Low Energy Transfer – Another way to fly to the Moon
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.
In 1990, the LET concept was put to the test on the Japanese spacecraft Hiten. The spacecraft, destined for the Moon, had completed its primary mission before trying out the LET concept. The LET used almost no delta-v or change in the spacecraft’s speed and direction. The technique used the gravity of the Earth and the Moon to slowly guide the spacecraft to a stable orbit.
While the method is effective, it is rather slow. Hiten took five months to enter orbit around the Moon instead of the usual three days. It had proved successful, and Hiten entered a temporary lunar orbit on October 2, 1991. Initially, the spacecraft traveled through the L4 and L5 Lagrange points (stable areas of gravity between the Earth and the Moon) before settling into a permanent lunar orbit on February 15, 1993. The longer spaceflight allowed Hiten to look for dust particles in the L4 and L5 locations as well as its lunar orbit.
After the successful extended Hiten mission, LET has been used on other spacecraft including SMART-1, Genesis, and GRAIL. The Indian Space Research Organization had successfully used the technique on its Mangalyaan or Mars Orbiter Mission (MOM). This mission was a low cost (US$73 million) space probe that is currently operating in Mars orbit, the first successful Asian Martian probe. The use of LET is now being considered for several other space missions such as Mars Direct.
Edward Belbruno now runs Innovative Orbital Design, Inc., based in Princeton, New Jersey. He has written several books on orbital mechanics including Fly Me to the Moon, and Capture Dynamics and Chaotic Motions in Celestial Mechanics. He is also a consultant working with NASA.
This story was produced for The Lunar Initiatives newsletter: The Lunar Initiatives – it includes the opinions of the author and does not necessarily reflect the views of SpaceFlight Insider
Joe Latrell is a life-long avid space enthusiast having created his own rocket company in Roswell, NM in addition to other consumer space endeavors. He continues to design, build and launch his own rockets and has a passion to see the next generation excited about the opportunities of space exploration. Joe lends his experiences from the corporate and small business arenas to organizations such as Teachers In Space, Inc. He is also actively engaged in his church investing his many skills to assist this and other non-profit endeavors.