by Gurbir Singh
Orbiter
The orbiter is designed to operate for a year in a 100-km lunar orbit. It has five instruments, all of which have been produced by ISRO. TMC-2 will continue the work of TMC-1 and fill in the gaps from the Chandrayaan-1 mission to provide a full 3-D map of the lunar surface. The Imaging Infra-Red Spectrometer will scan the lunar surface using a wider wavelength range (extended from 3 microns in Chandrayaan-1 to 5 microns in Chandrayaan-2) to once again map the distribution of minerals, water molecules and hydroxyl present on the lunar surface.[866]
A SAR operating in a dual frequency is expected to detect further evidence in support of water ice located in the first few tens of metres below the lunar surface and in the regions permanently in shadow. CHACE-2 will look at the composition of the tenuous lunar atmosphere during descent. This is similar to the instrument carried aboard the MIP that detached from Chandrayaan-1and captured data as it descended to its lunar surface impact.
Data from a joint instrument, Chandrayaan-2 Large Area Soft X-ray Spectrometer and Solar X-ray Monitor, will be used to map the distribution and relative concentration of elements, such as aluminium, silicon, calcium and magnesium, on the lunar surface. Combining this data with that collected by C1XS on Chandrayaan-1 will continue building a complete picture of the chemical constituents of lunar surface material.
Lander
The lander will be ISRO’s first attempt to soft land on an extra-terrestrial body. It will be equipped with a high-resolution descent camera, a throttleable liquid fuel main engine with attitude control thrusters and a high degree of automation to ensure a soft landing in an obstacle-free environment. Given the time delay between Moon-Earth communication, the lander is designed with built-in autonomy to land using real-time data from the altimeter and descent camera without intervention from Earth. Following separation, an initial de-orbit burn will bring the lander down from 100 km to a 15-km altitude. A further series of engine burns will bring the lander above the landing site with zero-horizontal velocity. From this point, the lander will descend vertically to the surface through another series of engine burns until it is at an altitude of 4 m above the surface. At 4 m, the engine will cut-off, and the lander will free fall to soft landing. In the 1/6 lunar gravity this is not a hard landing as it would be from 4 m on Earth.[867] Since the Moon is devoid of any substantial atmosphere, parachutes do not work. The engine has been subjected to high-altitude tests lasting over 500 seconds. Following a successful landing, the rover will exit from the lander and explore the immediate area of the landing zone. The lander will remain stationary for the duration of the two-week mission.
The lander’s onboard science payload consists of a seismometer for studying moonquakes, a thermal probe (thermometer) to measure the temperature at the landing site over the duration of the mission and two experiments (Langmuir Probe and Radio Occultation) to detect electrons emanating from a few centimetres above the lunar surface. On Earth, the high-energy radiation from the Sun is deflected by the Earth’s magnetic field or absorbed by its atmosphere. In the absence of both on the Moon, this high-energy radiation from the Sun reaches its surface, and the interaction of the radiation with particles of dust causes atoms to release electrons resulting in a thin layer of plasma over the sunlit side of the Moon.
Two experiments on the lander will measure the Total Electron Count (TEC) and its variation in density and temperature. The Langmuir probe on the lander will directly collect these data and transmit it from the lander to the orbiter. The radio communication link itself can also be used to infer TEC. The Orbiter will orbit the Moon once every hour. As it rises and sets over the horizon, the radio signals will travel through this layer of plasma. The variation in the intensity of the signals received by the orbiter when it is close to the lunar horizon will help to measure the density of this plasma.[868]
Rover
At 20 kg, the rover has a limited payload. The onboard cameras, antennae and communication systems are powered by a battery, which is charged by a single solar panel. It communicates its data to Earth via the orbiter a 100 km above and not the lander a few metres away. It has six wheels controlled by 10 motors guided by a pair of stereoscopic cameras providing a 3D view of its environment. To test the traction of its wheels and driving performance on Earth, a helium balloon is attached to the rover to simulate the reduced lunar gravity. The rover’s science payload consists of two instruments. A Laser Induced Breakdown Spectroscope will look at the same plasma that the Langmuir probe on the lander will examine.
Figure 15‑6 Chandrayaan-2 Rover. Credit ISRO
The second instrument, Alpha Particle Induced X-ray Spectroscope, will also identify chemical compositions but use a radioactive sample, typically Curium 244 that emits Alpha particles, X-rays and photons. The Alpha particles emitted are deflected by the atomic nuclei in the lunar dust. By measuring this deflection, the mass of the deflecting nuclei and its chemical composition can be determined. These Alpha Particle Induced X-Ray Spectroscopes are small, lightweight instruments that have been used to collect data on the Moon, Mars and a comet.
Both the rover and the lander will communicate with the Earth via the orbiter, which will be contactable above the horizon for about 30 minutes during each orbit. Chandrayaan-2 is scheduled for a launch sometime in early 2018.
Why India Went to Mars
India went to Mars in 2013 because both Japan and China had tried and failed. If India could succeed where Japan and China had not, it would be next after the US, USSR/Russia and ESA to orbit Mars. India had published its intensions for the exploration of Mars, the solar system and human spaceflight but the decision to go to Mars in 2013 was in response to external events. While India’s journey to the Moon in 2008 was planned and matured over time, the mission to Mars was more opportunistic and finalised in haste. India always had plans for a mission to Mars, the decision to go in 2013 was in response to the failure of China’s mission to Mars in 2011. Publicly, ISRO had declared that there “is no race with anybody” but later conceded that the decision to go to Mars was taken following the failure of the Chinese mission.[869]
Orbital mechanics dictate that the opportunity for a journey to Mars comes around only once every two years. After 2011, the next opportunity would be the autumn of 2013. Neither Japan or China could attempt a Mars mission for 2013 but India could. Japan had unsuccessfully attempted to fly its spacecraft Naomi to Mars in 1999, and although eventually, it got to Mars in 2003, Nozomi failed to enter the Martian orbit and flew past instead. Exactly when ISRO began seriously contemplating a mission to Mars is unclear. Ajey Lele, a Senior Fellow at the Institute for Defence Studies and Analyses in New Delhi, considers that this could have been an outcome of the 2006 meeting of leading Indian scientists to discuss the future of India’s space programme.[870] That was the time when India’s first mission to the Moon was taking shape. Through this scientific collaboration, India wanted the US to start to ease its sanctions on high technology. In March 2006, India and the US signed a deal that paved the way for scientific instruments from the US to be installed on Chandrayaan-1. In media reports of that agreement, the NASA chief stated, “We are also looking at sharing of data for Earth sciences and Earth resources and broader scientific cooperation in exploring beyond the Earth.”[871]
By November 2008, ISRO’s first Moon mission Chandrayaan-1 had been in lunar orbit for two weeks and had successfully deployed its MIP to the surface. Riding high on this success, ISRO’s then chairman Madhavan Nair publicly announced that “the study for Mars exploration has already started.”[872] During the 2010 Indian Science Congress, Nair confirmed India’s plans for a Mars mission to a visiting journalist from the UK.[873] During the same Congress, Abdul Kalam, who had been instrumental in incorporating the MIP on Chandrayaan-1, raised the idea of Mars as a mission objective.[874] By the time the Chandrayaan-1 mission came to a premature end in 2009, India may not have had a specific timeline, but it did have a firm conviction that Mars was the next destination
.
Prior to the failure of the Chinese space mission to Mars in November 2011, China had chalked up a series of successes in space. It successfully completed three human spaceflights, including one with three astronauts; delivered a module for its space station to orbit and placed its Chang’e 1 spacecraft in lunar orbit in October 2007, a year before Chandrayaan-1. Although its first Mars mission failed in 2011, China was not culpable for the failure. Its spacecraft, Yinghuo-1, was designed to enter Martian orbit after hitching a ride on an ambitious Russian space mission called Phobos-Grunt. This Russian spacecraft was to land on the Martian moon Phobos, collect a sample of material from the surface of Phobos and return it to Earth. Russia signed an agreement with China that Yinghuo-1 would be launched on the same rocket as Phobos-Grunt. However, neither Phobos-Grunt nor Yinghuo-1 left Earth orbit following a launch vehicle failure. The fourth stage of the launch vehicle, along with both spacecraft, disintegrated during Earth re-entry several weeks after launch.[875] Following this failure in 2011, China could not have readied another Mars mission for November 2013, but India could.
Further, while India’s PSLV-XL launcher could just about get a spacecraft to Mars in 2013, for the next two opportunities in 2016 and 2018, an additional velocity of at least 380 m/s would be required compared to 2013. That was just outside the reach of the PSLV-XL.[876] So, India had to go in 2013 or wait until 2020. India saw 2013 as the best opportunity to get to Mars before China and took it.
From Sriharikota to Mars
India’s first Mars mission called Mars Orbiter Mission (MOM) or Mangalyaan (in Sanskrit, Mangala = Mars and Yaan = vehicle) was launched at 14:38 IST on 5 November 2013, not towards Mars, but a 50-km cubic volume of space near where Mars would be at 07:27 IST on 24 September 2014.[877] Not only was Mars a target moving around the Sun at 24 km/s, but MOM would be launched from Earth moving at 30 km/s around the Sun. In addition, MOM’s launch pad at Sriharikota) was moving at about 0.5 km/s (Earth’s rotation). MOM would have to hit that small target after a journey of 667 million kilometres 300 days after launch. Timing was critical. If MOM arrived a little earlier or later, it would either crash into Mars or pass it by. Once it arrived at the precise time and place, there was one more critical step; MOM had to fire its engine to slow down to enter orbit around Mars, a key manoeuvre that ISRO had not conducted before.
Figure 15‑7 Mars Orbiter Mission and Its Science Payload. Credit Adapted from ISRO
On 24 September, everything went to plan. MOM arrived at precisely the right spot, its engine fired, and it entered the planned highly elliptical orbit of 421 by 77,000 km. With that, India became the fourth space agency to arrive at Mars, and it had done it on its first attempt. The Indian Prime Minister was present at Sriharikota watching MOM arrive and embraced the ISRO chairman on live TV in celebration. In some ways, India’s arrival at Mars was tantamount to a rebirth of the Indian Space Programme. With the widespread national and international press coverage that followed, many within India, as well as outside, realised for the first time that India had an active and effective space programme.
As part of the planning process for MOM, ISRO had studied historical Mars missions, particularly the 29 missions that had failed of the total 55 until then. Each of the 29 failures were “carefully addressed during the preparation for MOM.”[878] ISRO had been designing, building, launching and operating satellites for over three decades. On the face of it, MOM was yet another mission to put a satellite in orbit, further away but conceptually the same. In practice, building and sending a satellite to Mars introduced a host of new engineering challenges.
Although launched on 5 November, MOM remained in Earth orbit until 1 December before embarking on a nine-month journey to Mars. During this extended period, MOM was exposed to potential harm from the intense radiation environment of space as it crossed the radiation fields around the Earth (Van Allen belt) 39 times. Japan’s attempt to reach Mars in 1999 with its Nozomi spacecraft had failed in part because of radiation damage to some of the control systems.[879] MOM’s radiation shielding was designed for protection against an accumulated dose of 9 krads for packages inside the cuboid and 15 krads for packages mounted on the outside.[880]
The vast Earth-Mars distance generated two additional challenges. While ISRO was familiar with operating satellites in Earth orbit at 36,000 km, Mars-Earth distance varied between about 50 million km and 400 million km. The round-trip communication interval between Earth and MOM could take up to 42 minutes (21 minutes each way at 400 million km or 5 minutes at 50 million km). ISRO not only had to develop the infrastructure to communicate over this vast distance, but it also had to build in additional level of autonomy, given the time lag, MOM needed to “think and do” by itself.[881]
The solar panel was MOM’s only source of electrical power. Mars orbits the Sun at a distance that is about 1.5 times that of the Earth, where the intensity of the Sun is only 42% compared to that at the Earth-Sun distance.[882] The battery and efficiency of its solar panel had to be designed to operate in that context. While the battery used was the same as in Chandrayaan-1, a single 36 Ah lithium-ion battery, MOM was fitted with a three-panel solar array (each panel 1.8 m by 1.4 m) generating 840 W instead of a single panel solar array required for Chandrayaan-1 which had generated 750 W in Lunar orbit.
To catch the 2013 Earth-to-Mars launch window (21 October – 19 November), ISRO worked to a hectic schedule with no contingency. The launch window is the period when a journey from Earth to Mars can take place with minimum energy. This window appears once every 780 days when the orbits of Mars and Earth line up so that the angle between Earth, Mars and the Sun is 40°. This configuration is necessary for a Hohmann Transfer Orbit, and all spacecraft that have ever gone to Mars from Earth have used it. MOM’s journey from Earth to Mars had three phases, Geocentric, Heliocentric and Martian, determined by which body is producing the dominant gravitational influence, the Earth, Sun or Mars. The Geocentric phase, where MOM was in the Earth’s gravitational field, lasted until it had travelled 918,347 km from the Earth. It entered the Martian phase once it got closer than 573,473 km to Mars. In between, MOM was predominantly under the Sun’s gravitational force, and thus in the heliocentric phase. The MOM spacecraft design was a modified version of that used for Chandrayaan-1 which in turn was based on the IRS/INSAT bus. ISRO chose a design with proven reliability given the tight timeline. On 21 September 2012, just a month after the formal announcement, ISAC in Bangalore took delivery of the structure of the MOM spacecraft built by Hindustan Aeronautics Limited (HAL). MOM’s sub-systems, some of which were already being developed, were then transferred to the structure, and the spacecraft began to take shape. Installation of science instruments started in April 2013, testing from August onwards, and on 3 October 2013, the spacecraft was delivered to Sriharikota to begin preparation for launch.
The PSLV-C25 launch from the FLP was initially scheduled for 28 October but then pushed to 5 November 2013. This was the first time Sriharikota conducted a launch in the month of November is avoided as the rainy season typically spans October and November. MOM was launched by the most powerful variant of ISRO’s reliable and in practice only operational launch vehicle, the PSLV-XL. Despite this, there was a velocity shortfall of around 1.5 km/s. This shortcoming in PSLV-XL’s power dictated the overall mission profile, the smaller science payload of 15 kg, the six Earth orbits (known as Gravity Assist) and the highly elliptical Martian orbit of 418 by 76,872 km when it got there. ISRO turned this highly elliptical orbit into an opportunity by configuring experiments that exploited such an orbit.[883] During the same launch window, NASA launched its spacecraft called Maven to Mars. With a launch vehicle that can put seven tonnes into GTO compared to PSLV’s 1.3 tonnes, Maven was launched two weeks after MOM and was on its way to Mars an hour after launch compared to MOM’s three weeks and arrived at Mars three days before MOM.
PSLV’s unusual destination called for a unique launch trajectory. Typically, a PSLV delivers its
payload to Earth orbit in about twenty minutes, from where the payload proceeds under its own power to the final destination. The fourth stage ignition and payload separation is monitored from ground stations in Port Blair in Brunei, Biak in Indonesia and Canberra in Australia. For MOM’s eventual trajectory to Mars, an interval of 24 minutes was required between the end of stage three and the ignition of stage four. By that time, the ground track of the fourth stage would have travelled beyond Australia. The nine-minute fourth stage burn would take place over the south Pacific, well beyond the reach of any of ISRO’s or its international partners’ ground stations.[884] To ensure that the telemetry of the fourth stage burn and MOM separation was recorded, ISRO commissioned two ships from the Shipping Corporation of India, Nalanda with a 4.6 m antenna and Yamuna with a 1.8 m antenna. Poor weather conditions at sea delayed these ships from getting to the operational sites as scheduled, resulting in the launch date moving from 28 October to 5 November. They arrived on location about three days before launch and positioned themselves approximately 2,000 km apart near Fiji.[885] One would monitor the fourth stage ignition and burn and the other the release of MOM from the spent fourth stage.