The comets are coming!

A pair of "dirty snowballs" are about to appear in a twilight sky near you.

Over the next month, our solar system is treating us to two visits from comets. Both may be visible to the naked eye - and surely with binoculars.

And that's not all the comet news. Last January, a small United States spacecraft named Stardust swept through a comet's cloak of dust and gas to collect samples of this material. Stardust - now working its way through the asteroid belt between Jupiter and Mars - is speeding back to Earth carrying comet dust for scientists to study. It's expected to drop its package of samples into the Utah desert in two years.

Meanwhile, scientists in Europe are tracking a comet-chasing spacecraft named Rosetta. They launched it in March. It won't meet up with a comet until 2014. But if everything works as planned, Rosetta will orbit the comet for about a year to study it. It also will send a lander to the comet's surface. This will give astronomers a rare look at a comet's core, or nucleus.

Why all the high-tech, high-priced interest in comets? These hefty clumps of dust and ice are thought to be the leftover material from which the planets formed some 4.6 billion years ago. By studying them, scientists hope to learn more about the huge disk of dust and gas that surrounded the sun before the planets began to assemble themselves - a bit like cosmic dust bunnies - from this material.

Studying comets in our solar system also is helping astronomers understand what they see when they look at nearby solar systems. (So far, planets the size of Jupiter or larger have been detected in orbit around some 60 nearby stars.)

Three years ago, for example, astronomers in the US found that a red giant star 500 light-years from Earth was surrounded by water vapor. What was the water vapor doing there? The star was too rich in the wrong kinds of elements to generate an envelope of water vapor. The astronomers concluded that the vapor must have come from comets and other icy objects. (As the star matured and expanded into a red giant, the astronomers theorized, it vaporized the comets. Our sun will expand into a red giant, too - in about 5 billion years.)

In our solar system, comets buzz around the sun, starting from two vast regions. One is the Oort cloud, named for a Dutch astronomer who first suggested that such a cloud existed. This "cloud" surrounds the sun in the farthest frontiers of the solar system. Its inner limit lies far beyond Pluto - some 50,000 Astronomical Units (AU) away. (Astronomers use AUs to measure some distances in space. One AU is the distance between Earth and the sun, or 93 million miles.) Nobody knows for sure how many objects the Oort cloud contains. It's too far away and the objects are too faint and small to see.

Astronomers figure that these Oort-cloud objects probably formed near Neptune and Uranus. The gravity from these planets probably kicked the "planetesimals" (planetary building blocks) out of the neighborhood. Some theorize that the Oort cloud represents only from 1 to 10 percent of the planetesimals ejected by Neptune and Uranus. The rest hurtled into deep space.

Comets that enter the inner solar system from the Oort cloud originate in the cloud's farthest reaches, where the sun's gravity is weaker and the comets may be tugged by the gravity of passing stars or the vast clouds of gas and dust that move between the stars. These comets can take from 200 to several thousand years to make just one round trip around the sun. David Jewitt, an astronomer at the University of Hawaii, says the best estimate astronomers have is that the Oort cloud contains around 1 trillion objects. The comets visible this month are thought to have come from the Oort cloud.

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