NASA’s Dawn probe set to begin orbiting the dwarf planet Ceres

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Big, dim, obscure – and just possibly a secret haven for alien life. That's Ceres in a nutshell.

At 950 kilometres across, it was once known as the largest asteroid, but it has lately been upgraded to the status of dwarf planet. Whatever it is, we are about to see it up close for the first time thanks to the imminent arrival of NASA's Dawn mission. After a 7 1/2-year journey, Dawn is set to begin orbiting Ceres on March 6.

"The approach has gone flawlessly so far," Dawn's project manager, Robert Mase, said during a news briefing held on Monday at the Jet Propulsion Laboratory near Pasadena, Calif.

Now comes the fun part, as planetary scientists test their ideas about Ceres against the reams of images and data that they hope the Dawn probe will radio back over the next 15 months.

Topping the list will be looking for evidence that Ceres is geologically – and possibly hydrologically – active, with pockets of briny subsurface water that may occasionally vent vapour into space and sustain colonies of microbes deep below the surface.

The odds of Dawn spotting signs of life would be a long shot, to say the least, but given that Ceres almost certainly has a thick layer of ice beneath its dusty surface, an abundance of carbon and some internal heat, it is clearly a destination of interest for astrobiologists.

The encounter also promises to plug one of the last remaining gaps in humanity's initial reconnaissance of the solar system. After Dawn's arrival at Ceres and the coming flyby of Pluto by the New Horizons spacecraft this July, every major object in orbit around the sun that was known before the era of spaceflight will have been explored by robot probes.

But for scientists, the $475-million Dawn mission is about more than ticking off all the boxes on the cosmic tour bus. Ceres is a puzzle they have been eager to unpack since it was first spotted more than two centuries ago.

Back then, astronomers were confounded by the large and apparently empty space between the orbits of Mars and Jupiter. Mathematically, it looked an awful lot like there should be a planet there, if only for aesthetic reasons.

When Giuseppe Piazzi, an Italian astronomer and Catholic priest, spotted a faint object orbiting in the gap in 1801, the mystery of the missing planet seemed settled.

Piazzi, whose observatory was based in Palermo, dubbed the object Ceres Ferdinandea after the patron goddess of Sicily and the island's reigning monarch at the time, King Ferdinand IV. Astronomers of the day adopted Ceres but dumped the King in keeping with naming conventions.

The story took a surprising twist one year later when another "planet" was found with roughly the same orbit Ceres, then another and another. Astronomer William Herschel coined the term "asteroid" to describe what was apparently a new class of celestial object.

Today, some 750,000 asteroids larger than one kilometre across are known to ply the vast swath of space called the asteroid belt. For the most part, they are rocky and primitive – leftover material from the formation of the planets that has not changed much over the past four billion years. But Ceres, the largest of them all, could prove to be an exception.

Rough estimates of Ceres's density indicate it cannot be made of solid rock. Instead, up to half of its mass is probably water.

"What we don't know is in which form the water is stored," said Julie Castillo-Rogez, a geophysicist and Dawn science team member.

The water may be chemically locked in the rock, she said. But a more likely scenario is that it forms a layer of ice that begins just a few metres below Ceres' cratered surface and extends down as much as 100 kilometres.

Ceres is 2.8 times farther from the sun than Earth and receives less than one seventh the sun's radiant energy. That would suggest that any ice that exists there is stone cold. But Ceres may also generate its own internal heat due to the decay of radioactive elements. That, plus some dissolved salts, might be enough to maintain a thin layer or isolated patches of water in liquid form. Whether anything could be alive down there is another question. Dr. Castillo-Rogez said a strong case can be made that Ceres had at least the capability of supporting microbes in the distant past. "It's difficult to assess," whether that's still true, she said.

Even with its battery of scientific instruments, Dawn may not provide a definitive answer to the life question, but the mission is likely to yield important clues to the state of Ceres's water. Last year, astronomers working with Europe's Herschel space telescope reported evidence for puffs of water vapour escaping from Ceres. More recently, Dawn has spotted two mysterious white spots on the surface that could be freshly exposed ice, or mineral just created around a hole that is venting water vapour from the interior.

The spots "suggest some kind of recent 'geological' activity on this small world," said Peter Brown, a planetary scientist at the University of Western Ontario, and so they will no doubt be objects of intense scrutiny once Dawn's closeups start streaming in over the coming weeks.

The official program of the coming Lunar and Planetary Science Conference, which is held every March near Houston, says it all. In this year's conference schedule, a session on Ceres is subtitled: "This is your last chance to talk about Ceres before Dawn data wreck your theories."