The sun regularly releases bubbles of gas particles called coronal mass ejections. Many CMEs hurtle harmlessly through space, but about 30 collide with Earth every year, many glancing off its atmosphere. A direct hit from a very large CME is a one-in-100-year event.
CMEs travel at up to six million miles per hour and can reach Earth in as little as 18 hours. As they move through space, CMEs produce shock waves that can damage satellites, and their high-energy proton radiation increases astronauts' risk of cancer.
How the CME strikes the planet's electromagnetic shield determines how disastrous the storm will be. If its magnetic field is parallel with the Earth's, the planet's shield will replace repel it. But if its magnetic field is aligned opposite the Earth's, the two magnetic fields connect, which allows the CME's charged particles to enter the atmosphere.