Dawn Arrives at Ceres
Mysterious Bright Spots and Ion Propulsion
Since leaving orbit of the asteroid Vesta in 2011, NASA's Dawn spacecraft has been slowly and steadily matching orbits with the dwarf planet Ceres, maneuvering itself for insertion into orbit around it. This morning, at around 7:30 EST, it was successfully captured by Ceres' gravity and began another slow approach trajectory that will culminate in its first circular mapping orbit in April. Its arrival makes it the first spacecraft to orbit a dwarf planet (just beating out New Horizons, which will arrive at Pluto in July), and the first spacecraft to orbit two different deep space bodies.
Dawn's purpose is to investigate Vesta and Ceres and evaluate what they can teach us about the formation of the solar system, as well as any possible role for asteroid belt objects in catalyzing life on Earth and elsewhere. Ceres is of particular interest because of its likely icy composition, possible geological activity, and the discovery by the Herschel infrared space observatory of plumes of water ejected into space by Ceres.
During the recent phases of Dawn's approach, however, some specific features have leapt to the forefront of scientific and public interest. As Dawn has gotten closer and imaged Ceres with increasing clarity, bright spots that were first noticed in an enhanced Hubble image from 2004 have become even more pronounced and puzzling. Most appear to reside in some of Ceres' many surface craters, and, even more strangely, what was once thought to be one bright spot now appears as two distinct bright spots, sharing the same crater.
Two views of Ceres: On the left, taken on 19 February from a distance of 46,000 km, the two distinct bright spots in a crater are clearly visible; on the right, taken on 12 February from a distance of 86,000 km, the wider distribution of bright spots can be seen
The most tantalizing possible explanation for these spots is probably that they are cryovolcanoes -- volcanoes that eject volatiles such as water, instead of molten rock -- which have been observed on other icy bodies in the solar system like Triton and Enceladus. This would be indicative of ongoing geological activity on Ceres, and a possible subsurface body of liquid water. However, mission scientists have characterized this as unlikely, noting that the spots don't appear to be mounds or elevations in the surface. It is more likely that the spots are sheets or deposits of water ice, though how they came to reside on the surface in the configuration we've observed would be a fascinating investigation in itself.
Whatever the case, deputy principal investigator Carol Raymond said in a press conference on Monday that the spots are "unique in the solar system," and that her team is still puzzled by them. This means an opportunity for true discovery awaits the spacecraft. We will likely have to wait until April to get our first answers; that is when Ceres will reach an orbit with an altitude at which clear imaging of the spots will be possible.
Dawn's Ion Propulsion
The trajectories available to Dawn for traveling through the solar system and approaching the Ceres dwarf planet are shaped by its unique mode of propulsion: electrostatic ion thrusters. Space agencies have experimented with ion-based propulsion since the early 1990s, but Dawn is the first spacecraft to employ it for a purely exploratory mission.
Conventional chemical propulsion, used by most manned and unmanned spacecraft, involves a chemical fuel (such as hydrazine, or liquid hydrogen) and an oxidizer (such as liquid oxygen, or nitrogen tetroxide). These are injected at high pressure into a chamber, where they are ignited and combusted. The resulting high energy exhaust is routed through a nozzle that accelerates it out of the back of the spacecraft; through Newton's third law of motion, the equal and opposite reaction is to accelerate the spacecraft forward. With this mode of propulsion, space vehicles change their velocity (and thus their orbits) by executing discrete engine burns that last usually only a few minutes, imparting high acceleration on the craft and consuming ample propellant.
In contrast, no combustion occurs in the operation of an ion propulsion system. Instead of a high energy fuel and oxidizer, the propellant in an ion thruster is an inert gas -- xenon, in the case of the Dawn spacecraft. The xenon atoms are injected into a chamber, where they are bombarded with electrons, creating ions. The ions can then be accelerated through simple natural forces. Electromagnetic ion thrusters use magnetic fields to accelerate the ions out of the back of the spacecraft. Electrostatic ion thrusters, like the ones Dawn flies with, use a simple set of positively and negatively charged grids to create a difference in electric potential (the positively charged ions are repelled from the positive grid and attracted to the negative grid), which speeds the ions along and ejects them from the spacecraft. In both cases the end result is the same as chemical rockets -- exhaust accelerating out of the engine creates an equal and opposite reaction that accelerates the spacecraft forward.
The key advantage of ion propulsion, though, is the velocity at which the exhaust exits the engine. In a conventional chemical rocket using hydrazine fuel and nitrogen tetroxide oxidizer, exhaust exits the engine with an effective velocity of around 3.5 kilometers per second. In contrast, ion exhaust exiting an electrostatic ion engine can reach from 50 up to 100 kilometers per second in velocity. This difference means the ion engine can impart the same impulse on the spacecraft as a chemical rocket while using dramatically less propellant -- the ion engine is much more fuel-efficient.
Over the course of its mission, Dawn has traveled to the asteroid Vesta, inserted into orbit around it, broken orbit from Vesta, traveled to Ceres, and entered orbit around it. To accomplish all this it has used its ion propulsion to change its velocity by more than 10 km/s. That's the delta-v it has produced, as you might recall from a previous primer. That's a huge amount of change in velocity for a space mission. For a chemical rocket to produce an equivalent delta-v, it would take an enormous reserve of propellant, such that the spacecraft would be unmanageably massive. The efficiency of the ion drive was key in enabling Dawn's ambitious mission.
The trade-off for the tremendous efficiency of the ion engine is extremely low thrust. It takes a lot longer for an ion propulsion system to create the large changes in velocity it is capable of. Whereas a high-thrust chemical rocket may complete an orbital maneuver burn in a matter of minutes, spacecraft propelled by low-thrust ion engines usually burn their engines continuously, over spans of weeks and months, slowly but steadily changing their velocity and orbit. They accelerate so slowly ("0 to 60 in about 2 days" is how one Dawn mission controller phrased it) that, unlike chemically-propelled spacecraft, they are moving significant distances over their constantly-changing orbit as they are thrusting. The result is that ion drives in continuous operation produce spiral-like trajectories, with the craft tracing out its orbit even as the orbit itself is made larger by the thrusting ion engine. Take Dawn's trajectory, for example, in this NASA diagram:
The blue and black dotted line is Dawn's spiraling trajectory into a higher and higher orbit around the sun, with blue segments indicating the engine thrusting and black segments indicating "coast" periods, with the ion engine inactive. These coast periods allow Dawn to rotate and point its high-gain antenna at Earth, for data downlink and uplink, and assessing the health of the engines and spacecraft. Note the difference between this low thrust trajectory and that of the Voyager missions, which used only conventional chemical propulsion and gravitational "slingshot" assists:
This brings us to the long, frustrating wait for better imagery of the enigmatic bright spots. Dawn is aiming for a high altitude polar orbit, that is, an orbit that is inclined 90 degrees to Ceres' equator. Put another way, a polar orbit moves "up" the face of Ceres on one side, over its north pole, "down" the opposite face, over the south pole, and so forth. The reason for this is simple: As Dawn orbits from pole to pole, Ceres rotates underneath it, allowing Dawn to eventually map the entire surface, without having to change its orbit to cover different regions.
Instead of one or two big and quick propellant-hungry engine burns to insert into its desired orbit, as a chemical rocket would perform, Dawn is slowly and gently nudging its orbit into the desired shape with efficient ion propulsion. It's why Dawn will not arrive in its first mapping orbit until late April, and we'll have to wait until then to start figuring out the bright spots. A stray cosmic ray interrupted one of Dawn's thrusting phases last year, so it's actually taking a different trajectory into the mapping orbit than originally planned. This video animates the approach. Note the blue exhaust indicating the thrusting ion engine.
Dawn proving the space-worthiness of a dedicated ion propulsion system is a critical milestone in unmanned space exploration. The trade off of thrust for very high efficiency should allow very ambitious deep space probe missions in the future, to other tantalizing targets in the asteroid belt and inner solar system.
I'll obviously have much more about the Dawn mission as more data comes in, but if you want to catch new images as they're released I recommend following their Twitter account.
*Image credits: Cover image - NASA; Two views of Ceres - NASA/JPL; Dawn trajectory - NASA; Voyager trajectories - NASA.