2 of Space Settlements: A Design Study on the NASA website. Abbreviations key Escape orbits with low pericentre – C3 = 0 Geostationary orbit – GEO Geostationary transfer orbit – GTO Earth–Moon Lagrangian point – L5 Low Earth orbit – LEO''' Lunar orbit'' means low lunar orbit Red arrows show where optional aerobraking/aerocapture can be performed in that particular direction, black numbers give delta-v in km/s that apply in either direction. Lower-delta-v transfers than shown can often be achieved, but involve rare transfer windows or take significantly longer, see: fuzzy orbital transfers. Electric propulsion vehicles going from Mars C3 = 0 to Earth C3 = 0 without using the Oberth effect need a larger Delta-v of between 2.6 km/s and 3.15 km/s. Not all possible links are shown. The Delta-v for C3 = 0 to Mars transfer must be applied at pericentre, i.e. immediately after accelerating to the escape trajectory, and do not agree with the formula above which gives 0.4 from Earth escape and 0.65 from Mars escape. The figures for LEO to GTO, GTO to GEO, and LEO to GEO are inconsistent. The figure of 30 for LEO to the Sun is also too high. Near-Earth objects Near-Earth objects are asteroids whose orbits can bring them within about 0.3 astronomical units of the Earth. There are thousands of such objects that are easier to reach than the Moon or Mars. Their one-way delta-v budgets from LEO range upwards from , which is less than 2/3 of the delta-v needed to reach the Moon's surface. But NEOs with low delta-v budgets have long synodic periods, and the intervals between times of closest approach to the Earth (and thus most efficient missions) can be decades long. The delta-v required to return from Near-Earth objects is usually quite small, sometimes as low as , with aerocapture using Earth's atmosphere. However, heat shields are required for this, which add mass and constrain spacecraft geometry. The orbital phasing can be problematic; once rendezvous has been achieved, low delta-v return windows can be fairly far apart (more than a year, often many years), depending on the body. In general, bodies that are much further away or closer to the Sun than Earth, have more frequent windows for travel, but usually require larger delta-vs. See also Bi-elliptic transfer Gravity assist Hohmann transfer Oberth effect Orbital speed Tsiolkovsky rocket equation Porkchop plot Synodic period Notes References