Blurred stars seem to cut through the green halo of comet 103P/Hartley 2 in a five-minute-exposure photograph taken through a backyard telescope in the United States on Saturday. In recent weeks the comet has been drawing closer to Earth, and Hartley 2 will make its closest pass tomorrow, offering prime viewing via binoculars and telescopes. (See comet pictures.)
Discovered in 1986 by Australian astronomer Malcolm Hartley, the mountain-size ball of ice and dust orbits the sun every 6.5 years. But comet Hartley 2 has remained shrouded in mystery, in part because gravitational interactions with Jupiter kept pushing the comet's path farther from Earth. That means Hartley 2 has been too faint to be seen using anything other than professional telescopes—until now.
This week the comet should already be visible to the naked eye as a faint fuzzy patch in the late-night sky high above the northeast horizon. Hartley 2 will be passing the brilliant star Capella in the constellation Auriga, making the comet easier to track down. Dark skies away from cities will offer the best views, and binoculars or small telescopes will heighten the details.
"I would recommend binoculars as the best way for the beginner to observe comet Hartley 2," Anthony Cook, an astronomer at the Griffith Observatory in California, recently told National Geographic News. "But through a telescope, the comet may fill the field of view, with more structure in the bright center, and a faint tail could become observable."
A backyard astronomer in Italy captured comet Hartley 2 sharing space with NGC 281, aka the Pacman nebula, on October 2 during the comet's journey toward the inner solar system.
The cosmic pair's proximity is an optical illusion—the starmaking nebula sits a whopping 9,500 light-years from Earth, while the comet was a mere 16 million miles (26 million kilometers) away when the picture was snapped.
Nearly a quarter century after its discovery, Hartley 2 is now poised to make its closest approach to Earth yet. On October 20 the comet will pass 11 million miles (17.7 million kilometers) from our planet—just 45 times the distance between Earth and the moon.
On November 4, EPOXI's Deep Impact spacecraft—famous for firing a projectile into comet Temple 1 in 2005—will make a close flyby of Hartley 2. The craft will dive within 600 miles (965 kilometers) of the comet's surface, getting close-up images of its craters and the sources of its dust and gas plumes.
Already, the uniform appearance of the comet's head suggests to astronomers that Hartley 2 may be a relatively young comet with a fresh surface.
In the infrared, the comet's crumb-like tail is clearly visible as a fuzzy streak to the right of Hartley 2. More than five months before the comet's closet approach to the sun, solar radiation had already begun to vaporize the icy surface of the nucleus, creating the dusty tail, which stretches for about 1.1 million miles (1.7 million kilometers).
Image courtesy NASA/JPL-Caltech/UCLA
En route to comet Hartley 2, NASA's Deep Impact spacecraft has also been taking pictures of the icy body, including this shot from September 5.
In the past, probes flying by comets such as Halley, Wild 2, Borrelly, and Temple 1 have found no standard appearance among the frigid dustballs. Now the EPOXI mission "may answer whether comet Hartley 2 has a family resemblance to one of these, or if it is also unique," Griffith Observatory's Cook said.