Date: March 12, 2010

Title: The Hunt For Vulcanoids


Podcaster: Bob Hirshon

Organization: American Association for the Advancement of Science (AAAS):

Description: For nearly a hundred years, scientists have wondered if there might be a band of asteroids as large as 60 km wide orbiting the Sun, in a gravitationally stable zone that’s closer to the sun than the orbit of Mercury. The MESSENGER mission to the planet Mercury is now hunting for the elusive objects, and could soon solve the mystery.

Bio: Bob Hirshon is Senior Project Director at the American Association for the Advancement of Science (AAAS) and host of the daily radio show and podcast Science Update. Now in its 23rd year, Science Update is heard on over 300 commercial stations nationwide. Hirshon also heads up Kinetic City, including the Peabody Award winning children’s radio drama, McGraw-Hill book series and Codie Award winning website and education program. He oversees the Science NetLinks project for K-12 science teachers, part of the Verizon Foundation Thinkfinity partnership. Hirshon is a Computerworld/Smithsonian Hero for a New Millenium laureate.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the Education and Outreach team for the MESSENGER mission to planet Mercury. Follow the mission as the spacecraft helps to unlock the secrets of the inner solar system at


Hello, and welcome to the 365 days of astronomy podcast. I’m Bob Hirshon, host of the AAAS radio show and podcast Science Update. Now if you glanced quickly at today’s podcast title and are expecting a report on the pointy-eared, human-like inhabitants of the planet Vulcan, from Star Trek, you may be disappointed. Today we’ll be reporting on the search for a very different variety of vulcanoid. These are hypothetical rocky bodies orbiting very close to the sun— in fact, inside the orbit of hte planet Mercury. Clark Chapman is senior scientist at the department of space studies at the Southwest Research Institute in Boulder, CO. He says vulcanoids were named not for any characters or planets on Star Trek, but for another mythical planet. Chapman: For a long time there’s been speculation that there might be a planet named Vulcan sort of in the earth’s orbit but on the opposite side of the sun where we couldn’t see it.

Hirshon: The existence of the planet Vulcan was disproved back in 1915. But it’s from that mythical planet that we get the name vulcanoids. Chapman says the vulcanoids scientists are searching for are remnants from the period billons of years ago when our solar system formed– remnants that today would be orbiting between Mercury and the Sun.

Chapman: They would be leftovers from the time when the solar system was forming out of swarms of small bodies called planetessimals. All of the planets are thought to have grown at least in part out of these smaller bodies. And it turns out that this region inside Mercury’s orbit, partway to the sun, is a stable region. So that bodies once orbiting there could still be orbiting there. Unlike the areas between Earth and Venus or Earth and Mars, where any original bodies would have not been able to last the age of the solar system.

Hirshon: So for decades, scientists have focused their attention on this region. But the intense glare of the sun has made observation difficult. Studies with telescopes have ruled out the existence of large bodies there, above 60 km in diameter. Now Chapman and his colleagues are using the cameras aboard the MESSENGER spacecraft to take a closer look.

Chapman: MESSENGER, on the spacecrafts’s route to go into orbit around Mercury, has been orbting the sun a quite a number of times, and it’s flown past Mercury itself three times. But every time it’s at its perhelion distance—that’s the closest distance in the orbit to the sun, which is roughly near the orbit of Mercury—there’s an opportunity to view part of the volume of space occupied by vulcanoids. MESSENGER’s instruments cannot point closer than a certain angle toward the sun. But we’re able to get a pretty good slice of the volume of space that vulcanoids might be orbiting in by sort of looking as close to the sun as we’re allowed to with the camera system. And we’ve been doing this for several of the past perhelion passages of the spacecract.

Hirshon: They’ve now begun to analyze the images to see if they can find any orbiting bodies. Chapman: We’ve seen some strange things in the pictures that turn out to be little pieces of dust and debris right near the spacecract. But they look pretty spectacular in the images. But we ruled out that they’re any kind of real significant celestial objects. So so far we haven’t found any, but we have plans to make more observations in the future.

Hirshon: He says that one thing complicating the search is that no one knows what the hypothetical vulcanoids are made of, how big they might have been originally, or what they might look like today.

Chapman: It’s uncertain how big original vulcanoids might have been, but since they go around the sun very rapidly and in a relatively confined space inside Mercury’s orbit, they run into each other and collide. So we would expect that there would be a distribution of sizes ranging from whatever the largest size would be all the way down to very small, where other forces like solar radiation pressure would sweep them out. So really the largest vulcanoid—there’s a limit set by the searches that have already been done, by rockets and observations from balloons, and observations during total eclipses and so on, and it’s several tens of kilometers in size. Things of 50 kilometers or 100 kilometers would already have been detected. So we’re talking about relatively small bodies that remain too small to have been ruled out by previous searches. But bodies that are very small would have been swept out by strong radiation forces from the sun.

Hirshon: And while they haven’t spotted any yet, they still have a lot of photo analysis to conduct. If they end up ruling out the existence of medium to large vulcanoids, that will lead to speculation as to what happened to them.

Chapman: One reason why they might not be there is that they sort of bumped into each other and collided with each other and over the course of four and a half billion years ground themselves down into nothing. But of course if they were able to do that, that would say something about how strong the material was. You know, if they were, for example, made of solid nickel iron, you might expect that they’d last despite all the collisions. Whereas if they were made of very weak materials, then they wouldn’t. So you can kind of set a bound on that.

Hirshon: If they do find them, that will lead to questions of what to do next.

Chapman: Certainly if they were found, they would be remnants from the solar nebula and the materials that were there at the very earliest times in solar system history and would presumably be made out of materials that can withstand the great heat that exist there that close to the sun. So it would be very interesting material to study. I don’t know how simple it would be to have a sample return mission to a vulcanoid, which is in a hotter region than Mercury itself. The MESSENGER spacecraft has been designed very carefully to withstand the heat of the sun at Mercury’s distance. So it would be a very touch mission. But sure, if we could get a sample of a vulcanoid, that would be very exciting.

Hirshon: Primordial material from the birth of our solar system. That would certainly be, to quote Mr. Spock, quite fascinating. For the 365 days of astronomy, I’m Bob Hirshon.

End of podcast:

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