An artist's concept of the Dawn mission. Credit: NASA

NASA is poised to launch an innovative spacecraft Thursday that will use ultra-efficient ion engines to reach and orbit the two largest members of the asteroid belt between Mars and Jupiter. The unprecedented mission is, quite literally, a "blue light special" in the realm of interplanetary exploration.

"It's a mission of firsts. Dawn will be the first mission to journey into and explore the heart of our asteroid belt," said Jim Adams, deputy director of planetary science at NASA headquarters. "It'll be the first to journey to and orbit around two celestial bodies and it'll be the first to visit a dwarf planet."

NASA's Near Earth Asteroid Rendezvous spacecraft orbited the asteroid Eros in 2000 and 2001, flying past another asteroid along the way. NASA's Galileo probe flew past two asteroids in route to Jupiter in 1991 and 1993 while the experimental Deep Space 1 ion-drive spacecraft and the Stardust probe flew by asteroids in 1999 and 2002 respectively.

But Dawn is the first mission aimed at orbiting two asteroids - and the two largest known at that - and the first to use ion propulsion on an operational basis.

Equipped with the largest set of solar arrays ever sent into deep space, the $420 million* Dawn mission will first orbit and map the 330-mile-wide asteroid Vesta starting in 2011, peering into a vast southern crater thought to be the result of an ancient collision that sent countless meteorites crashing to Earth. If Earth had a relatively sized crater, it would be the size of the Pacific Ocean.

After nine months mapping and analyzing Vesta's rocky surface with three powerful instruments, one of Dawn's three engines will be restarted, blasting out electrically charged xenon ions at speeds up to 90,000 mph and emitting a characteristic blue exhaust plume.

Three years later, if all goes well, Dawn will slip into orbit around the dwarf planet Ceres in 2015. Vesta is the second largest asteroid while 585-mile-wide Ceres is the largest.

Ceres, Pluto and the recently discovered Eris in the extreme outer solar system are the only currently known members of the dwarf planet family, a new designation that emerged from the recent debate over how Pluto and other similar bodies should be classified.

Like Jupiter's moon Europa, Ceres may harbor a vast reservoir of water under its frozen crust, a possible abode for primitive life - one closer and easier to reach from Earth.

"This is comparative planetology at its best," said Dawn program scientist David Lindstrom. "We are going to the two largest asteroids, but they are very different. Vesta is a very dense, igneous object from which we get plentiful meteorites. About 5 percent of the meteorites that fall on Earth come from the asteroid Vesta.

"We know that some of these rocks are (lava-like) basalts that crystallized 4.5 or (4.6) billion years ago, within a few million years of the earliest dated materials in the solar system. So we truly are going back in time, back to the dawn of the solar system.

"In contrast, Ceres, which is the largest asteroid and makes up about a quarter of the total mass in the asteroid belt, has much lower density indicating substantial amounts of ice. We have no known meteorites from Ceres so there's even more to learn by going to Ceres."

Liftoff from the Cape Canaveral Air Force Station atop a United Launch Alliance Delta 2 rocket is scheduled for 7:20 a.m. Thursday. Sixty two minutes later, after a final push from a solid-fuel third-stage motor, the 2,685-pound spacecraft will be released on its lonely trajectory away from Earth.

Engineers will spend several weeks checking out Dawn's systems and slowly firing up its ion propulsion system. Loaded with nearly 1,000 pounds of xenon propellant, Dawn is equipped with three ion thrusters. But only one will be fired at any one time.

It is the ion propulsion system that makes it possible for Dawn to venture to Vesta, brake into orbit and later depart for Ceres. A more traditional chemical-fuel rocket would require several tons of fuel to accomplish the same thing, boosting the cost of the spacecraft and requiring a much more powerful - and expensive - launcher.

Ion propulsion has been used in the past in Earth orbit and was tested aboard the Deep Space 1 technology demonstration spacecraft. Dawn marks NASA's first use of the technology in an operational planetary mission.

Instead of burning liquid propellants in short, high-power bursts, Dawn's propulsion system works by using electrical power to ionize and accelerate electrically charged xenon to velocities 10 times greater than the exhaust from chemical rockets. One of Dawn's 12-inch-wide ion thrusters uses about 10 ounces of fuel over 24 hours of operation.

The resulting thrust is very low compared to chemical rockets, but it can be maintained for months at a time. Over the course of Dawn's mission, the ion engines will fire some 2,000 days, exceeding the capability of the entire Delta 2 rocket.

Dawn will get the necessary electrical power from two huge 27-foot-wide solar arrays that will stretch 65 feet from tip to tip. The arrays are the most powerful ever sent into deep space, capable of generating 11,000 watts of power - enough for 10 average American homes.

Dawn doesn't need that much power, but the arrays were sized to provide sufficient energy when the spacecraft is orbiting Ceres, which is three times farther from the sun than Earth.

"This is NASA's first mission to use ion propulsion since it was proven on Deep Space 1 and it really is the key to why we can undertake such an ambitious and potentially rewarding journey in which we're going to be visiting some of the last unexplored worlds in the inner solar system," said Marc Rayman, Dawn project systems engineer. "The ion propulsion system is 10 times more efficient than conventional propulsion, so it allows us to undertake missions that are really far beyond the capability of conventional systems.

"It works by ionizing, or giving an electric charge, to atoms of xenon gas and then it uses a high voltage to shoot these ions through this metal grid at very high speed, up to almost 90,000 miles per hour. And the action of the ions leaving the thruster causes a reaction that pushes the spacecraft in the other direction. ... It really does emit this cool, blue glow like in the science fiction movies."

Holding up a piece of paper, he said "the thruster pushes on the spacecraft about as hard as this single piece of paper pushes on my hand. But in space, the effect of this gentle, almost whisper-like thrust gradually builds up over time until we can achieve very high speeds."

It would take Dawn four days to accelerate from zero to 60 mph. But it would only use two pounds of fuel in the process.

"Ultimately we can achieve fantastically high speed," Rayman said. "Over the course of its mission, Dawn will change its own speed by the same amount as the entire Delta 2 heavy launch vehicle with its nine solid motors, it's first stage, plus its second stage, plus its third stage. It's what I like to call 'acceleration with patience.'"

But getting there is just half the fun. The goal is to learn more about the formation of the solar system and the forces that have shaped its evolution.

The main asteroid belt between Mars and Jupiter is made up of hundreds of thousands of small bodies left over from the birth of the solar system. Enough material is present to form a small planet, but scientists believe Jupiter's gravity continuously churns the asteroid belt, stirring the debris and preventing it from coming together.

Vesta was discovered in 1807 by Heinrich Wilhelm Olbers, the German astronomer who pointed the way to modern cosmology with "Olber's Paradox," which questioned why, if the universe was infinitely large and old, the night sky did not blaze like the surface of a star (it doesn't, it turns out, because the universe is not infinitely old and is expanding; there is not a star along every possible line of sight).

Vesta measures about 359 by 358 by 285 miles and rotates once every five hours and 20 minutes. It features a huge 285-mile wide crater in its southern hemisphere that is eight miles deep. The ancient collision that dug the crater blasted some 200,000 cubic miles of rock into space.

Holding up a small meteorite believed to have originated from Vesta, Lucy McFadden, a Dawn co-investigator at the University of Maryland, said analysis shows "Vesta did indeed get heated to the melting point of silicate rock and coooled slowly and formed these minerals of these compositions.

"It is this signature of Vesta, which we can see in reflected sunlight, combined with this detailed minerology that we can study in laboratories here on Earth that gives us an idea that we know something about Vesta. But we need to go and see what Vesta is on a global scale."

Ceres was discovered in 1801 by the Italian astronomer Giuseppe Piazzi. it was the first asteroid ever discovered and it is the largest, a roughly spherical body measuring 606 by 565 miles.

"Ceres is a small planet," said co-investigator Mark Sykes of the Planetary Science Institute at the University of Arizona. "In fact, it's the smallest known planet in the solar system. ... Ceres contains about half the mass of all the materials in (the asteroid belt). Unlike all the other asteroids we've been to that show irregular fragmented shapes ... Ceres is round, it's not irregular. It's massive enough to overcome the mechanical strength of its material and force it into a round shape. This distinguishes it as a planetary body."

As with other terrestrial planets, Ceres has a differentiated interior with heavier materials near the core and a lighter, icy crust.

"We think there may be a subsurface ocean beneath Ceres' surface covered by a layer of ice," Sykes said.

While flying through the asteroid belt conjures up images of tumbling boulders from movies like "2001: A Space Odyssey" and others, there is little chance Dawn might run into something and create a crater of its own.

"Space is a lonely place," Sykes said. "Unlike 'Star Wars' where they go to the asteroid belt and they're dodging around all those asteroids, bumping into them and stuff, Dawn won't be doing that. If you were to put yourself out in the asteroid belt and look around you wouldn't be seeing asteroids floating around, it would look iike empty space.

"But we're in that space for a long period of time and we calculate along our trajectory how close do we get to anything else. aAtually in our case, we're looking at it because hey, maybe we could see something more!"

NASA originally hoped to launch Dawn in June, but the flight was delayed by a variety of booster-related issues, bad weather that interrupted launch processing and trouble with downrange tracking systems. NASA ultimately delayed the flight from July to September to avoid any possible conflict with launch of the Mars Phoenix lander in August.

* NASA's press kit puts the cost of the Dawn mission at $357.5 million, but that does not include the cost of the Delta 2 rocket. The booster is believed to cost around $75 million.