Slingshot success

Isro has already achieved a great deal on Mars mission

The Indian Space Research Organisation's (Isro's) Mars Orbiter Mission, or Mangalyaan, embarked on the third stage of its five-stage mission in the early hours of Sunday morning. The orbiter was pushed into a Mars transfer orbit, using a gravity-assisted slingshot manoeuvre. The fourth stage begins 10 months hence, when the vessel makes its rendezvous with Mars and gets into orbit around the red planet. The fifth stage would be the launch of the scientific experiments. Each stage presents different and progressively more complex challenges. From the second stage onwards, Isro has had to demonstrate new capabilities. The second stage involved firing thrusts from the rockets to tweak Earth's orbit into a curved hyperbolic path enabling gravity assist. Isro had never done this. There were minor alarms: one of the series of planned rocket thrusts failed and a compensatory firing was set up using extra fuel. The compensation was successful. The vessel described a progressively more curved hyperbola around Earth to generate speed using Earth's gravitational field. The third stage is more tricky. The vessel had to be pushed out of Earth's orbit at the perfect moment to start the journey to Mars while employing as much of the gravity slingshot effect as possible.

Earth, Mars and other planets orbit the sun in elliptical, roughly egg-shaped orbits. The transfer orbit puts Mangalyaan into another, roughly egg-shaped orbit around the sun. That transfer orbit has to be calculated to intersect the Martian "egg" at exactly the right place and time to rendezvous with the planet. The gravity assist, using Earth's field to impart velocity (velocity refers to speed in a specific direction), is designed to conserve fuel rather than get to Mars quickly. A minor error could mean that Mars is missed by millions of kilometres. Slingshots are difficult to calculate and each transfer orbit presents its own challenges. That Isro got it right in the first shot is a major achievement.

The chosen transfer orbit involves four course-correction rocket firings over the next 10 months. Each of those will be increasingly delicate. As distance from Earth increases, real-time telemetering becomes impossible and the course corrections will have to be pre-programmed with contingency measures built in to perform autonomously. If all goes well, the fourth stage will be almost totally autonomous, with the vessel firing its rockets to force itself into Mars orbit. Again, this manoeuvre will be calculated to minimise fuel expenditure. Even with minimal fuel consumption, it takes a 350-tonne rocket to deliver a 1,350-kilogram orbiter with a 13-kg scientific payload. Prior to Sunday, only the National Aeronautics and Space Administration (Nasa), the Russians and the European Space Agency had succeeded in demonstrating slingshots. The completion of the third and fourth stages will mean facing up to even more complex challenges. On their track record so far, it appears that the boffins at the Satish Dhawan Space Centre will be up to the task.