gives a value for of 6.54 km/s. The turn angle can be calculated from: Which gives , meaning that we have an turn. The radius of closest approach to Earth will be given by: Where is the gravitational constant of the Earth. Substituting the values gives = , which is bad because the radius of the Earth is . A correction would therefore be required to comfortably avoid the planet. Proposed use Aldrin proposed a pair of Mars cycler vehicles providing regular transport between Earth and Mars. While astronauts can tolerate traveling to the Moon in relatively cramped spacecraft for a few days, a mission to Mars, lasting several months, would require much more habitable accommodations for the much longer journey: Astronauts would need a facility with ample living space, life support, and heavy radiation shielding. A 1999 NASA study estimated that a mission to Mars would require lifting about into space, of which was propellant. Aldrin proposed that the costs of Mars missions could be greatly reduced by use of large space stations in cyclic orbits called castles. Once established in their orbits, they would make regular trips between Earth and Mars without requiring any propellant. Other than consumables, cargo would therefore only have to be launched once. Two castles would be used, an outbound one on an Aldrin cycler with a fast transfer to Mars and long trip back, and an inbound one with fast trip to Earth and long return to Mars, which Aldrin called up and down escalators. The astronauts would meet up with the cycler in Earth orbit and later Mars orbit in specialised craft called taxis. One cycler would travel an outbound route from Earth to Mars in about five months. Another Mars cycler in a complementary trajectory would travel from Mars to Earth, also in about five months. Taxi and cargo vehicles would attach to the cycler at one planet and detach upon reaching the other. The cycler concept would therefore provide for routine, safe, and economical transport between Earth and Mars. A significant drawback of the cycler concept was that the Aldrin cycler flies by both planets at high speed. A taxi would need to accelerate to around Earth, and near Mars. To get around this, Aldrin proposed what he called a semi-cycler, in which the castle would slow down around Mars, orbiting it, and later resume the cycler orbit. This would