Darkness looms for NASA’s Dawn

Sometime soon, NASA’s Dawn spacecraft, which has been exploring the Asteroid Belt since 2007, will run out of fuel, bringing its 11-year mission to an abrupt end.

Already, its manoeuvring thrusters are practically running on fumes, with so little left that nobody is quite sure when their last reserves will sputter out. When that happens, most likely between mid-September and mid-October, the spacecraft will lose its ability to orient its solar panels to the sun and point its sensors at interesting targets. It will lose communication with Earth, unable to aim its antenna in the right direction.

For the moment, however, the spacecraft is continuing to return spectacular close-ups of the solar system’s largest asteroid, Ceres, as every 27 hours it dives within 35 kilometres of the surface, then soars 4000 kilometres out before swooping back, across another part of that 1000-kilometre-diameter world.

It’s the final phase of a mission that, by visiting not only Ceres but also the solar system’s second largest asteroid, Vesta, has revolutionised not only our understanding of the asteroids, but also the technology of space travel.

Key to making this possible was the use of ion for its main engine – not to be confused with its now almost drained thrusters.{%recommended 4728%}

Ion propulsion uses solar-electric power to eject small amounts of propellant at extremely high velocities. That makes it extremely efficient. Dawn’s mission director and chief engineer, Marc Rayman of NASA’s Jet Propulsion Laboratory (JPL) in California, US, compares it to a car that gets 130 kilometres to the litre.

Ion propulsion doesn’t have a lot of thrust. “It would take Dawn four days to accelerate from zero to 60 miles per hour,” Rayman says.

But over time, it builds up. “This is what I like to think of as acceleration with patience,” he says. “That’s what allowed us to undertake a uniquely ambitious mission.”

No other spacecraft, he adds, has travelled to one distant body, Vesta, gone into orbit around it, manoeuvred around, then broken out of that orbit, travelled to another alien world, Ceres, and then gone into orbit about it.

“And it does that with ion propulsion, which I first heard of in a Star Trek episode,” Rayman enthuses. “[And] it really does produce this cool blue glow, just like in science fiction movies.”

The scientific findings have been equally dramatic.

Even though Vesta and Ceres are large — containing, in combination, about 45% of the entire mass of the main Asteroid Belt, they are hundreds of millions of kilometres away — so far that prior to Dawn’s arrival, the best images we had of them, from the Hubble Space Telescope, looked like slightly splotchy, fuzzy balls.

Dawn revealed them to be entire worlds in their own rights. It also found them to be dramatically different.

Vesta is dry and rocky, like the Earth’s moon. Ceres is less dense and therefore water-rich, like some of the moons of Saturn and Jupiter. Vesta became hot enough early in its life to melt and “differentiate,” as heavy materials such as iron settled to its core, just as happened on Earth, Mars, and the Moon. Ceres does not contain an iron core.

“This is almost like night and day,” says Jim Green, NASA’s chief scientist. Vesta and Ceres, he says, must not only have had different histories, but must have formed in quite different parts of the solar system.

Carol Raymond, principal investigator for the mission, also of JPL, adds that Ceres must have formed farther out from the sun than it’s present location, then been transported inward, later on. Vesta probably formed closer in, and is more like a mini-version of the terrestrial planets. Vesta also appears to have formed very early in the solar system’s history — probably within the first one million to 1.25 million years, Raymond says. Otherwise, there wouldn’t have been enough short-lived radionuclides left around to create the radioactive heat that allowed to completely melt.

Vesta, she says, is a time capsule that tells us that such bodies started forming very, very quickly. “These objects were popping up out of the [protoplanetary nebula] and growing very big, very fast,” she explains.

Ceres had even bigger surprises, the biggest of which was that it appears to be geologically active, with a 4000-metre mountain that probably formed very recently in geologic time. It also has bright white salty deposits, mainly composed of sodium carbonate that has somehow been squeezed to the surface from the slushy, briny remnants of what was probably once a large subsurface ocean.

When the end comes, Dawn will remain parked in its present orbit, as what Rayman calls an “inert, celestial monument in orbit around the dwarf planet that it unveiled”. Not that it will stay in that orbit forever. But the trajectory was chosen to be stable enough that it shouldn’t crash for at least 20 years — and almost certainly for at least half a century, if not longer, Rayman says.

That way, there’s time to plan a return mission to Ceres to land and search for signs of life before there is any risk of Dawn crashing into and contaminating it with Earth microbes. If Ceres had proven to be as dead and inactive as the moon, Green says, scientists wouldn’t be as worried about keeping it pristine. But the discovery that it’s so geologically active is a game changer. “[Ceres] is so exciting and intriguing that we’ve just got to go back,” he says.

In fact, he says, NASA has already convened a group of scientists to start the process of figuring out how best to return. “We’ll get the results probably in nine months or so,” he says.