|Fig. 1: An artist's conception of MOM orbiting Mars. (Source: Wikimedia Commons)|
Mangalyan, India's Mars Orbiter Mission (MOM), lifted off on November 5th, 2013. After almost a 300 day long journey, the interplanetary probe was successfully inserted in orbit around Mars on September 24, 2014. With this, India's space agency ISRO, became the fourth space agency to reach Mars, and the first one to do so in the maiden attempt.  India's mission is even more impressive given its modest price tag of $76 million, significantly less than NASA's $671 million Maven spacecraft, which entered Mars orbit just 2 days before MOM.  Media reports have also noted that MOM cost less than the $100 million Hollywood spent to produce the movie "Gravity". 
Clearly, multiple factors influence the cost of mission like MOM, including cost of the equipment, fuel, human resource, testing, etc. In this report, we investigate if fuel efficiencies used had any role to play in cost reduction of the MOM Mars mission. We use NASA's MAVEN mission for comparison.
One could naively imagine that reaching Mars is same as reaching any other target and one could launch a spacecraft towards Mars. But, Mars is a moving target! So, interplanetary journeys follow what is called the Hohmann orbit or minimum energy transfer orbit, to cut down on energy costs.  In short, the spacecraft is transferred from Earth's orbit around the Sun to Mars' orbit around the Sun, on a trajectory that forms an elliptical orbit which is tangential to orbits of both planets. The spacecraft is raised above Earth's atmosphere and accelerates in the direction opposite to the direction of Earth's revolution around the Sun, and coasts away to Mars' orbit around the Sun.
This journey from Earth to Mars can be divided into 3 phases; (a) from the launch pad to the point where it is placed on transfer orbit, or the Geocentric phase, (b) journey on the transfer orbit, the Heliocentric phase, and finally, (c) from transfer orbit to final orbit around mars, the Areocentric phase.
In the Geocentric phase, the MoM was placed in an orbit around Earth by Polar Satellite Launch Vehicle.  The PSLV was not powerful enough to place the MoM directly on to Mars transfer orbit, and the spacecraft was injected into a highly elliptical parking orbit around Earth. The MoM's closest point to Earth in this orbit (the perigee) was 250Km and the farthest point was 23,500 km (the apogee). 
From this initial elliptical geocentric orbit, the spacecraft was gradually raised into the transfer orbit through multiple orbit raising maneuvers. The engine employed by MoM for orbit raising maneuvers had 440N thrust, and had to be fired repeatedly so as to gradually build up velocity required to escape from Earth's gravitational pull.  A total of 6 burns were performed, each time increasing the apogee by several thousand kilometers, till the final orbit with an apogee of 192,918 km of reached. The total burn time for these six maneuvers was approximately 23 minutes.  This was followed by a final 7th burn to insert the spacecraft in a Heliocentric orbit, breaking away from Earth's gravity and on course to Mars.
The Heliocentric phase, the spacecraft follows a trajectory that is roughly one half of an ellipse around sun. Since both endpoints of the flight path are moving around the Sun, the transfer orbit is planned such that both the spacecraft and Mars are at the intersection point of the transfer orbit and Mars' orbit. During the Heliocentric phase, the spacecraft travelled a distance of 660,000,000km, in which only 2 out of 4 planned trajectory correction maneuvers were performed. 
On September 24th, 2014, the Mars Orbiter was inserted in orbit around Mars. For the final orbit insertion, a 440N thrust motor was fired for approximately 24 minutes. 
MAVEN took a more straight forward route from Earth to Mars. It was launched on an Atlas V rocket, which placed the spacecraft with the powerful Centaur upper stage in Earth orbit. The Centaur upper stage was briefly fired to place itself and the spacecraft on a circular orbit around Earth.  Just after 27 minutes of being in this orbit, the Centaur upper stage fired again, breaking free from Earth's gravitational pull, and taking the MAVEN spacecraft with it. The spacecraft separated from Centaur, and deployed its solar panels. In the upper Earth orbit, Centaur fired for around 9 minutes to place MAVEN on the interplanetary trajectory. 
The short Geocentric phase for MAVEN was followed by a long Heliocentric phase in which the spacecraft coasted away towards Mars on the transfer orbit for approximately 10 months. Only 2 of the 4 planned course corrections were performed in the Heliocentric phase. 
During the final orbit insertion around Mars, 6 170N main engines and 8 22N thrust attitude controls were employed for a duration of approximately 33 minutes. 
As explained earlier, MOM had a long geocentric phase, in which it went around Earth multiple times, and in each revolution the engine was briefly fired to raise the orbit and to gradually reach the Earth's escape velocity. This approach meant that engine required for placing the spacecraft in transfer orbit need not be very powerful, and in turn, even the launch vehicle can be less powerful, as it does not have to carry a heavy upper stage.
In contrast, MAVEN employed a powerful Atlas V launch vehicle and Centaur upper stage which together were capable of putting the spacecraft on a direct path to Mars on interplanetary transfer orbit, with only a short detour on orbit around Earth. This difference between MOM and MAVEN missions contributed significantly in cost savings in launch vehicle and fuel for MOM mission. 
© Kanthi Nagaraj. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 J. Burke, India's Mars Satellite Successfully Enters Orbit, Bringing Country into Space Elite," The Guardian, 24 Sep 14.
 J. Amos, "Why India's Mars Mission Is So Cheap - and Thrilling," BBC News, 24 Sep 14.
 J. Rogers, "Experts: Cheap Mars Mission Could Prove Lucrative for India," Fox News, 26 Sep 14.
 W. Hohmann, "The Attainability of Heavenly Bodies," U.S. National Aeronautics and Space Administration, NASA TT F-44, November 1980 [Translation from Die Erieichbarkeit der Himmelskörper (R. Oldenbourg, 1925)].
 "India Launches Maiden Mars Mission, Placed into Earth Orbit," New Indian Express, 5 Nov 13.
 "India's Mars Orbiter Leaves Earth, Enters Sun Orbit," New Indian Express, 1 Dec 13.
 "India to Enter Mars Orbit on September 24," The Hindu, 22 Sep 14.
 "India's First Mars Satellite 'Mangalyaan' Enters Orbit," BBC News, 24 Sep 14.
 "Mars MAVEN Probe Launched on $671 Million Mission," CBS News, 18 Nov 13.
 L. Keeney, "NASA's Mars MAVEN Orbit Insertion Was 'Right on the Money'," Denver Post, 22 Sep 14.
 "NASA Maven Orbit insertion Fact sheet," U.S. National Aeronautics and Space Administration, FS-2014-0-177-GSFC, Septemper 2014.