Seven Things to Know About CometsS

After offering a unique show to sky-watchers, Comet ISON will come just 750,000 miles from the Sun this Thursday, November 28—its perihelion, or closest approach to the Sun.

Image: Taken on Nov. 19, 2013, this image shows a composite "stacked" image of C/2012 S1 (ISON). NASA/MSFC/MEO/Cameron McCarty

The comet won't be visible from Earth for a time as it slingshots around the Sun, but if it survives its trip (many comets break up on this journey), it will once again become visible in the Northern Hemisphere starting as early perhaps as mid-December.

Comet ISON was discovered only in late 2012, but astronomers surmised immediately, "Oh, this could be big," says Denton Ebel, chair of the Division of Physical Sciences, since "ISON was very bright and active even while it was in the outer solar system." Dr. Ebel works on samples from NASA's Stardust mission, which collected dust from Comet Wild2 and returned it to Earth.

Seven Things to Know About CometsS

Comet ISON passes through the constellation of Virgo. NASA/MSFC/Aaron Kingery

Need a refresher course on comets? Here are seven things to know about these exciting but enigmatic Sun-orbiting space objects.

1. Like asteroids, comets are likely remnants of the formation of the solar system about 4.6 billion years ago.

2. Comets' main components are rock, ices, and dust.

3. Comets orbit the Sun. As they near the Sun, they develop a coma—a thin cloud of gas that forms directly from ice by "sublimation," explains Ebel. They also develop two tails—one of dust, which can be millions of miles long, and one of ionized (electrically charges) gases. It's the coma and the tails that can make comets such spectacular night-sky sightings.

4. While most asteroids orbit in the Asteroid Belt between Mars and Jupiter, comets come from farther away.

Short-period comets—those with orbits that last less than 200 years—originate in the Kuiper Belt. (Many dwarf planets, including Pluto, also reside in the Kuiper Belt.) Long-period comets, and some that will never return, like comet ISON, likely come from the more distant Oort cloud, which is thought to extend almost a light year from the Sun.

Seven Things to Know About CometsS

Comet Hale-Bopp, March 1997. Philipp Salzgeber

5. The Oort Cloud is hypothetical at this point. Spacecraft have not traveled there; the Voyager spacecraft, for instance, have only gone to 16 light-hours from the Sun, and astronomers cannot observe it directly—but in 1950 astronomer Jan Oort hypothesized that such an area must exist. This isn't as far-fetched as it may sound. In 1951, astronomer Gerard Kuiper hypothesized the Kuiper Belt, and it was only in 1992 that the first confirmed Kuiper Belt object was discovered.

6. Comets from the Oort Cloud only rarely enter the inner solar system. "That's fortunate," says Ebel, because comets can be big (ISON is over 1 kilometer in size) and fast," and potentially dangerous to Earth. As comets near Earth, they travel much faster than asteroids do, sometimes approaching 100 times the speed of sound on Earth—because they all obey Kepler's laws of planetary motion.

7. There is still a lot to learn about comets. "We know that comets are made of rock and ice," explains Denton Ebel, "but we don't know the proportions of each or their internal structure." And that's why it's hard to predict how a comet will react as it nears the heat of the Sun. Some may be densely packed, while others may be, says Ebel, "like Swiss cheese!" Researchers are trying to learn more, however. After 2015, the unmanned New Horizons mission will enter the Kuiper Belt, in part to learn more about comets.

Learn more about the work of Dr. Ebel in a video.