What is a gravity assist or a swingby for a spacecraft? I've heard about spacecraft gaining velocity from passing close to planets and being 'sling shot'-ed into a higher orbit. Where does this energy come from?

Gravity assist is used to speed up, slow down or change the direction of a spacecraft as it approaches a planet within our solar system. This saves fuel that would otherwise have to be used to make these changes.

Here are the basics! The spacecraft approaches the planet. Because the planet is moving too (while orbiting the Sun), it actually can boost the velocity of the spacecraft. What the spacecraft gains in velocity, the planet loses.

Here's a real example...Galileo increased its speed at Earth by 11,620 mph in 1990 and 8,280 mph in 1992. The Earth slowed down in its orbit by a speed of about 5 billionths of an inch per year from the two events.

The technique of gravity assist for a spacecraft was used as early as the Mariner 10 mission in 1973-1974 and has been used recently to create unique orbits such as with the Ulysses spacecraft.

Gravity assist is also referred to as the swingby approach or the slingshot approach because of the way the spacecraft is swung around the planet and 'let go' like a slingshot.

Submitted by Mark (Maryland, USA)
Submitted by Jean-Marc (Oullins, France)
(September 3, 1998)

You might also be interested in:

VHM/FGM Instrument Page

Ulysses has a unique orbit. By going past Jupiter, Ulysses was able to go into an inclined orbit that has never been achieved before (this is the sling shot approach to getting a spacecraft into orbit)....more

What are the retrograde motions of planets in the sky?

It depends on which type of motion you are asking about. If you take a birds-eye view from the top of the solar system all the planets orbit around the Sun in a counter-clockwise (or direct) direction....more

How do Astronauts Live in Space?

Almost everyone has a question or two about living in space. What do astronauts do in space? How do they do everyday things like eat, sleep and go to the bathroom? It's important to note that astronauts...more

How far is the Earth from the Sun, the Moon and all the other planets? How far are all of the planets from the Sun? Do you know of a software that tracks the planets in real-time?

There is a really neat internet program called Solar System Live that shows the position of all of the planets and the Sun for any given day. If you go to that page, you'll see an image similar to the...more

Is it really true that man never really walked on the Moon?

The picture of the American Flag (the one put there by the Apollo astronauts) is waving (or straight out) in the wind. How could that be possible if there is no atmosphere on the Moon? Was it some sort...more

How many planets orbit the sun?

I was wondering if there is a new planet? Are there planets (a tenth planet?) after Pluto belonging to our solar system? What are the names of the new planets discovered in the solar system? Are there...more

According to Stephen Hawking, any object with an energy which equals Plank's energy has to become a black hole.

If that is so, the energy released during the Big Bang must have created many such black holes. Therefore most of the Energy of the Big bang must have disappeared in that form. Then how did the Universe...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA