‘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.