Cosmic Ray Hit Causes Small Delay at Dawn's Rendezvous With Ceres Early Next Year

An artist's concept of NASA's Dawn spacecraft exploring Vesta and Ceres in the asteroid belt between Mars and Jupiter. Dawn had recently experienced an anomaly in its ion propulsion system resulting from a cosmic ray hit, which will cause a small delay to the spacecraft's rendezvous with Ceres next year. Image Credit: NASA

An artist’s concept of NASA’s Dawn spacecraft exploring Vesta and Ceres in the asteroid belt between Mars and Jupiter. Dawn had recently experienced an anomaly in its ion propulsion system resulting from a cosmic ray hit, which will cause a small delay to the spacecraft’s rendezvous with Ceres next year. Image Credit: NASA

Next year can rightfully be described as that of the dwarf planets. Having recently passed the orbit of Neptune, NASA’s New Horizons spacecraft will fly by Pluto and its ensemble of moons in July 2015, finally lifting the veil on this mystifying dwarf planet in the distant reaches of the Kuiper Belt. In another part of the Solar System, a few months prior to that historic event the space agency’s Dawn spacecraft will become the first man-made object to orbit dwarf planet Ceres in the asteroid belt between Mars and Jupiter, allowing scientists to shed more light to the processes that drove the formation and evolution of planets early in the Solar System’s history. Yet, despite Dawn’s ongoing progress to reach its ultimate destination, it now seems that the latter will have to wait a little longer.

Developed under the agency’s low-cost Discovery series of missions, Dawn is one of NASA’s most historic and innovative interplanetary robotic missions to have ever flown, tasked with conducting a series of important firsts in the annals of robotic space exploration. At the conclusion of its mission, should it be successful, Dawn will become the first spacecraft ever to orbit two different extraterrestrial objects—a large asteroid and a dwarf planet—while utilising an innovative system of ion thrusters as its main means of propulsion. The objects of its study, Vesta and Ceres, are not only the most massive objects in the asteroid belt, but they also represent the last surviving protoplanets in the Solar System today—left-over members of the primordial planetary embryos which eventually grew to form all the major planets, approximately 4.5 billion years ago. The detailed study and comparison of these very different objects will allow scientists to answer fundamental questions regarding the conditions and processes that eventually gave rise to our Solar System.

The giant asteroid Vesta is shown here as the smallest body among other similar bodies in the solar system: Mars, Mercury, Earth's moon and the dwarf planet Ceres. Image Credit: NASA/JPL-Caltech/UCLA

The giant asteroid Vesta is shown here as the smallest body among other similar bodies in the solar system: Mars, Mercury, Earth’s moon, and the dwarf planet Ceres. Image Credit: NASA/JPL-Caltech/UCLA

Having been cancelled no fewer than two times during its development, due to a series of technical and programmatic issues, the 1,240-kg spacecraft was eventually launched 7 years ago this week, on 27 September 2007. After successfully completing a detailed 14-month study of Vesta between July 2011 and September 2012, which revealed a stunning world that bore more similarities to terrestrial planets than asteroids, rich in impressive geologic formations, Dawn began spiralling out of the former’s orbit and set sail for its rendezvous with Ceres, which was originally scheduled for late February/early March 2015. Yet a recent anomaly in the spacecraft’s ion thrusters has pushed back that schedule by about one month. During a planned communications session on 11 September, ground controllers found out that Dawn’s propulsion system had stopped operating and the spacecraft had gone into safe mode. Engineers quickly assembled in order to assess the situation and restore the spacecraft back to its normal operating status. Following an analysis of the situation, it was finally determined that the likely cause that triggered the safe mode was a hit of high-energy cosmic rays particles which disabled some of the electronic components on Dawn’s propulsion system. “This anomaly presented the team with an intricate and elaborate puzzle to solve,” says Robert Mase, Project Manager for the mission at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. The mission’s team was further aided in its efforts by the fact that a similar event had occurred in 2011, prior to the spacecraft’s approach to Vesta. “We followed the same strategy that we implemented three years ago to recover from a similar radiation strike – to swap to one of the other ion engines and a different electronic controller so we could resume thrusting quickly,” says Marc Rayman, Dawn’s Mission Director and Chief Engineer at JPL. “We have a plan in place to revive this disabled component later this year.”

Such events are nothing new for interplanetary space missions. Every spacecraft that travels out of the Earth’s protective magnetic field is exposed to the constant bombardment of the high-energy solar and galactic cosmic radiation that permeates all of space. Even though various radiation-hardening techniques are implemented to protect sensitive spacecraft electronics from the damage caused by cosmic rays, the latter can often penetrate a spacecraft’s radiation shielding, posing a great risk to its mission.

The 2.3 kW NSTAR ion thruster for the Deep Space 1 spacecraft, during a hot fire test in a vacuum chamber at NASA's Jet Propulsion Laboratory. Dawn uses a similar propulsion system. Image Credit: NASA/JPL

The 2.3-kW NSTAR ion thruster for the Deep Space 1 spacecraft, during a hot-fire test in a vacuum chamber at NASA’s Jet Propulsion Laboratory. Dawn uses a similar propulsion system. Image Credit: NASA/JPL

In addition to disabling Dawn’s propulsion system, engineers also suspected that cosmic rays were behind another issue that had cropped up as well. Dawn’s onboard 1.52-meter high-gain parabolic antenna seemed unable to effectively point toward Earth during the safe mode, thus preventing any high-speed communications with ground controllers who were forced instead to relay commands to the spacecraft using its set of three low-gain antennas which slowed their overall progress. Nevertheless, even at this slower rate the mission’s team was able to reset the spacecraft’s onboard main RAD6000 computer, restoring the normal operation of the high-gain antenna.

At the end of the recovery operations, which lasted until 15 September, Dawn successfully re-ignited its ion thruster in order to resume its journey toward Ceres. That small period of inactivity according to NASA, will cause the spacecraft to delay its rendezvous with the dwarf planet by a about one month, arriving there sometime during April 2015. This delay is due to the nature of Dawn’s ion propulsion, which must operate constantly during the interplanetary cruise phase of the mission in order to produce the required thrust that is needed for the spacecraft to reach Ceres.

Cosmic ray hits are not the only obstacles that Dawn has faced thus far. In June 2010, when the spacecraft was still on its way to Vesta, telemetry data had shown that one of the four onboard reaction wheels had stopped operating due to increased friction. Reaction wheels are the attitude control system of choice by mission planners, allowing spacecraft to properly maintain their orientation in space at a very high degree of accuracy. To that end, a simultaneous operation of three reaction wheels is needed, while most spacecraft are equipped with a fourth spare unit which acts as a backup in the case of a reaction wheel failure. Yet, as Dawn was preparing to depart Vesta in August 2012, the mission’s team discovered that a second one had also malfunctioned due to excessive friction, leaving the spacecraft to operate on just two reaction wheels. Not willing to let this predicament jeopardize the mission, as was the case with the exoplanet-hunting Kepler space telescope which had also faced similar problems, Dawn’s mission team had already created a software update following the failure of the first reaction wheel in 2010 that would allow the spacecraft to successfully carry on its mission while using only two reaction wheels, in conjunction with its onboard set of twelve 0.9-newton hydrazine thrusters. As an added level of redundancy, the new software was designed to keep Dawn going even in the case of a complete reaction wheel failure, allowing it to maintain its position in space by using only its onboard hydrazine thrusters.

A team of astronomers using ESA's Herschel Space Observatory, have discovered clear signals of water vapour coming off Ceres. Image Credit: ESA/ATG medialab/Küppers et al.

A team of astronomers using ESA’s Herschel Space Observatory, have discovered clear signals of water vapour coming off Ceres. Image Credit: ESA/ATG medialab/Küppers et al.

Despite the various bumps on the road to Ceres, Dawn has managed to persevere while bringing itself ever-closer to the enigmatic dwarf planet. The timing of its arrival there early next spring couldn’t have been better. A series of observations by ESA’s Herschel space telescope has revealed the presence of water vapour around the distant world, hinting at the exciting possibility of water ice outgassing from localized pockets at its surface, similar to the geysers observed on Saturn’s moon Enceladus. If confirmed, such a finding would further showcase the unique nature of dwarf planets compared to the rest of the Solar System’s minor bodies like Vesta. “Vesta in many ways has been the low-hanging fruit,” said Britney Schmidt, a Dawn science team liaison from the Georgia Institute of Technology, Atlanta, during a Google Hangout. “When we went to Vesta, we really confirmed things we thought we knew. It was ‘idea-affirming’. And I think when we get to Ceres, it will just going to be absolutely a game-changer – a new window into the Solar System that we wouldn’t have without going there. It’s just another reason why planetary exploration is so important: it’s telling us about ourselves and our history and it’s developing new technologies, new understanding and new knowledge. If it does nothing else than inspire us to get up in the morning, smile and have a better day, then I’m happy.”

Our first close-up views of some of the Solar System’s most interesting dwarf planets next year certainly promise to be a great source of just such an inspiration.


Video Credit: NASA/JPL


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