Dawn spacecraft begins science orbits of Vesta

August 2, 2011 By Dwayne C. Brown and Priscilla Vega, JPL/NASA
NASA's Dawn spacecraft obtained this image of the giant asteroid Vesta with its framing camera on July 24, 2011. Credit: NASA

NASA's Dawn spacecraft, the first ever to orbit an object in the main asteroid belt, is spiraling towards its first of four intensive science orbits. That initial orbit of the rocky world Vesta begins Aug. 11, at an altitude of nearly 1,700 miles (2,700 kilometers) and will provide in-depth analysis of the asteroid. Vesta is the brightest object in the asteroid belt as seen from Earth and is thought to be the source of a large number of meteorites that fall to Earth.

The Dawn team unveiled the first full-frame image of Vesta taken on July 24: www.nasa.gov/mission_pages/daw … imedia/pia14317.html

This image was taken at a distance of 3,200 miles (5,200 kilometers). Images from Dawn's framing camera, taken for navigation purposes and as preparation for scientific observations, are revealing the first surface details of the giant asteroid. These images go all the way around Vesta, since the giant asteroid turns on its axis once every five hours and 20 minutes.

"Now that we are in orbit around one of the last unexplored worlds in the , we can see that it's a unique and fascinating place," said Marc Rayman, Dawn's chief engineer and mission manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

This image is a mosaic of Vesta's equatorial region, composed of observations taken through the panchromatic filter on the framing camera onboard NASA's Dawn spacecraft on July 24, 2011. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

After traveling nearly four years and 1.7 billion miles (2.8 billion kilometers), Dawn has been captured by Vesta's gravity, and there currently are 1,800 miles (2,900 kilometers) between the asteroid and the spacecraft. The giant asteroid and its new neighbor are approximately 114 million miles (184 million kilometers) away from Earth.

"We have been calling Vesta the smallest terrestrial planet," said Chris Russell, Dawn's principal investigator at UCLA. "The latest imagery provides much justification for our expectations. They show that a variety of processes were once at work on the surface of Vesta and provide extensive evidence for Vesta's planetary aspirations."

Engineers still are working to determine the exact time that Dawn entered Vesta's orbit, but the team has reported an approximate orbit insertion time of 9:47 p.m. PDT on July 15 (12:47 a.m. EDT on July 16).

In addition to the framing camera, Dawn's instruments include the gamma ray and neutron detector and the visible and infrared mapping spectrometer. The gamma ray and neutron detector uses 21 sensors with a very wide field of view to measure the energy of subatomic particles emitted by the elements in the upper yard (meter) of the asteroid's surface. The visible and infrared mapping spectrometer will measure the surface mineralogy of both Vesta and Dawn's next target, the dwarf planet Ceres. The spectrometer is a modification of a similar one flying on the European Space Agency's Rosetta and Venus Express missions.

Dawn also will make another set of scientific measurements at Vesta and Ceres using the spacecraft's radio transmitter in tandem with sensitive antennas on Earth. Scientists will monitor signals from Dawn and later Ceres to detect subtle variations in the objects' gravity fields. These variations will provide clues about the interior structure of these bodies by studying the mass distributed in each gravity field.

"The new observations of Vesta are an inspirational reminder of the wonders unveiled through ongoing exploration of our solar system," said Jim Green, planetary division director at Headquarters in Washington.

In this image, obtained by the framing camera on NASA's Dawn spacecraft, a set of three craters, informally nicknamed "Snowman" by the camera's team members, is located in the northern hemisphere of Vesta. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn launched in September 2007. Following a year at Vesta, the spacecraft will depart in July 2012 for Ceres, where it will arrive in 2015. Dawn's mission to Vesta and Ceres is managed by JPL for NASA's Science Mission Directorate in Washington. JPL is a division of the California Institute of Technology in Pasadena. Dawn is a project of the directorate's Discovery Program, managed by NASA's Marshall Space Flight Center in Huntsville, Ala.

UCLA is responsible for overall Dawn mission science. Orbital Sciences Corp. in Dulles, Va., designed and built the spacecraft. The German Aerospace Center, the Max Planck Institute for Solar System Research, the Italian Space Agency and the Italian National Astrophysical Institute are international partners on the mission team.

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5 / 5 (1) Aug 02, 2011
since the giant asteroid turns on its axis once every five hours and 20 minutes

I wonder what the ratio is at the surface, along the rotational equator, between the pull of gravity from Vesta's mass, and the inertial vector from its spin. Gravity has to be bigger or Vesta would tear itself apart, but I wonder how much bigger it is. There could be a noticeable difference between the equator and the rotational north and south pole. If you dropped an object at each location, you might have a detectable difference in the rate of accelleration. I doubt you'd feel it, but it might be measurable by even a simple test.
5 / 5 (1) Aug 02, 2011
Surface gravity acceleration .22 m/s^2. Centripetal acceleration = w^2/r where w = instantaneous velocity, P is revolution period and r is radius. = (2 pi r/P)^2 / r = 4 pi ^2 r / P ^2 = 4 pi^2 * 529,000 m /(5.342 * 3600 s)^2 = .056 m/s^2

So over a quarter.. not bad.
not rated yet Aug 02, 2011
Can someone tell me how fast the spacecraft is moving, relative to Vesta? I'm curious, and nobody has published it, and one cannot calculate it because the density of Vesta has hitherto been unknown!
4 / 5 (1) Aug 03, 2011
Looking at the second image (equatorial region), it seems Vesta is an amalgam of smaller bodies acquired through accretion. Is this right? Or is the image color distorted and I'm not seeing what's actually there, as the first image shows impact scars.
5 / 5 (1) Aug 03, 2011
Looking at the second image (equatorial region), it seems Vesta is an amalgam of smaller bodies acquired through accretion. Is this right?

That's a great question. The answer is somewhat vague though. We know that Vesta is a gravitationally bound object, meaning that it has enough gravity to hold itself together, as opposed to a mechanically bound object like our satellites, which would fall appart without bolts. The impact craters suggest that it is solid similar to the way that the Earth and Moon are solid. It may be composed of a mixture of bedrock and regolith. It could have a stratified layer and core structure too. It could be a fragment of a failed planet that came apart when it was still molten. That would make Vesta a lot different than a simple chunk of rock. That's kinda why we're there now; to find out just what this thing is like.
5 / 5 (2) Aug 03, 2011
Can someone tell me how fast the spacecraft is moving, relative to Vesta?

At the orbit distance of 3000 km it should be going 80 meters/second. (That's 1900 miles and 170 miles/hour in English measure.) So, about the speed of a race car, which is practically stopped in space terms. I found it on the Dawn mission page at JPL:


By comparison, a geosynchronous satellite orbits earth at a lazy 6800 miles/hour, which is still slow in space terms.

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