Scientists have long theorized about a population of small, asteroidlike objects roaming around the sun inside the orbit of Mercury. None has been found, but the search for these so-called Vulcanoids was elevated earlier this month from jet airplanes to a camera mounted on a suborbital rocket.
The project was led by the Southwest Research Institute in Boulder, Colo., and funded by NASA and the Planetary Society. Future Vulcanoid searches might be carried out on private space planes, such as whichever one wins the $10 million X Prize competition.
The latest mission also conducted the first-ever "ultraviolet spectrograph" examination of Mercury.
Both aspects of the mission were primarily tests of new ways of looking at hard-to-observe objects in the inner solar system, explained SwRI director Alan Stern. It is possible the observations will yield a Vulcanoid, but neither set of data has been examined yet.
The case for VulcanoidsVulcanoids are named for the Roman god of fire and metalworking. There is good reason to suspect they exist.
A region of space inside Mercury's orbit is gravitationally stable in such a way, theorists say, that space rocks could survive there. All similarly stable areas of the solar system do contain objects. Examples include the asteroid belt between Mars and Jupiter and the Kuiper Belt out beyond Neptune.
Further, the heavily pockmarked surface of Mercury shows it had many run-ins in the past.
"Mercury's craters mean there used to be a lot of Vulcanoids," Stern explained in a telephone interview. "We know there can't be very many today, but there could be hundreds still left."
If they are there, Vulcanoids could provide a primordial sample of the material out of which Mercury was made, Stern said. If not, then astronomers might have some explaining to do. The region should have retained some bodies since the solar system's earliest days, computer modeling shows.
"If that whole region is cleaned out, something had to clean it out," Stern said.
At least two possibilities would then have to be considered: Either there are some misunderstood physics, he said, or perhaps a planet long ago migrated through the inner solar system "and swept it clean."
Better view neededBecause the rocks would be so near the sun, conventional telescopes cannot search for them without risking damage to their optics. Past Vulcanoid searches have been conducted from the ground during eclipses and at morning and evening twilight.
Astronomers seek to rise above Earth's atmosphere for better views from crisper and darker skies.
Stern and his colleagues have also hunted for the potential targets from high-flying jets. And they've surveyed images of the sun's environment taken by the Solar and Heliospheric Observatory. So far, the lack of any discoveries has only served to put loose constraints on the size and quantity of possible Vulcanoids. Any that remain are probably not more than about 37 miles (60 kilometers) in diameter.
The rocket-mounted VULCAM, as the 3-pound instrument is called, is the most far-reaching effort yet to hunt down these elusive space rocks.
"VULCAM is a derivative of an imaging instrument we have flown many times on F-18 aircraft," said Dan Durda, also of SwRI. The camera "has the potential to become an even more powerful tool for searching for Vulcanoids" at higher altitudes, up to 160 miles (260 kilometers) or more where NASA suborbital missions can reach.
Next upIf the new data do not yield a discovery, the search won't end. Several missions over the span of a few weeks would be needed to check all the space within which Vulcanoids could exist.
Stern said their existence could be determined firmly with a few piloted, suborbital missions. He's considering approaching the eventual winner of the , a competition among entrepreneurs to build a re-usable, piloted spaceship for suborbital excursions.
A handful of flights, each allowing about a minute of data-gathering time, would be needed. Stern said the X Prize winner will have the needed capability.
"You can nail this problem," he said. "It'll be done forever. We will either discover them or rule them out."
New look at MercuryThe rocket mission was primarily designed to test another instrument. A nearly 500-pound ultraviolet spectrograph gathered UV light from Mercury and split it into its constituent colors, so researchers can analyze what sorts of minerals are on the planet's surface.
For the same reasons that Vulcanoid searches are difficult in the glare of the sun, this is the first ultraviolet spectrum ever obtained of Mercury.
"The whole inner solar system is basically denied to the great observatories," Stern said.
The Mercury data will be analyzed over the next few weeks.
The technique, once proven as expected, could open up whole new and relatively inexpensive way to explore Mercury, Venus and also comets as they approach the Sun and are at their most active. The observations should also yield some science of their own and help in planning the work of another spacecraft, NASA's mission to Mercury.