Paths to Space

Getting to the International Space Station, the moon, Mars, or Venus is not simply a matter of aiming directly at the object and pushing a firing button. Because these bodies, like our own planet, move in their own elliptical orbits around the sun, engineers must plan the orbits of spacecraft so that they arrive at the same point in space and time as their targets. Hitting a “target” in space is a complicated mathematical challenge, similar to shooting a high-flying duck from a racing speedboat.

During the Apollo missions, spacecraft were first placed in a parking orbit around Earth before being "boosted" out and into a trajectory toward the moon.

During the Apollo program launches, the moon was the “duck,” traveling around Earth at an average speed of 2,268 miles per hour. Earth, or “the speedboat,” was revolving on its own axis as well as orbiting the sun. The launch point, Kennedy Space Center in Florida, was moving eastward at 910 miles per hour because of Earth’s rotation. Earth itself was orbiting the sun at 66,000 miles per hour. Moreover, gravitational fields of the moon, sun, and Earth all were tugging at the “bullet,” the Saturn rocket-launched Apollo spacecraft.

During the Apollo missions, Apollo spacecraft were boosted into earth-parking orbits, where the spacecraft coasted until they were in proper position to begin the second leg of their journeys. At the appointed time, the spacecraft were boosted out of a parking orbit and into a trajectory or path to their target, the moon.

Orbital paths rarely form a perfect circle. In many cases, the paths of objects around the sun are elliptical. The farthest point from Earth in an orbit is called the apogee; the nearest point is called the perigee.

Unmanned spacecraft have landed on Mars and Venus and have flown past Mercury, Jupiter, Saturn, Uranus, and Neptune. The travel plans for long journeys into space are devised using mathematical calculations similar to those used to get a spacecraft to the moon. These calculations are far more involved, however, because the gravitational influences of the sun, the planets, and other objects that occupy our universe, such as asteroids and comets, exert their own gravitational and orbital influences on spacecraft during extended journeys from Earth out into space.