Today MESSENGER will pass within 0.308 astronomical units (AU) of the Sun (one AU is Earth’s distance from the Sun, approximately 150 million kilometers or 93 million miles), providing MESSENGER scientists with another opportunity to search for vulcanoids.
Named after the hypothetical planet Vulcan, whose existence was disproven in 1915, vulcanoids are asteroids that orbit the Sun inside the orbit of the planet Mercury.
No vulcanoids have yet been discovered, and it is not known if any exist. But should they be found, these small, rocky asteroids may yield insights into the formation and early evolution of the solar system. They might contain material left over from the earliest period of planet formation and help determine the conditions under which the terrestrial planets, particularly Mercury, formed. Vulcanoids would also represent an additional population of impactors that contributed to the cratering history of Mercury much more than that of any other body. Impacts by vulcanoids would make the planet’s surface appear older, relative to the surfaces of the Moon and other inner planets, than it actually is.
If they do exist, the vulcanoids would be difficult to spot. First, they would be very small – less than 60 kilometers (37 miles) in diameter (a limit set by Earth-based observations) – and their reflected light would generally be drowned out by the bright glare of the nearby Sun. Because of their proximity to the Sun, searches for vulcanoids from the ground can be carried out only during twilight or or dawn or during solar eclipses.
The mission’s imaging team is taking advantage of the probe’s proximity to the region of space inside Mercury’s orbit during this perihelion to continue their search for vulcanoids. The latest search started on August 14 and continues through today.
“Our searches for vulcanoids may not turn up any objects,” says MESSENGER principal investigator Sean Solomon, of the Carnegie Institution of Washington, “but a discovery of even one vulcanoid would change our thinking about the evolution of Mercury. The solar system still has many surprises in store for us, so it makes sense for us to be ready for the unexpected.”