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 protoplanets. 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.
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 left Earth orbit in September 2007 depicted below. It used its engine to speed up 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. It stayed in orbit around Vesta until July 2012. See the yellow line portion of this NASA graphic.
Dawn arrived at Vesta in July 2011. It departed Vesta a year later in August 2012 after successfully completing that phase of the mission. Arrival at Ceres is scheduled for February 2015. The end of the primary mission is scheduled for July 2015.
This video gives a unique perspective on the Vesta encounter. Presented as a Greatest Hits music video, see some of the highlights of the visit to Vesta.
Dawn Now Heading For Ceres
The next phase of the Dawn mission takes the spacecraft to the largest asteroid Ceres in February 2015. One thing which makes Dawn unique is its rocket propulsion engine. Dawn uses an Ion Engine, not a conventional combustion engine. Dawn was placed into Earth orbit in September 2007. Conventional chemical rockets placed it in Earth orbit with a speed relative to Earth of about 7,800 m/s. For those not familiar with the metric system, that is nearly 5 miles/sec. Dawn then needed to increase its 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 speed boost and then to Vesta. It used its ion engines for thrust. The graphic below shows in light green when the engines are thrusting. Blue is coasting. It also shows the current location of Dawn and the other bodies. At this time, Dawn is at the far left very slowly catching up to Ceres. It will take a little more than a year from now to be close enough to Ceres for it to gravitationally capture Dawn and place it in orbit. Arrival is shown at the bottom of the graphic.
Why use an ion engine? What is the advantage? Why not use conventional rockets?
Ion Propulsion Engines
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. But, it can exert this thrust for very long periods of time like weeks or months. 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 low friction skate board. Aim a low powered repeating BB gun horizontally so you will roll away from the 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 much 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.
Historically, ion engines have been developed over the last 50-60 years. The principle behind the ion engine is akin to removing socks out of the clothes dryer on a dry day. They repel each other because they are electro-statically charged. Like charges repel. Unlike charges attract. Ion engines charge a fluid so its atoms can be expelled in one direction out the nozzle and drive the spacecraft in the other direction. Dawn’s ion engine uses xenon gas that is more than 4 times heavier than air.
When the ion engine is running, electrons are emitted from a hollow tube called a cathode. These electrons enter a magnet-ringed chamber, where they strike the xenon atoms. The impact of an electron on a xenon atom knocks away one of xenon’s 54 electrons. This results in a xenon atom with a positive charge, or what is known as an ion. Xenon ions shoot out the back of the engine at a speed of 100,000 km/h (60,000 mph). At full throttle, the ion engine will consume 2,500 watts of electrical power, and put out 1/50th of a pound of thrust. That’s far less than the thrust of even small chemical rockets. But an ion engine can run for months or even years.