The knowledge that objects of some kind could comfortably carve out a life between Mercury and the sun was enough to keep some astronomers wondering. Perhaps the problem was that people had been thinking too big. Instead of a planet Vulcan, maybe it made more sense to look for a whole bunch of Vulcanettes. Or, as scientists have since named the members of this hypothetical population of asteroid-like objects, vulcanoids.
To refocus the search, the most likely location of these objects had to be pinpointed. A big planet would have been relatively obvious, but perhaps smaller bodies could hide in the glare of the sun. Any object that strayed too close to the solar furnace would vaporize over the lifetime of the solar system, like a marshmallow held too close to a campfire. On the other hand, any object whose orbit took it too close to Mercury would be affected by that planet’s gravity. Over several million years, Mercury’s pull could boost such a body out of the hot zone or even steal enough energy from it to plunge it into the sun.
These limitations define a ring of space that starts about 6.5 million miles from the sun and extends out to just under 20 million miles—an area comprising about 1 quadrillion square miles. An object orbiting in that Goldilocks region could survive billions of years. But sitting securely in that stable zone between Mercury and the sun is not quite enough to guarantee a decent life span for a vulcanoid. There is also the matter of size.
Vulcanoids have a lower size limit, because very small things (think grains of dust) would be swept clean out of the innermost solar system by the wind of subatomic particles blowing off the sun’s surface. Even light itself exerts pressure, and anything smaller than a few hundred yards across would be long gone from the inner solar system by now. There is an upper size limit as well. The bigger the object, the brighter it would appear from Earth. Anything beefier than about 40 miles across would have been found by now. Astronomers don’t see such things, so they must not be there.
By the middle to late 20th century, these upper and lower bounds for both size and location were well defined. A new generation of astronomers could get serious about the search for vulcanoids—a search that has now heated up all over again.
The difficulty in hunting for vulcanoids, if they exist at all, is that they orbit so close to the sun. From our vantage point 93 million miles out, a vulcanoid would never wander more than 12 degrees from the sun in the sky, so it would be swallowed up by the glare. The only hope of finding one would be to observe it just after sunset or just before sunrise, when the sun is slightly below the horizon and the hypothetical vulcanoid is slightly above.
That is a very thin slice of time, mere minutes long, making any search extremely challenging. And the sky is bright enough at that moment to easily wash out the feeble light from the target. (Observations during total solar eclipses fare no better, for the same reason.) Looking near the horizon means peering out through miles of Earth’s turbulent, hazy, and sometimes polluted atmosphere, which would blur and dim the vulcanoid’s appearance even more.
Searching for vulcanoids is a Herculean task, but one that a few scientists have gladly taken on. Stern, now at the Southwest Research Institute, and his collaborator Dan Durda—both friends and colleagues of mine—have been peering carefully at the hot desert between the sun and Mercury for more than a decade. “I didn’t think it would be a 10- or 12-year quest,” Stern says wryly. “But we’re going to chase them down to the ground. We’re going to find them or eliminate the possibility that they’re there.”
Recognizing the difficulties imposed by atmospheric interference, Stern and Durda took the search in a new direction: up above most of Earth’s atmosphere. They built a special camera and in 2002 flew with it on an F-18 fighter jet at 49,000 feet, where the sky is much clearer. It was a valiant effort, but unfortunately at that height the sky is still too bright to find vulcanoids—even at twilight, when they tried.
Earth-orbiting spacecraft might seem the next obvious vantage point. However, even from 300 miles above the surface of our planet, the search would still be nearly impossible. In a space shuttle orbiting at five miles per second, for example, the period between sunset and the time any vulcanoids would dip below the rim of the Earth can be measured in seconds. Putting a dedicated spacecraft in orbit would be prohibitively expensive, as well. And so this approach was abandoned.
Space probes beyond Earth orbit, designed for other uses, have been tasked with the vulcanoid search. The Solar Dynamics Observatory (SDO), a NASA spacecraft launched in February to monitor the sun’s magnetic activity, should be able to spot any objects at the upper end of the size range. It has taken a preliminary look but found nothing, Stern says, narrowing the search to smaller bodies. Messenger, another NASA craft that will settle into orbit around Mercury in March 2011, has been scanning for vulcanoids too. So has the Solar Terrestrial Relations Observatory, or Stereo, a pair of satellites tagging along with Earth in its orbit around the sun—one just ahead of our planet, one just behind. Designed in part to examine the space around the sun for the effects of massive solar eruptions, Stereo is a good platform from which to search for the brighter end of the potential vulcanoid population. Stern, Durda, and a few colleagues have calculated that the twin satellites would be able to detect vulcanoids as small as 1.5 to 4 miles in diameter, but they haven’t found any so far. “These results are disappointing,” Durda says, “but we haven’t given up hope yet.”
The best hope may now rest on a new method of reaching the limits of our atmosphere. If airplanes are too low and satellites move too quickly to make an effective search tool, then how about a compromise? Enter suborbital rocket flights.