Kids ask: When will a comet hit Earth?

Brandon Sebeh of Beach Park asked: "When will the next comet impact the Earth?" Brandon, 11, is a sixth-grader at Gurnee's Viking School.

A comet is an icy potato-shaped nucleus surrounded by a cloud of gas dust particles called a coma. Heat from the Sun causes gas and dust to stream away from the comet and form one or two tails. Like Earth, these space objects orbit the sun, but in such huge trajectories that it can take hundreds or thousands of years to make one full trip around the sun.

Scientists know quite a lot about comets. When Halley's comet most recently passed near Earth, NASA sent space probes to examine it. The comet's icy nucleus is 9 miles long and made mostly of ice mixed with dust particles. Experts believe the sun warms the icy nucleus and releases gas, creating the spectacular show that makes comets visible. It'll be more than 50 years before Halley's comet returns.

About comets colliding with Earth, Dr. Geza Gyuk, Adler Planetarium director, says it's not likely to happen soon. "While none of the known comets have orbits that will cause them to crash into the earth, most of the comets we see are only very occasional visitors to the inner solar system. The last time they visited our environs may have been many thousands of years ago." However, it is likely to happen in the long term, maybe thousands of years into the future.

Only 15 years ago, a comet blasted into Jupiter's atmosphere and exploded. Scientists witnessed Shoemaker-Levy 9 split into many pieces when it collided with the planet.

Just a month ago another object, perhaps a comet, hit Jupiter again.

"There may be a comet out there that we have not discovered yet that is on a collision course with the earth," Dr. Gyuk said. "Since comets spend most of their time far away from the sun where they are very dim, it is very difficult to detect them much in advance. We will probably have about a year's warning if not less."

What is likely to strike the earth is a meteoroid, which is a tiny hunk from an asteroid. Millions penetrate the Earth's atmosphere each day; however, only a very few actually survive and fall to the earth's surface as meteorites.

Find out more about comets, meteorites and asteroids at the Adler Planetarium's movie "Cosmic Collisions." See adlernet.org for more information.

Check these out

The Warren Newport Public Library in Gurnee suggests these titles on comets and cosmic debris:

• "Asteroids, Comets & Meteors," by Giles Sparrow

• "Comets and Asteroids," by Ian Graham

• "Exploring Comets," by Jennifer Way

• "A Look at Comets," by Ray Spangenburg and Kit Moser

• "Space Leftovers: A Book About Comets, Asteroids and Meteoroids," by Dana Meachen Rau