The asteroid belt's largest bodies have drifted virtually alone since their creation about 4.5 billion years ago, but each will soon receive a visit from NASA's Dawn spacecraft.
Mission scientists hope to use Dawn's observations of the dwarf planet Ceres and asteroid Vesta to investigate the solar system's structure and evolution, as well as shed new light on secrets of planet formation.
"We truly are going back in time, back to the dawn of the solar system," said David Lindstrom, NASA's Dawn program scientist at the agency's Washington, D.C., headquarters, during a July mission briefing.
Dawn is set to rocket into space from Florida's Cape Canaveral Air Force Station on Thursday at 7:20 a.m. EDT (1120 GMT) aboard a Delta 2 booster. The launch will send the 2,685-pound (1,218-kilogram) Dawn spacecraft on an eight-year journey to the asteroid-rich zone between Mars and Jupiter.
Its launch was scheduled for Wednesday, but bad weather prevented crews from fueling the second stage of the probe's Delta 2 rocket on Sunday, prompting the 24-hour delay. Delays launching Dawn's $449 million mission earlier this summer were expected to add about $25 million to the probe's total cost, mission managers have said.
Together, Ceres and Vesta make up more than a third of the asteroid belt's mass. During Dawn's total 3-billion-mile (4.9-billion-kilometer) trip, it will make an orbital pit stop at each of the two rocky bodies before the mission ends in July 2015.
The spacecraft's first visit is set for August 2011, when it will park itself around Vesta — an oblong asteroid 330 miles (530 kilometers) in diameter that hides an iron core beneath its rocky surface. The spacecraft will gradually scan the dry rock's exterior during a 10-month visit, helping scientists to investigate how planets like Mars might have looked early in their formation.
"It's sort of like an archaeological trip where we're going to the ruins of an ancient civilization," said Chris Russell, Dawn's principal investigator at the University of California, Los Angeles.
Once Dawn has fully scoped out the arid asteroid, it will put its lightweight ion drive in full gear to reach the Ceres, a Texas-sized dwarf planet roughly 585 miles (942 kilometers) in diameter, by February 2015.
Based on observations made by the Hubble Space Telescope, astronomers think Ceres may harbor not only a thin atmosphere, but also a thick layer of water ice beneath its surface. If true, Ceres may boast nearly six times as much fresh water than is found on Earth.
"If we discover evidence of a subsurface ocean, than certainly it would be a high priority to go back and explore in more detail," said Mark Sykes, a Dawn co-investigator at the Planetary Science Institute in Tucson, Ariz.
In addition to providing for life on Earth, astronomers think such sources of water could have played integral roles in planet formation.
To study the asteroid belt's two biggest satellites, Dawn will use three different instruments: a large optical camera, a mapping spectrometer and a neutron and gamma-ray detector (GRaND).
The optical camera will create detailed maps of each body down to 230 feet (70 meters) per pixel for Vesta and 410 feet (125 meters) per pixel for Ceres. Working in less detail, Dawn's mapping spectrometer will scan each object's surface for patches of minerals.
The GRaND instrument will use celestial radiation to deduce the basic elements present in both Ceres and Vesta. If the instrument can peer deep enough below Ceres' surface, signs of frozen or liquid water may be detected.
Highs and lows
NASA first approved the Dawn mission in 2001, but the program has since suffered cancellations, postponements and several launch delays. Attempts to launch the probe in July went without success due to bad weather, rocket booster glitches and difficulties arranging tracking aircraft and ships to monitor the planned liftoff.
Technical and financial woes prompted NASA to order the mission to stand in October 2005 and ultimately scrap the flight in March 2006. But a reevaluation of the Dawn mission's difficulties by the space agency led to its reinstatement a few weeks after its cancellation.
In spite of recent accidental damage to the spacecraft's solar panel array and several delays this year, mission managers are confident Dawn will successfully begin its journey on Thursday. Should mission managers delay the probe's launch again, they will have until late October to send Dawn spaceward; otherwise, the first launch opportunity to visit both Ceres and Vesta won't open again until about 2022.