The highest resolution image obtained by the Stardust spacecraft as it flew by Comet Wild 2 in January has just been released by the US space agency.
By Dr David Whitehouse
BBC News Online science editor
The image shows a pockmarked "flying mountain" strewn with jets of gas and dust that billow in the solar wind.
As well as taking pictures, Stardust managed to capture and store thousands of fresh cometary dust particles released from the object's surface.
The probe will return this cargo to Earth for analysis in January 2006.
The probe passed within 240km of Comet 81P/Wild 2 on Friday 2 January, snapping 72 images and collecting a mass of data which scientists continue to scrutinise.
Now, the mission team has put out a remarkable composite image of the comet nucleus.
It is a short exposure image that shows tremendous surface detail, overlain on a long exposure image taken just 10 seconds later that reveals jets.
Stardust will return its precious cargo in 2006
"This spectacular composite image shows a surface feature unlike any other planetary surface seen to date in our Solar System," says Professor Donald Brownlee, from the University of Washington.
"Other than our Sun, this is currently the most active planetary surface in our Solar System, jetting dust and gas streams into space and leaving a trail millions of km long.
"The overall shape of the nucleus resembles a thick hamburger with a few bites taken out," says Thomas Duxbury, the Stardust Project Manager, from Nasa's Jet Propulsion Laboratory.
"The surface has significant relief on top of this overall shape that reflects billions of years of resurfacing from crater impacts and out-gassing."
Stardust will bring samples of comet dust back to Earth to help answer fundamental questions about the origins of the Solar System.
The remarkable image provides some clues about what it would be like to walk on the surface of a comet.
Astronomers do not know the strength of the surface. When the comet is active, some regions might be strong enough to support an astronaut; elsewhere this individual could sink into a fluffy "snow".
Any visitor would not see very far because on such a small world the horizon is very close.
Looking around, the astronaut would see a landscape of small hills, of crumbling dust and occasionally huge depressions that are the entrances to a vast subterranean network of constantly shifting caverns.
On a comet, geysers of gas and dust erupt into space, many starting their journey below the horizon. When they reach up into the sky, they are twisted into curious spirals by the nucleus's spin.
Some geysers start in shadow and then dramatically burst into sunlight as the jets are caught by the solar wind and are blown away from the Sun.
At this stage of the comet's orbit, no stars would be visible to a surface astronaut - the sky would be too full of glowing gas.
Stars would only be visible when the comet receded into the cold outer Solar System where it would freeze and become a small inert world.
Gravity is slight, so an astronaut could jump into space and take weeks to come back down again. They could watch the nucleus rotate beneath them.
As well as gravity, other forces come into play because of the comet nucleus's small size and rapid spin. The astronaut could stride to the cometary equator where so-called Coriolis forces are apparent.
Throwing snowballs of cometary material into the air, the astronaut would see these missiles curve in seemingly illogical directions. They could even be tossed against the comet's spin and appear to hang motionless in space.
No manned cometary landings are planned and probably never will be because of the dangers involved. However, the unmanned Rosetta spacecraft will release a small probe to touch down on Comet 67P/Churyumov-Gerasimenko in 2014.