An F-18B aircraft in flight. Photo: NASA

Flight research at NASA's Dryden Flight Research Center, Edwards, Calif., took on a new dimension recently when two of the center's high-performance F-18 mission support aircraft served as flying observatories for an astronomical occultation experiment.

Scientists from the Southwest Research Institute (SwRI), Boulder, Colo., in collaboration with the NASA field center, successfully demonstrated the ability to navigate the aircraft to a position where the astronomer could observe a predicted momentary occultation, or eclipse, of a star as an asteroid passed in front of it. Astronomers Dr. S. Alan Stern and Dr. Daniel Durda of the SwRI Department of Space Studies used the specially-designed Southwest Ultraviolet Imaging System-Airborne (SWUIS-A) mounted above the instrument panel in the rear cockpit of an F-18B for the observations.

Five nighttime astronomy flights were flown over the course of the past year. Although the astronomers had limited success in predicting the exact location of the asteroid-star eclipse during the first four missions, Dryden research pilot Ed Schneider navigated the F-18 to within two seconds of the expected occultation point on the final flight Jan. 9 and the SWUIS-A captured the image of asteroid 308 Polyxo as it passed in front of a star.

"This observation will reveal the size of 308 Polyxo, which, like most asteroids, is too small for even the Hubble Space Telescope to resolve," said Stern, director of the SwRI Department of Space Studies in Boulder and principal investigator for the project.

"Our observation with SWUIS-A, obtained from above the clouds that obscured the sky over Dryden during the event, will be combined with three ground-based observations from other locations to deduce the asteroid's shape," added Durda, flight astronomer on the successful occultation mission.

"Knowing the sizes of asteroids, we can then use their apparent brightness in the sky to tell us how bright or reflective their surface minerals are, which in turn tells us about the composition of the asteroids," said Durda. "Some are as dark as an asphalt parking lot, others as bright as a dirt road. By combining the size and shape information, we can learn a little about their collisional history and internal strength. That ties back to the collisional history of the entire asteroid belt, and helps us understand the formation of the planets."

Stern and Durda worked with Mike Smith, acting chief of Dryden's Operations Engineering branch, and Dr. Paul Herrick of the University of Alaska to design the installation of the SWUIS-A imager and connect it to the aircraft electrical power and video recording systems. Although the device was mounted in an F-18, the design allows its use in other aircraft with a bubble canopy affording a clear view of the sky.

"We tried for five different events," said Dryden research pilot Dana Purifoy, who coordinated the NASA effort. "The SwRI astronomers would provide information for the ground track for the shadow that would be created by the occultation, they would predict the angle for the camera, and that would give us the location and the time that we had to fly the aircraft to. Most of the events we went after were fairly faint-small asteroids against small stars-and their ability to precisely predict the shadow still needed some work. There were times when we were on time and on target, but the event wasn't. The final one that culminated the series in January was a large event, and the asteroid occulted the star exactly where they had predicted it would," Purifoy added.

Observation from F-18B aircraft. Photo: SwRI

For decades, airborne astronomy and geophysical observations have proven to be useful adjuncts to ground-based and space-based instrumentation, particularly for optical and infrared studies, Durda explained. Compared to ground-based instruments, airborne research platforms offer superior atmospheric transmission and the mobility to reach remote or otherwise unreachable locations, such as over oceans. Airborne platforms also virtually guarantee good weather for observing the sky since they can fly far above the atmospheric pollution that often obscures the sky above Earth-bound observatories, and are far less expensive to operate than spacecraft.

Durda said a key advantage of smaller, high-performance aircraft like Dryden's F-18s is that their per-hour operating cost is only a fraction of that of larger transport-type aircraft typically used as airborne observatories. Other advantages include faster reaction times to transient astronomical events, along with the F-18's ability to navigate to the path of the occultation shadow.

Although the F-18's flown in the recent astronomy research cannot carry as large a payload or as sophisticated telescopes as larger airborne observatories, the SwRI/NASA demonstration project enabled astronomers to fly along with their instruments, providing a space shuttle-like "payload specialist" capability.

To date, 14 SwRI/NASA airborne astronomy missions have been flown using the sensitive but rugged SWUIS-A imager. Earlier missions included high-altitude flights in a NASA WB-57 based at the Johnson Space Center, Houston, Texas, and on Space Shuttle mission STS-85 at the same time in 1997 to observe comet Hale-Bopp, and in NASA Dryden's F-18s to perfect techniques for observing asteroid and planetary occultations.

Future SWUIS-A missions aboard high-altitude aircraft, such as a two-seater trainer version of the U.S. Air Force U-2 flying at more than 70,000 feet altitude, will capitalize on the ability of the instrument to look near the sun to search for vulcanoids-an assumed population of small asteroids circling the sun inside Mercury's orbit-and to observe breakup mechanics in sun-grazing comets. SWUIS-A may also be used to detect and track space debris that might pose a hazard to satellites, the space shuttle, and the International Space Station. Using SWUIS-A to study a variety of terrestrial phenomena, including lightning, ozone depletion and meteoroid showers, is also envisioned.

Data from the observations recorded aboard Dryden's F-18Bs are being forwarded to the International Occultation Timing Association for inclusion in its library of information about the formation and collisional history of the asteroid belt.

Purifoy noted the project was supported by Dryden's Flight Operations branch on a "time available" basis during what would otherwise have been pilot proficiency flights. Flight training for the astronomers was funded largely through the SwRI internal research program, with no direct NASA funding expended for the project.