Another reason why so few missions have targeted Mercury is that it is very difficult to obtain a satellite orbit around the planet on account of its proximity to the Sun, which causes the Sun's gravitational field to pull on any satellite that would be set into Mercury's orbit. The planet's orbit is inclined to the ecliptic by 7°, and its orbital velocity varies from 24.25 miles per second (39.03 km/s) at aphelion to almost 30 miles per second (48 km/s) at perihelion. Spacecraft would be even faster, because they accelerate as they approach the greater gravitational pull of the Sun, but must slow down for orbit insertion, so this entails considerable fuel requirements.