Gravity assist

[3] A close terrestrial analogy is provided by a tennis ball bouncing off the front of a moving train.

[2] This oversimplified example cannot be refined without additional details regarding the orbit, but if the spaceship travels in a path which forms a hyperbola, it can leave the planet in the opposite direction without firing its engine.

The same principles apply as above except adding the planet's velocity to that of the spacecraft requires vector addition as shown below.

In his paper "To Those Who Will Be Reading in Order to Build" ("Тем, кто будет читать, чтобы строить"),[6] published in 1938 but dated 1918–1919,[a] Yuri Kondratyuk suggested that a spacecraft traveling between two planets could be accelerated at the beginning and end of its trajectory by using the gravity of the two planets' moons.

[7] In his 1925 paper "Problems of Flight by Jet Propulsion: Interplanetary Flights" ("Проблема полета при помощи реактивных аппаратов: межпланетные полеты"),[8] Friedrich Zander showed a deep understanding of the physics behind the concept of gravity assist and its potential for the interplanetary exploration of the solar system.

[9][10][11][12] In 1961, Michael Minovitch, UCLA graduate student who worked at NASA's Jet Propulsion Laboratory (JPL), developed a gravity assist technique, that would later be used for the Gary Flandro's Planetary Grand Tour idea.

[13][14] During the summer of 1964 at the NASA JPL, Gary Flandro was assigned the task of studying techniques for exploring the outer planets of the solar system.

In this study he discovered the rare alignment of the outer planets (Jupiter, Saturn, Uranus, and Neptune) and conceived the Planetary Grand Tour multi-planet mission utilizing gravity assist to reduce mission duration from forty years to less than ten.

The main practical limit to the use of a gravity assist maneuver is that planets and other large masses are seldom in the right places to enable a voyage to a particular destination.

For example, the Voyager missions which started in the late 1970s were made possible by the "Grand Tour" alignment of Jupiter, Saturn, Uranus and Neptune.

That is an extreme case, but even for less ambitious missions there are years when the planets are scattered in unsuitable parts of their orbits.

However, if a spacecraft gets too deep into the atmosphere, the energy lost to drag can exceed that gained from the planet's velocity.

This maneuver, called an aerogravity assist, could bend the trajectory through a larger angle than gravity alone, and hence increase the gain in energy.

[citation needed] A rotating black hole might provide additional assistance, if its spin axis is aligned the right way.

Although attempts to measure frame dragging about the Sun have produced no clear evidence, experiments performed by Gravity Probe B have detected frame-dragging effects caused by Earth.

[citation needed] The gravity assist maneuver was first attempted in 1959 for Luna 3, to photograph the far side of the Moon.

[19] Thereafter, Pioneer 10 became the first of five artificial objects to achieve the escape velocity needed to leave the Solar System.

In December 1973, Pioneer 10 spacecraft was the first one to use the gravitational slingshot effect to reach escape velocity to leave Solar System.

[20][21] Pioneer 11 was launched by NASA in 1973, to study the asteroid belt, the environment around Jupiter and Saturn, solar winds, and cosmic rays.

It gained the energy to escape the Sun's gravity by performing slingshot maneuvers around Jupiter and Saturn.

[4] Having operated for 47 years, 5 months and 7 days as of February 13, 2025 UTC [refresh], the spacecraft still communicates with the Deep Space Network to receive routine commands and to transmit data to Earth.

Its trajectory took longer to reach Jupiter and Saturn than its twin spacecraft but enabled further encounters with Uranus and Neptune.

[32][33] The MESSENGER mission (launched in August 2004) made extensive use of gravity assists to slow its speed before orbiting Mercury.

[38] After entering orbit around Saturn, the Cassini spacecraft used multiple Titan gravity assists to achieve significant changes in the inclination of its orbit as well so that instead of staying nearly in the equatorial plane, the spacecraft's flight path was inclined well out of the plane of the rings.

That enabled it to flyby the asteroids 21 Lutetia and 2867 Šteins as well as eventually match the velocity of the 67P/Churyumov–Gerasimenko comet at the rendezvous point in August 2014.

In fiction In the novel 2001: A Space Odyssey – but not the movie – Discovery performs such a manoeuvre to gain speed as it goes around Jupiter.

As Arthur C. Clarke made clear at various times, the location of TMA-2 was switched from near Saturn (in the novel) to near Jupiter (in the movie).