Dawn (spacecraft)

• 1. The Quiet Thrust of History

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  Harold R. Kaufman built the first working ion thruster in 1959 at NASA's Glenn Research Center in Ohio. This early engine used mercury as its propellant and resembled a gridded electrostatic design. Suborbital tests followed during the 1960s, leading to the Space Electric Rocket Test 1 mission on the 20th of July 1964. The engine operated for 31 minutes before falling back to Earth. A second orbital test called SERT-2 occurred in 1970. Deep Space 1 launched in 1998 demonstrated long-duration use of xenon-propelled ion thrusters. That mission validated technologies like the NSTAR electrostatic ion thruster while performing flybys of an asteroid and a comet. The Dawn mission would later rely on this heritage for its unique ability to orbit multiple bodies.

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• 2. A Budget Battle And Rebirth

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  Twenty-six proposals were submitted to the Discovery Program solicitation with an initial budget target of US$300 million. Three semi-finalists emerged in January 2001 including Dawn, Kepler, and INSIDE Jupiter. NASA selected both Kepler and Dawn for the program in December 2001 with a planned launch date of 2006. The project status changed repeatedly over the next few years. It was cancelled in December 2003 then reinstated in February 2004. Work entered stand down mode in October 2005 and discussions labeled it indefinitely postponed by January 2006. On the 2nd of March 2006, NASA cancelled the mission again. Orbital Sciences Corporation appealed the decision offering to build the spacecraft at cost without profit. This appeal led to a review and a final announcement on the 27th of March 2006 that the mission would proceed. Cost overruns inflated the final price from US$373 million to US$446 million in 2007. Christopher T. Russell took charge as team leader.

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• 3. Two Worlds One Mission

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  The Dawn mission targeted two large bodies in the asteroid belt to answer questions about Solar System formation. Ceres and Vesta were chosen as contrasting protoplanets where one appeared wet and icy while the other remained dry and rocky. Their accretion stopped due to the formation of Jupiter. These bodies provide a bridge between rocky planet formation and icy body evolution. Ceres comprises a third of the total mass of the asteroid belt with spectral characteristics suggesting water-rich carbonaceous chondrite composition. Vesta is smaller yet accounts for a tenth of the belt's mass and shows signs of significant heating and differentiation. Evidence indicates both formed very early retaining records from terrestrial planet times. Radionuclide dating suggests Vesta differentiated quickly within three million years or less. Thermal studies suggest Ceres formed more than three million years after CAIs. Five percent of meteoritic samples found on Earth are thought to originate from collisions with Vesta.

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• 4. A Long Spiral To The Belt

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  Dawn launched from Space Launch Complex 17B at Cape Canaveral Air Force Station on the 27th of September 2007. A Delta 7925-H rocket provided escape velocity before ion thrusters took over. Initial testing accumulated more than 11 days 14 hours of operation by the 17th of December 2007. Dawn completed its first thrusting phase on the 31st of October 2008 sending it toward Mars for a gravity assist in February 2009. During this cruise phase the spacecraft spent 270 days using its thrusters. It expended less than 72 kilograms of xenon propellant for a total change in velocity of 1.81 km/s. The closest approach to Mars occurred on the 17th of February 2009 at a distance of 549 kilometers. This flyby slowed Mars's orbital velocity slightly while the spacecraft entered safe mode briefly due to software errors. The mission then began an elongated outward spiral trajectory toward the asteroid belt.

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• 5. Mapping The Rocky Giant

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  Dawn entered orbit around Vesta on the 16th of July 2011 making it the first spacecraft to orbit an object in the asteroid belt. The spacecraft moved to lower orbits by running its xenon-ion engine using solar power. An initial survey orbit lasted 69 hours at an altitude of 2,700 kilometers. A high-altitude mapping orbit followed on September 27 at 830 kilometers. The final low-altitude mapping orbit was reached on December 8 at 200 kilometers. Preliminary results published in May 2012 estimated Vesta's metal-rich core size at 220 kilometers across. Scientists described Vesta as the last remaining example of large planetoids that formed rocky planets. Further results from October 2012 explained anomalous dark spots likely deposited by ancient impacts. Reports from December 2012 noted gullies interpreted as eroded by transiently flowing liquid water. The probe departed Vesta on the 5th of September 2012 after a delay caused by reaction wheel failures.

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• 6. The Icy Dwarf Planet

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  Dawn began photographing Ceres on the 1st of December 2014 with images released as animations showing partial rotations. Images taken after the 26th of January 2015 exceeded resolution limits set by the Hubble Space Telescope. The spacecraft entered Ceres orbit on the 6th of March 2015 four months before New Horizons arrived at Pluto. This made Dawn the first mission to study a dwarf planet at close range. Initial operations included an RC3 orbit lasting 15 days at 15 kilometers altitude. A Survey orbit followed starting the 6th of June 2015 at 470 kilometers altitude where the craft circled every three Earth days. High-Altitude Mapping Orbit began the 17th of August 2015 at 1,380 kilometers allowing stereo imaging for 3D surface resolution. Low-Altitude Mapping Orbit started the 23rd of October 2015 at 380 kilometers enabling gamma-ray and neutron detection of surface composition. The final science orbits climbed back to higher altitudes to complete observations from different angles while reducing propellant consumption.

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• 7. Engineering Around Failure

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  During its time in orbit around Vesta the probe experienced several failures of its reaction wheels. Investigators planned to modify activities upon arrival at Ceres using hybrid mode combining reaction wheels with ion thrusters. Engineers completed a 27-hour test series of this hybrid mode on the 13th of November 2013. On the 11th of September 2014 an ion thruster unexpectedly ceased firing triggering safe mode operation. The team exchanged the active engine and controller with another unit to avoid propulsion lapses. The disabled component was revived later in 2014 after determining damage likely came from high-energy particles. Another issue prevented aiming the main communications antenna toward Earth requiring temporary use of a weaker antenna. A computer reset restored the aiming mechanism of the main antenna. These technical hurdles required innovative engineering solutions throughout the transit phases.

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• 8. A Monument In Orbit

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  NASA announced the end of operations on the 1st of November 2018 when Dawn depleted its hydrazine fuel supply. The spacecraft had run out of propellant by the 31st of October 2018 while remaining in an uncontrolled but stable orbit around Ceres. An extended mission proposal for May 2019 involving asteroid 145 Adeona was declined by NASA's Planetary Mission Senior Review Panel in May 2016. The one-year extension expired the 30th of June 2017 leaving the craft to remain as a monument for at least 20 years. The derelict probe continues to orbit Ceres without power or control. It stands as the first spacecraft to have orbited two extraterrestrial bodies and the first to visit either Vesta or Ceres. The mission achieved a velocity change of approximately 11 km/s over nine years using ion propulsion.

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