Manager Says Latest JWST Technical Issues Have Been Resolved
OTTOBRUNN, Germany — The acting project manager for NASA’s James Webb Space Telescope (JWST) on Sept. 6 said the program team has resolved issues related to malfunctioning micro-shutters, cryo-cooler valve leaks and degraded infrared-light detectors that had been highlighted by the U.S. Government Accountability Office (GAO) in December as threats to the flagship observatory.
Eric Smith, addressing a briefing here to mark the delivery of the last of the four main JWST instruments, also said the spacecraft’s weight gain of a year ago has been reversed by trimming kilograms from several areas of the structure and taking advantage of the fact that JWST’s launch vehicle, Europe’s Ariane 5 heavy-lift rocket, has more power than it did when it was selected for the mission.
Under regular scrutiny by the U.S. Congress following repeated cost increases, JWST is entering a series of key environmental test campaigns that have started without all four instruments to maintain the program’s schedule in the face of instrument-delivery delays.
The satellite is expected to weigh 6,500 kilograms at launch. Going into the cycle of multiple tests, it is holding to its current schedule of a launch in October 2018 from Europe’s Guiana Space Center on the northeast coast of South America, Smith said.
Its current cost to NASA is estimated at $8.8 billion, a figure that has remained constant since a major program review in 2010, Smith said.
The figure does not include a contribution valued at more than 600 million euros ($800 million) from the 20-nation European Space Agency (ESA) and several individual ESA governments. Europe is furnishing two of the four observing instruments in addition to JWST’s launch.
One of the two Europe-built instruments, the Mid-InfraRed Instrument imager and spectrometer (MIRI), was delivered to NASA in 2012.
The second, the Near-Infrared Spectrograph, or NIRSpec, is a year late following the late-2011 discovery by prime contractor Astrium GmbH, based here, of cracks in its support structure.
A 2012 review concluded that the cracks in the silicon carbide structure were due to insufficient caution in the way the structure was mounted. The basic material was determined to be sound, and Astrium was able to replace the cracked structure with the spare flight model.
Eric Beranger, chief executive of Astrium Satellites, said the cracks were a consequence of dealing with materials and a structure that “is on the edge of what is feasible. We are dealing with processes, and even with tools, that have yet to be invented,” Beranger said during the briefing here.
Alvaro Gimenez, ESA’s director of science and robotic exploration, said the 12-month delay in NIRSpec’s delivery added about 12 percent to the instrument’s cost to ESA. The total NIRSpec program cost is now estimated at about 170 million euros, including Astrium-provided services to be delivered after NIRSpec arrives at NASA’s Goddard Space Flight Center in Greenbelt, Md.
The 200-kilogram NIRSpec is being prepared for flight to Goddard on Sept. 20.
NIRSpec’s delivery follows the late-July arrival at Goddard of the Near-Infrared camera, built by Lockheed Martin Advanced Technology of Palo Alto, Calif., which like NIRSpec was nearly a year late.
The GAO report in December highlighted several issues that could threaten JWST’s schedule in addition to the NIRSpec cracks.
A sophisticated assembly of some 250,000 micro-shutters to block out light and give JWST a selection of what to focus its camera on arrived to be integrated into NIRSpec with a defect that made them unable to open after exposure to the acoustic environment expected under the Ariane 5 fairing as the vehicle traverses the atmosphere.
Smith said “a small design change” in the shutters has been tested successfully and that the full assembly of a replacement set is underway and on schedule.
A second issue was with the light detectors used on three of the four JWST instruments whose performance degraded over time, threatening to cut short JWST’s planned 10-year mission.
Smith said the detectors have been used on other U.S. government missions and had the same problem. He said contractor Teledyne has developed new detectors that do not have this problem and that the company is a year ahead of schedule of making them ready for JWST.
A final problem dealt with valves on the cryo-cooling system for the MIRI instrument. GAO reported, and Smith confirmed, that the original set of valves did not fully close, producing small leaks. A new set of valves that do not have this problem has been produced.
These equipment delays have forced NASA to start cryogenic vacuum tests — JWST is designed to operate at minus 235 degrees Celsius to pick up faint infrared sources — without the full complement of instruments.
The agency has added a new set of tests as a consequence.
The cryogenic vacuum tests — three are now planned as a result of the delays — take three months each, and if they are interrupted they must be repeated from the beginning.