The human spirit of adventure and exploration has been rekindled by the advent of the space era, which has opened up exciting possibilities of exploring our solar system by sending spacecraft to different planets, their moons, asteroids and even comets. India entered the space age in a modest way almost five decades ago with the launch of a sounding rocket (from Thumba Equatorial Rocket launching Station) to investigate the behaviour of the upper atmosphere and ionosphere over the geomagnetic equator. Since then, the programme has evolved along several dimensions, thanks to the vision of our greatest space pioneers, Dr Vikram Sarabhai and Dr Satish Dhawan. Today, the country has developed capabilities for building sophisticated remote sensing satellites for Earth-resource survey as well as communication and broadcasting satellites.
Consistent with policies to attain autonomy in accessing space, India developed and operationalised Polar Satellite launch and Geo-synchronous Satellite launch Vehicles for meeting a substantial fraction of our country’s launch needs. In planning space activities, science has also received special attention due to the exciting possibilities of using satellites for astronomical, geophysical as well as solar and planetary studies. Spurred by the success of the Chandrayaan-I mission, ISRO decided to be more ambitious by planning a mission to Mars called Mangalyaan.
Planetary exploration is viewed today in the context of understanding the origin and evolution of the solar system, and also to develop future strategy for human visits and habitation. Since 1960, there have been 42 missions to Mars undertaken by US, USSR, Russia/China, Europe, Japan, and now India. These missions have had a mixed success rate, with India being the only nation to succeed in the first attempt. In spite of such a large number of missions, many scientific questions on Mars remain unanswered. For example, is there unambiguous evidence that Mars had a warmer and wetter pastIJ Such conditions could be important for life to evolve. Understanding the nature of Martian climate change, including the behaviour of its polar caps, may yield better insights into climate change on Earth. Further, a study of Martian geology is important to inventory the planet’s resources. Against this backdrop, India’s Mangalyaan mission seeks to throw more light on some of these unanswered scientific questions even as we develop mastery over complex aspects of a planetary mission management.
Mangalyaan had a mass of 1,337 kg, including a propellant mass of 852 kg at the time of launch. Besides the normal elements of the spacecraft that included structural, mechanical, propulsion, thermal, power, attitude and orbit control, as well as telemetry tracking and command and radio frequency communication system, it also carried five scientific payloads.
It is difficult to fully appreciate the challenges ISRO faced in accomplishing even the primary goal of orbiting the spacecraft around Mars. To begin with, PSlV is among the least powerful launch vehicles used for a Mars mission of this sophistication. Directly raising the orbit of the spacecraft from 1 AU (1 Astronomical Unit is the distance between Earth and Sun, which is about 150 million km) to 1.5 AU (the distance between the Sun and Mars) would have put an impracticable demand on the fuel available for propulsion. It is here that our scientists used techniques which were highly innovative, of which three examples are particularly important.
First, in order to reach Mars using minimal amount of fuel, the best moment to escape Earth’s gravity happens to be when the angle between Earth and Mars (when viewed from the Sun) is 44.6 degrees. This geometry occurs just once in 26 months, which restricted the launch window. Second, in order to ensure that the spacecraft escaped from Earth’s orbit on the target date based on this optimal angle, it had to be released at a particular point that also depends on the time and date of the launch. To reach this so-called argument of perigee, an enhanced version of the PSlV launch vehicle (PSlV-Xl) was employed, whose coast phase was stretched from the conventional duration of 680 seconds to 1,700 seconds. Third, by successively phasing the orbit of the spacecraft around Earth in five steps, the velocity was boosted by several hundred metres per second using minimal fuel. With a final propulsive boost to escape Earth’s orbit, the spacecraft was put on the long interplanetary course needed to rendezvous with Mars after 300 days of travel.
The spacecraft approached Mars in a hyperbolic trajectory, with a velocity of 6.5 km/s with respect to the planet. Since the escape velocity for Mars is just 5.8 km/s, the last major manoeuvre was to retard the spacecraft by about 1.8 km/s. This reduction would enable the spacecraft to enter an elliptical orbit around Mars (80,000 km in Apoapsis and 500 km in Periapsis), which scientists endorsed for its uniqueness for observing the planet comprehensively during its mission life. Minor deviations from this target could have caused the spacecraft to either evade capture by Mars or crash onto its surface.
It is interesting to get a flavour of the perfection and precision demanded by these manoeuvres. In the initial five-phase orbital manoeuvres around Earth, the calculated velocity gain was 875 m/s, whereas the actual value realised was 874 m/s. This close match was achieved thanks to the accuracy of the ISRO designed and built ceramic servo accelerometer. This sensor is sensitive to changes in acceleration at the level of one millionth of the gravity force we experience on Earth. This accelerometer was also used in several subsequent trajectory manoeuvres, thereby enabling accurate velocity computation and position. As the spacecraft left Earth, the accuracy called for in terms of the overall attitude (orientation) targeting requirement to reach Mars was of the order of 0.01 arc seconds. In simpler terms, this is equivalent to shooting a 1 cm diameter coin placed at a distance of 200 km! Further, a propulsive error of a mere 1m/s in the velocity given to the spacecraft by the propulsion system leaving for Mars can generate as much as 200,000 km error in the position when the spacecraft approaches Mars. Closer to Mars, a delay of 30 seconds in initiating the retarding burn would have resulted in a Periapsis of 3,63,083 km (the realised value was of 480 km was close to the target).
Coming to another dimension of complexity, the mission by its very nature had to make use of a significant level of autonomy in spacecraft operations. This is because of the unacceptable transit time of electromagnetic signals between the spacecraft and Earth, making it impossible to deal with contingencies during critical activities using real time ground interventions. For example, when the spacecraft reaches Mars around insertion time, the one-way time signal delay between ground and the spacecraft is about 12.5 minutes. So right from start, the spacecraft design teams had envisaged a well-planned set of onboard features to deal with contingencies. It is to the credit of the design team that the various features envisaged and incorporated have been found to be adequate as validated by the successful handling of the activities up to Mars Orbit Insertion without any problem.
On the scientific side, of these five instruments, two are providing information about the atmosphere of Mars, the next two about the Martian surface, and the fifth measures the particle environment in the Martian exosphere. The Mars Colour Camera (MCC) is an electro-optical sensor imaging the surface of Mars in three colours, with varying spatial resolution between 25 m-3 km. The Thermal Infrared Spectrometer (TIS) observes heat emission from Mars’ surface to detect its temperature and hot-spot regions or hydrothermal vents. TIS measures emitted infrared radiation from the Martian environment in the 7-13 m region of electromagnetic spectrum using a device called micro bolometer. The Methane Sensor for Mars (MSM) is a differential radiometer that can measure columnar methane in the Martian atmosphere at sensitivity of several parts per billion (ppb) levels. The possible detection of methane in the Martian atmosphere will provide clues about the presence of life on Mars. The lyman Alpha Photometer (lAP) measures the relative abundance of deuterium (D) and hydrogen (H) from lyman Alpha emissions in the upper atmosphere of Mars, allowing us to understand the water loss process from Mars’ surface through the atmosphere. Finally, the Mars Exospheric Neutral Composition Analyzer (MENCA) is a quadruple mass spectrometer covering the mass range of 1-300 atomic mass units (amu), with a mass resolution of 0.5 amu. MENCA provides in-situ measurement of the neutral composition and density distribution of Martian exosphere (the outermost reaches of the atmosphere). All five instruments will continue to make extensive measurements during the expected mission time of six months. MCC, MSM and TIS also provide complementary information to interpret data, eg MCC is used for dust optical thickness estimation to correct for atmospheric scattering in MSM data, for accurate estimation of methane. Similarly, TIS gives information about surface temperature to analyse MSM data.
Even though India’s space endeavour has focused primarily on social and developmental applications, the quest for scientific investigations in astronomy, geosciences and planetary science have never been lost sight of. The first step of going to the Moon and now to Mars has established our credentials not only to undertake even more advanced planetary missions by ourselves, but more importantly to qualify India as being an active partner in future major international missions. Further, studies of the science of space are usually fascinating to the young minds and thus encourage them to pursue a career in science, a unique investment for the future. Another consideration relates to the exacting demands on technology, which most science missions call for, a strategy to push the levels of technology that can be used to sophisticate the future application missions.
The then Prime Minister Atal Bihari Vajpayee declared from the ramparts of the Red Fort on August 15, 2003, that India had decided to enter the planetary era and as the first step, his Government had decided to undertake a mission to the Moon, called Chandrayaan-I. Even though we had at that time suggested to the Prime Minister to designate the mission as Somyaan, the Prime Minister in his wisdom considered it more appropriate to call it Chandrayaan and equally importantly, to treat this mission as the first among a series of future planetary endeavours — what an extraordinary vision!
On September 24, 2014, the day of reckoning when the Mangalyaan was to be inserted into the orbit of Mars, our present Prime Minister Narendra Modi decided to be with the scientists at the ISRO Control Centre in Bangalore, unmindful of the success or failure of the penultimate phase of the mission. What a wonderful gesture; we were all touched! Further our PM went on to say that the success of the space programme was a shining example of what we can achieve as a nation. It was a clear signal from the Prime Minister to our countrymen to join his incessant drive to set higher benchmarks in every sphere of national endeavour through excellence and perfection.
The Mangalyaan saga is sure to be etched in golden letters in the annals of Indian space history.
