Ballistic capture

Ballistic capture is a method of achieving orbit around a planet or moon. A spacecraft moving at a lower orbital velocity than the target celestial body is inserted into an orbit similar to that of a planet or moon, allowing it to move toward it and gravitationally snag it into orbit with no need for an insertion burn.

It was first used by the Japanese spacecraft Hiten in 1991 as a method to get to the Moon.

It is predicted to be:

• safer, as there is no time critical insertion burn,
• launchable at almost any time, rather than having to wait for a narrow window of opportunity,
• more fuel efficient for some missions.

The conventional method is a Hohmann transfer orbit, which does involve an insertion burn.

Low-energy transfer

Main article: Low-energy transfer

Low energy transfer (LET) was proposed in the 80s. The spacecraft is launched into a transfer orbit that places it ahead of the target planet's orbital path. As the planet approaches the spacecraft, the planet's gravity draws the craft into a high circular orbit eliminating the need for an insertion burn. This orbit is a temporary one, eventually the spacecraft would leave it again - but then it uses low power thrusters to get into a lower orbit. It can use the much more efficient ion thrusters at this stage as there is no need for a high level of thrust.

With some target orbits the total fuel is less than for Hohmann transfer. Hohmann transfer is more fuel efficient if an orbit closer to Mars is targeted, but can only be done at particular times. Ballistic capture, on the other hand, can be used at any time.

In 2014, low-energy transfer was proposed as an improved orbital maneuver for future Mars missions. It can be performed anytime, not only once per 26 months as in other maneuvers and does not involve dangerous and expensive (fuel cost) braking. But it takes up to one year, instead of nine months for a Hohmann transfer.