NASA's Dawn spacecraft is on track to begin the first extended visit to a large asteroid. The mission expects to go into orbit around Vesta on July 16 and begin gathering science data in early August. After a year, Dawn will venture forth to orbit Ceres.
After traveling nearly four years and 1.7 billion miles (2.7 billion kilometers), Vesta captures Dawn into its orbit on July 16. There will be approximately 9,900 miles (16,000 kilometers) between them. When orbit is achieved, they will be approximately 117 million miles (188 million kilometers) away from Earth.
July 7, 2011 - PASADENA, Calif. -- NASA's Dawn spacecraft obtained this image with its framing camera on July 1, 2011. It was taken from a distance of about 62,000 miles (100,000 kilometers) away from the protoplanet Vesta. Each pixel in the image corresponds to roughly 5.8 miles (9.3 kilometers).
More about the Dawn mission below the squiggle. ⤵
What is the role of the size of a planet or protoplanet, and the role of water, in determining the evolution of those bodies? Ceres and Vesta are the two most massive residents of the asteroid belt. Vesta is very rocky. Ceres is believed to contain large quantities of ice. The differences between these two protoplanets will be studied using measurements by Dawn. How did they form? What does their formation tell us about Mercury, Venus, Earth, and Mars? What can they tell us about the many planets being found around other stars as in the Kepler mission.
Planets grew in the early solar system from a process called accretion. Gravity attracted the gas and dust into bodies that formed spherical shape. The strength of Jupiter's gravity is thought to have interfered with this accretion in the space between it and Mars. Instead of a single planet, that region is populated by a belt of smaller objects called protoplanets. Both Vesta and Ceres have lived through the history and collisions of the early solar system. They retain physical and chemical records of that planet forming time.
Vesta appears to be a dry, differentiated body, with evidence of lava flows. Telescopic observations reveal mineralogical variations across its surface. An apparent impact crater 460 km in diameter centered near its south pole demonstrates that impacts likely played an important role in its history.
Ceres, the largest body in the asteroid belt and only slightly farther from the Sun than Vesta, is very different. It does not reveal the rich reﬂectance spectrum that Vesta does, and no meteorites have been linked to it. Microwave observations have been interpreted to mean that it is covered with a material like clay, which would indicate water played a role in Ceres’ history.
The Dawn spacecraft reaches about 20 meters across with the solar panels deployed. There are 4 main parts to the science payload.
- Framing Camera will gather images of Vesta and Ceres in three colors and black and white.
- Visible & Infrared Spectrometer (VIR) is a full surface mapping spectrometer in three bands, 0.35 to 0.9 micron, 0.8 to 2.5 micron and 2.4 to 5.0 micron.
- Gamma Ray and Neutron Spectrometer (GRaND) measures elemental abundances on the surface of Vesta and Ceres.
- Gravity Science allows accurate measurements of the spacecraft's orbit from which scientists can determine the gravity fields of Vesta and Ceres.
THE JOURNEY TO VESTA AND CERES
The journey started 27Sept2007. Follow the path counter-clockwise from Earth. Notice 3 things. First, the path to Vesta and then to Ceres is sort of a spiral outward from the Sun. Second, Dawn passed close to Mars and received a gravity assist which boosted its speed. A boost at that point caused it to go into a farther orbit from the Sun. Third, the path of Dawn is marked by a dashed line pattern. Those indicate when the spacecraft engine was thrusting and coasting. Click this link to see the current location of Dawn.
What is Gravity Assist? The link has an extensive explanation for those interested. Basically, what happened at Mars is simple physics. Dawn approached Mars from behind as they orbited the Sun. Its path intersected the path of Mars but was not quite on a collision course. As Dawn drew nearer, the force of gravity from Mars speeded up Dawn with respect to the Sun. That placed Dawn in a different and larger orbit. The force of gravity from Dawn slowed Mars ever so slightly with respect to the Sun. That place Mars in an orbit slightly smaller. The total angular momentum of the two with respect to the Sun was unchanged. Dawn 'borrowed' some momentum from Mars.
ION PROPULSION ENGINES
Ion propulsion is solid science, but for years has been in the domains of Star Trek, Star Wars, and other stories. NASAs Deep Space 1 mission helped bring ion propulsion to the forefront of scientific reality. It launched in 1998 and was equipped with an ion propulsion engine. The extended mission successfully visited Comet Borelly and sent back images as well as testing a dozen other technologies.
Conventional propulsion engines use high pressure or temperature to push a gas through a nozzle. The action of the gas forced backward is equaled by the reaction of the rocket and engine and payload forced forward. This is Newton's 3rd Law. Ion engines use the same principle. But, the way the gas is forced backward is different. In the words of Marc Rayman JPL.....
The inert gas xenon, which is similar to helium and neon but heavier, is used as propellant. The composition of xenon is simple: each atom consists of a tiny and dense nucleus surrounded by a cloud of electrons. The nucleus is 54 positively charged protons plus about 76 neutral neutrons. (Xenon gas is a mixture of 9 isotopes, meaning there are 9 different values for the number of neutrons. From a low of 70 to a high of 82, the number of neutrons makes only very modest differences in the behaviors of the atoms.) The 54 positive charges in the nucleus are precisely balanced by 54 negatively charged electrons, rendering the atom electrically neutral -- until the ion propulsion system gets in the act.
Inside the ion thruster, an electron beam, somewhat like the beam that illuminates the screen in a television, bombards the xenon atoms. When this beam knocks an electron out of an atom, the result is an electrically unbalanced atom: 54 positive charges and 53 negative charges. Now with a net electrical charge of 1 unit, such an atom is known as an "ion." Because it is electrically charged, the xenon ion can feel the effect of an electrical field, which is simply a voltage. So the thruster applies more than 1000 volts to accelerate the xenon ions, expelling them at speeds as high as 40 kilometers/second (89,000 miles/hour). Each ion, tiny though it is, pushes back on the thruster as it leaves, and this reaction force is what propels the spacecraft. The ions are shot from the thruster at roughly 10 times the speed of the propellants expelled by rockets on typical spacecraft, and this is the source of ion propulsion's extraordinary efficacy.
This very high velocity of exhaust gas is what allows an ion propelled craft to achieve speeds much faster than conventional ones and with far less fuel. The force exerted by the thrust is about that of the weight of a sheet of paper. As a result, the force needs to act for very long periods of time. One must be patient. At maximum throttle, Dawn can accelerate or decelerate at a rate of 15 mph/day. It would take 4 days to go from 0 - 60 mph.
When spacecraft arrive at a planet, they have to burn their engines to drop into orbit. For conventionally powered craft, the speed change might be about 1000 meters/second (2200 miles/hour) and consume about 300 kilograms (660 pounds) of propellants, and take about 20 minutes. With its ion engine, Dawn could accomplish that change in speed with less than 30 kilograms of xenon. But, Dawn might require more than 3 months. You have to be patient and plan ahead.
By July 16, Dawn will be traveling no faster toward Vesta than you can drive in a car. The probe will be close enough and slow enough that the protoplanet’s gravity will tenderly take the approaching explorer in its grasp.
WHAT'S AHEAD FOR DAWN?
After a year in orbit around Vesta gathering data, Dawn will fire the ion engine to begin the journey to Ceres. Arrival there is set for February 2015 and is set to last for 6 months. No future journeys are scheduled for Dawn after that. NASA has extended missions before. Case in point, Spirit and Opportunity Rovers on Mars were 90 day missions. Today, after more than 7 years on Mars, Opportunity is still trekking across the planet doing a remarkable job. Spirit got permanently stuck and was not heard from again after the Martian winter.
Who knows what might be ahead for Dawn if the mission can be extended.