Courtesy of NASA
The best global view of the heavily cratered surface of Mercury—a mosaic of more than 140 images snapped by Mariner 10 in March 1974—reveals expansive plains that may have been created by volcanic activity. Because of Mercury’s proximity to the sun, Earth-based telescopes don’t offer a clear look at the planet. Even Mariner 10 managed to photograph only about half the surface.
It’s 2:30 in the afternoon, on a blustery winter day in Holtsville, New York, and Frank Melillo’s housebound beagle, Princess, has been barking incessantly for an hour and a half. That’s how long Melillo has been standing in the shadow of his apartment building, his windbreaker zipped up to his Adam’s apple, trying to get a bead on the planet Mercury with his Celestron 8 telescope. Except for a few clouds, the sky is blue and bright, and it seems an odd time for an amateur astronomer to be planet hunting. But Mercury, the planet closest to the sun, shows its face at odd times. So for the umpteenth time, Melillo stretches his right hand up toward the southern sky and sights the sun with his pinkie finger. Then he picks out wispy cloud details beyond the tip of his thumb—about 20 degrees, more or less, to the east. Since the clouds he’s using for reference are moving pretty quickly in the wind, he hurriedly trains his telescope on the slice of sky between the building and a white pine tree. “It should be right there,” he says.
Getting Mercury in the crosshairs of a telescope is as problematic for professional astronomers as it is for amateurs like Melillo. It’s easy to sight planets farther than Earth from the sun—Mars, Jupiter, Saturn—because they sit smack in the middle of the night sky. But to see Mercury and Venus, one must look more toward the sun. Venus is often bright enough and far enough from the sun to stand out in the evening or morning sky, but Mercury is notoriously elusive and dangerous to sight with a telescope that could easily fry a retina if too much sunlight gets in. Mercury has never been photographed by the Hubble Space Telescope, because stray light from the sun could ruin the instrument’s electronics. There are only 30 or 40 days a year when Mercury isn’t too close to the sun to be seen by any means, and that doesn’t count days lost to bad weather. For a planetary scientist applying a year in advance for time on the best telescopes, Mercury can be frustrating in the extreme. That’s why Melillo and other amateurs often snag the best Mercury photographs. “I tell Frank to let me see every image he gets,” says Ann Sprague, a planetary scientist at the Lunar and Planetary Laboratory at the University of Arizona in Tucson. “I would be foolish not to look at them.”
Distance from sun: 28.6 to 43.4 million miles
Length of year:
88 Earth days
Length of one day (sunrise to sunrise): 176 Earth days
Tilt of axis:
Mercury is not just hard to view from Earth. A major challenge facing NASA’s scrappy new Messenger
probe, scheduled for blastoff from Cape Canaveral next month, is to compensate for the sun’s huge gravitational pull. Rocketing to Mercury is a bit like running down a hill and then trying to stop suddenly. NASA hopes to use Venus as a brake, taking advantage of its gravitational field to keep Messenger
from careering past Mercury and into orbit around the sun. If all goes well, earthbound scientists will have an orbiter around the inside planet by July 2009.
The timing couldn’t be better. As astronomers home in on finding an Earth-like planet in another solar system, they need to find out more about Mercury so they can better understand the precise signature of a solar system like ours. So far we’ve detected only huge planets in other solar systems, most orbiting very close to their suns. Understanding our own solar system better will allow us to infer the existence of smaller planets in other systems. Many people assume that Mars is the destination of choice for finding extraterrestrial life. But Mercury may hold the secrets that count.
At twilight on a good day, Mercury briefly appears as a flickering dot on the horizon. Sky watchers, first with the naked eye and later with telescopes and radar and other gear, have assembled strange facts about the planet hiding in the sun’s blaze. Fact one: Mercury’s orbit is highly elliptical, so it passes about 29 million miles from the sun on its closest approach, or perihelion, and swings out to about 44 million miles at its farthest point, or aphelion. Fact two: Mercury completes its orbit around the sun in only 88 Earth days. Fact three: It takes 58.6 Earth days, exactly two-thirds of an orbit, for Mercury to revolve once on its axis. The combination of those motions means that one Mercurial day—sunrise to sunrise—requires three full rotations and two orbits, or 176 Earth days. Mercury and Venus are the only known worlds where the day is longer than the year.