By Lisa Kaspin-Powell
On Thursday evening, August 21, the American Institute of Aeronautics and Astronautics Los Angeles–Las Vegas Section hosted another in its series of speaker dinners. Attendees gathered at the Crowne Plaza LAX to hear Dr. Jonathan Arenberg of Northrop Grumman Aerospace Systems speak on “The James Webb Space Telescope: Mission, Design and Progress to Launch.”
Arenberg, the James Webb Space Telescope (JWST) Chief Engineer, began his talk by describing the motivations for the project. He showed slides of two of his favorite paintings: works by Rembrandt. He used the paintings to illustrate the combination of so many elements into a striking work of art, and likewise with the JWST – a “work of collective genius spanning 250,000 components and about 12,000 man hours of effort to date.” Arenberg expressed his goal that his audience would gain a substantial appreciation of JWST’s beauty. As he noted, “JWST will change mankind’s view of the universe and our place in it.”
The telescope is named after James E. Webb, the second administrator of NASA, because of his integral role in the Apollo program. Arenberg made the important point that the JWST is one of NASA’s 3 top priorities.
The description of JWST’s scientific mission provided context for the details of its architecture and design. The telescope will look back 13.8 billion years—farther than any other telescope—to find galaxies that existed shortly after the Big Bang. JWST’s infrared imaging instruments and giant mirror will also help us to see what happened just after the dark ages, at the first light and the deionization of hydrogen; to understand how stars and protoplanetary systems are born; and to determine the origins of life.
The JWST will carry a sophisticated package of instruments to achieve those goals. The Near-Infrared Imager and Slitless Spectrograph (NIRISS) will be used to investigate first light detection, exoplanet detection and characterization, and exoplanet transit spectroscopy. Sparse aperture interferometry is provided by coronagraphy and a non-redundant mask; these reveal planets otherwise hidden by the light of the stars they orbit. The planets’ thermal emissions can then be detected and studied for atmosphere analysis. The NIRCam (Near Infrared Camera) will detect light from the earliest stars and galaxies in the process of formation, stars in nearby galaxies, and Kuiper Belt objects. The NIRSpec (Near-Infrared Spectrometer), like NIRCam, will be used to observe star formation, but in addition will help determine the chemical composition of distant galaxies. The MIRI (Mid-Infrared Instrument) will be used to investigate distant stellar populations, the physics of newly forming stars, and the sizes of faintly visible comets and Kuiper Belt objects, or “Pluto’s cousins”. These instruments will reside within the ISIM (Integrated Science Instrument Model). The spacecraft bus will be in the sun, providing the steering, propulsion, communications and thermal control. The Fine Guidance Sensor, which is packaged with NIRISS, will allow the telescope to point in the right direction. The momentum flap, like a trim flap in sailing, will help to stabilize the telescope.
JWST is truly an international program. In 2018, it will launch on an Ariane 5 from French Guiana. MIRI is provided by a consortium of European countries, the European Space Agency, and the Jet Propulsion Laboratory. The detectors are from Raytheon Vision Systems; the NIRSpec from ESA; the NIRCam built by the University of Arizona working with Lockheed-Martin; and the NIRISS from the Canadian Space Agency. Fourteen countries are involved in building the James Webb Space Telescope: Austria, Belgium, Canada, Denmark, France, Germany, Ireland, Italy, the Netherlands, Spain, Sweden, Switzerland, the United Kingdom and the United States of America.
The JWST mirror will be the biggest ever carried on a space telescope; in turn, the entire JWST will be the largest space telescope. JWST will send far sharper images compared with Hubble or Spitzer, and will show more of the early universe in one day than Hubble or Spitzer have captured over their working lives. With its large aperture and stable position, the JWST will have access to 35% of the sky at any one time, and the whole sky in a year. With a 6.4 m diameter, JWST’s mirror area is 7 times that of the Hubble’s, yet is only half its mass (6400 kg vs 11110 kg) due to its being made of beryllium, 95% of which has been machined off. In addition, the mirror is in hexagonal sections so it can be folded for flight and unfolded when needed. The 18 mirror sections will be coated with gold, which is an excellent reflector of red light and IR. Beryllium was chosen as opposed to glass not only because of its weight but because it has a relatively low coefficient of thermal expansion, compared with glass. This will enable it to withstand the change from room temperature to the approximate 40K at its planned position- an orbit around the L2 Lagrange point. Within that orbit, the JWST will remain in roughly the same location relative to the Earth and the Sun (1 million miles from Earth), well away from the Earth’s and Sun’s heat. The extreme cold will minimize background IR from the telescope itself. The positioning will have the added benefit of keeping JWST out of the Earth’s shadow, helping to maintain a steady light exposure level.
One disadvantage of that distance is that servicing the mission will not be possible – at least not with today’s technology–which will limit the life of the mission. Another limiting factor is the propulsion system’s fuel capacity – enough for 10 years of operation.
Additional cooling will be provided by what Arenberg described as “the world’s largest thermos bottle.” The sunshield would protect against 250,000 watts and let only 1 watt through, and will also protect the optics and ISIM from any heat from the spacecraft bus electronics. The shield is made up of 5 layers, separated by vacuum to provide insulation. A 1/3-scale sunshield passed thermal testing back in 2010, and in addition was shown to be able to withstand 11 years of micrometeorite bombardment. Testing was carried out initially at room temperature, then under cryogenic conditions, using a Space Environment Simulator (SES). SES testing showed that the instruments were able to function at room and cryogenic temperatures. The historic Chamber A, used in the Apollo program, was used for this testing. Arenberg showed a photo of the Johnson Space Center’s huge vacuum chamber.
Arenberg showed the audience several striking images to illustrate the magnitude of the telescope. One image showed a crowd around the sunshield, with 15 people to a side. Another image showed 18 canisters that each held a section of the mirror, which Arenberg called his “mirror in a can” photo. The gold coating amounted to a 50 g golf ball. Arenberg also showed a video of a simulation of the JWST’s deployment, featuring the sunshield coming out at 454,000 km altitude. Another video showed a time lapse of the deployment and stowage of the secondary mirror.
JWST is at the end of the design phase, and at the beginning of integration and testing. Telescope integration hardware is complete and installed. The spacecraft is in manufacture. More testing is in progress to manufacture and verify the primary mirror segment with independent tools.
Arenberg closed the formal presentation by again showing the two paintings, and by saying that “working on the JWST was the job of a lifetime.” After he answered several questions, the audience came away with an inspiring example of how an overview of the science behind a mission can drive the details of system design.