Mission to Mars

ON November 5, the workhorse launcher of the Indian Space Research Organisation (ISRO), the Polar Satellite Launch Vehicle (PSLV), will launch the spacecraft for the organisation’s maiden mission to Mars in its uprated and extended configuration PSLV-XL. Just as the orbiter mission around the moon, Chandrayaan-1, which was also launched by the first version of PSLV-XL in 2008, this too will be a mission aimed at only an on-orbit study of the red planet. But unlike the circular orbit of Chandrayaan-1 at an altitude of 100-200 kilometres, the Mars orbiter will locate itself in a highly elliptical 377 km x 80,000 km orbit around the planet.

Although the mission to Mars had been mentioned off and on by ISRO officials since 2007 and studies within ISRO for such a possible mission seem to have begun around that time, it had never been projected as a definitive planetary mission in the short term. “It is expected to take at least four years to complete the initial studies,” G. Madhavan Nair, the then ISRO Chairman, had been quoted in 2007 by a newspaper. But we now have a full-fledged mission to Mars within just a year after the initial study phase.

The official announcement for this mission was made on August 15, 2012, by Prime Minister Manmohan Singh in his Independence Day speech. In fact, according to K. Radhakrishnan, the present ISRO Chairman, the financial sanction for the project came only in July 2012. Thus, despite the technological complexity of any mission to Mars, ISRO has got the Mars Orbiter Mission (MOM), as it has been named notwithstanding the name Mangalyaan gaining currency in the popular media, launch-ready in perhaps record time among all the ISRO missions so far.

“In July we had the sanction, on August 15 we had the announcement and today we are ready. That is an achievement,” said Radhakrishnan during an interview in September to this correspondent. He said the moon, the Mars and the sun were always in the scheme of things that the Advisory Committee for Space Sciences (ADCOS) had envisioned for ISRO. “On that basis,” Radhakrishnan added, “in August 2010 we appointed a study team with Adimurty, who is a mission man, as Chairman. He came up with a feasibility study in three months. Immediately, we energised the science community.” (V. Adimurty is Senior Adviser (Interplanetary Missions), ISRO, and Dean (R&D) at the Indian Institute of Space Science and Technology (IIST).)

However, this does not really explain the rush with which the mission has been pursued. The explanation may, however, be found in essentially the following two factors—one, setbacks to another planned mission, and, two, purely technical. If all had gone as ISRO had planned, the organisation would have been busy this year getting ready for the launch of the orbiter-rover-lander mission to the moon, Chandrayaan-2. In fact, all the scientific payloads for Chandrayaan-2 are ready. But this project has been delayed, and now postponed to 2017, for the following reasons. One, Russia, which was to provide the lander for the mission, has expressed its inability to be ready with the same even for a revised launch date in 2015 because of a complete review of its planetary mission programmes following the failure of its Phobos-Grunt mission to Mars in 2011. The other cause of delay is the failure to develop the indigenous Geosynchronous Satellite Launch Vehicle (GSLV), which was meant to carry the Chandrayaan-2 spacecraft.

So, with no other major space programme on the anvil, the ISRO may have taken a decision to go to Mars. The failure of China’s Mars mission, Yinghuo-1, in 2011 could be an added factor, which gives India an opportunity to be one up on China in space exploration. But a Mars launch in the immediate term came with severe time constraints on the available launch window.

As in the case of Chandrayaan-1, here too the launch vehicle will deliver the orbiter in a similar-sized elliptical 250 km x 23,500 km parking orbit around the earth. The apogee of the orbit will gradually be raised, again just as it was done for Chandrayaan-1, with five burns of the spacecraft’s main engine, the Liquid Apogee Motor (LAM), which would bring the spacecraft to the final earth-bound orbit of about 600 km x 2,00,000 km with an orbital period of 95.5 hours (see Table). From here the spacecraft will enter a Mars transfer trajectory (MTT) at an appropriate time so that Mars can be reached with minimum fuel consumption. The mission life is targeted for six months. However, there is a possibility of extending this lifetime.

But the mission to Mars is technologically a different ball game. The success rate of international missions to Mars is only 42 per cent. The crucial difference arises from the fact that with the fifth burn, Chandrayaan was within 500 km of its orbit around the earth, which could be traversed under 25 minutes. But here, given the much greater earth-Mars distance and the constantly changing relative positions in their respective orbits around the sun, even in the most efficient transfer trajectory, the spacecraft has to travel a distance of about 690 million km (Mkm) to reach Mars, which will take 300 days. This brings with it far greater challenges in propulsion, navigation and communication as compared to Chandrayaan-1. Though the lunar mission, too, was called a deep-space mission, this, one may say, is truly the first deep-space mission for ISRO.

Technology mission As stated in ISRO’s information brochure, considering the critical mission operations and stringent requirements on propulsion and other systems of the spacecraft, MOM is primarily a technology mission. “It has been configured to carry out observations of physical features of Mars and carry out limited study of Martian atmosphere [emphasis added],” says the brochure. Radhakrishnan said once the feasibility study was done, ISRO called for ideas from its scientific community and got about 30 ideas. From these, 11 experiments that were achievable and were of relevance were short-listed. This gave a maximum payload capacity of 22 kilograms. But considerations of power, weight and volume, as well as maturity, brought this down to about 15 kg and the payload list to five. “That is why people talk of 22 kg,” he points out and adds, “This is only a technology demonstrator. But when you go for the next mission you certainly need a scientific objective.”

One could, of course, argue that, given the complexity of a mission to Mars, and the high failure rate of past missions of other countries, ISRO could have waited until 2016, when probably even the GSLV would have been ready with its larger payload capacity. But that would mean technology demonstration itself would have to wait for another three years, as after a lunar mission this is the next most complex inter-planetary mission that one could take up. Further, Radhakrishnan said, the 2016 opportunity was not as energy efficient as a November 2013 launch. As a result, the launch of a Mars orbiter in 2016 using PSLV-XL was not feasible. “Moreover, ISRO has demonstrated in Chandrayaan-1 mission that an unmanned mission to a celestial body could be realised without major modifications to the existing configuration of PSLV-XL, which is a proven launcher. The MOM will place India among the select few countries which have achieved this rare feat.”

Accordingly, the main stated objectives of the mission are to develop the technologies required for design, planning, management and operations of an interplanetary mission, including:

Orbit manoeuvres to transfer the spacecraft from an earth-bound orbit into a heliocentric trajectory and finally enable its capture into a Martian orbit;

Development of force models and algorithms for orbit and attitude computations and analyses;

Deep-space communication and navigation in all phases;

Maintaining the spacecraft in all phases of the mission by meeting power, communications, thermal and payload operation requirements; and,

Incorporation of autonomous features to handle contingency situations.

The scientific objectives, given the five on-board experiments, include:

Exploration of the Martian surface by studying its features , morphology, topography and mineralogy and the Martian atmosphere (constituents such as methane and carbon dioxide) using indigenous scientific instruments; and

Study of the dynamics of the upper atmosphere of Mars, effects of solar wind and radiation and the escape of volatiles to space.

Given the limited launch capacity of the PSLV, and the limited objectives of the Mars mission, the mass of the orbiter satellite is constrained to be 1,350 kg, the configuration of which is a balanced mix of flight-proven Indian Remote-sensing Satellite (IRS), INSAT and Chandrayaan bus. Modifications in the bus configurations for MOM have been in the areas of communication, power, propulsion system and on-board autonomy. The 390-litre capacity propellant tank on board the spacecraft can carry a maximum of 852 kg of propellant, which is adequate for the mission’s propulsion requirements with sufficient margins. After accounting for the various structural elements, bus systems, power supply and other components, the available dry mass of 500 kg has, as mentioned earlier, constrained the instrumental payload to 15 kg only.

A primary consideration in all Mars missions is reaching the planet by expending the least amount of energy or fuel. This is achieved by sending the spacecraft along a trajectory called the Hohmann Transfer Orbit or Minimum Energy Transfer Orbit. From the final earth-bound orbit, the spacecraft will leave the earth in a hyperbolic trajectory in a direction tangential to the earth’s orbit. This is the MTT along which the spacecraft will escape from the earth’s sphere of influence (SOI) with a velocity equal to the earth’s orbital velocity plus the cumulative boost (Δv) of about 1.5 km/s given by the five LAM firings (880 m/s) and the sixth LAM firing into trans-Mars injection (640 m/s).

The earth’s SOI extends up to about 1 Mkm and that of Mars extends up to about 0.6 Mkm. In this trans-Mars trajectory, the spacecraft will be primarily under the influence of the sun; that is, from the geocentric phase it will now be in the heliocentric phase. It will take 10 months of journey in this phase before it enters the Mars’ orbit tangentially.

The possibility of encountering Mars at that exact moment of the spacecraft’s intersection with Mars’ orbit depends on the relative positions of the earth, Mars and the sun. When the configuration of these three bodies is such that they form an angle of approximately 44°, this becomes possible, and this occurs a few days before or after the time of closest approach of Mars to the earth, which distance is about 55 Mkm. Such a configuration recurs periodically at intervals of about 780 days (about 26 months). In the case of the earth-Mars system, minimum energy opportunities occur only if the spacecraft launch takes place in November 2013, January 2016 or May 2018.

The fact that the spacecraft has to travel a distance of 690 Mkm in the MTT, when the distance of closest earth-Mars approach is only 55 Mkm, needs explanation. The optimum distance between the two planets for trans-Mars injection is, as mentioned earlier, when earth-Mars-sun make 44°. Further, a straight line may not be the most energy-efficient way to reach Mars. This is because the straight line translates into a huge, inefficient orbit around the sun. To put the spacecraft in such an extreme solar orbit would require enormous amounts of energy and fuel. The minimum energy transfer path is a much longer one (See Figure).

The propulsion requirements and associated challenges for the minimum energy transfer to Mars and subsequent capture include orbit raising, trans-Mars injection, three mid-course corrections and finally arresting the spacecraft for capture. Once injected into the MTT, the mission sequence requires three mid-course corrections to be made to the trajectory and the last correction to be carried out about 15 days before the spacecraft’s capture into the Martian orbit so that an accuracy of ±50 km is achieved in the rendezvous. The final Mars Orbit Injection (MOI) is achieved by a braking or de-boost manoeuvre of about 1.1 km (a negative Δv) at the periapsis (closest approach to Mars) of the hyperbolic MTT. This, in fact, is the largest incremental (albeit negative) velocity, which means the MOI will demand the longest retro firing of LAM and it will have to deliver after lying idle for 300 days. Together, with the incremental velocity of 1.5 km/s given up to trans-Mars injection, the magnitude of the cumulative incremental velocity required of LAM is thus 2.6 km/s. The spacecraft will enter the Martian orbit in September 2014. The size of the spacecraft’s Martian orbit will be, as mentioned earlier, 377 km x 80,000 km and its orbital period will be 3.66 days.

The LAM that will be used in this mission, both for orbit raising and MOI, is the same 440 Newton thruster that is used in geostationary satellite launches by ISRO. The first operation of orbit raisings is limited to the first one week. But MOI is only after 300 plus days of MTT. Once the valves get wetted by the propellant, they can swell a little bit and the performance will come down. They may also begin to leak. So the strategy that has been adopted is to close this path after orbit raisings, isolate the engine by operating pyro valves and open additional flow lines and valves when restarting the engine 10 months later to take care of the problem. The engine has been tested for its performance for a given number of days after use.

“In Chandrayaan-1 the engine was qualified for 30 days. Now we are talking of 300 days,” pointed out Radhakrishnan. “The performance deterioration in propulsion efficiency, which means specific impulse, is about 2 per cent. So we know it a priori . When you finally want to calculate how much the engine should fire to impart a given retro boost to capture a Martian orbit, this information is important but not very crucial at the same time because it is done in the closed loop mode. It will be looking at the accelerometers and then adjusting automatically. Also, the trans-Martian injection being very complex, you may miss this capture. We have kept fuel for one more try,” he added.

The PSLV-XL will deliver the spacecraft in an elliptical orbit of 250 km x 23,500 km. This orbit, by design, will have an inclination of 17° and an argument of perigee (AOP) of about 280°. (AOP is the angle between the spacecraft orbit’s perigee, the point of closest approach from the earth, and the orbit’s ascending node, the point where the body crosses the plane of the Equator from south to north. The angle is measured in the orbital plane and in the direction of motion. Essentially, it is the relative orientation of the spacecraft’s elliptical orbit with respect to the equatorial plane.) Unlike normal launches where the AOP is 180°, this highly unusual orientation is dictated by the following consideration pertaining to the Mars mission.From the perspective of the mission plan and objectives, the desired inclination of the spacecraft’s Martian orbit is about 30° with respect to the Martian equator. To reach that along the minimum energy path, the spacecraft needs to be launched in its parking orbit around the earth with the correct AOP. If the spacecraft’s orbit around the earth does not have this correct orientation at launch, the desired Martian orbit can still be achieved, but at the expense of more energy, pointed out Radhakrishnan. Since every day the relative positions of the planets are changing, depending on the day and time of the launch, the argument of perigee will also change. “For the mission management it is going to be a challenge,” he said. The mission plan has, therefore, a catalogue of programmes to steer the launch profile accordingly. For the November 5 launch, the AOP will be 282°.

Because of the considerations of a minimum energy transfer into the Martian orbit, the date for trans-Mars injection from the spacecraft’s earth-bound orbit is fixed. This date is November 27 and it will get out of the earth’s SOI on November 30. The launch window available for the mission was October 21 to November 19. The spacecraft will, therefore, have to make several earth-bound orbits before its injection into the MTT. Therefore, the spacecraft will pass through the high-radiation environment of high-energy electrons in the Van Allen belts surrounding the earth, twice for each orbit. And, the earlier the launch, the more will be the number of such orbits and transits through the Van Allen belts. For the earliest possible launch date, ISRO had calculated that it would make about 60 such passes.

Accordingly, the spacecraft’s components have been designed for the maximum cumulative radiation dose expected on each component. For a maximum of 60 passes through the radiation belts, it has been worked out to be 6 krad. Thus, radiation hardening or radiation shielding has been provided to the components such that they can withstand 12 krad (margin of a factor of 2) with a 22 AWG aluminium shielding. It should, however, be mentioned here that these values are of the same order as the total radiation dose received by electronic components (with a standard radiation shielding of 4-5 mm aluminium) in low-earth orbits during typical mission lives of five years but an order of magnitude less than the total dose received in geostationary orbits over a 15-year-mission life.

Originally, the launch date had been fixed as October 28. But because of adverse weather conditions over the Bay of Bengal, it was postponed by a week to November 5. This would, of course, mean that the spacecraft will have to make fewer transits through the Van Allen belts and, correspondingly, the radiation dose will be lower.

According to Koteswara Rao, Scientific Secretary, ISRO, the number of earth-bound orbits will now be only 19, which means 38 passes through the radiation belts. The flip side of this is that if for any reason, including adverse weather conditions, the launch needs to be postponed, there is an outside chance that ISRO might be forced to miss the launch window altogether and abandon the launch. But Radhakrishnan is quick to allay any apprehension on this count.

Compared to the previous PSLV missions, here there is a long coasting phase between the third stage (PS3) burn-out and the fourth stage (PS4) ignition. This is to achieve the correct argument of perigee at the time of the spacecraft’s injection from PS4. The coasting phase is increased by about 20 minutes. “This is the largest coasting phase that we are going to have,” Radhakrishnan said. Owing to the long coasting phase, ground stations are required in the Pacific to monitor the PS4 ignition, its burn-out and the spacecraft injection. For this, two ship-borne terminals (Nalanda and Yamuna) have been placed in the Pacific, at about 3,000 nautical miles from Fiji. (It is the delay in one of these ships reaching its destination due to bad weather that led to the postponement of the October 28 launch.) In these 20 minutes there is also a phase where, because of the position, there will be no telemetry in real time for about a few minutes, but the ship-borne stations will take charge. Also, there will be some cooling, and mechanisms such as the solar panel deployment, soon after the spacecraft’s injection, have to operate at negative temperatures of -20 °Celsius. The mechanisms themselves have been tested for -60 °C.

Communication challenges Besides, the radio-silence period during the launch and the communication challenges at crucial phases of the mission arise from the distances involved. For instance, at the time of capture into the Martian orbit, the communication range or the line of sight distance from the earth is about 230 Mkm. Once in the Martian orbit, given the fact that the distance between the earth and Mars vary, and also because the spacecraft orbit is highly elliptical, the communication range varies from a minimum of 60 Mkm to a maximum of 380 Mkm.

Correspondingly, time delays in two-way communication will vary from 6.8 min to 43 min. This is the time that will be taken for a command from the earth to reach the spacecraft and receive the response. A command cannot be given and corrected in real time, so some storage is required. What has been done, therefore, is to provide the satellite with an on-board three-level in-built autonomous capability, which is a new challenge for ISRO. These include autonomously switching over from primary to redundant systems, self-generation of appropriate commands when a certain expected command is not received from the earth, and bringing the spacecraft to a “safe mode” when it is in a non-normal condition and enable intervention from the earth (see interview with K. Radhakrishnan on page 11).

Challenges from power requirements also arise because the orbit of Mars is farther from the sun. The average solar flux at Mars is 598 W/m (watts per square metre), which is 42 per cent of what the earth receives in its orbit. Also, because of the highly eccentric orbit, the solar flux varies by ± 19 per cent during a Martian year compared with 3.5 per cent on the earth. To compensate for this lower solar irradiance, the orbiter spacecraft is equipped with three solar panels of size 1.8 m x 1.4 m generating a total of 840 watts in the Martian orbit. A single 36 Ah (ampere hour) lithium ion battery will provide power during eclipse phases during the geocentric phase and in the Martian orbit.

Scientific payload Coming to the scientific payload of a limited mass of 15 kg that MOM will carry, it comprises three instrument packages with a total of five instruments. Notwithstanding the modest scientific objectives of the mission, ISRO scientists, too, have claimed that they conceptualised these in such a way that they included experiments that had not been carried out before by other missions. In particular, the methane sensor has not been carried by other nations in the past and the National Aeronautics and Space Administration’s (NASA) MAVEN, which will travel to Mars almost simultaneously with MOM, too, does not carry one. Methane would provide the evidence for biological processes on the planet.

Recent reports, based on data collected by NASA’s Mars rover Curiosity, that there is no evidence of methane on Mars does not mean that there is no methane on the planet because Curiosity is only exploring one region of the planet. But, more pertinently, some theoretical studies have argued about the impossibility of detecting methane even if it had been there at some point of time. But it should not come as a surprise if MOM does return evidence for methane. Besides there are so many unanswered questions about this mysterious planet that ISRO’s mission has the potential to return interesting results. But most significantly, given the complexity of the mission, ISRO would have demonstrated its technological capability to conduct a deep-space mission and the mission can be termed successful even if it just succeeds in delivering the spacecraft into the Martian orbit.