Probe built to visit asteroids killed in budget snarl
BY STEPHEN CLARK
Posted: March 3, 2006
A robotic mission to study two of the solar system's largest asteroids has been killed by NASA after months of uncertainty while extensive reviews probed the mission's funding and technical credentials.
The cancellation of Dawn comes amid other proposed cuts in the agency's science budget in an effort to fulfill the Vision for Space Exploration, which calls for completing the assembly of the space station, retiring the space shuttle fleet, and developing the next-generation Crew Exploration Vehicle.
However, Dawn's cancellation is a rarity. Most missions under consideration for termination or deferment are relatively early in the development and design phases, but Dawn's spacecraft is currently sitting at a contractor facility at Orbital Sciences where it was undergoing final assembly last year.
Missions such as Triana - a politically charged Earth observation satellite - have also found their way onto the chopping block as construction neared completion. In 1998, a NASA remote sensing satellite named Clark also fell victim to budget concerns and launch delays.
NASA has tried in the past to re-use parts and instruments from abandoned spacecraft on other missions. The future of the Dawn hardware is currently unclear.
Managers of the Dawn mission were first warned of trouble last October, when NASA officials ordered a halt to operations as final testing was getting underway before shipment of the craft to its Florida launch site in advance of a then-planned June 2006 blastoff. NASA simultaneously launched a thorough review of the cost overruns and technical problems facing the mission.
At that point, workers were bolting on the last boxes and testing the assembled spacecraft. After the stand down, the vehicle was "safed" and attention was focused on paperwork items and answering questions from the independent assessment team, Dawn Principal Investigator Christopher Russell told Spaceflight Now.
"My first reaction to the news of the stand down was shock and a feeling that there must have been a better way to accomplish the confidence building that NASA obviously needed," Russell said.
"It is emotionally very hard to stop a race when you see the finish line in sight. And it is very hard on those people who were laid off when the stand down took place. A stand down is a very big hammer. It is not for finishing nails."
Dawn was to have been the ninth mission of the Discovery program, which attempts to fly a higher number of missions that are lower in cost and smaller in scope than earlier NASA programs. It was selected for implementation in 2001.
Plans originally called for Dawn to rocket into space aboard a Delta 2 booster as early as this June to begin its circuitous trek through the solar system that would have included a fly-by of Mars in February 2009. Dawn would have then arrived at asteroid Vesta in late 2011, where a stay of at least six months was anticipated. After departing Vesta, Dawn's ion engines would have navigated the probe toward asteroid Ceres, where it would have entered orbit in August 2015 and stayed until the end of the mission.
The stand down initially made the June launch impossible, and postponements to November 2006 and early 2007 followed. Officials say Dawn had until October 2007 to launch and still reach Mars for a critical gravity assist maneuver.
Dawn's science payload consisted of a framing camera provided by German scientists at the Max Planck Institute and the German Aerospace Center, DLR. The Italian Space Agency was responsible for the visible and infrared mapping spectrometer. The Los Alamos National Laboratory in New Mexico built a gamma ray and neutron detector.
The two asteroids targeted by Dawn are believed to have remained intact since their formation in the very early stages of the solar system. Scientists expected to learn the chemical composition of both asteroids, search for water-bearing minerals and a metallic core, and determine their precise mass, shape, volume, rotation rate, and gravity.
The Discovery program had capped the costs of Dawn at $371 million, but project officials saw the first indication of going over-budget in early 2005, according to Russell. A new cost analysis system alerted management of a potential $7 million deficit.
"We then did a grass roots estimate of what it would take to launch successfully," Russell said in January. "So everyone on the project was asked to look carefully at the work to go and provide their best estimate of the cost. This number was higher, $17 million, as one might expect when (giving) people a chance to re-estimate."
"Then we called in a committee of experts who just took a top level look at the costs and schedule and recommended that we add more cost and schedule reserve and fund it. This number was $40 million. It was a worst-case number but it was the number that NASA used to decide that Dawn needed to be stopped for a while so that an assessment of readiness for launch could be made."
In the period leading up to the stand down, worries also spread concerning several key spacecraft issues. Relying on a solar electric ion propulsion system, Dawn was to have carried a tank for the xenon gas propellant required by the three cutting edge engines. The xenon tank - composed of a titanium liner covered with composite wrapping - is located deep inside the spacecraft bus, and other pieces were added around it during the manufacturing process.
The flight tank inside Dawn passed pre-flight tests that included taking it to pressures far above those necessary for ground or space operations. Yet when similar tanks were put through more stringent tests, they ruptured at pressures lower than the expected design limits. By finding that the tank was not as strong as first thought, engineers were forced to compensate the weakness by forming a new strategy that included not filling the tank to its full capacity.
Trouble with the Power Processing Units was also plaguing the project. Built by L3 Communications, the units provide the electrical power to the thrusters in the ion propulsion system. Officials say the delivery of at least one PPU was delayed and that there may have been workmanship problems.
Another concern with the stress on the craft's materials during the "bakeout" procedure was also raised shortly before the work stoppage was ordered.
"It was not any one problem with the spacecraft, but the problems continued to occur as the mission development continued," Russell said before the cancellation. "I would say this is the normal course of events in this business. Those holding the purse strings did not have my confidence. I do not believe any one problem such as a blown fuse, or over-heating of a part, or some contamination on a connector was the trigger. It was that we reported every problem as it occurred, so there was a series of problems."
Despite these issues, scientists close to the project felt that Dawn's problems were not all that different from those encountered during in preparation for the launch of any deep space probe.
"I have always felt that Dawn was in better shape than previous missions with which I was involved," Russell said earlier. "At a minimum, we are 'in family' with the other missions in terms of open paperwork, schedule delays, and technical issues."
As recently as four weeks ago, Russell still held out hope that Dawn would be cleared to proceed toward launch.
The Discovery program is currently accepting new proposals for a new mission that must launch by 2013 with costs under $425 million. Next in line for the program is the Kepler mission that will seek Earth-size planets around nearby stars in the Milky Way galaxy. Its launch is scheduled for June 2008.
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