'Fly-bys', or 'gravity assist' manoeuvres, are now a standard part of spaceflight and are used by almost all ESA interplanetary missions. Imagine if every time you drove by a city, your car mysteriously picked up speed or slowed down. Substitute a spacecraft and a planet for the car and the city, and this is called a 'gravity assist'. These manoeuvres take advantage of the fact that the gravitational attraction of the planets can be used to change the trajectories, or the speed and direction, of our spacecraft on long interplanetary journeys. As a spacecraft sets off towards its target, it first follows an orbit around the Sun. When the spacecraft approaches another planet, the gravity of that planet takes over, pulling the spacecraft in and altering its speed. The amount by which the spacecraft speeds up or slows down is determined by the direction of approach, whether passing behind or in front of the planet.
When the spacecraft leaves the influence of the planet, it once again follows an orbit around the Sun, but a different one from before, either on course for the original target or heading for another fly-by. 'Slingshot' effect The first spacecraft to experience a gravity assist was NASA’s Pioneer 10. In December 1973, it approached a rendezvous with Jupiter, the largest planet in the Solar System, travelling at 9.8 kilometres per second. Following its passage through Jupiter’s gravitational field, it sped off into deep space at 22.4 kilometres a second – like when you let go of a spinning merry-go-round and fly off in one direction. This kind of acceleration is also called the ‘slingshot effect’. Mission: Impossible? Even before this encounter, Italian astronomer Giuseppe ‘Bepi’ Colombo had realised the potential of such manoeuvres and had used them to design a ‘Mission: Impossible’ to Mercury, the innermost planet of our Solar System. To reach Mercury, a spacecraft launched from Earth needed to lose more energy than a conventional rocket would allow.
Colombo’s brilliant idea was to realise that gravity assists could also be used to slow a spacecraft. On 10 March 1974, the NASA Mariner 10 spacecraft flew past Venus, lost speed and fell into its rendezvous orbit with Mercury. Extraordinary manoeuvre The ESA/NASA Ulysses mission used one of the most extraordinary gravity assists to allow it to see the polar regions of the Sun, places that are forever hidden from any observing location on Earth.
In October 1990, the Ulysses spacecraft left Earth to voyage towards Jupiter. There, it used a gravity assist to throw it out of the plane of the planets into a gigantic loop that passed over the south pole of the Sun in 1994, and then the north pole 13 months later.
More manoeuvres coming up Also in 2004, ESA’s Huygens probe will arrive at the Saturn’s moon Titan. It is carried on the NASA spacecraft Cassini which used four gravity assists (one with Earth, two with Venus and one with Jupiter) to accelerate it towards Saturn. ESA’s comet-chaser Rosetta will use a similar number of gravity assists to speed it to Comet Churyumov-Gerasimenko. Over the next eighteen months ESA’s lunar scout SMART-1 will become the first spacecraft to use gravity assists in conjunction with a revolutionary propulsion system, the solar-electric ion engine. This will pave the way for ESA’s Mercury mapper, appropriately called BepiColombo, which will use the same technique to orbit the inner planet early in the next decade. As well as affecting spacecraft, the gravitational influence of planets also affects the distribution of asteroids and comets. There are families of small bodies, for example the Apollo and the Plutino asteroids, which converges on a particular shape and size of orbit because their members have been repeatedly subjected to small gravitational attractions from the planets.
There are also individual, one-off gravitational effects that can send objects such as comets either plummeting into the inner Solar System or hurtling out beyond the planets. Watching for these ‘wild cards’ is a prime area of study for ESA, as the geological record on Earth shows that asteroids have occasionally collided with our planet in the past.
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