JWST instrument repairs on schedule for next year
BY STEPHEN CLARK
Posted: September 25, 2013
GREENBELT, Md. -- The infrared eyes of the James Webb Space Telescope, the successor to Hubble, are poised begin a two-year stretch of unprecedented ground testing after the delivery of the observatory's final instrument to NASA, according to mission managers.
NIRSpec is the last of the mission's four science packages to arrive at Goddard from builders in Europe, Canada and California.
With NASA more fixated than ever on JWST's budget and schedule, the end of instrument deliveries mark an inflection point for the costly observatory, which promises to rewrite science textbooks and make untold discoveries over its 10-year life, much like its predecessor, the Hubble Space Telescope.
"This represents the retirement of a fairly significant risk to us," said Bill Ochs, JWST project manager at Goddard.
Next year, NIRSpec and another newly-arrived instrument - the Near-Infrared Camera, or NIRCam - will be added to the telescope's scientific heart, a module crammed with instrumentation and wiring designed to withstand the unforgiving loads of launch and the frigid vacuum of space.
Engineers are putting JWST's instrument module, packed with five sensors and nine support subsystems, through a deep freeze test chamber chilled to minus 400 degrees Fahrenheit. And they will do it three times over the next two years.
The observatory must operate at such low temperatures because it is designed to detect infrared light, or the heat radiating through space from some of the most distant, coolest objects in the universe, many of which are enshrouded in clouds of dust, rendering the stars and galaxies invisible to conventional visible telescopes.
The three rounds of cryogenic testing on JWST's instrument module - one more than originally planned - mark the first time different components the telescope have been assembled and tested as a unit.
Ochs said the next year could strain the mission's budget and schedule reserves, which have remained fairly stable since the program was put on solid footing in 2011 following what some mission officials have termed a "near-death experience" for the flagship telescope.
An independent review of the JWST mission revealed NASA failed to provide adequate cost and schedule reserves, and the space agency rescheduled the mission's launch from 2014 to 2018 and upped JWST's projected cost to $8.8 billion.
Ochs was named project manager in late 2010 as NASA shuffled the mission's leadership.
"A lot of the activity is ramping up in 2014," Ochs said. "A lot of things are coming to a head."
Och outlined major milestones coming up in fiscal year 2014, which begins Oct. 1. They include:
"We know what's ahead of us," Ochs said. "It's the unknowns that concern us, but we can minimize those through risk mitigation."
JWST's preflight testing, designed to chill and shake the observatory to prove it can survive its mission, is planned in steps - first at the instrument and mirror level, then of the combined telescope.
"This is a very complex test setup," said Matt Greenhouse, JWST's instrument project scientist at NASA's Goddard Space Flight Center. "It's really the most complicated deep cryogenic test that NASA's ever done."
The telescope's instruments are bolted inside a car-sized chassis called the Integrated Science Instrument Module, or ISIM, a load-bearing composite structure made of carbon fiber and cyanate-ester resin, a unique combination designed to retain alignment in super-cold temperatures and survive the extreme forces of launch.
Greenhouse said ISIM, including the sensors, structure, and engineering subsystems, is the culmination of an approximately $1 billion investment, more than most of NASA's robotic space missions.
"Two of the tests are required to fully verify the [ISIM] element for flight," Greenhouse said. "We have to go through a cryogenic cycle and then a vibration and acoustics test, and then another cryogenic cycle to fully ensure that the payload will survive the trip to orbit."
The ISIM is already undergoing the first of three rounds of cryogenic testing. The ongoing cryogenic test, which includes two flight instruments and ground stand-ins for NIRSpec and NIRCam, will conclude in November, and Greenhouse said the first test sequence is geared toward wringing out potential problems with the test facility at Goddard.
Next year, when the ISIM is pulled out of its test chamber after its first exposure to cryogenic temperatures, engineers will install the NIRSpec and NIRCam instruments into the chassis. JWST's other observing payloads - the European-built Mid-Infrared Instrument and Canadian-provided Fine Guidance Sensor and Near Infrared Imager and Slitless Spectrograph - were delivered to Goddard last year and are already installed in the ISIM.
The infrared detector arrays were manufactured by Teledyne Imaging Sensors, which is building another set of detectors after engineers diagnosed and fixed the problem.
"The original set of detectors had a design flaw which caused them to slowly degrade over time at ambient [temperatures], and in this program we spend lots and lots of time at ambient," Greenhouse said.
In order to simplify and streamline the repair work, Ochs said four quadrants of microshutters inside NIRSpec will be replaced while technicians are changing out the instrument's detectors.
The microshutters, each the width of three to six human hairs, are designed to open and close to block the light of bright nearby objects while allowing in light from faint targets.
The 250,000 programmable microshutters are arranged in a waffle-like grid.
"Some of the shutters were getting stuck closed," Ochs said.
Ochs said the first quadrant of fresh microshutters is complete, with three other sections due to be finished by the middle of next year.
Another issue with NIRSpec - cracks in its silicon carbide support structure - was responsible for a one-year delay in the instrument's delivery to NASA. That issue was resolved by replacing the cracked unit with a spare.
The last cryogenic cycle in 2015 will include all of JWST's flight instrument hardware, including the new detectors and redesigned microshutters.
Ochs said some of JWST's cost and schedule margin, freshly renewed in 2011 after the mission's overhaul, has been consumed by the delays and instrument troubles.
"That is what the reserves are there for," Ochs said. "We still have seven-and-a-half months of reserve of the ISIM schedule before its due to be integrated on the telescope."
Ochs said JWST has 14 months of overall schedule slack and a "healthy" cost reserve.
"We're reaching the point of the program once we get into [integration and testing] where we may eat into that a little bit," Ochs said. "But I feel pretty stingy about giving up schedule reserves."
Ochs said some of the margin was consumed by a scheduling conflict with the joint U.S.-Japanese Global Precipitation Mission, a satellite due for launch in 2014 which needed access to Goddard's test facilities before JWST.
The next phase of JWST's step-by-step assembly will be the attachment of the instrument module to the telescope, which includes a 21.3-foot-diameter primary mirror and a thermally-stable backbone. That milestone is scheduled for late 2015.
Then the combined telescope and instrument package will be shipped to Houston for another round of cryogenic testing at NASA's Johnson Space Center before it is transported to Northrop Grumman's facility in Southern California for mating to the spacecraft bus and sunshield.
Some time in mid-2018, the completed JWST will ride a ship to the European Space Agency's Guiana Space Center in South America for final preparations for launch on an Ariane 5 rocket in October 2018 toward an observing post at a gravitationally-neutral Lagrange point a million miles from Earth.
Ochs said JWST will ride on a proven version of the workhorse rocket known as the Ariane 5 ECA, not an evolved configuration with a new upper stage engine set to debut in 2017.
"I was pretty adament that we want to launch on a rocket with a great track record and not something that's only flown a couple of times, even if it is just an upgrade," Ochs said.