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.
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.
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.
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.
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.
The November 5 launch, had the AOP of 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.
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.
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.
The number of earth-bound orbits will now be only 19, which means 38 passes through the radiation belts.
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 ISRO has ever had.
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.
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
The scientific payload has limited mass of 15 kg that MOM carried three instrument packages with a total of five instruments.
MOM includes 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.
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 it should not come as a surprise if MOM does return evidence for methane.
Mars has two satellites and one of them called Phobos, is likely to be in the vicinity of the Mars orbiter and we will study Phobos from our orbiter.
There is a comet likely to pass by the side of Mars at a distance of 50,000 km. NASA [National Aeronautics and Space Administration] scientists have forecast this. If this should happen, it will be another opportunity to observe the comet.
- The success rate of international missions to Mars is only 42 per cent.
Russia's Phobos Grunt Mission to Mars was a failue in 2011.
China's Mars mMission,Yinghuo-1 in 2011 was also a failure.
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