A decade of operations in deep space is no longer a unique understanding, as we have seen in recent weeks, as Cassini ended 13 years of spectacular science at Saturn and the international Dawn mission celebrated ten years exploring the asteroid belt—which lies between Mars and Jupiter—and became the first emissary from Earth to enter orbit around two dwarf planets, Vesta and Ceres. Running such complex spacecraft for long periods of time, with all instruments fully operational, is notoriously difficult. Added to the mix is the complexity of co-ordinating between several nations and tackling technical obstacles far from home. Joe Makowski, Orbital ATK’s Phase E Project Manager for Dawn, recently outlined for AmericaSpace the challenges and the triumphs of this remarkable mission.
From the outset, Dawn was intended to be an international voyage, with the spacecraft “bus” provided by Orbital Sciences Corp.—now Orbital ATK—and instrumentation and power furnished by various European partners. Dawn’s twin solar arrays, which give it a total length of 65 feet (19.7 meters) when fully unfurled, were built by DutchSpace (now Airbus Defence and Space Netherlands), whilst Germany supplied a framing camera and Italy provided a visible and infrared mapping spectrometer. Star trackers were provided by France. Other instrumentation, including Dawn’s gamma-ray and neutron detector, came from Los Alamos National Laboratory in Los Alamos, N.M.
“Dawn was conceived as a mission with multiple international partners for hardware and science, so we could bring the best experience to the mission,” Mr. Makowski told AmericaSpace. As a lower-cost mission of exploration, belonging to the “Discovery” program, it received a ceiling of around $300 million and Germany and Italy were both considered “partners”, rather than “suppliers”. “The international science instrument contributions helped mitigate the cost constraints of a Discovery-class mission,” he added, “and provided a path for outstanding scientists to contribute to the mission.”
Although Orbital ATK has a wealth of historical experience working with non-U.S. suppliers, the company was obliged to contend with challenges imposed by the International Traffic in Arms Regulations (ITAR), which limits information-sharing beyond the United States. “The implications of ITAR were perhaps more noticeable when dealing with our international mission partners,” explained Mr. Makowski. “It again imposed significant constraints on what information we could share with our non-U.S. partners, as well as how quickly we could do so. The limitations on our part could understandably result in frustration for our mission partners. Due to time and space logistics, issue resolution may have taken a little longer than domestic interfaces, but the team’s determination to resolve problems overcame most of those constraints quickly.”
Down-selected from 26 Discovery proposals in December 2001, Dawn was originally targeted for launch in mid-2006 to visit and orbit both Vesta and Ceres, two quite distinct dwarf planets in the asteroid belt. However, technical challenges and spiraling costs made the mission’s early development a tortured time. In October 2003, Dawn passed its Preliminary Design Review (PDR)…and was formally canceled just weeks later, in December. “NASA re-defined a cost parameter, mandating a “total-cost-accounting” method to be applied,” Mr. Makowski told AmericaSpace. “That had the effect of reducing the project-cost margin at PDR below the mandated value.” The deletion of a laser altimeter from Dawn’s science payload did nothing to mitigate this problem.
In hopes of saving Dawn, the mission was re-defined in two segments, with a “prime” component targeted to visit Vesta and an “extended” option to continue to Ceres, thereby ensuring that mandated funding margins could be maintained. “The plan was presented that way at the Mission Confirmation Review in December 2003, but was rejected,” said Mr. Makowski. NASA’s stance was that Dawn had been selected on the basis of performing comparative science on both dwarf planets and, if funding to visit Ceres did not prove to be available, the option placed the entire mission in serious jeopardy.
Dawn was therefore barred from progressing beyond Phase B (the post-contract-award stage of its development) into the next phase. Over the following weeks, efforts by Orbital and NASA’s Jet Propulsion Laboratory (JPL)—which carried project management oversight—were made to save the mission. Orbital CEO David Thompson oversaw the investment of more financial resources to ensure continuation and several other cost-cutting measures, including the removal of a magnetometer from Dawn’s science payload, enabled the Ceres half of the mission to be restored. Early in 2004, NASA confirmed that Dawn could progress to its implementation phase (Phase C/D).
The delays had unavoidably pushed Dawn’s launch date further to the right, with departure from Earth not anticipated before mid-2007. However, all was still not well, for costs continued to increase from $373 million to over $446 million. In early March 2006, an Independent Assessment Team (IAT) reviewed the program and noted development issues with Dawn’s xenon tank, some of its power and digital electronics and harness delays. “The cancelation was announced before JPL had an opportunity to fully present the Project response to the IAT report,” said Mr. Makowski. A timely intervention from JPL Director Charles Elachi, and new management procedures established by incoming NASA Administrator Michael Griffin, led to a reassessment and by the end of March Dawn had once again been restored to flight status.
The original launch date in June 2007 slipped into July, before teams stepped down to make room for the August launch of NASA’s Phoenix mission to Mars. Dawn eventually settled on a revised target date in late September. “I vividly recall sitting on console at JPL on 27 September 2007, when a Delta II Heavy lofted the spacecraft on its planetary journey,” said Mr. Makowski, “fulfilling a vision of asteroid exploration and culminating a very strenuous 5.5-year effort by the team to produce a perfect spacecraft. Humans are not perfect, so we build-in protections to catch and protect against our faults. This could never be done alone, but required the combined efforts of hundreds of skilled people.”
In ten years of operations, Dawn has uncovered the faces of two worlds, which were previously barely resolvable in images acquired from Earth. On the whole, the spacecraft has performed admirably, tempered by intermittent issues with its four reaction wheels, which form an integral part of its pointing mechanism. Problems with the wheels had been an industry-wide issue, before Dawn’s launch, but far from Earth engineers had to devise ways of handling them. “The industry used detailed fault trees and ground testing to assess the possible failure mechanisms and incorporated changes, in an attempt to resolve them,” Mr. Makowski told AmericaSpace. “The consensus at the time was to fly the wheels and seek to limit their total revolutions below values where issues were encountered.”
One of the reaction wheels sustained excess friction and was powered-down in June 2010, a full year before the spacecraft reached Vesta. Although Dawn needed only three of the wheels in order to achieve pointing, its restoration was imperative. “Orbital ATK recommended a software enhancement to JPL for Dawn, called Hybrid Mode,” said Mr. Makowski. “This change allowed any two remaining wheels and the reaction control jets to provide full three-axis control for the spacecraft. Its value was that it would save substantial amounts of hydrazine fuel over an all-jet solution. The software was developed, tested and uploaded on the spacecraft in April 2011, ready for use at Vesta, if needed.”
Dawn’s operations at Vesta proceeded smoothly, although a second reaction wheel failed in early August 2012, shortly before the spacecraft was due to depart the dwarf planet and head for Ceres. Within days, it was brought back online and the mission continued. In addition to the reaction wheels, Dawn’s supply of hydrazine maneuvering fuel also came under threat. “Noting that hydrazine had become our most precious resource, the project embarked on a very rigorous effort to preserve hydrazine by reducing attitude turns and planning the Ceres operations very efficiently,” said Mr. Makowski. “Since wheels were not required for ion thrusting or the high orbits at Ceres, the two-wheel Hybrid Mode was held in reserve, for use at Ceres Low-Altitude Mapping Orbit, where the hydrazine savings was the greatest. That combined strategy worked perfectly.”
Hybrid Mode was invoked in December 2015 and remained in use until April 2017, when the third reaction-wheel failure occurred. “By that time, Dawn had accomplished all its Prime Mission objectives and was over nine months into its extended mission,” said Mr. Makowski. “The fuel saved by these actions proved to be a vital piece in enabling Dawn to achieve all of its NASA requirements for the mission, established long before launch.”
With perhaps another year of operations at Ceres ahead of it, Dawn has proven one of the most spectacular scientific missions ever conducted. As well as being the first deep-space exploratory voyage to employ xenon-ion propulsion, it has been the first to enter orbit around two different worlds. “The science data gathered at Vesta and Ceres will be of value for decades and has enlarged our understanding of the system in which our Earth resides,” concluded Mr. Makowski. “It took the best efforts by everyone, ranging over a span of nearly 25 years, but “Mission Accomplished” was achieved, because we agreed to do it together.”
The author would like to express sincere thanks to Mr. Joe Makowski, Orbital ATK Dawn Phase E Project Manager, for his time in providing the above comments to AmericaSpace. Thanks are also due to Ms. Vicki Cox, Orbital ATK Director of Communications, Space Systems Group, for her support and for arranging the correspondence.