NASA scientists hope to celebrate Independence Day a few days late this year, with a spectacular launch of the Dawn mission Saturday -- the world's first flight to the mysterious dwarf planet Ceres and the asteroid Vesta.
It is an ambitious enterprise -- the first from Earth to orbit two celestial bodies in succession. The journey is possible only because of Dawn's exotic ion-propulsion engine. Its astonishing efficiency will carry it 3 billion miles on less than 72 gallons of fuel. That has led some to dub the spacecraft "the Prius of space."
"I'm excited, but I don't want to get too excited until I see the rocket take off," said University of Maryland astronomer Lucy McFadden, a co-investigator and education director for the $449 million project.
The launch window for Dawn opens at 4:09 p.m. Saturday at the Cape Canaveral Air Force Station in Florida, with more opportunities each day until July 23.
A successful liftoff would cap five years of planning and construction, interrupted twice by cancellations and again in 2006 by a "stand-down" because of budget worries.
Delays and technical issues since 2004 have added $56 million to the mission's price tag, NASA officials said.
"It's been a rocky road, but we're ready for launch," McFadden said. "I just hope the rains will cease in Florida long enough for us to get off the ground."
She and other mission planners have been studying the best-ever photos of Ceres and Vesta, snapped in May by the Hubble Space Telescope.
They reveal a variety of light and dark surface features on both objects. NASA hopes to get close-ups when Dawn slips into orbit around Vesta in 2011, and Ceres in 2015.
The Hubble imagery is tantalizing, McFadden said. "There are light basins and dark basins. Are they topographic features, or regions where something was extruded from the interior and flowed out on the surface? It's encouraged us that we are doing the right thing by going to explore these new worlds."
Ceres and Vesta are the two largest and brightest "minor planets" in the solar system.
When Ceres was discovered in 1801, astronomers at first thought it was a new planet. But as they began to find more objects between the orbits of Mars and Jupiter (including Vesta in 1807), they concluded they were seeing fragments of a demolished, or unconsolidated, planet. They eventually named the region the "asteroid belt" for the myriad, tiny star-like objects observers kept finding there.
Some turned out to be pretty big. Ceres, for example, is a Texas-sized 600 miles across.
Last year, the International Astronomical Union reclassified Ceres as a "dwarf planet," along with Pluto and Eris (an icy orb beyond Pluto). That means each is large enough so that its gravity has pulled it into a spherical shape but too small to have cleared the rubble from its orbit.
Vesta, just 320 miles in diameter, is irregularly shaped and still classified as an asteroid.
Scientists see Vesta and Ceres as relics from the construction of the solar system 4.6 billion years ago. It was a time when all the dust and gas swirling around the sun was consolidating into the inner, rocky planets (Mercury, Venus, Earth and Mars), and the outer gas giants (Jupiter, Saturn, Uranus and Neptune).
The rubble in the asteroid belt might have consolidated, too, McFadden said. But Jupiter became so big that its gravity -- and the sun's -- kept the region too unstable for a planet to form.
Ceres and Vesta just stopped growing, and scientists want to learn the identities and relative abundance of their elements, minerals and ices. That knowledge, they hope, will yield a snapshot of how the rest of the planets in the solar system were sorting themselves out at the time.
The two asteroids seem surprisingly different, based on telescopic views and examination of lava-like meteorites, called "basaltic achondrites," thought to have come from Vesta.
"One [Ceres] is water-rich, and the other appears to be dry," McFadden said. That suggests Ceres is more like the icy moons of the outer planets, while Vesta is more like the rocky inner planets.
"What caused the two bodies to take different paths in their evolution? We want to understand how that relates to the formation of the terrestrial [rocky] planets," she said.
Dawn is equipped with two 30-foot solar panels for electric power. It carries two German-made cameras to photograph the surface, and spectrometers to analyze and map the elements and minerals on the surface.
The spacecraft will also measure the asteroids' gravity fields for clues to their interior structure.
Getting Dawn from here to there is the job of its solar-electric, ion propulsion engine.
With just under 72 gallons of xenon gas for propellant, Dawn will slowly accelerate by about 24,500 mph after leaving Earth orbit. That's far more acceleration than any other spacecraft has managed on its own, said systems engineer Marc D. Rayman of NASA's Jet Propulsion Laboratory in California.
Unlike the Galileo mission to Jupiter or the Cassini mission to Saturn, Dawn can't tap the gravity of a giant planet for a big boost in speed. A Mars fly-by in 2009 wouldn't add enough gravitational oomph to do the job, either.
Reaching Vesta, orbiting there, then moving on to an orbit around Ceres would have required more chemical fuel than any available rocket could carry.
"Without ion propulsion, it really would be impossible to do the mission Dawn is doing," Rayman said.
NASA first tested ion propulsion technology in 1998 on its Deep Space 1 mission.
Instead of a rearward blast from a chemical rocket, Dawn will press forward in space using a feeble but constant rearward beam of electrically charged atoms of xenon gas. "It's a low-impulse thing, but it can make a lot happen, as long as you're patient," McFadden said.
It works like this: Dawn will use solar-generated electric power to knock electrons loose from xenon atoms, giving them a positive electric charge. The charge causes these atoms -- now called ions -- to accelerate toward a negatively charged grid in the back of the engine, then zip out the back at 68,000 mph.
The same force that attracts the xenon atoms to the negative grid also draws the grid toward the approaching atoms, and that moves the whole craft forward in space.
The push is laughably weak, equal to the weight of a sheet of paper. In fact, it would take Dawn four days to accelerate from zero to 60 mph, and require two pounds of xenon.
However, "If you have the time, this is a terrific way to travel," Rayman said. "The job is really too demanding for conventional propulsion systems."
Just getting to Vesta would have required 2.5 tons of chemical propellant, he said. But with plenty of time to accelerate and an ion engine, Dawn can get to Vesta, and then Ceres, for less than the 937 pounds of xenon gas on board.
NASA officials are eager to launch Dawn on time Saturday. A significant delay would crowd a time-critical August launch window for the Phoenix Mars lander, and the planned Aug. 9 launch of space shuttle Endeavour to the International Space Station.
A last-minute delay could postpone Dawn until September, adding as much as $25 million to the mission's expenses.
"We're trying to manage Dawn so we're ready to go on the launch window so it's off and on its way by the 23rd," said Todd May, a deputy associate administrator for NASA.
Once Dawn does take wing, the eight-year mission will be managed by JPL. Scientists will keep watch on the spacecraft's health and progress, but they won't see data from Vesta until 2011, and their instruments won't reach Ceres until 2015. It's one of the drawbacks of extended space flights, even if the humans involved are waiting safely back on Earth.
"It's a long time," McFadden sighed. "I'm eligible for retirement by then."
Dawn's 8-year itinerary
July 2007: Launch from Cape Canaveral
March 2009: Mars fly-by
October 2011: Orbit Vesta, as near as 124 miles
April 2012: Depart Vesta
Feb. 2015: Orbit Ceres, as near as 435 miles
July 2015: End of mission
Total distance traveled: 2.9 billion miles
Mission cost: $449 million
Cost per mile: $6.46 The spacecraft
Length: 5.8 feet
Solar panels, tip-to-tip: 65 feet, generating 10 kW
Weight: 1,647 pounds before fueling
Fuel: 937 pounds (72 gallons) xenon propellant for ion engine; 100 pounds of hydrazine for thrusters
Acceleration: 0-60 mph in 4 days
Estimated engine operations: 2,000 days
Miles per gallon (post-orbital): 40.3 million
Second-biggest asteroid in solar system and fourth discovered, in 1807, by Heinrich Wilhelm Olbers, Germany
Size: 359 by 348 by 285 miles, about the size of Arizona
Shape: spheroid, with 8-mile-deep crater at south pole
Distance from sun: 213 million miles
Rotation: 5 hours, 20 minutes
Orbit around sun: 3.6 Earth years
Composition: mainly basaltic rock - frozen lava Ceres:
Largest asteroid in solar system, the first to be discovered, on Jan. 1, 1801, by Giuseppe Piazzi, of Italy. Reclassified in 2006 as a "dwarf planet."
Size: 606 by 565 miles, the size of Texas. Its mass is one-third of the total mass of all asteroids in the solar system
Shape: Nearly spherical
Distance from sun: 251 million miles
Rotation: 9 hours, 4.5 minutes
Orbit: 4.6 years
Composition: A dense core, with lighter minerals and water ice near surface