A reminder that tonight is the peak of the annual Geminid meteor shower (Dec. 13-14). The Geminids will stream from a point in the sky in the constellation Gemini the Twins, located above Orion and his famous belt. You can watch the meteor display beginning as early as 9 o’clock local time, but the greatest number of meteors is expected in the early morning hours between 1 a.m. and dawn. A big, bright moon also occupies Gemini and will wash out many of the fainter meteors. Whenever you choose to watch and wait for Geminids be sure to face away from the moon to preserve your night vision.
Comet 2I/Borisov passed closest to the sun on Dec. 8, took a bow and began its exit from our solar system. As it bids adieu, the comet will pass closest to Earth on Dec. 28 at a distance of 180 million miles (290 million km). Astronomers used the Hubble Space Telescope in November and December to take new photos of the one-time visitor. The first shows the comet near a distant background spiral galaxy, which looks smeared because the telescope tracked the comet during the time exposure.
Borisov was about 202.6 million miles (326 million km) from the Earth at the time and shows a bright central condensation called the “false nucleus.” The true nucleus, a small, icy body about 0.6 miles (1 km) across, sits invisibly at the center of that bright spot, shrouded by dust ejected by the comet from solar heating. A dusty tail fans off to the upper right. Like our own solar system’s comets, Borisov’s tail consists of minute dust particles about the size of those in cigarette smoke. The pressure of sunlight pushes the motes away from the comet’s head to form a tail the way a breeze blows back your hair.
In the second photo taken on Dec. 9, we see the comet near its closest approach to the sun when it was heated to higher temperatures. At the time it was located near the inner edge of the asteroid belt. The icy-dusty nucleus still remains hidden, but if you look closely subtle changes in the tail are evident.
“Surprisingly, our Hubble images show that its nucleus is more than 15 times smaller than earlier investigations suggested it might be,” said David Jewitt, a professor of planetary science and astronomy at the University of California Los Angeles. “The radius is smaller than half a kilometer. This is important because knowing the size helps us to determine the total number, and mass, of such objects in the solar system, and in the Milky Way. Borisov is the first known interstellar comet, and we would like to know how many others there are.”
Observations by Hubble and other telescopes have shown that rings and shells of icy debris surround young stars when planet formation is underway. A gravitational interaction between these comet-like objects and other massive bodies — perhaps newly-formed planets — could slingshot them into deep space where they drift between the stars for eons before an encounter like Borisov’s. There may be thousands of such interstellar objects bobbing around up there, but most are too faint to detect with present-day telescopes.