 NASA has speculated on the feasibility of mining the atmospheres of the outer planets , particularly for helium @-@ 3 , an isotope of helium that is rare on Earth and could have a very high value per unit mass as thermonuclear fuel . <unk> stationed in orbit could mine the gas and deliver it to visiting craft . However , the Jovian system in general poses particular disadvantages for colonization because of the severe radiation conditions prevailing in Jupiter 's magnetosphere and the planet 's particularly deep gravitational well . Jupiter would deliver about 36 Sv ( <unk> rem ) per day to <unk> colonists at Io and about 5 @.@ 4 Sv ( 540 <unk> ) per day to <unk> colonists at Europa , which is a decisive aspect due to the fact that already an exposure to about 0 @.@ 75 Sv over a period of a few days is enough to cause radiation poisoning , and about 5 Sv over a few days is fatal . 
