONTOUR Continues

Mission operators continue to listen for a signal from CONTOUR.

Using its 34-meter antennas, NASA's Deep Space Network stations are scanning the spacecraft's expected path beyond Earth's orbit, attempting to pick up radio signals from CONTOUR's transmitters. The CONTOUR team is also awaiting feedback from several NASA-sponsored and other optical and radar sites that have been searching the skies for signs of the spacecraft.

CONTOUR's STAR 30 solid-propellant rocket motor was programmed to ignite at 4:49 a.m. EDT on Aug 15, boosting the spacecraft out of an Earth parking orbit and onto a trajectory to encounter two comets over the next four years. The spacecraft was too low for DSN antennas to track it during the burn - about 140 miles (225 kilometers) above the Indian Ocean - and the CONTOUR mission operations team at the Johns Hopkins University Applied Physics Laboratory expected to regain contact about 45 minutes later to confirm the burn. No signal was received, and the team has been working through plans to find the craft along the predicted trajectories for a successful burn.

CONTOUR's onboard computer was carrying a command that, starting at 6 a.m. EDT today, would have turned the spacecraft and pointed another of its four antennas toward Earth. So far, however, no signal has been received.

CONTOUR, a Discovery-class mission to explore the nucleus of comets, was built and managed by the John Hopkins University Applied Physics Laboratory, Laurel, Md., for NASA. Additional information about CONTOUR is available on the Internet at: http://www.contour2002.org.

Here's the latest press release from the CONTOUR people, which can be found here:

CONTOUR: Latest News
August 16, 2002 -- 1 p.m. (EDT)
Search for CONTOUR Continues

Mission operators continue to listen for a signal from CONTOUR.

Using its 34-meter antennas, NASA's Deep Space Network stations are scanning the spacecraft's expected path beyond Earth's orbit, attempting to pick up radio signals from CONTOUR's transmitters. The CONTOUR team is also awaiting feedback from several NASA-sponsored and other optical and radar sites that have been searching the skies for signs of the spacecraft.

CONTOUR's STAR 30 solid-propellant rocket motor was programmed to ignite at 4:49 a.m. EDT on Aug 15, boosting the spacecraft out of an Earth parking orbit and onto a trajectory to encounter two comets over the next four years. The spacecraft was too low for DSN antennas to track it during the burn - about 140 miles (225 kilometers) above the Indian Ocean - and the CONTOUR mission operations team at the Johns Hopkins University Applied Physics Laboratory expected to regain contact about 45 minutes later to confirm the burn. No signal was received, and the team has been working through plans to find the craft along the predicted trajectories for a successful burn.

CONTOUR's onboard computer was carrying a command that, starting at 6 a.m. EDT today, would have turned the spacecraft and pointed another of its four antennas toward Earth. So far, however, no signal has been received.

CONTOUR, a Discovery-class mission to explore the nucleus of comets, was built and managed by the John Hopkins University Applied Physics Laboratory, Laurel, Md., for NASA. Additional information about CONTOUR is available on the Internet at: http://www.contour2002.org.

I must finish with a shameless plug for the exciting computer animation I created to illustrate CONTOUR's mission, available at the CONTOUR website

Asteroids are held together by gravity... that is why NEAR could land on one, and why these photos from the NEAR probe show a boulder field strewn with SEVERAL rocks.

The limiting factor on how big an asteriod can get without falling apart is a combination of centrifugal force caused by its rotation, and the Roche limit of how far away it is from a larger body that inflictes tidal forces upon it. More on the Roche limit here.

Comets may be a single cohesive body of some sort, we don't know yet. Hopefully the Contour mission will tell us what comets really are made of.

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