Devangshu Datta: On a slingshot and a prayer

Isro's real test on the Mars mission now lies in successfully completing the three remaining course correction firings

The first course correction of the Mangalyaan Trans-Mars transfer orbit has been successful. News flashes about the mission are likely to recede into the background now, with occasional updates as the vessel cruises towards the fourth rock from the sun for the next 10 months. If this phase is uneventful, the next burst of excitement will occur when the vessel gets into orbit around Mars in September 2014.

However, the Indian Space Research Organisation (Isro) has already demonstrated great technological competence in ensuring that the Mars Orbiter Mission (MoM) has got this far. Only four space agencies - NASA, the European Space Agency, Russia's Space Agency (which is the successor to the USSR''s SA) and now Isro, have ever managed transfer orbits.

The vehicle is now well beyond the range of real-time monitoring. It would take about 10 seconds for a radio signal to reach from Earth to MoM. There are three more planned course corrections before the MoM intersects Mars' orbit. All of these will be performed autonomously according to pre-programmed instructions. If there are contingency issues, or failures, recoveries would also have to be pre-programmed and done autonomously.

The first phase of the mission - putting the rocket into Earth orbit - took only 44 minutes. Isro had done this multiple times before. The second phase - gradually turning the Earth orbit into an extreme hyperbolic orbit in order to carry out a slingshot was in itself, very tricky. Most Mars missions have failed at this stage.

The concept of a slingshot manoeuvre is simple according to the laws of mechanics. However, it requires perfect timing. The first time NASA managed it was with the Mariner missions in the 1970s. Mariner 1 - a planned fly-by of Venus failed at slingshot stage.

But Mariner 2 worked and since then, all Mars missions have used slingshots.

Think of a batsman turning his wrists to change the trajectory of a fast delivery, to beat the fielders and reach the boundary by using pace imparted by the bowler. Now, think of a model aircraft the size of a fly or smaller, flying around the boundary and the batsman hitting that aircraft on demand. Multiply the difficulty of this absurdly difficult task by many orders of magnitude and you have some idea of the precision required for a slingshot.

In this analogy, the orbiter is the ball. The fielders represent random celestial objects, which the orbiter must miss on its journey. The fast bowler imparting pace is the Earth's gravity field. The model aircraft is Mars. The batsman's wrists represent the MoM's rockets.

A slingshot or gravity-assisted transfer orbit as it's properly called, uses the gravitational field of the Earth to impart pace. An object falling towards the Earth accelerates at 9.8 metres per second. If it's dropped from greater height, it gains more velocity. If a course correction is applied at the right moment, the trajectory can be diverted to miss the Earth while keeping most of the speed imparted by gravity even as the object flies off onto a new path.

Isro raised the MoM to 192,000 km away from the Earth by a judicious sequence of rocket firings. It then "dropped" the MoM to a distance of "just" 240 km away from the Earth before firing rockets again, to direct it away. Earth's gravity imparted a great deal of speed as this hyperbolic orbit was performed, and the MoM retained most of that as it was directed away.

The orbiter is now free of the Earth's gravity field but not that of the Sun. Hence, the MoM is now orbiting the sun in a vast elliptical orbit. Mars is also orbiting the sun in a different elliptical orbit. Those two orbits will intersect in September 2014, if all goes well, and the three remaining course correction firings are executed precisely.

The underlying reason for this complicated series of manoeuvres is to preserve fuel. Rather than being designed as a minimum distance or minimum time journey, the Earth-Mars trip is planned as a minimum energy journey. Even in a well calculated and executed minimum energy trip, the fuel and engine component is massive. A 350 tonne PSLV is required to propel the 1.350 tonne orbiter. The net weight of the five scientific experiments it carries is less than 15 kgs (1,000 kg=1 tonne).

The MoM will travel about 680 million km to reach Mars although the red planet is never more than about 400 million km from the Earth and often closer. However, except for those four firings, (each lasting less than a minute), it will not spend fuel since it will cruise along using the gravity-assist.

The next major fuel expenditure will come when it does reach Mars. At that stage, it will have to be forced out of the solar orbit and into an orbit around Mars. This will require a fair amount of rocket firing to change velocity. The process of getting into Mars orbit will have to be completely autonomous since light and radio waves will take over 15 minutes to bridge the Earth-Mars distance at that stage. It will be over and done with, one way or another, before Isro has confirmation.

Isro has already proved it has developed new competencies in orbit and trajectory calculation, tele-metering ability and autonomous programming and those are enough in the way of positive takeaways. A successful completion of the mission would be a terrific shot in the arm.