Four teams of scientists and engineers of the Indian Space Research Organisation worked tirelessly for 18 months to get the orbiter and its payloads on board. By T.S. SUBRAMANIAN
M. Annadurai, Programme Director, Indian Remote-sensing Satellites (IRS) and Small Satellites Systems (SSS), is proud of two things about India’s Mars Orbiter Mission (MOM). One, the “autonomy” it enjoys to take corrective decisions if any anomaly occurs on board the spacecraft and, two, the Indian Space Research Organisation’s (ISRO) “unparalleled job” in building the spacecraft, which is “nearly as perfect as possible”, within the allotted money and the “tight schedule” of 18 months. For, “basically, we have gone through the successes and failures of the missions to Mars undertaken by other countries and also India’s own Chandrayaan-1 mission to moon, and incorporated the lessons learnt from them in the Mars orbiter,” he said. (The MOM and Chandrayaan-2 come under ISRO’s IRS programme.)
Annadurai said the MOM was “a logical extension of Chandrayaan-1”. Not only are their initial mission profiles similar but ISRO’s XL version of the Polar Satellite Launch Vehicle (PSLV), which put Chandrayaan-1 into orbit, will launch MOM as well. Just as Chandrayaan-1 was initially put into a highly elliptical orbit, the PSLV-XL will put the Mars orbiter into a highly elliptical orbit of 250 kilometres by 23,500 km on November 5. However, while the moon is about four lakh kilometres away, “the Mars orbiter has to go a very long way—200 million to 400 million km—to reach Mars,” Annadurai said. This voyage will take about 300 days.
In order to achieve this, ISRO will initially fire the 440 Newton engine on board at the orbiter’s perigee and keep incrementally increasing its apogee. A prolonged firing of this engine on November 29 will catapult the spacecraft out of its geocentric orbit into a heliocentric orbit. That is, it would get out of its earth-bound orbit, and go round the sun in such a way as to coast along for nine months and then “rendezvous with Mars”, Annadurai said. “Mars may be 200 million km from the earth in September 2014. The Mars orbiter will approach Mars at that time. We will slow down the orbiter by firing the 440 Newton engine [to enable the spacecraft to get into the Martian orbit]. There will be a gap of 300 days between the first firing of the 440 Newton engine and the second,” he added.
“This is the first time that we are doing this kind of an operation in orbit”, that is, firing the 440 Newton engine after the engine has remained idle for 300 days, he said. “We will basically reorient the spacecraft and fire the engine. That firing will make the orbiter attain an orbit of 377 km by 80,000 km around Mars,” he explained. The spacecraft will be reoriented in such a manner that minimum energy will be used to put it into the Martian orbit. This calls for a special manoeuvre by the PSLV-XL by suitably delaying the ignition of its fourth stage on the launch date.
In the six months after the orbiter is put into the Martian orbit, the spacecraft will use the five instruments on board to perform various scientific experiments. It is equipped with a colour camera for optical imaging of Mars’ surface; a methane sensor to detect possible life on the red planet, a thermal infrared camera to study its geological activity, a Lyman Alpha Photometer to study the Martian atmosphere; and a payload to study the neutral composition of the planet’s upper atmosphere.
“These instruments will be operated in various positions of the spacecraft’s orbit in such a manner that they will have the advantage of not only having a near-look at Mars but also a global view of the planet. The orbiter will operate like our remote-sensing satellites when it is close to its perigee and it will work like INSATs and geo-imaging satellites near the apogee. This is good for inter-planetary missions that have remote-sensing payloads,” he said.
The Mars orbiter stands out for the complexity of the space communication its operation involves. The huge distance covered to reach Mars entails a space communication delay. It will take 20 minutes for a command from the ground station to reach the spacecraft and it will take another 20 minutes to receive the telemetry from the orbiter. Besides, the signals coming from the orbiter will be feeble. “When we reach the Martian orbit, we have to pump more radio-frequency power to communicate with the orbiter. So the Mars orbiter should be able to cater to this complexity,” Annadurai said.
Since there will be “a round-trip delay of 40 minutes” to communicate with the orbiter, controlling the spacecraft in a conventional manner, as in the case of INSAT or Chandrayaan-1, is difficult. That is the reason why ISRO wanted to build a spacecraft that would enjoy detailed autonomy. If any anomaly or “misbehaviour” were to occur on board the orbiter, it would take 20 minutes for the ground stations to come to know about it. Annadurai explained: “When you give the command to the spacecraft to switch over to a redundant system [after the anomaly is noticed], it will reach the orbiter 20 minutes later. So you can switch over to the redundant system only 40 minutes after the misbehaviour occurs. By then the spacecraft would have more problems. Even the ground equipment will not be effective. So you have sensors and transmitters on board the orbiter that can sense the failure and reconfigure to the redundant system on the spacecraft on their own. The reconfiguration will take place autonomously in the orbiter.”
For instance, if the 440 newton engine does not perform well for the spacecraft’s Mars orbit injection as it did near the earth-bound orbit, it will be sensed by the performance of the accelerometer. Additional thrusters on board will come into play and the spacecraft will acquire the minimum velocity needed to enter the Martian orbit. “If we do not do that, we will not get into the Martian orbit. The orbiter will vanish into space. These are the basic lessons we have learnt from the other missions. So the autonomy of spacecraft is a must. We have put a good amount of autonomy in our spacecraft. Without autonomy, it [this mission] is not possible,” Annadurai said.
Another highlight of the MOM is its circumambulation of the earth for about three weeks after it is first put into an orbit of 250 km by 23,500 km. Here, the thermal environment is two to three times hotter than near Mars because the earth is closer to the sun than Mars, and the sun load on the earth is pretty high. In the Martian orbit, the sun’s heat is felt much less. “The implication is that the power generation from the orbiter’s solar panels has to take care of both these conditions,” he said.
ISRO satellite engineers and scientists were racing against time to get the five scientific instruments ready and to integrate them into the spacecraft bus within a span of 18 months. The deadline could be met because ISRO Chairman K. Radhakrishnan set up four teams that worked simultaneously, Annadurai said. While one team built the spacecraft, another assembled the launch vehicle. Two other teams worked on getting the ground systems ready and doing their calibration. These teams “worked right from the beginning not sequentially but performed parallel operations,” he said.
Meanwhile, a committee headed by Professor U.R. Rao, former ISRO Chairman, selected the five instruments that would go into the spacecraft. The teams were aware that PSLV-XL, which is capable of putting a spacecraft weighing more than 1,350 kg into orbit, was the rocket to be used in the mission.
“We worked backwards, with the experience gained from Chandrayaan-1 and Mars missions of other countries, to calculate how much dry mass will be left in the bus system [after it enters its Martian orbit]. We calculated that only 15 kg of dry mass would remain, and so we ensured that the five instruments have a combined weight of 15 kg,” Annadurai said. The instruments needed to perform scientific experiments that match the orbit. The various ISRO centres that built the instruments were asked to send the payloads to the clean room of the ISRO Satellite Centre, Bangalore, by April/May 2013, for integration into the spacecraft’s bus. There were periodic reviews of all the payloads and their development. A fifth team, headed by U.R. Rao, monitored the progress in the development of the payloads.
The spacecraft, with its five payloads, was readied at the ISRO Satellite Centre, Bangalore. It underwent a series of tests both at the ISRO Satellite Centre and at the ISRO Satellite Integration and Test Establishment [ISITE], also in Bangalore. Then it was transported by road to Sriharikota. It reached Sriharikota on October 3 for integration with the PSLV-XL. The integration was completed on October 20.
“The number of calendar days these teams have worked may be small. But the effective manpower that went into the making of the orbiter and the payloads is more than five years of manpower. This is how we realised the mission of Mars orbiter,” said Annadurai.