Today’s the day: on March 6, 2015, at about 7:40 EST, (12:39 GMT) NASA’s Dawn spacecraft arrived at Ceres, becoming the first spacecraft ever to orbit a dwarf planet. NASA officials got a signal Dawn confirming that it’s healthy and in orbit at about 8:36 a.m. EST (1336 GMT) today. Ceres is the largest body in the Asteroid Belt. It’s about 590 miles in diameter, big enough and massive enough to be pulled into a roughly spherical shape. Its density is lower than that of Earth for the same volume, though, so it has a gravity scarcely 3% that of Earth. It’s got an atmosphere – barely. Most of it seems to be water vapor, and the source could be two or three large jets of vapor erupting continuously from its surface.
Humans have only known about Ceres since its discovery in 1801. We knew it was in the Asteroid Belt, between the orbits of rocky Mars and the gas giant Jupiter. We estimated its size, and its approximate mass. That, however, was about the extent of it. First it was classified as a planet, then an asteroid, and now we’re calling it a dwarf planet. Now that Dawn has completed its 3.1 billion mile, 7.5 year journey, we can really study this relic from the earliest times of our star system’s history.
Even from 30,000 miles out (48,000 kilometers), the images had already begun to show detail that mystified. It’s gray, and cratered, like our own Moon. However, it also has intensely bright spots, and two of them, nestled in the center of a crater in its northern hemisphere, look like a pair of tiny eyeballs staring back at us. What are they? Initial guesses made them cryovolcanic eruptions of water from the dwarf planet’s mantle of water, but on closer examination we can already tell that they’re missing some of the key physical features that would need to be there. The current best guess is that they’re simply very very reflective ice. We won’t know for sure until we start getting some more detailed pictures from closer in, and we should begin to see those pictures in the coming days and weeks.
This is the end of Dawn’s voyage through space; its first stop was Vesta, the next largest object in the Asteroid Belt. Vesta’s diameter is only about 330 miles, and it’s far enough from Ceres that the journey from Vesta to Ceres took the Dawn probe almost three years to make the trip. The milestone comes just four months ahead of another highly anticipated dwarf-planet encounter: On July 14, NASA’s New Horizons probe will zoom through the Pluto system, giving scientists their first good looks at that faraway dwarf planet and its five known moons.
The two bodies are “intact protoplanets from the very dawn of the solar system,” Dawn Deputy Principal Investigator Carol Raymond, also of JPL, said during a news conference Monday (March 2).” So they’re literally fossils that we can investigate to really understand the processes that were going on at that time.”
The mission’s spaceflight feats are made possible by Dawn’s innovative propulsion system, which accelerates xenon ions out the back of the spacecraft. This process generates tiny amounts of thrust; it would take Dawn four days to go from 0 to 60 mph (97 km/h), team members have said. Even so, Dawn’s ion drive is about 10 times more efficient than traditional chemical systems. The engines can keep firing for weeks, months and years, accelerating Dawn to tremendous speeds.
“With the 1,000 lbs. of xenon propellant that was loaded on board, Dawn has already accomplished more than 24,000 mph of velocity change,” Dawn project manager Robert Mase of JPL said during Monday’s news conference. “To put that in context: That’s more than it takes to get a vehicle from the surface of the Earth up to the International Space Station.” Thanks to ion propulsion, Dawn crept up on Ceres slowly and gradually. The probe eased into orbit today without the need for any harrowing make-or-break maneuvers.
Ceres is an intriguing world that in many ways looks more like the icy moons of the outer solar system, such as Jupiter’s satellite Europa and the Saturn moon Enceladus, than its rocky neighbors in the asteroid belt. For example, the dwarf planet is thought to consist of 25 to 30 percent water by mass, mostly in the form of ice. Ceres may also once have had (and might even still possess) an ocean of liquid water beneath its surface, as Europa and Enceladus do. Indeed, some researchers think Ceres may be capable of supporting microbial life.
“It’s really going to be exciting to see what this exotic, alien world looks like,” Rayman told Space.com in late January. “We’re finally going to learn about this place.”
Dawn can’t scan for alien life forms. It doesn’t have the necessary equipment to do that. However, if Ceres’ hypothesized underground ocean exists, Ceres may be able to spot evidence of it. They may even be able to figure out how heat is transferred through Ceres, which could shed some light on whether the dwarf planet has underground liquid water.
Overall, Dawn will characterize the dwarf planet in detail, mapping out its surface and determining what Ceres is made of.
Dawn will spend the next six weeks spiraling down to its initial science orbit, getting there on April 23. The probe will then begin taking Ceres’ measure from an altitude of 8,400 miles (13,500 km). Dawn will study the dwarf planet from a series of increasingly closer-in orbits until the mission ends in June 2016.
As space exploration goes, this is some pretty intriguing stuff. We’ll keep you posted as we learn more.
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