Isro’s Mars Orbiter to be catapulted to the red planet

New Delhi: With one last swing propelled by Earth’s gravity, India’s Mars Orbiter spacecraft will be catapulted out of Earth’s orbit towards the red planet on Sunday.

“The show is just starting,” said Ajey Lele, research fellow at the Institute for Defence Studies and Analyses, as scientists at the Indian Space Research Organisation (Isro) prepared to fire the spacecraft’s liquid engine just past midnight on Sunday to give it the thrust to leave Earth’s sphere of influence and start on its long journey to Mars.

After travelling a distance of 680 million km for 280 days, the orbiter is expected to reach the Martian trajectory in September 2014.

“Isro is using the slingshot method. It has slowly been raising the orbit of the orbiter through raising manoeuvres, and when it finally reaches its last apogee, it will shoot upwards towards Mars,” said Lele. “The orbiter will then enter the heliocentric atmosphere, which is the intermediary between Earth and Mars.

An Isro spokesperson said, “It will be on a minimum energy trajectory path, on which the spacecraft will travel on its own with trajectory corrections. To reach Mars, the launch has to take place at the specified time, which comes once in 26 months.”

For the trans-Mars injection, Isro is using the Hohmann transfer orbit, an elliptical orbit that can be utilized to transfer the spacecraft between the orbits of the two planets. The Hohmann transfer orbit is known to use the least amount of energy.

After being launched aboard the PSLV C25 from Sriharikota in Andhra Pradesh on 5 November, the spacecraft was injected into an elliptical orbit around Earth. After five successful orbit-raising manoeuvres over the past 25 days, this will be the sixth engine burn, after which the spacecraft will be gradually manoeuvred into a hyperbolic trajectory for it to escape Earth’s sphere of influence, ending at 918,347km from the surface of Earth, according to Isro.

For interplanetary missions, in order to save fuel, time and expense, space agencies use gravity assists to catapult orbiters towards the destination.

“The slingshot trick is that we’re using the earth’s gravity to shoot the orbiter towards Mars with adequate velocity,” said Dr. Mayank Vahia, Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research (TIFR) in Mumbai.

“After the five orbit-raising manoeuvres, every time the orbiter came closest to Earth, the Earth would make it swing faster. On Sunday, after the sixth engine burn, Earth’s gravitation would swing it fast enough to escape Earth’s sphere of influence.”

There are three satellites presently orbiting Mars, two sent by the US and one by the European Space Agency. India’s Mars Orbiter Mission and the National Aeronautics and Space Administration’s (NASA) MAVEN are expected to reach the Martian orbit next year.

India’s orbiter will face three major challenges after leaving Earth’s sphere of influence, the Isro spokesperson said. “First of all, we have to fire the engine for a very precise amount of time for it to reach the Mars trajectory. After it leaves the SOI, there will be a communication delay of up to 40 minutes; hence, sometimes the spacecraft will have to take decisions on its own, which is a major challenge.”

He added, “When approaching Mars, velocity has to be reduced precisely and sufficiently to reach the Martian orbit. It is a very tough challenge, because it is a very dynamic situation where both the planets will be moving and conditions will be changing.”

“That is the tricky part—60% of Mars missions have failed at that point. If you don’t slow it down enough, then it will fall towards Mars, and if it is slowed down too much it will fly right past Mars,” said Vahia.

“The navigational challenge is the equivalent of sending a ping-pong ball from Bangalore into an exact-sized tube placed on the moon.”

The spacecraft is expected to intersect the orbit of Mars at the exact moment that Mars is there too. This will be possible when the relative position of Earth, Mars and Sun form an angle of about 44 degrees.

After 280 days when the spacecraft arrives at the Mars sphere of influence, it will reach a point in the orbit closest to Mars. When the spacecraft comes closest to Mars, it will be captured into a planned orbit around Mars by implementing the Mars orbit insertion (MOI) manoeuvre. The liquid engine will be restarted after 10 months for the MOI manoeuvre.