Good question! If a comet or asteroid (a comet is made out of ice and rock, and an asteroid if mostly rock) passes too close to Earth it could potentially be “trapped” by earth’s gravity field. This is probably unlikely because comets move through space at very fast speeds (faster than a spacecraft that launches from earth). It is more likely that the comet will be “flung” by earth’s gravity.

Basically, earth’s gravity would force the comet to change direction. It’s like giving it a little pull without even touching it. If a comet or asteroid passes right through earth’s orbit at the right time, it could collide with earth. The chances of a big comet passing right through earth’s orbit at the right time is very small, so it is unlikely that we will experience a big impact in our lifetime. Early in the earth’s life (~4.5 billion years ago), big impacts were a lot more common, but that is because there were many big comet, asteroids, and tiny planets called planetesimals flying around our solar system at that time.

Very tiny meteoroids (shooting stars) enter earth’s atmosphere all the time, but they actually burn up by friction before they hit the ground.

If the comet were to hit the Earth, it would cause a huge explosion much as if an asteroid were to hit the Earth. A comet merely coming close to the Earth would not do much to the Earth, because the Earth is so much bigger than the comet, but the gravity of the Earth might tear the comet into chunks because of the same force that causes tides.

Many things could happen, and it is something that used to frighten me when I was your age. Keep in mind that the odds of this happening are roughly on par with winning a lottery. If a comet passes too close to Earth, it COULD hit the Earth and and cause a mass-extinction event, like when the dinosaurs died. Far more likely, if it is close to Earth but not on a direct course, we could end up having many meteors as the comet breaks apart when it gets closer to the sun.

The tail of comets is rocks and ice and gas coming from the back. The effects really depend on the size of the chunk of rock hitting Earth. Last year in Russia, a very small meteor entered the Earth's atmosphere and was caught on many car cameras. Click here to watch, please is a collection of some of the best footage. It looks scary, but it burned up almost entirely in the atmosphere.

The Earth is constantly being hit by small meteors that never even make it to the ground. In 1918, a very famous meteor hit Russia (called the Tunguska event), but since much of Russia is uninhabited, nobody was ultimately hurt. It was a pretty powerful impact, and knocked down about 80 million trees.

This all can sound very scary, and I definitely stayed up late worrying about it when I was younger, but the odds of a major event are small. Scientists at NASA track the movements of all known comets and asteroids, and do a very good job of making sure we would have advance warning and enough time to try to prevent any big impact from happening. Also, we owe a big thank you to the planet Jupiter. Because it is so big, it pulls many comets into it rather than letting them into the inner part of the solar system, and so it saves Earth from most potential threats. Some scientists even believe that without Jupiter, impacts on Earth would be too frequent for intelligent life to form over the millions of years. I hope I was able to calm any fears of this!