Hohmann Transfer Orbit
Hohmann Transfer Orbit is a manoeuvre that was invented by a German scientist, Wolfgang Hohmann, in 1925. It is considered to be the most fuel efficient way of transferring a satellite or a spacecraft from one circular orbit to another. Though fuel efficient, the Hohmann Transfer Orbit takes more time than other modes of orbital transfer.
While travelling from Earth to any other destination such as the moon, Mars etc, a spacecraft doesn’t directly move from point A to point B. The spacecraft has to move in an orbit, hence, the spacecraft must be inserted from the Earth’s orbit to the destination planet like the Mars’ orbit. The Hohmann Transfer Orbit is how the spacecraft is transferred from one orbit to another. Taking the instance of a spacecraft being launched towards Mars, the spacecraft is launched from Earth into an elliptical orbit around the Earth; once on that orbit, the spacecraft accelerates again to place itself on a circular orbit along which Mars is revolving. It is to be noted that a slight acceleration or deceleration while in orbit completely changes the shape of the orbit itself. So, every acceleration or deceleration impacts the spacecraft’s trajectory.
The calculations regarding quantum and timing of accelerations are made with the help of Kepler’s Third Law so that the spacecraft leaves Earth at the appropriate time so it may reach Mars’s orbit when Earth and Mars align. Mars and Earth align only once in 26 months, and the Hohmann Transfer has to take place at that point of time.
MOM and Hohmann Transfer
India’s Mars Orbiter Mission (MOM) used the Hohmann Transfer Orbit to put its spacecraft on Mars’ orbit. Instead of a direct exit from Earth, a slow exit with the Hohmann Transfer Orbit made the MOM more economical.
India’s MOM and NASA’s Mars Atmosphere and Volatile EvolutioN (MAVEN) mission have certain key differences when it comes to the trajectories they took to reach Mars. MAVEN utilised an Atlas 5 rocket which is significantly more powerful from the PSLV that launched the MOM. The rocket that launched MOM is smaller and has much lesser fuel capacity than Atlas 5. While the Atlas 5 can lift approximately 7,000 kgs into geostationary transfer orbit, the PSLV can approximately life 1,300 kgs into geostationary transfer orbit. So, while MAVEN is injected directly into a trans-Mars trajectory from launch, the MOM first launches into an Earth orbit, then gradually after multiple steps is put on a trans-Mars trajectory. The MAVEN is put in a Hohmann Transfer Orbit in an hour from its launch, while the MOM spent more than four weeks just orbiting around Earth. So, the MOM modifies its trajectory very slowly in a manner that increases fuel efficiency (which is crucial because it has very limited fuel holding capacity) and puts it on the right trajectory towards Mars’ orbit.