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,
Vulcan (hypothetical planet) circular orbits of Venus and Earth. Einstein's theory was empirically verified during the solar eclipse of May 29, 1919 by measuring the bending of sunlight. Astronomers generally quickly accepted that a large planet inside the orbit of Mercury could not exist, given the corrected equation of gravity. Observing a planet inside the orbit of Mercury is difficult, since the telescope must be pointed very close to the Sun, where the sky is only dark during a solar eclipse. Also, an error in pointing the telescope can result in damage for the optics, and injury to the observer if viewing directly.