NASA ‘time machine’ lifts off
NASA’s Dawn spacecraft launched Thursday on a 3.2-billion-mile journey to the asteroid belt, where scientists hope to find clues to the formation of the solar system.
The spacecraft, atop a Delta 2 rocket, took off from Cape Canaveral in Florida at 4:34 a.m. PDT. “We have our time machine up and flying,” said UCLA space physics professor Christopher T. Russell, the lead scientist on the project.
Scientists at the Jet Propulsion Laboratory in La Cañada Flintridge waited anxiously for word from the spacecraft that it was on its way. The message arrived two hours after launch. Dawn also signaled that it had deployed its solar panels that were providing power to the spacecraft.
Most NASA spacecraft have been express rockets to a single solar system moon or planet. By contrast, Dawn is a commuter tour bus.
Powered by slow-go ion engines that take four days to go from zero to 60 mph, the spacecraft will make two stops on an eight-year journey to the asteroid belt between Mars and Jupiter. Once the craft is there, scientists plan to study some of the oldest objects in the solar system, lumps of stone that are thought to have changed very little in 4.5 billion years.
Dawn’s first destination is Vesta, a lump of rock with a big hole at its south pole. After seven months of orbiting the 350-mile-wide rock, it will travel to the dwarf planet Ceres, which some scientists think could have more fresh water than Earth, locked up in an icy underground vault.
By observing them close up, scientists at JPL hope to better understand the process that allows some objects to become planets while others remain stunted as solar system flotsam.
Scientists used to think that the asteroid belt, which contains hundreds of thousands of small rocks and a few big ones, was the remains of a planet destroyed in an apocalyptic collision.
Now the prevailing theory is that Ceres and Vesta, which together contain about half the mass of the entire asteroid belt, are the biggest chunks of a proto-planet that failed to form because of interference from Jupiter’s strong gravity.
“What we’re seeing is arrested development,” Russell said. “Jupiter got to them before they had the ability to form a planet.”
This will not be the first mission to an asteroid. Deep Space 1 visited the asteroid Braille on July 28, 1999. But the $446-million Dawn mission will be the first time a spacecraft has the ability to go to two separate destinations.
“In science fiction you see ships all the time going from one place to another, but we haven’t done that,” Russell said.
What makes that possible now are the craft’s three JPL- and Boeing-developed ion engines, which are powered electrically by the largest set of solar panels ever launched on a U.S. planetary spacecraft. Each is 27 feet long.
The thrusters use an electrical charge to accelerate ions from 937 pounds of xenon fuel, which should be enough to last the eight-year mission. Altogether, the engines will fire a total of more than 2,100 days.
A conventional rocket would quickly run out of fuel if it fired its engines that long. But the ion engines use only a tiny amount of fuel at any one time. Though this makes for pretty slow acceleration -- the amount of thrust is equal to the weight of a piece of paper resting on the palm of your hand -- over time the speed builds up enough that the craft is to reach its first target on Oct. 1, 2011.
It is to arrive at Ceres nearly four years later, in July 2015.
When the mission was first conceived 15 years ago, scientists were most interested in Vesta, which is thought to be the origin of nearly 20% of the meteorites that strike Earth.
It has a bizarre surface feature that they want a closer look at: a crater 285 miles across and 8 miles deep that was gouged by an ancient collision.
Vesta remains an object of curiosity. But the more scientists have learned about Ceres, the more fascinated they have become, Russell said. That’s because the 600-mile-wide object may have a buried ocean, as well as a thick layer of ice.
Analysis by the Hubble Space Telescope and other instruments show that it is less dense than Earth’s crust. Its vast stores of water have remained hidden by a layer of dirt.
“This is a very dusty part of the solar system,” Russell said.
For decades, astronomers have lusted after a mission to Europa, the Jupiter moon covered in a thick layer of ice. Scientists have constructed a number of scenarios under which some rudimentary form of life might be able to get started under the ice.
Russell thinks Ceres is a more hospitable place for life to get a start. For one thing, Ceres is closer to the sun, and therefore warmer. Its dust covering could also serve as a kind of insulation against freezing surface temperatures.
Finally, it doesn’t have to contend with the potentially life- killing effects of Jupiter’s radiation belts.
“I am now thinking that Ceres gives us our biggest surprises,” Russell said.
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