Fighter Jet Hunts for "Vulcanoid" Asteroids

By John Roach
for National Geographic News

May 6, 2002

Space scientists hitched a ride earlier this year aboard an F/A-18B fighter jet traveling through the stratosphere at 0.92 Mach. From the cockpit, the night sky was inky black and pierced with diamond-like planets, streaks of comet dust, and a zodiac of light.

Not out merely for a night ride, the scientists had their eyes trained on the western horizon, where twilight hung low in a range from deep blue to glowing red.

There, in the twilight space, is where a cluster of small asteroids known as vulcanoids are thought to exist, if they exist at all, said Daniel Durda, a senior research scientist at the Southwest Research Institute in Boulder, Colorado.

"Finding such objects and being able to study their physical properties would help astronomers better understand the conditions in the solar nebula from which the planets—our own Earth included—formed," he said.

Durda and his colleague Alan Stern, director of the institute's Space Studies Department, have enlisted the help of a high-tech camera and high-flying aircraft to better search the twilight sky for vulcanoids. That region of the sky is difficult to search from the ground because of atmospheric hazes, turbulence, and its proximity to the sun.

Vulcanoid Theory

When space scientists in the early 1900s were unable to account for all of the gravitational forces in the planet Mercury's orbit, they theorized that an undiscovered planet even closer to the Sun—called Vulcan—could cause the unaccounted-for gravitational pull.

In his general theory of relativity in 1915, Albert Einstein provided a more complete description of gravity and how it behaves close to a massive body such as the Sun. As a result, astronomers were able to account for all the tugs and pulls on Mercury's orbit, and astronomers abandoned their search for Vulcan.

"However, there remained some interest in looking for smaller debris—small asteroids—in the region, since such material might have remained from the disk of planet-forming material after the planets grew to their final size," said Durda.

Other astronomers theorize that Mercury suffered a major impact late in its formation that stripped the planet of its outer rocky layers. That debris might still be in the vicinity of the planet as a population of small asteroid-like bodies.

"Our motivation for looking for vulcanoids today is no longer linked in any real way with the original motivation for looking for Vulcan," said Durda. "Today, we're only treating the region as a more or less unexplored region in the solar system that could have undiscovered objects there."

Rationing Space

Until now, the search for vulcanoids has been conducted solely from the ground. Observations have been confined to the small windows of time that occur during solar eclipses and in the moments just after sunset and just before sunrise.

To date, these observations have resulted only in speculation about how many vulcanoids might exist, with none actually found, said Durda.

Theoretical models based on these searches suggest that a population of a few hundred vulcanoids about a kilometer (0.6 miles) in size could have survived the harsh dynamic environment of the inner solar system. Asteroids of that size, however, are exceedingly difficult to observe in the twilight sky with ground-based telescopes. "In the glare of twilight, only big—thus fairly bright—objects can be seen," said Stern. "From 50,000 feet (15,240 meters), the sky is much darker at twilight. Thus, we should do better than one can do from the ground."

That was the basis of a funding proposal that Durda and Stern submitted to NASA's Planetary Astronomy Program. It bought the space scientists and their high-tech camera a ticket for three rides in the F/A-18B fighter jet earlier this year, and possibly a seat in a U-2 aircraft in September.

"This observation campaign presents a major improvement over previous unsuccessful ground-based surveys," said Serge Tabachnik, an astrophysicist at Princeton University in New Jersey whose calculations indicate that vulcanoids could exist in stable orbits near the Sun.

In-Flight Movie

To aid their search for the vulcanoids, Stern and Durda took a very sensitive camera along for their ride in the fighter jet at NASA's Dryden Flight Research Center in Edwards, California. The camera, called the Southwest Ultraviolet Imaging System–Airborne (SWUIS-A), allows the researchers to obtain images of objects that are 600 times fainter than what is visible to the naked eye.

The SWUIS-A, which was designed at Southwest Research Institute in Boulder, has been used previously in high-performance aircraft to observe comets and asteroids. It records images 60 times a second and sends the data to a video recorder, explained Durda.

From these images, the scientists assemble a deep and clear image of the stars.

The imaging process eliminates any movement in the images due to the motion of the aircraft. It also allows the scientists to, in effect, take a time exposure—"to look fainter than we could in any individual 1/60-second exposure," said Durda, who is currently processing the image data collected on the research flights.

Durda and his colleagues will analyze the images from the three flights to search for any moving objects that might be vulcanoids. The orbits of any candidate asteroids will be calculated to see if they are in the region where vulcanoids have been proposed to exist.

"The data analysis is tricky and we have over 100,000 images to sort through," said Stern. The task will take several months.

Are They There or Not?

The success of the search mission depends not on whether the researchers actually find vulcanoids, but in determining whether they exist. The answer will help astronomers' understanding of the universe.

If the researchers do discover convincing evidence of vulcanoids, the next logical step would be to study the composition of the space rocks. Scientists speculate that the asteroids would probably be very rich in minerals with a very high melting point, such as iron and nickel or rarer metals like tungsten, osmium, and zirconium.

If the vulcanoids formed as debris from Mercury, said Durda, researchers will have to wait for the results from the MESSENGER spacecraft, which is scheduled to launch in March 2004 on a mission to Mercury.

"If we do not find vulcanoids, that is important too, since that can help place constraints on the processes which could have removed material from the inner solar system after the planets formed," said Durda.

One theory is that the material in the inner solar system could have spiraled into or away from the Sun due to the so-called Yarkovsky Effect, which is a term used to describe how an asteroid's trajectory can be influenced by its heat radiation.

The influence of the Yarkovsky Effect is quite small and is strongest on asteroids with a rough, fragmented surface and smaller than one kilometer (0.6 miles) in diameter. Dusty, fragmented materials heat up and cool down a lot faster than smooth, solid-rock materials, making asteroids with a "regolith" surface more mobile and likely to be long gone, explained Durda.

"If we see no vulcanoids at all, it might be because on average objects there had a regolith cover," he said. "That would be a nice thing to know."

Durda and Stern's research is funded in part by a grant from the National Geographic Society.

Join the National Geographic Society, the world's largest nonprofit scientific and educational organization, and help further our mission to increase and diffuse knowledge of the world and all that is in it. Membership dues are used to fund exploration and educational projects and members also receive 12 annual issues of the Society's official journal, National Geographic. Click here for details of our latest subscription offer: Go>>