Colonization of the outer Solar System the Solar System's gas giant planets for exploration and possibly mining of helium-3, which could have a very high value per unit mass as a thermonuclear fuel. The Jovian system in general has particular disadvantages for colonization, including its severe radiation environment and its particularly deep gravity well. Its radiation would deliver about 3,600 rems per day to unshielded colonists on Io and about 540 rems per day to unshielded colonists on Europa. Exposure to about 75 rems over a few days is enough to cause radiation poisoning, and about 500 rems over a few days is fatal. Jupiter itself,
Formation and evolution of the Solar System impact event, the Tunguska event, the Chelyabinsk meteor and the impact feature Meteor Crater in Arizona. The process of accretion, therefore, is not complete, and may still pose a threat to life on Earth. Over the course of the Solar System's evolution, comets were ejected out of the inner Solar System by the gravity of the giant planets, and sent thousands of AU outward to form the Oort cloud, a spherical outer swarm of cometary nuclei at the farthest extent of the Sun's gravitational pull. Eventually, after about 800 million years, the gravitational disruption caused by galactic tides, passing stars