Last week, the European Space Agency (ESA) successfully landed an unmanned probe (Philae) on a comet, a feat heretofore unmatched in human history. Lost in the amazement over Philae’s landing, though, is the equally amazing process required to catch up to the comet in the first place. In order to reach the comet, the probe relied on boosts from the gravity of different planets in order to gain enough speed. The maneuver is often incorrectly described as a gravity slingshot, which implies that a spacecraft somehow loops around a planet before being launched away. But that’s not really how it works.
Some of the first missions to use the “gravity assist” were the Voyager missions. The Voyager missions so excited the public imagination that in 1970, seven years before the Voyagers launched, the American Scientist published a detailed explanation of the mission, its goals, and how it would work. The author, William Pickering, explained that a gravity assist would be required for any Earth-launched craft to reach the outer planets, since rocket fuel simply lacked the energy required to escape the Sun’s gravity. Pickering correctly predicted that if properly done, the gravity of Jupiter and Saturn could be used to send Voyager out to Uranus and Neptune as well—all four outer planets in one trip.
In order to understand how the gravity assist works, it is necessary to understand velocity. An object’s velocity includes the direction of movement of that object as well as its speed. Velocity is a vector, or anything that has both magnitude (speed in this case) and direction, and vectors can be added. The sum of two vectors has a greater magnitude than either one alone. In a gravity assist, the object, in this case a space probe, has its own velocity (VspacecraftBefore) as it meets the assisting planet. The crucial point is that the assisting planet is in motion orbiting the sun, with velocity (Vplanet) of its own. Earth, for example, is moving through space at approximately 68,000 mph. In a gravity assist, the spacecraft first enters the outer limits of the planet’s gravity as it moves away from the sun. The planet “catches” the spacecraft, carrying it along and increasing its speed, until it spits the spacecraft out in a new direction, different from both the spacecraft’s original direction and the direction of the planet’s motion. Since vectors can be added and velocity is a vector, VspacecraftBefore +Vplanet = VspacecraftAfter, and magnitude VspacecraftBefore < magnitude VspacecraftAfter. Thus, the probe has increased its speed. It’s not really a slingshot at all, more like a relay throw.
Pickering explains the gravity boost in a lot more detail, but the short version above gives the general idea. For the comet mission, which required four gravity assists, the launch date and time had to be exactly calculated so that each change in velocity would allow the Rosetta spacecraft to correctly rendezvous with the next boosting planet. In the case of Voyager, available computing power was very limited, so these same calculations were mostly done by hand. Think on that.