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Week of 30 August 2002 · Vol. VI, No. 1

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Data-gathering space missions SPIDR satellite will map universe's cosmic web

By Brian Fitzgerald

How do galaxies form, evolve, and cluster with other galaxies? Hoping to provide answers to these questions, this summer NASA chose the Center for Space Physics' SPIDR satellite as the next mission in the agency's Small Explorer (SMEX) program.

A model of the SPIDR satellite. Photo courtesy of Draper Lab


A model of the SPIDR satellite. Photo courtesy of Draper Lab


"This is a major stepping-stone for us," says Supriya Chakrabarti, director of the center, a CAS astronomy professor, and the principal investigator for the $90 million project, which will be launched in 2005. This is the largest research grant thus far in the University's history.

SPIDR, which stands for spectroscopy and photometry of the intergalactic medium's diffuse radiation, will map the "cosmic web" of hot gases that span the universe. Current cosmological theory holds that gases produced by the Big Bang, which brought the universe into existence, condensed into a giant filamentary web along which the galaxies and clusters of galaxies eventually formed. SPIDR's findings will both test this basic understanding of the nature of the universe and provide new information to refine this understanding further.

SPIDR is one of two missions chosen by NASA from 46 proposals originally submitted in February 2000. Chakrabarti says that the SMEX program's preference for tightly focused, low-cost missions with small to midsized spacecraft was a major reason that BU received the grant.

"NASA is seeking answers to major, fundamental, profoundquestions," he says. "We tried to figure out what was achievable given the available funds, and we developed a plan that was feasible and low-risk." After the list was pared down to seven proposals, the researchers wrote a concept study report, which NASA reviewed. The space agency then sent some 20 experts to spend a day at each institution, scrutinizing different technical, engineering, and management aspects of its program. Finally, Chakrabarti and representatives from the six others under consideration traveled to NASA in late June.

"We were each given 20 minutes to make our case," says Chakrabarti, "and then, on July 2, NASA announced the selection of its next two missions." The other mission, to be launched in 2006, is Hampton University's Aeronomy of Ice in the Mesosphere Explorer, which will determine the causes of the highest altitude clouds in the earth's atmosphere.

A third of the SPIDR grant will enable BU scientists, who have overall responsibility for the entire program, to design and build the six innovative and sensitive ultraviolet spectrographs that will travel on board the satellite. "Students will be involved in conducting mission and science operations and performing data analysis," says Chakrabarti.

Roughly one meter wide and two meters long, the satellite will be launched from the bottom of an airplane flown out of Vandenberg Air Force Base in California and travel in an elliptical high earth orbit, using spectral imagery to capture data on gases ranging in temperature from a few hundred thousand to a million degrees.

The SPIDR project, which includes coinvestigators from eight other universities and collaborators from Draper Lab in Cambridge, is planned to have a mission lifetime of at least three years. "About one-third of the funds will go to Draper to build a 'spacecraft bus,' which is a structure that houses the electronics and provides pointing control, communicates with the mission control, and has the necessary computing to carry out all essential tasks," says Chakrabarti. "The other third of the funds will go to a private company, which will launch the satellite."

Members of the Center for Space Physics, including Tim Cook, a CAS research assistant professor and the satellite mission instrument scientist, and John Lapington, a research associate and the science payload manager, will send commands to the satellite in space and gather data that should answer fundamental questions concerning the formation and evolution of galaxies, clusters of galaxies, and other large structures in the universe, as well as address a number of questions related to hot gases in our own galaxy.

According to the Big Bang theory, the universe started about 16 billion years ago from an infinitesimally small point with infinitely large mass. This point violently expanded, creating space, time, and everything else. Some of this matter coalesced into stars and galaxies, although it is thought that much of the matter that makes up the universe is unobserved. "It must have gone somewhere," says Cook. "The question is: where? Theorists think that the matter heated up and condensed into these gas filaments." The SPIDR satellite is designed to detect and measure these gases, which cannot be seen with the naked eye.

SPIDR isn't BU's first satellite - in 1999 the center's TERRIERS satellite was launched to survey the earth's ionosphere and thermosphere in three dimensions. However, because of a flaw in the pointing system built by the center's industry partner, it wasn't able to orient itself so that its solar panels fully faced the sun, and it ran out of battery power.

Only six SMEX missions have made it into space since 1992, five of which are still operating and returning data. "From the time Explorer 1 was launched more than 40 years ago and discovered the Van Allen radiation belts, Explorer satellites have made impressive discoveries by obtaining significant science at the lowest cost," says Edward Weiler, NASA's associate administrator for space science. SPIDR "will continue in the Explorer tradition by investigating some of the most fundamental questions raised in space science."


30 August 2002
Boston University
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