However, at 1.9 billion kilometres, this object is very far from the Sun. Another mysterious feature is that much more gas and dust is escaping from the breakaway fragment than from the parent body. The disintegration has created a dust cloud more than 100,000 km across and which is several times brighter than the original object was before the event.

The object, called 60558 Echeclus, was discovered in 2000 and is a "centaur" - part rocky asteroid and part icy comet. Its new activity, revealed in images taken on 2 April, makes it look "really strange", says William Romanishen of the University of Oklahoma, US, one of the team that took the images. "The first thing that came to mind was a collision."

Earlier observations showed Echeclus rotates about once every 26 hours, so a fragment would need a push to escape its gravity, says Paul Weissman of the Jet Propulsion Laboratory, who spotted the original cloud of gas and dust around Echeclus on 30 December 2005.
Explosive sublimation

Such an impact on a comet-like body has not been observed before. But there are other possibilities, says Steve Tegler of Northern Arizona University, US, another team member. A likely one is that event was caused by explosive sublimation of volatile ices in Echeclus, resulting in material being blasted off, he says.

Tegler says the evaporating ice is probably carbon monoxide, with vaporises at about that distance from the Sun, where the object's temperature is about 80 degrees Kelvin, close to the sublimation temperature. No one has yet analysed the gas composition.

Another puzzle is the difference in activity between the main nucleus and the fragment. Freshly exposed ices normally sublimate, so "you'd expect equal activity from both pieces", Wiessman says. But the nucleus does not look very active.
Unstable orbits

Echeclus was discovered by the Spacewatch telescope in 2000, and at first looked like an asteroid. Then Weissman found it was surrounded by a coma, so astronomers also classed it as a periodic comet, 174P. The photos from 2 April show the coma has now spread out.

Echeclus belongs to a group of more than 100 centaurs with orbits well outside the main asteroid belt. Although originally from the distant Kuiper belt, they now orbit the Sun between Jupiter and Neptune, but will be ejected from those unstable orbits within tens of millions of years. Cometary clouds have bee reported around three other centaurs too.

Echeclus is currently moving towards the Sun on its 35-year orbit, and will pass closest to our star - about 880 million km - in April 2015. Other centaurs have become active as they moved inward, Tegler says. But none have shown such dramatic activity.

That is the view of a senior scientist working on the Cassini spacecraft, which has been studying Saturn and its moons for nearly two years.

Dr Bob Brown told a major conference in Vienna, Austria, Enceladus contains simple organic molecules, water and heat, the ingredients for life.

He raised the possibility of future missions to probe inside the moon.

Other research presented at the European Geosciences Union (EGU) annual meeting suggests that Enceladus may have a core of molten rock reaching temperatures of 1,400K (above 1,100C).

Jets and rings

In July 2005 Cassini completed a spectacularly close flyby of Enceladus, passing just 173km above its surface.

From this flyby came confirmation that the moon has an atmosphere, and strong evidence that the gases which make up the atmosphere are coming from cracks in the surface, nick-named "tiger stripes", near the south pole.

It appears that the gases are being forced through the surface, as they emerge in jets which shoot upwards for hundreds of kilometres before dispersing, eventually forming Saturn's E-ring.

Most of the gas is water vapour, suggesting strongly that liquid water lies under the moon's icy surface.

From his base at the University of Arizona, Tucson, Bob Brown leads the scientific team for Cassini's visual and infrared mapping spectrometer (Vims) which analysed the chemical composition of Enceladus's atmosphere and mapped the distribution of various gases.

"We very clearly saw water; there's water everywhere on Enceladus, it's 99.9% water ice in general at the surface, and we've known that for years, so it wasn't a big surprise," he told the BBC News website.

"But when we started looking at our spectra we saw absorption bands from a compound that had to have carbon and hydrogen bonded together.

"And when we mapped the location, it was right in these 'tiger stripes' - right where the jets are coming out, and right where it's hot - and it's pretty hard to imagine it's getting there from anywhere but inside."

The organic molecules appear to be quite simple, he said, probably largely methane.

The jets also contain nitrogen; and putting all this together means, said Dr Brown, that Enceladus contains all the ingredients necessary for the development of life, or of precursors to it.

"What you need to put microbes together of the kind that we're familiar with is carbon, hydrogen, nitrogen and oxygen, and water to act as an intermediary for metabolism," he said.

"You've got a rock core that's hot as hell; you've got all the conditions that we think gave rise to the first self-replicating molecules and eventually to life on this planet.

"So Enceladus in a very real sense becomes a stronger candidate for life than [Jupiter's moon] Europa, for instance."

One of the puzzling facets of Enceladus is how and why it is hot enough that it can generate liquid water and spew vapour into space.

Most of its surface has a temperature of about 80 Kelvin (minus 193C). But in the "tiger stripes" it soars to 140 Kelvin (minus 123C), and the interior must be considerably hotter.

Computer models have been produced which try to explain just how hot the interior needs to be, and examine the processes which could produce and maintain the temperatures observed today.

Dr Dennis Matson from Nasa's Jet Propulsion Laboratory (JPL) took EGU delegates through a model which envisages energy coming from two sources, radioactive decay and tidal heating, where differences in the gravitational forces exerted by a nearby body (in this case the giant planet Saturn) cause churning inside the moon, producing heat through friction.

"Down here [in the centre] we have molten magma," he said. "In this model, in the present day, it's entering a cooling phase which may go on for another billion years or so; but at depth you still have high temperatures."

Temperatures at the centre could reach 1,400 Kelvin, he said.

But there are still puzzles. Radioactive decay would have produced the vast majority of its heat shortly after the solar system's formation; somehow, Enceladus has retained some of that energy.

"We think there's a thermostatic mechanism going on in the magma," observed Dr Matson.

If the magma were to cool, he said, it would become more viscous, increasing friction from tidal churning and so producing more heat. But if temperatures veered higher, the magma would flow more easily, and tidal heat production would reduce accordingly.

Exceptional world

Along with all the other Cassini mission findings, the research presented here emphasises what an unexpected treasure trove of scientific novelty researchers have discovered on Enceladus.

Its tiger stripes amount to a "water volcano", the only one seen in the solar system other than on Earth.

Among our neighbours, it is the only known geophysically active world other than Jupiter's moon Io.

But as always with space missions, one set of answers leads to another set of questions.

The way to answer some is a further flyby in about two years' time, shortly before the end of Cassini's scheduled mission, which could take the $3.2bn craft just 25km above the tiger stripes and through their jets.

"There's a little bit of a danger, because observations suggest that the particles get larger as you get in closer," said Dr Brown.

"If they're only 20 or 30 microns [in diameter] they won't hurt the spacecraft; but if they're a millimetre or two, and hit the spacecraft in the wrong place, we're dead."

If there is enough fuel left on board Cassini and enough money in the coffers of its masters, the US, European and Italian space agencies (Nasa, Esa and Asi), the mission may gain an extension to its scheduled life, which could yield further flybys of the tiny moon.

But investigations aimed at looking for self-replicating molecules or even primitive forms of life would have to wait for a further mission.

For Europa, landers have been proposed which would burrow down through the top layer of ice into liquid water below, perhaps using heat from radioactive decay to penetrate the surface.

The same approach could potentially work on Enceladus; but Bob Brown believes there may be another, simpler way in.

"You could target the cracks; they clearly give you a way to get down inside, into the reservoir," he said.

"Now whether we can make something smart enough to do that robotically I don't know. But if there are bugs, they don't have to be in the ocean; they could live inside the vents, they just have to be somewhere where it's hot enough and they have enough energy to conduct metabolism.

"My guess is that if stuff has evolved in this ocean, it's figured out a way to work itself up into these vents; and maybe it's not completely crazy to think some of this stuff is sitting there near the surface."

First, the craft will direct the upper stage used to leave Earth orbit to crash into a permanently-shadowed crater at the lunar south pole, creating a plume visible to Earth-based observatories. Next, the satellite will observe the plume and fly through it using several instruments to look for water. Then the satellite will itself become an impactor, creating a second plume visible to lunar-orbiting spacecraft and Earth-based observatories.

"This type of payload is not new to NASA," said Associate Administrator for the Exploration Systems Mission Directorate Scott Horowitz, who made the selection. "We are taking advantage of the payload capability of the launch vehicle to conduct additional high risk/high payoff science to meet Vision for Space Exploration goals. It also signals to our workforce that innovative and competitive, low-cost approaches will be rewarded," he said.

The Lunar Reconnaissance Orbiter is the first of many robotic missions NASA will conduct between 2008 and 2016 to study, map, and learn about the lunar surface to prepare for the return of astronauts to the moon. These early missions will help determine lunar landing sites and whether resources, such as oxygen, hydrogen, and metals, are available for use in NASA's long-term lunar exploration objectives.

NASA's requirements for the secondary payload were that it benefits the robotic lunar program, cost no more than $80 million and not exceed 2,205 pounds (1000 kilograms).

On January 10, 2006, NASA issued a request for information to industry to allow businesses to provide secondary payload concepts to NASA. NASA encouraged its field centers to team with industry to develop proposals. Each NASA center reviewed ideas from industry, as well as secondary payload concepts developed internally. Several proposers, such as the winning spacecraft, took advantage of a new secondary payload adapter developed by the Air Force Research Laboratory, Kirkland Air Force Base, N.M.

NASA asked that the concepts advance the Vision for Space Exploration by advancing lunar science, characterizing the lunar environment, and identifying of sites for future human missions NASA was also looking for missions that would advance commercial opportunities and collect engineering data to support the Constellation program, which is developing NASA's new spaceship, the Crew Exploration Vehicle.

The European Space Agency (ESA) probe fired its main engine today for a 50-minute braking maneuver to allow itself to be captured by the gravitational pull of Venus.

Venus Express managers at ESA's mission control center here had loaded the engine-firing commands to the satellite several days before, and had little more to do than watch their screens and hope the sequence went as planned.

It did. Using large ground antennas located in Australia and Spain, ground teams confirmed that the engine firing was a success. With the satellite 74 million miles (120 million kilometers) from Earth, it took seven minutes for the satellite's signal to reach ground teams, creating some tense moments here as the science and mission teams waited for the satellite's signal.

"It's a great day for ESA," said ESA director-general Jean-Jacques Dordain after Venus Express' successful orbital arrival. "It's a happy day for all of us."

Tense times

During the orbital insertion engine burn, Venus Express' trajectory took it behind its target planet, meaning that for 10 minutes there were no radio signals available to ground teams. The mission control center went silent as ground teams waited to hear the return of the weak S-band signal from the satellite.

"We were sweating for a few minutes," said Manfred Warhaut, Venus Express flight operations for the ESA, in a post-orbit arrival press conference.

Applause followed the confirmation that the signal had been acquired anew. But there were 12 more minutes of engine burn that were needed to further slow the satellite's speed and confirm its capture by Venus.

Mission controllers applauded when the signal confirmed that the engine had completed its burn and shut down, making Venus Express the first dedicated orbiter to study its cloudy target since NASA's Magellan mapping probe plunged into the planet's atmosphere in 1994.

"It's a fantastic moment," said Don McCoy, Venus Express project manager at ESA. "We're finally around Venus. The S-band signal tells us we are in orbit. If we didn't get at least 46 minutes of engine burn, we basically would just fly past the planet."

Venus Express' initial nine-day orbit is designed to be highly elliptical ranging from an altitude of between 248 miles (400 kilometers) and 217,479 miles (350,000 kilometers) above Venus' surface, and gives the probe its only view of the entire planet's disk during its mission, mission managers have said. Two more burns will be conducted in the coming days to bring the orbit into its operational altitude of between 155 miles (250 kilometers) and 41,010 miles (66,000 kilometers).

"This is really a fantastic experience to see that we ended up in exactly the position we wanted to be in," Venus Express project scientist Håkan Svedhem said in the press conference. "Now our work starts."

Launched in November 2005, the ESA's $264 million (220 million euros) Venus Express mission is not expected to begin full operations until mid-May, but mission managers will begin switching the probe's seven observing instruments by Wednesday with the first image to arrive on Thursday, ESA officials said.

A cloudy world

While Venus is about the same age and size as Earth, it is 30 percent closer to the Sun.

Evidence from past satellite missions - the United States and Russia have sent more than a dozen satellites to Venus since the 1960s - shows that Venus once featured surface water.

"We have many questions surrounding Venus," Svedhem said, adding that the planet appears to have shared much in common with Earth during its early days. "Why is Venus the way it is, why is it not like Earth?"

For reasons not yet understood, Venus' atmosphere built up huge amounts of carbon dioxide, which trapped the sun's heat and drive temperatures to around 890 degrees Fahrenheit (477 degrees Celsius). It is an extreme example of the same greenhouse effect that operates on Earth, and scientists want to learn lessons from Venus that might be of use on this planet.

"If you want to find out about things on our own little planet, you have to look out," said David Southwood, ESA's science director.

Venus Express was built by a European consortium led by EADS Astrium, using hardware that was initially designed for Europe's Mars Express satellite. Instruments built for Mars Express and for Europe's Rosetta comet-chaser satellite were reused on Venus Express as well.

Taking advantage of previously used hardware helped speed Venus Express development. It took just three years from the time the manufacturing contract was signed to the launch of the satellite.

Initially designed for a 243-Earth day - or two Venusian day - mission, Venus Express has enough fuel remaining to double its time at the second planet from the Sun thanks to a spot on launch from Earth, mission managers said.

"We have quite a good margin of fuel on board," McCoy said, adding that Venus Express could study its cloudy target for up to six years if its extended mission is approved. "Right from the very instance of separation from our launcher, we've had an excellent trajectory to Venus."