Called "vulcanoids" after the Roman god of fire and metallurgy, this hypothesized population of small asteroids is exceedingly difficult to observe from the ground because they orbit so near the sun.

Researchers have made previous ground-based searches for vulcanoids during total solar eclipses, during the brief twilight period after sunset before the vulcanoids themselves set, and just before sunrise after the vulcanoids have peaked above the horizon.

So far, those observations have succeeded only in placing limits on how many vulcanoids might exist, with no vulcanoids yet found.

SwRI theoretical models suggest that a modest population of a few hundred kilometer-size and larger vulcanoids could have survived the harsh dynamic environment of the solar system, far interior to the orbit of the planet Mercury, from primordial times to the present.

The relative faintness of the vulcanoids against a twilight sky, along with atmospheric hazes and turbulence, have restricted ground-based searches to fairly bright limiting magnitudes corresponding to objects at least 12 to 37 miles (20-60 kilometers) across.

During the three-flight observation campaign, two SwRI astronomers took a sophisticated digital imaging system, the Southwest Universal Imaging System--Airborne (SWUIS-A), into the stratosphere in a high-performance F/A-18 jet aircraft used by NASA Dryden to support flight research missions.

This was the first phase of two observation series this year, with the second scheduled about the time of the autumnal equinox.

"Our vulcanoids search program, conducted from an altitude of 49,000 feet over the Mojave Desert, gave us a view of the twilight sky near the sun that is far darker and clearer than can be obtained from the ground," said principal investigator Dr. Daniel D. Durda, a senior research scientist in the SwRI Space Studies Department in Boulder, Colo.

"Our observations with the SWUIS-A imaging system will result in the most comprehensive, constraining search yet conducted for these objects," added co-investigator Dr. Alan Stern, director of the SwRI Space Studies Department.

"SWUIS-A is a versatile and capable, low-cost astronomical imaging system developed by SwRI that operates in the in the broadband visible light and near-infrared spectrums.

It has been successfully flown since 1997 in cooperation with NASA Johnson Space Center and NASA Dryden Flight Research Center. Another version was flown on two space shuttle missions during the late 1990s, focusing on cometary, lunar and planetary observations in the ultraviolet regions of the spectrum."

NASA research pilot Rick Searfoss, a former space shuttle astronaut and Dryden's project manager for this year's airborne astronomy missions, said the opportunity to fly SwRI's SWUIS-A instrument on one of Dryden's two-seat F/A-18B aircraft is a unique win-win project.

"In employing high-performance operational aerospace vehicles like a space shuttle orbiter or F/A-18 Hornet, sensors and telescopes aboard the vehicles can obtain better data than equivalent ground-based systems," he said.

"While space shuttle-based astronomy missions have produced phenomenal results, any space mission is a very expensive and difficult undertaking.

"This small airborne astronomy project is being done at absolutely no additional cost to NASA or the public," Searfoss added. "Our research pilots must fly a certain number of night flights anyway for our proficiency requirements, but in working with SwRI, we can also contribute to an exciting pure research goal. It's the perfect blend of science and flight operations."

The vulcanoids zone is one of the few dynamically stable niches of the solar system that remains largely unexplored. Researchers believe this presumed reservoir of small objects could contain a sample of planet-building material left over from the earliest days of the inner solar system.

Because of the intense thermal conditions and comparatively harsh collisional environment characteristic of this region, the vulcanoids population also might contain unique mineral assemblages not seen in other populations farther from the sun, such as asteroids and comets.

In addition, because vulcanoids would impact and crater Mercury yet spare nearby bodies, knowing how many vulcanoids may exist will aid researchers' understanding of that planet's surface chronology and the impact history of the other planets in the inner solar system, including Earth.

This innovative asteroid observation program is funded by grants from the NASA Planetary Astronomy Program, the National Geographic Society, the American Astronomical Society, and the Fund for Astrophysical Research, Inc.

Initial results from the spring observation phase are expected in late April.

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The search for vulcanoids and the SWUIS-A system
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