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The solar system is like clockwork, with planets orbiting the Sun year after year, and moons orbiting their planets.
Scientists can predict where and when many objects will be positioned in space. But not always, because sometimes accidents happen.
For example, many asteroids cross the orbits of planets, and if they happen to cross when a planet is in the way, you can guess what would happen. Asteroids are small, and planets are huge. In a head-on collision, the planet would probably “win,” and the asteroid would probably not exist anymore.
Crashes and near-misses are rare, which is why scientists (and other folks) get so excited about them.
▲ Comet orbits are elliptical, which means they’re long and skinny and oval-shaped. Their stretched-out orbits make them both the farthest objects away from the Sun—and the closest ones to it.
◀ Halley’s comet was last seen around Earth’s neighborhood in February 1986, and it will be a while before we see it again. It comes by approximately every 75 years, traveling past Earth in its orbit. The comet was named after Edmund Halley (1656–1742). In 1705, he correctly demonstrated that the comet returns regularly.
▲ Jupiter’s gravity ripped Comet Shoemaker-Levy 9 to dozens of pieces as the comet flew past it, and the pieces made another orbit around the Sun. Then, Jupiter’s gravity struck again. It pulled all the pieces straight into the giant planet. They went crashing through the clouds one at a time, sending up blobs of hot gas.
Check It Out!
Comets only develop tails when they’re close to the Sun. Why does a comet’s tail sometimes appear in front of the comet instead of behind it?
The Sun sends out a strong solar wind, which is made of charged particles. The wind blows the tail away from the Sun. As the comet moves around the Sun, its tail always points directly away from the Sun, which sometimes means the tail gets blown in front of the comet.
Solar System Mystery
◀ Many meteorites have been found on Earth, so that’s nothing out of the ordinary. But when scientists tested the rocks, they learned that some meteorites came from Mars! How long did those space rocks take to journey 140 million miles?
Visitors from space arrive on Earth all the time—that is, hundreds of meteorites fall to Earth each year. Most typically range between the size of a pebble and a fist, but the very rare big ones make huge holes in the ground. Barringer Meteorite Crater in Arizona is the biggest in the U.S. An asteroid traveling at 27,000 mph and weighing about 300,000 tons crashed there 50,000 years ago. ▶
◀ In 2013, a 10,000-ton asteroid entered Earth’s atmosphere above Russia. Moving at 40,000 mph, it exploded with the energy of 20 to 30 atomic bombs. The light from the meteor was brighter than the Sun. In the city of Chelyabinsk, about 1,700 people were injured, mostly by flying glass from windows blown in by the shockwave. This meteor is the largest known natural object to have come into Earth’s atmosphere since 1908.
Check It Out!
Why do some meteor showers appear at the same time every year?
The comet fields that create meteor showers cross Earth’s orbit at different points. A year is one Earth orbit around the Sun. So when Earth passes through a certain comet’s field, it’s always at the same time of year.
Space probe Galileo transmitted images of a small asteroid named Ida, and they revealed that tiny Ida has an even tinier moon, named Dactyl! It’s only a mile in diameter. You could walk from one end to the other end in minutes. ▶
