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The Mars Reconnaissance Orbiter using its Mars Climate Sounder instrument. Credit: JPL
A transfer orbit is a maneuver where a spacecraft shifts from one orbit to another on the same plane or angle. This process requires a temporary shift in the craft’s speed caused by two short engine impulses. This method has many applications to spaceflight. First for satellites and and second for space probes.
Satellites depend on a stable orbit to properly send and receive messages. The main goal of a satellite is to get into a geosynchronous orbit. That is an orbit that matches the Earth’s period of rotation called a sidereal day. A majority of satellites need a more specific type of orbit called a geostationary orbit, an orbit that is directly over the equator and and causes a satellite to seem stationary to an observer on Earth. Getting into geostationary orbit is not simple and requires gradual adjustment of the satellite’s orbit until it reaches it. This is where transfer orbit come in. The satellite is launched into an initial orbit and as it completes its revolution the on-board engines fires timed impulses that push into a slightly bigger orbit closer to the desired geostationary orbit. If done correctly the satellite will shift from its original orbit into a transition orbit. Another time burst from the engines finally push it into place in its final orbit. The process varies from satellite to satellite. Some are launched directly into geosynchronous or geostationary orbit, while others are launched into the transfer orbit first then make the needed adjustments.
The transfer orbit is also used for space probes that are sent on interplanetary mission. The biggest challenge for sending spacecraft to other planets and beyond is fuel. It takes a lot to launch a spacecraft into space and even more to get it to its final destination. Scientist have come up with several solutions in the interim to more efficiently use fuel. One idea is electric propulsion. To date, this is one of the most efficient as the amount of thrust can be controlled to meet unique challenges during different stages of a space probe’s voyage. The other solution used is the transfer orbit taking advantage of the Sun. The concept is brilliant in its simplicity. Rather than launching a probe directly at its target, it is sent into an extremely elliptical orbit around the sun. The probe uses the gravitational pull of the sun to accelerate and when it reaches the furthest point of its solar orbit or perihelion it use shorts bursts from its engines to transfer to the orbit of the nearest planet. This maneuver is repeated as many times as necessary to reach its target planet.
If you enjoyed this article there are several others on the Universe Today website you might enjoy. There is a great article on the Chandrayaan-1 a lunar orbiter taking pictures of the moon. There is also a great article on the length of a trip to Mars and the role transfer orbit plays in it.
There are also great resources on the web. There is a great article on the NASA website about interplanetary trajectories. There is also a great article about orbit transfers on the Davidson University website.
You can also listen to Astronomy Cast. Episode 84 “Getting around the Solar System” is an interesting show with some information pertinent to this article.
Sources:
http://en.wikipedia.org/wiki/Geostationary_transfer_orbit
http://en.wikipedia.org/wiki/Hohmann_transfer_orbit
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