Dawn (spacecraft)

• 1. Introduction

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  Dawn, a NASA space probe, launched in September 2007. It carries a memory chip stamped with more than 360,000 names submitted by members of the public, mounted just above one of its ion thrusters. It became the first spacecraft to orbit two extraterrestrial bodies, the first to visit Vesta or Ceres, and the first to orbit a dwarf planet. How did a mission that NASA cancelled twice before launch end up making both those trips, and what did it find once it got there?

• 2. An Engine Without Rockets

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  Harold R. Kaufman built the first working ion thruster in the United States in 1959, at NASA's Glenn Research Center in Ohio. It used mercury as its propellant. In 1964, that design flew on a suborbital test called Space Electric Rocket Test 1, running for its planned 31 minutes before falling to Earth. A follow-up orbital test, SERT-2, flew in 1970. NASA's Deep Space 1 probe launched in 1998. It proved that a xenon-fueled ion thruster called NSTAR could run for years, and it flew past an asteroid and a comet. Deep Space 1 also validated the Small Deep Space Transponder, the same long-range communication hardware that Dawn later carried.

  Three xenon ion thrusters, descended from that NSTAR design, propelled Dawn one at a time. Each produced just 90 millinewtons of thrust, with a specific impulse of 3,100 seconds. It took four full days of continuous thrust to push the spacecraft from a standstill to 60 miles per hour. A 10-kilowatt solar array built by Dutch Space, using triple-junction gallium arsenide cells, supplied the power. Dawn carried 247 kilograms of xenon for the approach to Vesta and another 112 kilograms for Ceres, out of 425 kilograms in total.

  That gentle push added up to a velocity change of roughly 11 kilometers per second over the whole mission. No earlier spacecraft had achieved that much propulsion with its own fuel after leaving its rocket. NASA credited the ion engines with a specific record, a solar-electric velocity change of 25,700 miles per hour, 2.7 times the previous record. By the 7th of September 2018, the engines had logged 5.9 years of running time, 54 percent of Dawn's years in space. None of that propulsion would have left the ground, though, without NASA first agreeing to keep building the spacecraft at all.

• 3. Cancelled Twice

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  Twenty-six proposals competed for a slot in NASA's Discovery Program, against an initial budget target of 300 million dollars. Three semi-finalists, Dawn, Kepler, and a mission called INSIDE Jupiter, were selected in January 2001 for detailed design studies. That December, NASA chose both Kepler and Dawn for the program, with launch then planned for 2006.

  The project was cancelled in December 2003 and reinstated two months later, in February 2004. Work was placed in stand down mode in October 2005. By January 2006, press reports called it indefinitely postponed even though NASA had made no formal announcement. On the 2nd of March 2006, NASA cancelled Dawn a second time.

  Orbital Sciences Corporation, the spacecraft's manufacturer, appealed the decision. It offered to build the probe at cost, forgoing any profit to gain experience in a new market. NASA reviewed the cancellation and reversed it on the 27th of March 2006. By the last week of September that year, the mission's instruments had reached full functional integration. Christopher T. Russell had been chosen to lead the Dawn mission team. The mission that had been projected to cost 373 million dollars would run to 446 million dollars by the time it reached launch in 2007.

• 4. Two Contrasting Worlds

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  Ceres and Vesta were chosen because the two protoplanets contrast sharply, one apparently wet and icy, the other dry and rocky. Both bodies stopped growing early, their accretion cut short when Jupiter formed nearby.

  Ceres alone accounts for a third of the total mass of the asteroid belt, and its spectral signature resembles a water-rich carbonaceous chondrite. Vesta, by comparison, holds only a tenth of the belt's mass. It shows a metallic core, a density close to Mars, and lava flows resembling those on the Moon.

  Radionuclide dating of meteorites believed to come from Vesta shows it separated into layers within three million years or less of the Solar System's birth. Ceres, by contrast, appears to have formed more than three million years after the oldest known solid material in the Solar System took shape. That material is known as calcium-aluminium-rich inclusions, or CAIs.

  Most V-type near-Earth asteroids share Vesta's spectral fingerprint and are classified as vestoids. Five percent of all meteorites found on Earth, the howardite-eucrite-diogenite group, are thought to result from collisions with Vesta. More than 1,400 of these HED meteorites have been catalogued. Ceres's own shape offers a clue to what lies beneath its surface. Its oblateness is too small for a body without internal layering, suggesting a rocky core wrapped in an icy mantle.

  The International Astronomical Union had only coined the term dwarf planet on the 24th of August 2006. Dawn reached Ceres in March 2015, just a few months before NASA's New Horizons probe reached Pluto in July that same year.

• 5. Instruments For Two Worlds

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  NASA's Jet Propulsion Laboratory managed the mission and provided its ion propulsion system. Orbital Sciences Corporation built the spacecraft itself, the company's first interplanetary project. European partners supplied key sensors. The Max Planck Institute for Solar System Research and the German Aerospace Center built the framing cameras. The Italian Space Agency built the mapping spectrometer. The Los Alamos National Laboratory in the United States contributed the gamma ray and neutron detector.

  Two identical framing cameras flew for redundancy, each fitted with a 150-millimeter lens. Each camera's detector, a Thomson TH7888A charge-coupled device, held 1024 by 1024 pixels. It could resolve features down to 17 meters per pixel at Vesta and 66 meters per pixel at Ceres.

  The visible and infrared spectrometer traced its design to instruments flown on the Rosetta and Venus Express probes, with heritage from Cassini's mapping spectrometer. It split incoming light into two channels fed by a single grating. One channel covered wavelengths from 0.25 to 1.0 micrometers. The other, cooled to about 70 kelvin, spanned 0.95 to 5.0 micrometers.

  The gamma ray and neutron detector was based on hardware previously flown to the Moon and Mars. It used 21 sensors with a wide field of view. The instrument measured elements such as oxygen, magnesium, and iron, along with potassium, thorium, uranium, and water, in the top meter of each world's surface. A magnetometer and a laser altimeter were both considered for the mission but never flew.

  Folded up for launch, Dawn measured 2.36 meters across. With its solar arrays fully extended, it spanned 19.7 meters, with a main antenna 1.24 meters wide. That instrument suite would soon face a harder test than expected, once Dawn reached deep space and its long journey began.

• 6. A Mission Interrupted

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  On the 17th of February 2009, Dawn passed within 549 kilometers of Mars during a gravity assist. The flyby slowed Mars's orbital velocity by roughly one inch every 180 million years. That same day, the spacecraft entered safe mode and lost some data. Engineers traced the cause to a software programming error and restored full operation two days later.

  Reaction wheels used to orient the spacecraft began failing during Dawn's time at Vesta. Engineers delayed its departure from the 26th of August to the 5th of September 2012. Later, at Ceres, the same failures pushed the team to adopt a hybrid mode, combining the remaining wheels with the ion thrusters to conserve fuel.

  On the 11th of September 2014, one of Dawn's ion thrusters unexpectedly stopped firing, triggering a safe mode. Engineers swapped in a backup engine and controller, suspecting the original had been damaged by a high-energy particle. Normal operation resumed on the 15th of September. The same incident left Dawn unable to aim its main antenna at Earth, until its computer was reset and the antenna's aiming mechanism restored.

  A further software glitch struck on the 30th of June 2015, this time traced to the gimbal mechanism on one of the ion engines. Engineers certified the spacecraft fit to continue only after switching to a different engine and testing it from the 14th to the 16th of July. None of these failures stopped Dawn from completing its survey of Ceres, work that continued for three more years before its fuel ran out.

• 7. A Monument In Orbit

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  In April 2016, the Dawn team proposed sending the spacecraft on from Ceres to fly past a third asteroid, 145 Adeona, in May 2019. They argued the extra science would outweigh the returns from simply staying at Ceres. NASA's Planetary Mission Senior Review Panel rejected the idea in May 2016. It approved a one-year extension instead, one that kept Dawn orbiting Ceres as it approached its closest point to the Sun.

  That extension expired on the 30th of June 2017, but NASA kept Dawn running anyway, announcing the mission would continue until its hydrazine ran dry. The hydrazine, used to control the spacecraft's orientation, was exhausted by the 31st of October 2018. NASA declared the mission over on the 1st of November.

  NASA expects the derelict spacecraft to remain in that uncontrolled orbit around Ceres, serving as what NASA called a monument, for at least twenty years.