How the Dawn Spacecraft Was Built and Developed to Explore Vesta and Ceres
They were once hailed as planets upon their discovery, put into the same class as the 8 planets we know of today. But as more and more objects like Vesta and Ceres were discovered, astronomers soon realized that they had a new type of object and labeled them asteroids. Vesta, Ceres, and many other asteroids that had been given planetary status had it revoked (sound familiar?). It is therefore truly ironic that these forgotten objects of history may end up shedding light on the formation of the rocky planets. The Dawn mission is tasked with this in mind.
Why Go To The Asteroid Belt?
Vesta and Ceres were not selected at random. Though the entire asteroid belt is a fascinating place to study, these two are by far the largest targets. Ceres is 585 miles wide and is ¼ the mass of the asteroid belt while Vesta is the 2nd most massive and has 1/48 the mass of the asteroid belt. These and the rest of the asteroids would have been enough to make a small planet were it not for Jupiter’s gravity ruining the show and pulling everything apart. Because of this history, the asteroid belt can be thought of as a time capsule of the building blocks of the early solar system. The larger the asteroid, the more the original conditions it formed under have survived collisions and time. So by understanding the members of this family we may gain a better picture of how the solar system formed (Guterl 49, Rayman 605).
For instance, we know of a special type of meteorite called the HED group. Based on chemical analysis, we know they came from Vesta after a collision at its south pole a billion years ago ejected about 1% of the volume it possessed and created a crater that is 460 kilometers wide. HED meteorites are high in nickel-iron and lack water, but some observational evidence showed the possibility of lava flows on the surface. Ceres is an even bigger enigma because we do not have any meteorites from it. It is also not too reflective (only a quarter as much as Vesta), a sign of water below the surface. Possible models hint at a mile deep ocean underneath a frozen surface. There is also evidence of OH being released in the northern hemisphere, which also hints at water. Of course, water brings the idea of life into play (Guterl 49, Rayman 605-7).
Dawn Gets Wings
The “principal investigator for the Dawn mission,” Chris Russell has had quite the uphill battle in getting Dawn secured. He knew that a mission to the asteroid belt would be difficult because of distance and the fuel that would be required. To go to two different targets with one probe would be even harder, requiring a lot of fuel. A traditional rocket would not be able to get the job done at a reasonable price, so an alternative was required. In 1992 Russell learned about ion engine technology, which had its origins in the 1960’s when NASA began to investigate it. It had dropped it in favor of funding the space shuttle but it was utilized on small satellites, allowing them to make small course corrections. It was the New Millennium Program that NASA instituted in the 1990’s that got serious applications for the engine designs going (Guterl 49).
Just what is an ion engine? It propels a spacecraft by taking energy away from atoms. Specifically, it strips the electrons away from a noble gas, like xenon, and thus creates a positive field (the nucleus of the atom) and a negative field (the electrons). A grid in the back of this tank creates a negative charge, attracting the positive ions to it. As they leave the grid, the transfer of momentum causes the craft to be propelled. The advantage to this type of propulsion is the low amount of fuel that is needed but it comes at the cost of fast thrust. It takes a long time to get going, so as long as you are not in a rush this is a great method for propulsion and a great way to cut the cost on fuel (49).
In 1998, the Deep Space 1 mission was launched as a test of ion technology and was a great success. Based on that proof on concept, JPL was given approval in December of 2001 to move forward and construct Dawn. The big selling point for the program was those engines reducing the costs and giving a longer life span. A plan that would have used traditional rockets would have required two separate launches and would have cost $750 million each, for a total of $1.5 billion. Dawn’s initial total projected cost was less than $500 million (49). It was a clear winner.
Yet as the project progressed costs began to go over the $373 million budget Dawn was awarded and by October of 2005 the project was $73 million over. On January 27, 2006 the project was cancelled by the Science Mission Directorate after worries over the financial situation, some concerns over the ion engines, and management issues became too much. It was also a cost-saving measure for the Vision for Space Exploration. JPL appealed the decision on March 6 and later that month Dawn was brought back to life. It was found that any engine problems were being fixed, that a change in personal resolved any staff issues, and that despite the cost of the project being almost 20% overboard a reasonable financial path was being developed. Besides, Dawn was over the half-way point to completion (Guterl 49, Geveden).
Dawn has a specific list of goals it hopes to accomplish on its mission, including
- Finding the density of each within 1%
- Finding the “spin axis orientation” of each within 0.5 degrees
- Finding the gravity field of each
- Imaging more than 80% of each at a high resolution (for Vesta at least 100 meters per pixel and 200 meters per pixel for Ceres)
- Mapping the topology of each with same specifications as above
- Finding out how much H, K, Th, and U are 1 meter deep on each
- Getting spectrographs of both (with a majority at 200 meters per pixel for Vesta and 400 meters per pixel for Ceres) (Rayman 607)
To help Dawn accomplish this, it will make use of three instruments. One of these is the camera, which has a focal length of 150 millimeters. A CCD is set at the focus and has 1024 by 1024 pixels. A total of 8 filters will allow the camera to observe between 430 and 980 nanometers. The gamma ray and neutron detector (GRaND) will be used to see rock elements such as O, Mg, Al, Si, Ca, Ti, and Fe while the gamma portion will be able to detect radioactive elements such as K, Th, and U. It will also be possible to see if hydrogen is present based on cosmic ray interactions at the surface/ The visual/infrared spectrometer is similar to the one used on Rosetta, Venus Express, and Cassini. The main slit for this instrument is 64 mrads and the CCD has a wavelength range from 0.25 to 1 micrometers (Rayman 607-8, Guterl 51).
The main body of Dawn is a “graphite composite cylinder” with much redundancy built into it to ensure all mission goals can be accomplished. It contains the hydrazine and xenon fuel tanks while all the instruments are on opposite faces of the body. The ion engine is just a variant on the Deep Space 1 model but with a bigger tank, containing 450 kilograms of xenon gas. 3 ion thrusters, each with 30 centimeter diameters, are the outlet for the xenon tank. The maximum throttle that Dawn can achieve is 92 milliNewtons at 2.6 kilowatts of power. At the smallest power level Dawn can be at (0.5 kilowatts), the thrust is 19 milliNewtons. To ensure that Dawn has sufficient power, solar panels will provide 10.3killowatts when at 3 AU from the sun and 1.3 kilowatts as the mission nears its conclusion. When fully extended, they will be 65 feet long and make use of “InGap/InGaAs/Ge triple-junction cells” for the power conversion (Rayman 608-10, Guterl 49).
Guterl, Fred. "Mission to the Forgotten Planets." Discover Mar. 2008: 49, 51.
Geveden, Rex D. "Dawn Cancellation Reclama." Letter to Associate Administrator for Science Mission Directorate. 27 Mar. 2006. MS. Office of the Administrator, Washington, DC.
Rayman, Marc D, Thomas C. Fraschetti, Carol A. Raymond, Christopher T. Russell. “Dawn: A mission in development for exploration of main belt asteroids Vesta and Ceres.” Acta Astronautica05 April 2006. Web. 27 Aug 2014.
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© 2014 Leonard Kelley