42 Years of Measuring the Sun, the Earth and the Energy in Between

By Denise Lineberry

On Jan. 31, 1958, Explorer 1 became the first satellite launched by the United States. Its primary science instrument, a cosmic ray detector, was designed to measure the radiation environment in Earth orbit. Though its final transmission was in May 1958, it continued to revolve around Earth more than 58,000 times. As those looping orbits continued, NASA was busy building other ground-breaking instruments to observe and better understand Earth’s systems.

By 1975, just five years after Explorer 1 burned up as it entered Earth’s atmosphere, NASA’s first Nimbus instrument launched, providing the first global, direct observations of the amount of solar radiation entering and exiting Earth. This helped confirm and improve the earliest climate models and laid the groundwork for NASA’s Earth Radiation Budget Experiment (ERBE).

By the 1970s, the ERBE team was beginning to plan for the next phase of Earth Radiation Budget measurements. Retired experiment scientist for ERBE, Bruce Barkstrom, recalled the very first ERBE science team meeting involved a full day of attempting to determine exactly where the top of the atmosphere was. After much debate, they assigned one person at NASA’s Langley Research Center in Hampton, Virginia, to develop the number, which ended up being about 18 miles (30 kilometers) above the sphere that forms the Earth.

“That was the level of detail we had to get into as a science team,” Barkstrom said.

In October 1984, ERBE launched aboard NASA’s Earth Radiation Budget Satellite (ERBS) from the space shuttle Challenger (STS-41G).

“We had to get up at 3:30 a.m. to watch the ERBS launch at 7:30 a.m., and what I remember about that particular morning was that we had an overcast sky. And when the shuttle lit up, it was such a bright exhaust that it lit up the whole sky from underneath,” Barkstrom recalled. “And then, of course, the shuttle went through the clouds, and the light dimmed, and probably about a minute later the sky lit up again because the sun was reflected off the exhaust.

“It’s impossible for me to describe this without getting a little emotional.”

For 10 years, ERBE provided invaluable data for scientists studying the energy interactions between the Sun, clouds and Earth. Its satellite measurements have provided new information on Earth’s radiation at the top of the atmosphere, including the important radiative effects of clouds on incoming and outgoing energy in the overall process.

In the late 1980s, satellite instruments provided the first direct observation that clouds cooled Earth’s climate. Former CERES Principal Investigator Bruce Wielicki developed an algorithm to apply to Nimbus and ERBE models to help quantify cloud forcing — the difference between the radiation budget components for average cloud conditions and cloud-free conditions.

With new knowledge about the important role that clouds play in Earth’s energy budget, the science team was anxious to gather more data. In 1997, the first in a new series of instruments, the Clouds and the Earth’s Radiant Energy System (CERES), launched, extending the important ERBE measurements.

Six other CERES instruments have since been activated in space to measure the solar energy reflected by Earth, the heat the planet emits, and the role of clouds in that process.

“The CERES instrument is small, it’s very elegant, it’s probably the most accurate radiometry that NASA has flown,” said CERES Principal Investigator Kory Priestley. “We’re trying to build the next generation of instrument now to meet the same requirements.”

The seventh and final CERES instrument launched aboard NOAA’s Joint Polar Satellite System (JPSS)-1 in November 2017. It has since been activated and first light is expected in January 2018.

For 42 years, NASA has observed Earth’s energy budget. NASA Langley’s Earth Radiation Budget Science Team is the only group producing ERB data globally. Though our understanding of Earth’s energy budget and the technology used to gather data has taken massive strides since Explorer 1 and Nimbus, that understanding is ever-evolving.

“With Earth observations, you never complete your understanding, so you’re always at the mercy of somebody discovering some new things,” Barkstrom said. “If you’re dealing with observational science, you never have that final escape into absolute certainty where you’ll never have to change things.”

Why Measure Earth’s Energy Budget?

According to Barkstrom, attempts to understand the radiation budget started in about 1880. Earth’s energy budget is a metaphor for the delicate equilibrium between energy from the Sun versus energy radiated back into space. Continuous, stable and accurate data records over decades are critical to understanding Earth’s energy balance.

The data collected improve models that provide seasonal and longer-term forecasts, which inform industry and policy makers to better plan for the future.

The Latest

NASA’s Total and Spectral Solar Irradiance Sensor (TSIS)-1 is currently on the International Space Station in a mission to measure the Sun’s energy input to Earth.

Various satellites have captured a continuous record of this solar energy input since 1978. TSIS-1 sensors advance previous measurements, enabling scientists to study the Sun’s natural influence on Earth’s ozone layer, atmospheric circulation, clouds and ecosystems.

These observations are essential for a scientific understanding of the effects of solar variability on the Earth system.

About the Missions: ERBE and CERES

ERBE

The radiation budget represents the balance between incoming energy from the Sun and outgoing thermal (longwave) and reflected (shortwave) energy from the Earth. In the 1970s, NASA recognized the importance of improving our understanding of the radiation budget and its effects on Earth’s climate. Langley Research Center was charged with developing a new generation of instrumentation to make accurate regional and global measurements of the components of the radiation budget. The Goddard Space Flight Center built the Earth Radiation Budget Satellite (ERBS) on which the first Earth Radiation Budget Experiment (ERBE) instruments were launched by the Space Shuttle Challenger in 1984. ERBE instruments were also launched on two National Oceanic and Atmospheric Administration weather monitoring satellites, NOAA 9 and NOAA 10, in 1984 and 1986.

CERES

The Clouds and Earth’s Radiant Energy System (CERES) experiment is one of the highest priority scientific satellite instruments developed for NASA’s Earth Observing System (EOS). The first CERES instrument was launched in December 1997 aboard NASA’s Tropical Rainfall Measurement Mission (TRMM), CERES instruments are collecting observations on three separate satellite missions, including the EOS Terra and Aqua observatories, the Suomi National Polar-orbiting Partnership (S-NPP) observatory, and soon, the Joint Polar Satellite System, a partnership between NASA and the National Oceanic and Atmospheric Administration (NOAA). In fall 2017, CERES FM6 launched on JPSS-1, becoming the last in a generation of successful CERES instruments that help us to better observe and study Earth’s interconnected natural systems with long-term data records.

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The Sky Belongs to All of Us

By Hashima Hasan

How did a little girl born in India soon after its independence from the British Empire, become a program scientist for NASA’s Hubble Space Telescope, and the first female program scientist for the James Webb Space Telescope, Stratospheric Observatory for Infrared Astronomy (SOFIA), Gravity Probe B, and other astrophysics flight missions?

The story starts in October 1957, when I was 7 years old, and my grandmother ordered the entire family, including my 3-year-old sister, all the servants and their families, to collect at dawn in the backyard of the home and watch Sputnik pass by the clear night skies of Lucknow.

That morning, as I saw Sputnik and the dark, starry sky, I dreamt the impossible dream that one day I would be a space scientist. The path was not easy. With determination and encouragement from my mother and school teachers, I forged ahead, won a scholarship to the University of Oxford, from where I earned a doctorate in theoretical nuclear physics in 1976. The path to a traditional academic career for a female scientist was fraught with challenges, exacerbated by social pressures. After pursuing post-doctoral research, a university faculty position, crisscrossing three continents and making a home across the Atlantic three times, I found myself again on the shores of the U.S. (1985) ― this time with a husband and two infant sons.

My research career had oscillated between nuclear physics and environmental science, preparing me for yet another scientific challenge, when I was offered a research position at the Space Telescope Science Institute (STScI), Baltimore, to write the software to simulate the optics of NASA’s newest (now legendary) telescope, the Hubble Space Telescope and its science instruments. I boldly accepted the job, and under the guidance of Dr. Christopher Burrows, wrote the Telescope Image Modeling (TIM) software.

Little did we know that after the launch of Hubble, TIM would be instrumental in our analysis of the first images, the identification and characterization of the spherical aberration, monitoring the focus of the telescope, and image simulations to enable scientists to analyze their aberrated data.

I was appointed as the Optical Telescope Assembly (OTA) scientist, and have the dubious distinction of being the first and only OTA scientist whose task was to keep the Hubble “in focus” until a fix could be designed. I regularly monitored the images to learn about the health of the telescope optics, degradation of filters in the Faint Object Camera, and image characteristics. The flaw in the primary mirror caused by shaving off glass from its edges no thicker than about a human hair, not only caused blurry images, but had a dramatic effect when there were minute movements of the mirror. We learned that the graphite epoxy truss that supported the primary and secondary mirrors, desorbed water faster and longer than calculations had predicted, causing minute shrinkage in the truss. This meant that approximately every 3 months the mirror had to be moved to bring it back to the “best focus” established by the science community. I also participated in the design and optical testing phase of the Corrective Optics Space Telescope Axial Replacement (COSTAR). During the first servicing mission, I did a final image analysis and focusing the telescope before COSTAR was deployed. I had been allowed three attempts to focus the telescope, but I achieved it in one attempt and COSTAR was deployed ahead of schedule. The following 2 years, I continued to work on the Hubble optics, a concept for an Advanced Camera for the Hubble, and astronomical research on barred galaxies.

I am proud to be a part of the NASA team that turned adversity to victory. The story of Hubble is a tribute to NASA’s “can do” attitude. The entire scientific, technology and human space flight community rallied around Hubble in the true “Explore as One” spirit to fix Hubble. The brave astronauts, who undertook the life-threatening job of servicing Hubble five times, helped make the observatory what it is today.

In 1994, I was ready for a new challenge and accepted a job as visiting senior scientist at NASA Headquarters, under the wing of the fabled, Dr. Edward Weiler. Under his tutelage, I rapidly learned how to manage flight missions and research programs, lead community working groups, strategic planning, international negotiations, and other skills. By 1999, I had achieved sufficient skills and experience to be appointed as a civil servant. During my 23 years at NASA, there have been numerous memorable moments. I would like to mention some.

In 1999, I was appointed as the program scientist for the Hubble, a position that I held till 2004. I provided scientific oversight to the science instruments, Wide Field Camera 3, and the Space Telescope Imaging Spectrograph (STIS), taking strategic decisions to enable development within cost and schedule. I participated in two servicing missions, SM3A and SM3B.

My involvement with the James Webb Space Telescope (JWST) started in 1995, when it was a mere concept referred to as the Next Generation Space Telescope (NGST), and Ed Weiler asked me to send a research grant to John Mather at Goddard Space Flight Center (GSFC) to study the concept for NGST. I was appointed NGST program scientist from 1999-2001 (and JWST program scientist from 2011-2015), and led the solicitation and selection of early technology development. I led the appointment of an Interim Science Working Group to develop the science requirement for NGST science instruments, and wrote the solicitation for the science instruments and Science Working Group. A particularly contentious negotiation we went through with our partners, the European Space Agency (ESA), and the Canadian Space Agency (CSA), was the partnership on the Mid-InfraRed Instrument (MIRI), ended amicably. Much negotiation was held with our partners, the European Space Agency (ESA) and the Canadian Space Agency (CSA), concerning the Mid-InfraRed Instrument (MIRI).

I developed a strategy for selecting a NASA center for management of the MIRI instrument. We were conducting a review of proposals for MIRI management on the fateful day, Sept. 11, 2001. Again, we did not let adversity stop us, and today MIRI and all the other science instruments are installed on JWST. Lessons learned from Hubble development have been applied to JWST development, including complete optical testing in a specially modified chamber at Johnson Space Center (JSC). The building of JWST is another example of “Explore as One,” where scientists, engineers, private industry and non-U.S. space agencies have come together with the ambitious goal of learning how the first stars and galaxies were born.

I would like all readers to follow their dreams as I have and not to get discouraged, as we continue exploring the Universe. The sky belongs to all of us, and NASA’s tremendous scientific journey can be followed through our space missions on  https://science.nasa.gov/.

About the Author

• Hashima Hasan

  Hashima Hasan is the NASA program scientist for the Keck Observatory, the SOFIA mission, ADCAR and is deputy program scientist for the James Webb Space Telescope. She also serves as the education lead for Astrophysics. Dr. Hasan has been the program scientist for many NASA missions, and from 2001-2006, she served as the lead for Astronomy and Physics Research and Analysis programs. Dr. Hasan received Her Ph.D. from the University of Oxford, U.K., in theoretical nuclear physics. She was the optical telescope assembly scientist at Space Telescope Science Institute, Baltimore, until 1994, when she joined NASA Headquarters.

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Measuring the Big Bang with the COBE satellite

By John Mather

The Cosmic Background Explorer satellite (COBE) went up on a Delta rocket on Nov. 18, 1989, into a polar sun-synchronous orbit 900 km up. Our team at NASA Goddard Space Flight Center (GSFC), Ball Aerospace, the Jet Propulsion Laboratory (JPL) and universities built it to look at the cosmic microwave and infrared background light that comes to us from the distant universe, so far away that it seems to be a nearly uniform glow. With it, we started the new subject of precision cosmology; before the COBE very little was known except the general idea of an expanding universe, misnamed the Big Bang. (It’s misnamed because the name conjures up the image of a firecracker, happening at a place and a time. Astronomers see an infinite universe expanding into itself, with no center, no edge and no first moment.)

Our team measured the spectrum of the cosmic heat ― more precisely the cosmic microwave background radiation― left over from times when the universe was compressed and hot, with a precision of 50 parts per million. The prediction was for a nearly perfect blackbody spectrum, and it matched. No other story of the universe was ever able to explain that. We also found the hot and cold spots of the heat radiation, known as anisotropy (Greek for not the same in every direction). Stephen Hawking said that was the most important scientific discovery of the century, if not of all time.

Now we know that: a.) the spots are responsible for our existence, because gravity acting on the regions of higher density was able to stop the matter from expanding; b.) most of the spots are caused by dark matter; and c.) if we ever know what made the spots, we might understand quantum gravity. In 2006, I got a call from Stockholm, and the Nobel Prize in Physics went to me and to George Smoot in recognition of the work of our team. Now the entire world knows what we know: it was really important.

We started in 1974, just 5 years after the first Apollo landing on the Moon, when NASA announced opportunities to propose new satellite missions. I had just finished my thesis project in January and taken a job with NASA’s Goddard Institute for Space Studies in New York City to become a radio astronomer. My thesis project at the University of California, Berkeley, was intended to measure that cosmic background radiation, but it failed to function properly. Yet only months after my arrival in New York, NASA announced the opportunity. My advisor Pat Thaddeus knew what to do: call up our friends and write a proposal. (One of those friends is Rainer Weiss of the Massachusetts Institute of Technology, who was also working on gravitational wave detection. He shared the 2017 Nobel Prize for detecting gravitational waves from merging black holes.)

I never expected our proposal to be chosen, but it was, thanks to far-seeing people at Headquarters like Nancy Boggess, and NASA created a new science team including people from two competing teams. Anticipating that choice, Mike Hauser recruited me to Goddard in Greenbelt, Maryland, and I was hoping to become the lead scientist. Soon Goddard assigned a brilliant team of engineers, who were just completing the IUE observatory, to help us along. We built up a team that eventually included 1,500 contributors, including a science team of 19 spread around the country.

The project was extraordinarily challenging, and became the largest in-house project Goddard has ever done. We brought the work in, because we were pushing so far beyond known engineering that it was impossible to write a contract specification; I spent much of my life in the offices of engineers seeking approaches to doing the impossible. I trusted my future to them, and they to me. In the end, our mission worked beautifully, after many changes, including a redesign after the Challenger loss made it clear we would not be launched on the shuttle.

NASA and its partner agencies like the European Space Agency and Canadian Space Agency are the only places in the known universe where space science and space engineering meet so intimately, where engineers build what has never been built before, so scientists may discover what has never been known before. I can only marvel at the works we have done, and imagine what we may yet do together.

About the Author

• John C. Mather

  John C. Mather is a senior astrophysicist in the Observational Cosmology Laboratory at NASA’s Goddard Space Flight Center (GSFC). His research centers on infrared astronomy and cosmology. As an NRC postdoctoral fellow at the Goddard Institute for Space Studies, New York City, he led the proposal efforts for the Cosmic Background Explorer (1974-1976), and came to GSFC to be the study scientist (1976-1988), project scientist (1988-1998), and also the principal investigator for the Far IR Absolute Spectrophotometer (FIRAS) on COBE. As senior project scientist (1995-present) for the James Webb Space Telescope, Dr. Mather leads the science team and represents scientific interests within the project management. He has received many awards including the 2006 Nobel Prize in Physics for his precise measurements of the cosmic microwave background radiation using the COBE satellite.

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Peering Homeward, 1972

By Laura Rocchio

On July 23, 1972 the first civilian satellite designed to image Earth’s land surfaces was launched from Vandenberg Air Force Base in California. On board the satellite, originally named the Earth Resources Technology Satellite (ERTS), but later known as Landsat 1, were two sensors. The primary sensor, called the Return Beam Vidicon (RBV), used three shuttered cameras to take photographs; the secondary sensor, the Multispectral Scanner System (MSS) was an experimental instrument.

Both sensors were packed onto a “butterfly-shaped” spacecraft repurposed from the successful Nimbus weather missions. There were strict size and weight limitations for the sensors, especially the experimental MSS that weighed less than the primary RBV sensor and the data recorder. (At over 150 pounds, the data recording system onboard was the biggest recording device ever orbited.)

The MSS technology was a novel way of looking at Earth. It used a scanning mirror to build up an image pixel-by-pixel with six scan lines sweeping across the satellite’s ground path 13.62 times per second as the satellite hurtled around Earth at over 14,400 mph. As the first civilian imaging scanner to orbit Earth, many of the scientists and engineers outside the small cadre of scanner enthusiasts wondered if the satellite’s MSS instrument would be able to successfully produce an image traveling at such a high velocity. This made for a harrowing day when the first imagery was transmitted back to Earth two days after launch.

A group of Landsat scientists and engineers gathered in the Landsat data processing facility at NASA’s Goddard Space Flight Center as the first MSS digital transmission was translated onto 70-mm film by an electron beam recorder and then displayed. As they watched the first imagery scroll by they saw clouds, more clouds, and finally land… but the black and white image had irregular wavy lines on it.

“It’s terrible. It has moiré patterns,” a technician lamented. Quickly those in the room figured out where the image was showing geographically—the Ouachita Mountain region of southeastern Oklahoma. Then the geologists in the room realized that they were seeing the curvilinear outcrops of the ancient mountains.

Anxiety transformed into excitement. NASA geologist Nicholas Short, who had been unconvinced of the utility of land remote sensing for geology, turned to the NASA Deputy Associate Administrator for Space Applications and said, “I was so wrong about this. I’m not going to eat crow. Not big enough. I’m going to eat raven.”

USGS cartographer Alden Colvocoresses, who had been cynical about any cartographically accurate data being collected with “a little mirror in space,” turned to his colleagues in the room and said simply, “Gentlemen, that’s a map.”

To the surprise of many, it was the ride-along secondary instrument of Landsat 1, the experimental Multispectral Scanner System that became the mission’s imaging powerhouse.

The MSS instrument represented many “firsts.” It was the first space-based sensor to digitally encode and transmit Earth surface data; the first Earth-observing instrument to obtain in orbit calibration data, which meant it was the first instrument Earth-scientists could use to make robust comparisons of changes to Earth’s surface over time and across geographies. It quickly proved itself better than the primary Return Beam Vidicon instrument—and a good thing too because just 15 days after launch a major electrical short associated with the RBV’s power-switching circuit caused enough problems that the RBV was shut down for the rest of the satellite’s mission.

The MSS data’s accurate geometric fidelity made it a major cartographic tool, and the low sun angle of Landsat’s mid-morning acquisition time accentuated shadows of topographic features making the images especially valuable to geologists; but many fields including agriculture, forest management and marine studies found the data useful.

The Explorer 1 mission had begun the U.S. forays into space, yet a striking realization that came from the space-bound missions that followed Explorer 1 in quick succession (Mercury, Gemini, Apollo) was that space offered a distinctive vantage point for observing our home planet.

A few months prior to the Landsat 1 launch, Secretary of the Interior and Landsat champion, Stuart Udall, had explained to The New York Times, “I thought an Earth applications program was a perfect means of bringing the benefits of space back to Earth.”

Once Landsat and its MSS instrument had proved itself after launch, NASA Administrator James C. Fletcher confirmed Udall’s belief, remarking that Landsat was “a second giant stride for mankind” because of the new technology’s potential to improve the understanding of environmental issues. He went on to say that Landsat had a “profound effect on the thinking of the world, particularly on our approach to emerging problems of protecting our environment and maintaining the quality of life for all of Earth’s people…not just clean air and water, but clean land.”

The First Space-Based GPS Satellite Tracking Experiment, 1982

On July 16, 1982 the fourth Landsat satellite—carrying “the most complex and pioneering Earth viewing instrument ever proposed for a NASA program” at the time—took to the sky.

Nearly everything about this second-generation Earth observation satellite had been upgraded from its Landsat 1, 2, and 3 predecessors. In addition to an MSS sensor, Landsat 4 carried a second-generation Earth-observing sensor, called the Thematic Mapper or TM instrument. The TM, a more advanced version of the MSS, was only one aspect of the mission’s radical redesign.

The Landsat 4 spacecraft was a custom-designed platform and not a re-purposed Nimbus weather satellite platform used for the first three Landsats. But the mission requirements were many—the satellite was required to be Space Shuttle rendezvous ready (for the concept of Shuttle-based repairs); to carry a large antenna (at the end of a long 12.5 foot boom) for communicating with NASA’s Tracking and Data Relay Satellite System (TDRSS); and to carry a GPS receiver.

Landsat 4 was the very first civilian satellite to carry a spaceborne GPS receiver package and to use GPS signals to calculate its position. The concept of GPS was so new at this time that in Landsat 4 press communications, the acronym “GPS” had to be written out and described as “a new US Air Force satellite navigation system involving orbiting navigational satellites to triangulate the exact position of other satellites which require navigation information as part of their data communication to Earth Stations.”

GPS receivers were used on both Landsat 4 and 5 satellites to assess if GPS could deliver more accurate position-location data than data gathered from traditional methods (ground-predicted ephemeris, or mathematically modeled locations).

GPS was in its infancy and only 4 of the planned 24 GPS constellation satellites were in orbit at the time of Landsat 4’s launch. So there were often times during Landsat 4’s orbit when no GPS satellites were in range.

Two researchers at NASA’s Goddard Space Flight Center, Howard Heuberger and Leonard Church, presented a paper on the Landsat 4 GPS navigation results demonstrating that GPS could establish Landsat 4’s position to within 50 meters, and its velocity within six centimeters per second—when the GPS satellites were in view. Though these error margins grew exponentially when GPS satellites were out of reach (because of lapses between measurements), Heuberger and Church concluded that GPS was a good alternative for supplying onboard ephemeris to future spacecraft systems even before the full GPS constellation was in orbit.

The experiment was largely a success, but deemed not ready for operational use. It was not until the launch of Landsat 8 in 2013—almost three decades after the Landsat 4 GPS experiment—that GPS receivers would become a routine part of Landsat spacecraft.

For an exhaustive technical history of the Landsat program, see the new book: Landsat’s Enduring Legacy: Pioneering Global Land Observations from Space.

About the Mission

Landsat

This joint NASA-U.S. Geological Survey program provides the longest continuous space-based record of Earth’s land in existence. Every day, Landsat satellites provide essential information to help land managers and policy makers make wise decisions about our resources and our environment.
For over 40 years, the Landsat program has collected spectral information from Earth’s surface, creating a historical archive unmatched in quality, detail, coverage, and length. Landsat sensors have a moderate spatial-resolution. You cannot see individual houses on a Landsat image, but you can see large man-made objects such as highways. This is an important spatial resolution because it is coarse enough for global coverage, yet detailed enough to characterize human-scale processes such as urban growth.

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My NASA Experience

By Marcia J. Rieke

The development of infrared detector arrays is intertwined with my experiences working on NASA projects. As an astronomer at a university, my interactions with NASA all start with a proposal in response to an opportunity. In 1983, near-infrared detector arrays were beginning to attract the attention of astronomers. At the suggestion of Nancy Boggess at NASA Headquarters, we wrote a proposal to the NASA Research and Analysis Program to obtain an array and test it. At the time, I was a member of the Infrared Astronomy Group working with George Rieke using a single light-sensing element (e.g. a 1 pixel array!) on ground-based telescopes, and I was only starting to become cognizant of astronomy opportunities with NASA.

In this initial proposal, we wrote that the array we were contemplating acquiring from what was then called Rockwell International (now Teledyne Imaging Systems), would potentially be useful for infrared instruments on HST. We were not thinking of proposing such an instrument ourselves as we were preoccupied with proposing an instrument for the SIRTF which was later re-named Spitzer.

Our proposal was selected, and we purchased a 32×32 HgCdTe array (wow, a whole kilopixel!). Taking a device to the telescope where one could actually take an infrared picture rather than creating a picture by scanning a single pixel back and forth made me feel even happier than a kid in the candy store. Some of my colleagues called it my “toy” camera, but it was so much fun. I remember characterizing the performance of this array, since performance would be of obvious great importance if such arrays were to be used on future NASA missions.

During testing, I discovered that the dark current of our first device was orders of magnitude less than what Rockwell had quoted. This needed to be understood because if my result was correct, then this class of infrared array would be a candidate for second generation HST instruments. I called Rockwell, and quizzed the staff about how they had measured the dark current on the array that they had sent us. The Rockwell test engineer explained that he had put a piece of aluminum foil over the dewar window to ensure that the array was in the dark. Well, that was the answer. Yes, the aluminum foil prevented visible light from entering the test dewar, but since it was at room temperature, it was emitting loads of infrared photons. Based on this discovery we decided to propose for a second generation HST instrument which eventually became “NICMOS.” As part of the development funding for that instrument, we moved all the way up to a 256×256 pixel array – 65.5 kilopixels but still not even 1 Mpixel camera. As a result of my involvement in the early steps of working with HgCdTe arrays, I became the Deputy PI for NICMOS, and became deeply involved in a NASA project. NICMOS was the first use in space of the style of near-infrared array that has now become the standard for infrared arrays.

Near the end of my involvement with NICMOS and before Spitzer was launched, another opportunity presented itself. People were discussing a “Next Generation Space Telescope” that would push the limits of detectability back to the first stage of galaxy formation. I replied to a letter soliciting members, and I set out to work on this new project. I stuck with it, and responded to the Announcement of Opportunity in 2001, and this triggered a change of events that has led to my being PI of the NIRCam instrument on the James Webb Space Telescope. The detector arrays in NIRCam are each 2028×2048 pixels (eg. 4 Megapixels) with the entire camera holding 40 Megapixels, a long way from my first 1 kilopixel array camera!

About the Author

• Marcia J. Rieke

  Marcia J. Rieke is a professor of Astronomy at the University of Arizona and is the principal investigator for the near-infrared camera (NIRCam) on the James Webb Space Telescope. Rieke came to the University of Arizona (UA) in 1976 and has made seminal contributions to infrared astronomy. She has served as the deputy principal investigator on the Near Infrared Camera and Multi-Object Spectrometer for the Hubble Space Telescope (NICMOS), and the outreach coordinator for the Spitzer Space Telescope. A fellow of the American Academy of Arts and Sciences, Rieke received her undergraduate and graduate degrees in physics from the Massachusetts Institute of Technology, Boston, Massachusetts.

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