Low Energy Transfer – Another way to fly me to the Moon

Hiten-Hagoromo (MUSES-A)
A depiction of Hiten (MUSES-A) in orbit around the Moon. Image Credit: CC / Go Miyazaki Enlarge

KENNEDY SPACE CENTER, Fla. — If you want to get anywhere in space, you need to learn about the complex process known as orbital mechanics. For years, the primary method of getting cargo from one orbital position to another relied on what is known as the “Hohmann Transfer Process”. That is, of course, unless your name is Edward Belbruno.

To get an idea of what is involved in moving around in orbit, consider the concept of moving from low Earth orbit to a geostationary position. When satellites are first sent spaceward, they establish a near-circular orbit anywhere between 200 and 800 miles (322–1,287 km) above the surface. The trick is to get that spacecraft to its home in geostationary orbit some 24,000 miles (38,624 km) up. The principle method used would be a Hohmann transfer.

Designed by the German scientist Walter Hohmann in 1925, the idea is to apply additional thrust at a specific point in the spacecraft orbit. This extra push will cause the spacecraft’s orbit to change from circular to elliptical. If you then apply thrust for a specific period at the apex (apogee in this case) of the ellipse, the orbit again becomes circular, albeit at the higher orbit. You can work your way down from a higher orbit by reversing the process, firing engines at the perigee of the ellipse.

The problem with Hohmann transfers is that, while the most expedient, they are not the most energy efficient means for moving spacecraft from one orbit to the other. The technique grows even more complicated when moving spacecraft between worlds such as the Earth and the Moon.

Edward Belbruno, a scientist working at the Jet Propulsion Laboratory (JPL), proposed a different technique than the Hohmann transfer, one he called Low Energy Transfer (LET) for minimizing fuel usage on spacecraft making the journey to other worlds. He had worked on missions such as Galileo, Magellan, and Cassini-Huygens as an orbital analyst and understood the mechanics of orbital transfers. The concept was not well received when proposed in 1988 for a JPL probe and was, initially, shunned.

The idea seemed straight out of science fiction. Instead of traveling faster than the orbital velocity of the target (essentially playing catch-up), the LET approach uses a lower orbital velocity than the target, letting the target’s gravity assist in the final capture of the spacecraft.

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