The NASA Dawn mission asks how the size of a body plays a role in the evolution of a planet. It also asks how water plays a role in that evolution. Asteroids Ceres and Vesta are the best bodies to study for answers to these questions. They are the most massive of the proto-planets. Yet, they are very different. Ceres is very primitive and wet. Vesta is more evolved and dry. This image compares Vesta at left, with Ceres and our Moon. Vesta is about as wide as my home state of Iowa. Our Moon has a diameter of 2,159 miles which is about 27% of the Earth. Clearly, Vesta and Ceres are not huge objects. But, they are the largest of the asteroid class of objects.
Some of our best ever views of Ceres were returned by Dawn February 4. Rotation and some surface features are clearly visible. The nature of the bright white spot has been a subject of speculation by many. Here is a humorous explanation offered by Randall Munroe at xkcd. Caution…the site can cause huge losses of productive time if you start clicking the Random button. You’ve been warned.
The Path to Vesta
Interplanetary trips are not along straight lines. Instead, they are along curving paths that are part of an orbit about the Sun. Dawn launched and left Earth orbit in September 2007. It then used its engine to speed up slowly but gradually to catch Mars. As it flew by Mars, the gravitational attraction sped up Dawn and allowed it to move a little farther from the Sun to reach Vesta in July 2011. It stayed in orbit around Vesta for a year until July 2012.
This brief video from NASA illustrates the journey very well.
After Dawn successfully completed the phase of the mission to Vesta, NASA presented a Greatest Hits music video showing some of the highlights of the visit. We are expecting a similar video from the Ceres visit that might be called Greatest Hits Part 2.
Dawn Now Near Ceres
The next phase of the Dawn mission places the spacecraft in orbit around the largest asteroid Ceres in early March 2015. Click the image for a bigger view. Dawn departed Vesta when at the the top center of this orbit diagram. It fired the engine almost continuously for over 2 years in order to speed up and move to a farther orbit from the Sun. This put it into a matching orbit with Ceres. Dawn is now adjusting its speed and direction to closely match that of Ceres so it can be captured in orbit around the asteroid for the next several months.
It is much like two cars traveling along a highway near each other in the same direction. But, Dawn and Ceres are moving nearly 39,000 mph or Warp 0.033 if you are Trekkie. The graphic below shows in light green when the engines were thrusting. Blue is for coasting.
Dawn will first orbit Ceres at 8400 miles altitude for a broad view. It will descend to 2750 miles to survey the asteroid. It then moves to much closer orbits for several scientific studies and mapping in detail at 920 miles and 230 miles.
Ion Propulsion Engines
Why use an ion engine? What is the advantage? Why not use conventional chemical rockets?
One thing which makes Dawn unique is the engine. Dawn uses an Ion Engine, not a conventional combustion engine. When Dawn was placed into Earth orbit in September 2007, conventional chemical rockets lifted it to a speed of about 7,800 m/s. For those not familiar with the metric system, that is nearly 5 miles/sec. A booster chemical rocket increased the speed to 11,000 m/s in order to move away from Earth and proceed on an interplanetary journey. It coasted close to Mars for a gravitational speed boost, and then to Vesta. It used its ion engines for thrust during the trip.
Ion engines are about ten (10) times more efficient than conventionally fueled engines. The Dawn engine has a low thrust value of 90 milli-Newtons. This is comparable to the force exerted by a single sheet of paper resting on the palm of a hand. Acceleration is very small. It would take Dawn 4 days to go from 0 to 60 mph. Some cars do that in 4 seconds.
It can exert this thrust for very long periods of time like months and years. The engines have operated more than 5 years. As a result, it uses much less fuel to achieve the same change in speed as a chemical rocket. Chemical rockets fire for brief times, but consume large amounts of fuel.
The key to this difference in efficiency has to do with the speed of the escaping gases at the engine nozzle. Chemical rocket exhaust speed is around 4,000 m/s. Ions escape from the nozzle of ion engines at speeds approaching 40,000 m/s. This can be illustrated by the following analogy. Stand on a frictionless skate board. Aim a low powered repeating BB gun horizontally so you will roll in the opposite direction of the fired BBs. Fire off a few seconds of BBs. You won’t go backward very fast. Replace the BB gun with one that weighs the same but fires BBs 10x faster. The faster the speed of the exhausted BBs, the faster you will recoil the opposite direction.
Ion engines deliver much faster exhaust speeds, and can fire for prolonged times. They deliver much greater changes in speed per kg of fuel than conventional chemical rocket engines. This is called Specific Impulse. The spacecraft carries along less mass in fuel. The downside is that maneuvers must be slow and planned far in advance. No sudden moves. So far, Dawn has executed all maneuvers perfectly.