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In Depth
Space
Mission to the asteroid belt
Dawn's new era
Last Updated July 9, 2007
CBC News Online

The spacecraft Dawn has been a long time coming. Postponed twice in early July, the asteroid probe's launch has now been put on hold until later this year.

Dawn was originally set for launch on July 6, but bad weather forced a delay to July 9. Then the new date was scrubbed when NASA decided that Dawn's big day could cause conflicts with other launches set for August, so it is now expected to take flight in September.

The launch will start an eight-year mission to the swirling asteroid soup between Mars and Jupiter, realizing a dream that at times felt impossible.

More than 15 years in the making, the mission has been dogged by technical issues and cost overruns that twice triggered its cancellation.

But now the $343.5-million US voyage to document two of the oldest planetary bodies — Ceres and Vesta — has a new start date for the planned 5.1 billion kilometre trip that will end in a holding orbit for up to 50 years.
Timetable

After separating from the Delta launch rocket, Dawn will embark on a lengthy interplanetary cruise planned to the vicinity of Mars.


Arrival there is expected in 2009, but it's not for a visit. There's just enough time to fly past and get a boost from Martian gravity and maybe do some scientific observations. Then it's off to the asteroid Vesta.

"After departing from Mars, the Dawn spacecraft will settle back into cruise for another orbit around the sun, during which it will gradually spiral outward into the asteroid belt between Mars and Jupiter," says NASA.

In late 2011, Dawn should arrive at Vesta. After seven months of observations near Vesta, it will be time for Ceres.
Are we there yet?

It'll take until early 2015 to reach the neighbourhood of the dwarf planet. It's another 1.7 billion kilometres, after all. By that summer, the scientific work there should be complete.

Dawn's mission will then be in its sunset phase. The spacecraft is to be set into a "quarantine" orbit that will keep it circling 700 kilometres above Ceres for at least 20 years, and likely 50 years, says NASA.
What's the mission?

"I think of Dawn as two journeys," said Christopher Russell of UCLA, the principal investigator who proposed the mission to NASA.


"One is a journey into space. This is analogous to what ancient explorers did … We're going to explore a region for the first time to find out what the conditions are today.

"Dawn is also a journey back in time," Russell added. "…The Earth hides its history, but we believe that Ceres and Vesta, formed more than 4.6 billion years ago, have preserved their early record … We're going back in time to the early solar system."

Dawn is expected to provide the most complete picture of the asteroid belt so far.
How is it equipped for research?

Dawn has three types of instruments to gather research data.

Two framing cameras will capture detailed images, with special filters to help study the minerals on Vesta's surface.

Measurements by a 21-sensor gamma ray and neutron detector are expected to reveal the composition of Vesta and Ceres down to a depth of one metre.

"Many scientists believe that Ceres may be rich in water; if that is the case, the signature of the water may be contained in this instrument's data," predicts NASA.

The minerals on the surface are to be measured by a visible and infrared mapping spectrometer.

Radio transmissions to Earth will also be monitored to detect variations in the gravity fields of Vesta and Ceres.

Fast Facts

* This is the first space mission to investigate Vesta and Ceres, the largest bodies in the asteroid belt.
* Vesta has a giant crater at its south pole. Scientists believe that about five per cent of the meteorites found on Earth came from the celestial crash that caused this crater.
* Ceres was considered a planet when it was discovered in 1801. Later demoted to asteroid, it was moved up to dwarf planet status in 2006 with Pluto and Eris.
* The names of 365,000 earthlings are heading into space with Dawn. The project had a contest to solicit names to be etched on an eight-millimetre square silicon chip that's going along for the ride.
* To adhere to a United Nations treaty on space exploration, NASA's planetary protection officer required that the remains of Dawn not impact Ceres for 20 years. NASA says its analysis shows the spacecraft will remain in orbit around Ceres for more than 50 years after the mission.

The pair are to have their "mass, shape, volume and spin state" measured to determine their minerals and elements and "provide insight into their origin and evolution," says NASA.
What's the spacecraft like?

The core of the spacecraft looks like a large box with dishes and gadgets attached.

It's 2.36 metres long, or as NASA says, about as long as a large motorcycle.

Spreading out of the box are two solar arrays, each 8.3 metres by 2.3 metres long. Solar power will run its onboard systems, backed up by a nickel-hydrogen battery when the arrays are pointed away from the sun.

Dawn stows its solar wings for the launch, but when they are extended in space it'll be about as long as a tractor-trailer, at almost 20 metres.

The craft weighs 1,217 kilograms at launch, including the propellant needed to power its voyage.

A key element of the mission is the ion propulsion used to power its three thrusters for years at a time. The thrusters are expected to take breaks a few hours each week to allow its antenna to transmit communications with Earth.

The thrusters are fuelled by xenon propellant, and take a slow-but-steady approach to acceleration. At maximum thrust each of the engines produces "about the amount of force involved in holding a single piece of notebook paper in your hand," explains NASA.

"You would not want to use ion propulsion to get on a freeway — at maximum throttle, it would take Dawn's system four days to accelerate from 0 to 60 miles per hour."