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The solar system is as regular as a clock. Planets orbit the Sun year after year, and moons orbit their planets.
Scientists can figure out where and when many things will be found in space. But not always, because accidents happen.
For example, many asteroids cross the orbits of planets. If they cross when the planet is in the way, you can guess what happens. Asteroids are small, and planets are big. In a head-on crash, the planet probably “wins.” And the asteroid probably becomes part of the planet.
Crashes and near-misses are rare. And that’s why scientists (and other folks, too) get so excited about them.
▲ Comets have long, skinny orbits. That makes them both the farthest objects from the Sun and the closest.
◀ Halley’s comet was last seen in February 1986. We can see it about every 75 years as it orbits past Earth. It was named after Edmund Halley (1656–1742). In 1705, he showed that the comet comes back regularly.
▲ Jupiter’s gravity tore Comet Shoemaker-Levy 9 to pieces as it flew by the planet. 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 crashed through the clouds one at a time. Each piece sent up blobs of hot gas.
Check It Out!
Comets only form tails when they’re close to the Sun. Sometimes the tails are in front of the comet, not behind it. Why?
The Sun sends out a strong solar wind. The wind is made of charged particles. It blows the comet tail away from the Sun. As the comet moves around the Sun, the tail always points directly away from the Sun. Sometimes that means it gets blown in front of the comet.
Solar System Mystery
◀ Many meteorites have been found on Earth, so that’s no big deal. But when scientists tested the rocks, guess what? Some meteorites came from Mars! How long did it take them to travel 140 million miles?
Visitors from space come to Earth a lot. That is, hundreds of meteorites fall to Earth each year. Most typically are between the size of a pebble and a fist, or smaller. 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. That was 50,000 years ago! ▶
◀ In 2013, a 10,000-ton asteroid entered Earth’s atmosphere above Russia. It was going 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 hurt. Many were injured by glass from windows the shockwave broke. This meteor is the largest known natural object to come into Earth’s atmosphere since 1908.
Check It Out!
Some meteor showers show up at the same time every year. Why?
A year is one Earth orbit around the Sun. The comet fields that cause meteor showers cross it at different points. So, Earth always goes through them at the same time of year.
Space probe Galileo took images of a small asteroid named Ida. The images showed that it has an even tinier moon. It’s named Dactyl, and it’s only a mile wide. You could walk from end to end in minutes! ▶
