Four years after launch out of Cape Canaveral, NASA's ion-drive Dawn spacecraft is finally in orbit around the asteroid Vesta, checking out the second largest body within the rubble-strewn belt in between Mars and Jupiter in unmatched detail. Photos unveiled today show a unusually tortured world along with enormous parallel lines separating the intensely cratered northern hemisphere from clearer terrain in the south covered with the chaotic remains of a devastating impact.
"All these photographs have been already a fantastic revelation to the team about what the surface is like," Christopher Russell, the mission's primary investigator, told journalists today. "We did not imagine the detail we're seeing and the various processes that we're seeing evidence of now. These are really insightful into this kind of building block of the premature solar system...It's actually a stunning and exciting little world sitting there in the center of the asteroid belt that we're going to learn a great deal about."
NASA's Dawn spacecraft, right now in orbit around the asteroid Vesta, is beaming back exciting photos displaying huge equatorial grooves, upper left, separating the heavily-cratered northern hemisphere through smoother terrain in the south, where a catastrophic impact took place the distant past.
Discovered in 1807, Vesta is just about spherical, measuring about 359 miles by 284 miles. It's the biggest member of the asteroid belt yet visited by a robotic spacecraft, 2nd in size only to Ceres, Dawn's following target and, together with Pluto, one of just five known dwarf planets. NASA is actually spending $466 million to understand more about Vesta and Ceres throughout the course of a 10-year mission.
Vesta's surface area is believed to be composed of basaltic rock that ran out of a presumably scorching interior when the asteroid formed some 4.5 billion years in the past. Because of its small size, the asteroid probably cooled rapidly and did not undergo subsequent resurfacing just like larger worlds.
As such, Vesta might have one of the oldest, most captivating surfaces in the solar system, a icy snapshot of the components and conditions that was around at the dawn of the solar system.
"We believe that this goes back to the initial 5 million years of the solar system," Russell suggested. "What occurred back then was that the material that was orbiting that was going to end up being the sun began to condense. We have evidence of what was taking place through very primitive meteorites which have fallen to the Earth.
"Right when Vesta began to come together, we believe there seemed to be a supernova that irradiated the material and added radioactive material to it that when the materials joined together, it had an additional heat source and several of these bodies which were formed melted and then differentiated, they provided a crust of lava and an iron core. Those are examples of the oldest bodies out there."
Previous observations by the Hubble Space Telescope demonstrated what appeared to be a huge impact crater near Vesta's south pole that expands some 285 miles across and measures some 8 miles deep. In excess of 200,000 cubic miles of debris were blasted into space, according to on the Dawn press package, a few of which ultimately made its way to Earth as meteorites.
To put the impact into perspective, a proportionately sized crater on Earth might possibly be around the size of the Pacific Ocean.
Images from Dawn show chaotic landscapes near the south pole covered with a magnificent central peak and huge ripple-like lines extending around the asteroid's equator. Terrain north of the lines is much more heavily cratered.
"Those lines are pretty much in the equatorial area and they're very much perpendicular to the direction we assume the impact was going when it struck Vesta," Russell said. "So one thing it could be is that when the compression of that amazing impact came, Vesta got smaller in that direction for a while and after that expanded which caused tectonic features around the equatorial region."
Launched on top of a United Launch Alliance Delta 2 rocket on September 27, 2007, Dawn is the first operational deep space mission to be designed with an ion propulsion system. Instead of burning up liquid propellants in short, high-power bursts, Dawn's propulsion system works by utilizing electrical power to ionize and accelerate electrically charged xenon to velocities ten times more than the exhaust from chemical type rockets.
Dawn's ion propulsion system "only pushes on the spacecraft as hard as this individual sheet of paper pushes on my hand," said Marc Rayman, mission supervisor and chief engineer, holding up a single piece of paper. "And yet little by little, over time, the result of this whisper-like thrust can develop and produce exceptionally high velocity. So this is what I like to call acceleration with patience. And now, our patience is paying off very handsomely indeed."
During final approach to Vesta, Dawn's final velocity was just 60 miles per hour relative to the asteroid. Finally, on July 15, the spacecraft was taken by Vesta's weak gravity, placing it in orbit.
"Thanks to all the thrusting we've been doing all along the way, Dawn was able to gradually creep up on Vesta and slip ever so gently into orbit with similar grace and elegance it's displayed in nearly 1000 days of inner planetary ion thrusting," Rayman explained.
Dawn's preliminary orbit around Vesta carries it over the asteroid's poles at an height of roughly 1,700 miles. The actual orbit's orientation keeps the spacecraft and its massive solar panels in direct sunlight to supply power for its instruments and its propulsion system.
Traveling from north to south, Dawn will accomplish one orbit every three Earth days. Simply because Vesta completes one rotation or "day," every five hours and twenty minutes, Dawn should be able to observe the asteroid's complete illuminated surface every orbit. During south-to-north passes above Vesta's night side, Dawn will transmit saved images and other data back to Earth.
Operating in the initially high survey orbit, Dawn's instruments can provide a global view of Vesta in ultraviolet, visible and infra-red wavelengths. A mapping spectrometer will observe the surface area with a image resolution of 2,300 feet per pixel while the spacecraft's visible-light cameras will resolve features as small as 820 feet across, around 150 times better than photographs taken by the Hubble Space Telescope.
Dawn's gamma ray and neutron detector will assist researchers characterize the surface composition of Vesta while evaluation of subtle changes in the spacecraft's radio signal, triggered by slight changes in Dawn's orbital velocity, will help scientists map out the actual asteroids gravity field and internal structure.
After seven orbits, or around three weeks in the survey orbit, Dawn's ion propulsion system will spend 30 days or so slowly decreasing the orbit to an altitude of about 420 miles, permitting the spacecraft to circle the asteroid twice each Earth day.
Dawn will complete around 60 trips around Vesta in this so-called high-altitude mapping orbit, or HAMO, surveying the asteroid in outstanding detail, permitting scientists to build topographic maps showing the elevation of hills and mountains and the absolute depths of the craters covering its surface.
After HAMO operations are complete, Dawn will spend about 6 weeks lowering down to a low-altitude mapping orbit, or LAMO, at less than one hundred and ten miles, circling Vesta every 4 hours.
While the lower orbit provides Dawn's cameras sharp views of Vesta's surface, the main goal of this 10-week stage of the mission is to permit the gamma ray and neutron detector to monitor cosmic ray impacts on the surface. High-energy cosmic radiation hitting atoms in the upper three feet of the surface area will generate gamma rays and neutrons that can be used to discover the identities of the original atoms.
The impact of Vesta's gravity on Dawn's orbit will be more noticeable in the low-altitude orbit, allowing scientists to map out its inner mass distribution.
When the low-altitude stage of the mission is complete, Dawn will spend an additional six weeks spiraling out for a 2nd set of observations at an altitude of 420 miles, comparable to the earlier high-altitude mapping orbit. Because approximately eight months will have passed between the 2 sets of observations, a greater portion of Vesta's northern hemisphere will be illuminated as the asteroid's seasons progress and areas not observed earlier will be mapped at the identical level of detail.
If all goes well, Dawn will commence its departure from Vesta in Summer of 2012, setting off on a two-and-a-half-year journey to Ceres. The mission is funded through July 2015.
- Published in Space