NASA's long-roaming Dawn today became the first spacecraft to orbit a dwarf planet, and the space agency is using the mission to test technology that may one day transport supplies for an outpost to shelter astronauts on Mars.
At 7:39 a.m. ET today, Dawn, which has traveled 3.1 billion miles, achieved orbit around Ceres, the largest object in the asteroid belt that lies between Mars and Jupiter, NASA said.
The spacecraft, which launched in September 2007, came within 38,000 miles of Ceres and was captured by the planet's gravity.
At 8:36 a.m., the space agency received a signal from Dawn indicating that its systems were healthy and working normally.
"Since its discovery in 1801, Ceres was known as a planet, then an asteroid and later a dwarf planet," wrote Marc Rayman, Dawn's chief engineer and mission director at NASA's Jet Propulsion Laboratory, in a blog post. "Now, after a journey of 3.1 billion miles and 7.5 years, Dawn calls Ceres, home."
Today's event is the second time the spacecraft has entered orbit around an extraterrestrial target, making Dawn the first spacecraft to orbit two objects.
Voyager 1, for instance, may have traveled to enter interstellar space and flown past planets in our solar system, but it has never entered another object's orbit, let alone leave one orbit to move on to another.
From 2011 to 2012, Dawn orbited and explored Vesta, the first asteroid to be visited by a spacecraft and the brightest asteroid in our sky.
The spacecraft, which uses two radiation-hardened RAD6000 processors, sent back images and data about Vesta, which is also in the asteroid belt between Mars and Jupiter. Dawn's work gave scientists a huge amount of information about the asteroid, which has a mountain two and a half times taller than Mount Everest and 90 canyons.
"Dawn's spectacular exploration of Vesta reveals a fascinating world that's more like a small planet than an asteroid," Rayman told Computerworld. "It's more like Earth than just a chunk of rock."
Now Dawn, which carries three scientific instruments, including a camera and a gamma ray detector, is focusing its sights on Ceres. It is looking for signs that the dwarf planet once had the conditions needed to support life.
So what has enabled Dawn to fly so far?
Rayman explained that the spacecraft, which measures 65 feet from one solar array wing tip to the other, uses an ion propulsion system that gives the spacecraft the thrust it needs. With a conventional propulsion system, Dawn and the rocket that launched it would have had to be much larger – and more expensive – to have even reached Vesta.
"It can thrust much, much more than any other spacecraft," said Rayman. "It's taken advantage of gravity boosts, but Dawn has the unique capability to propel itself far more than any other spacecraft ever has. It can fly to a destination, break into orbit and then break out of orbit and fly somewhere else and break into orbit there... this has never been attempted until the Dawn mission."
According to Rayman, this would be impossible with conventional propulsion.
Unlike propulsion systems that heat standard gas or put gas under pressure to produce thrust, an ion-based system uses xenon gas and gives it an electric charge. Once charged, it becomes an ion, which can be moved by a simple voltage charge. When voltage is added to ions, it shoots them out of the engine at great speeds, pushing the spacecraft in the opposite direction.
Ion propulsion has 10 times the efficiency of conventional propulsion. "That means we can take on missions that are much more ambitious," Rayman said. "When we eventually send humans to Mars, ion propulsion may figure into that mission simply by sending the cargo that the crews will need to Mars with this propulsion system."
Dawn is not the first NASA spacecraft to use ion propulsion. The agency's Deep Space 1 spacecraft, launched in 1998 to test new technologies and capture data about a comet, was the first to use an alternative to traditional propulsion. The Dawn spacecraft, however, has used ion propulsion to travel much farther and to study two extraterrestrial objects.
'Acceleration with patience'
It's doubtful that an ion-based system would be used in a spacecraft that's transporting astronauts because ion propulsion doesn't move very fast.
"We only flow a very small amount of xenon through the engine at one time," explained Rayman. "It's very efficient but it's very gentle. That one piece of paper in your hand pushes on your hand as much as the xenon pushes on that spacecraft. But in the zero gravity of space flight, it can move a spacecraft."
It would take four days for ion propulsion to move a spacecraft from zero to 60 mph at full throttle, he said.
"No spacecraft has ever spent so much time in powered flight, but I like to call it acceleration with patience," Rayman said. "And if you're patient, it's a great way to explore the solar system."
To send a large payload to Mars, though, this propulsion system would do the trick. "You can get there faster with conventional propulsion, but if you want to send a massive payload to support humans, who require a lot of food, a lot of water and a lot of air and supplies, this would save you from having to use unaffordably large or unavailable rockets. It would be a slow trip, but there's no hurry."
The Dawn spacecraft also is using two identical processors – one for main use and one for backup. It also has two identical radio transmitters, two receivers and two star trackers, which are used to orient the craft.
The spacecraft doesn't use artificial intelligence but it does have self-monitoring software. The software, built by NASA, monitors the system's software and hardware, along with conditions like temperatures, currents and pressures, and will take active steps to protect the spacecraft in the event of a problem.
"If the radio transmitter doesn't seem to be working correctly, it will power that transmitter off and power the other one on," explained Rayman. "If it determines it's been too long since it's received a command from Earth, it will then turn its receiver off and turn the other one on. It also may swap to another computer in case that's the problem. It will take an action, point to Earth sending a signal and requesting a signal and then, if it still doesn't receive a signal back, it will take another action."
Engineers coded into the system a series of possible explanations for various problems so the system itself can work through them on its own.
Dawn has had trouble with its reaction wheels, which control the spacecraft's orientation, and is expected to stop working early next year.
NASA engineers have figured out a way to keep the spacecraft going even without the requisite number of reaction wheels, but that fix has been quickly using up the craft's supply of conventional propellant, which is used to rotate it in one direction or another.
The spacecraft will run out of its rotation propellant by early next year and will remain in permanent orbit around Ceres.
"It will just become an inert monument to human ingenuity orbiting around the first dwarf planet to ever be orbited," Rayman said. "What more fitting tribute to the spacecraft itself than to leave it around a body that no other spacecraft has had the capability of reaching?"