An afterthought

Published : Dec 19, 2008 00:00 IST

The Indian flag painted on the Moon Impact Probe.-ISRO

The Indian flag painted on the Moon Impact Probe.-ISRO

The Moon Impact Probe was never a part of Chandrayaan-1s original configuration.

THE primary objective of the Moon Impact Probe (MIP), according to the Indian Space Research Organisation (ISRO), was to demonstrate the technologies required for landing a probe at the desired location on the moon. It was also intended to qualify some of the technologies related to future soft landing missions and to carry out a scientific exploration of the moon at close distances. In contrast to the other four Indian experiments, which were well conceived and evolved over time, the MIP, it is argued, was not the optimal experiment to achieve the above.

The MIP actually was never part of Chandrayaan-1s original configuration, which included payloads from abroad in response to ISROs announcement of opportunity (AO) for proposals from elsewhere. This clearly indicates its lesser importance. Its inclusion probably became imperative because it was mooted by former President A.P.J. Abdul Kalam. This was subsequently endorsed (uncritically though) in November 2004 by ISRO Chairman G. Madhavan Nair at the International Conference on the Exploration and Utilisation of the Moon in Udaipur.

The probes mass of 35 kg is more than one-third of the total mass (of around 100 kilogram) of the 11 payloads on board and is the highest of all. This would seem to be highly disproportionate to what was achieved during its 24-minute descent from the mother satellite to self-destruction. However, once the MIPs inclusion was decided, ISRO scientists tried to make the best of a bad bargain without sacrificing any of the missions other, already planned, objectives. In fact, the initial estimate was only about 24 kg the final mass marks an increase by nearly 50 per cent. K. Thyagarajan, formerly of the ISRO Satellite Centre (ISAC), agreed that the payload mass could have been optimised better.

The nominal amount of on-board propellant required for the maintenance and orbit and attitude control of the lunar-orbiting 675 kg spacecraft over its lifetime of two years is about 100 kg. One could, therefore, argue that if the launcher (PSLV-XL) could deliver a 1,380-kg satellite (as against the originally planned 1,300 kg) in the appropriate earth-bound initial orbit (IO), its lifetime could have been extended by four to six months, and the period more gainfully utilised by the other experiments, if an equivalent amount of additional propellant had been carried instead of the MIP.

However, according to Thyagarajan, any additional propellant loading was not possible for the following reason. Given its high mass, the launcher, PSLV-XL, could inject the satellite not into a 36,000-kilometre-apogee Geosynchronous Transfer Orbit (GTO) (like the 1,050 kg METSAT/Kalpana-1) but only into an IO with an apogee of 22,000-23,000 km. This necessitated a subsequent lunar transfer through a series of firings of the satellites liquid apogee motor (LAM). To enable this, the satellites fuel tank was apparently filled to its capacity. So, it could not have carried any additional fuel even if one desired. But that only begs the question: couldnt the satellite have been reconfigured to carry a higher capacity tank?

Indeed, Chandrayaan-1 had to be reconfigured significantly to accommodate the MIP and that too within six months. For instance, the original plan of having 16 on-board thrusters was changed to eight, according to M. Annadurai, Chandrayaan-1 project director. Similarly, instead of the usual four star sensors (used for attitude control) only two were used. This reconfiguration exercise also seems to have necessitated some innovation. Instead of deploying the antenna at the end of a boom, as is usually done in such deep-space missions, the antenna was re-engineered for it to be deployed without a boom.

Annadurai, however, prefered to take a positive view of the whole exercise. I would not say we paid a price. It was a trade-off, he said. This forced us to optimise the mission to the maximum without giving up system redundancy. This challenge has resulted in an improved overall mission performance. We could carry out all operations with great precision. This has given confidence for efficient execution of future missions, he added. According to him, the satellite now has about 150 kg of the propellant, which is 50 per cent more than what is required (including the margin provided for in the fuel budget) for a lifetime of two years.

In its final form, the TV monitor-sized MIP payload was like an autonomous mini-satellite. It included three on-board scientific instruments: a C-band radar altimeter to measure the instantaneous height of the probe from the moon surface, a CCD video camera to acquire images of the surface during its descent and an off-the-shelf mass spectrometer to sense the transient lunar atmosphere (particularly for the sporadic and localised volatile emissions of helium-4, radon-222 and argon-40 from the surface). Besides the instruments, the MIP carried a small solid motor and mini solid thrusters, on-board electronics for communication with the orbiting mother satellite, an antenna, a thermal control system and a data storage and read-out system for relaying to the orbiter.

The solid motor provided the small de-boost velocity (of about 62 metre/second) to make the probes orbit sub-optimal so that it would crash on the surface (instead of going around with the orbiter after separation). The de-boost was kept small so that the orbiter and the probe had nearly the same horizontal velocity (of about 1.6 km/s) and the former could track the latter right until its demise.

Before the de-boost operation, the probe was spun (at 60 rpm) using the spin-thruster to stabilise it so that the on-board antenna remained steady during the firing. Thus the video imaging was actually done by a spinning camera, which is devolved to get images with the correct perspective. In the distance that the MIP traversed before crashing, it captured about 800 images, according to T.A. Alex, Director, ISAC.

According to Annadurai, the probe crashed near the rim of the Shackleton Crater on the south pole as targeted. The crash was signalled by a sudden break in the transmitted data. The crater itself is in permanent darkness whereas the sun shines in the adjacent regions (including the Malapert Mountain Range) nearly all the time. The evidence on where it crashed came from the final few video images which became progressively pitch dark from one side, as against the earlier lighted images, because of the adjoining craters darkness.

A valid argument against the MIPs inclusion is that the orbiter Chandrayaan-1 itself will be de-orbited at the end of its two-year life and allowed to crash on the moon. It is, therefore, conceivable that science and validation of technologies could be done during the final crash of the orbiter itself by a deliberate manoeuvre (as was done for SMART-1) and by adding suitable instruments on board. As regards the operations done using the MIP, all except one could have been carried out during the final descent of the orbiter.

An important feature of future lunar missions will be a lander/rover that would separate from an orbiter and descend to the surface, with the two moving objects remaining in constant radio communication. This technology could not have been validated using the final descent of the orbiter alone. The MIP was the test bed for this, but only to the extent that this was being done for the first time; not for overcoming any inherent technological limitations. The primary objective of landing the probe at a desired spot could have been better achieved during the final crash of the higher mass orbiter by including a pre-programmed trajectory and appropriate spin and de-boost, which, in fact, could have been made larger to achieve a slower descent.

Similarly, video images, instantaneous altitude information and data on the lunar atmosphere from close quarters could have been obtained during the final descent, by including a video camera, an altimeter and a mass spectrometer in the suite of main payloads, and transmitted directly to the ground station.

Besides qualifying the pyro-activated device that separated the probe from the orbiter and the communication link between the two, it is not clear what other technologies this experiment could have validated for future soft-landing missions. Unlike the hard-landing of the MIP at 1.6 km/s, a soft-landing mission is a different technology altogether. (At 6,000 km/hour, the probe, the flag on it included, would have blown to smithereens.)

According to a paper by R.V. Ramanan and Madan Lal of the Vikram Sarabhai Space Centre (VSSC), the optimal strategy for landing on the moon from a lunar parking orbit requires a powered braking (at an intermediate altitude) to bring the horizontal velocity to zero and the vertical velocity to a few m/s so that the probe has vertical soft touchdown with a near-zero velocity. This requires an optimum braking thrust of about 700 Newton. The thrusters that ISRO currently has are only of 440 N, and a new thruster has to be developed. They also point out that the landing mass is not optimal if two 440 N thrusters are used.

The MIP could not have validated any of the above. It is, therefore, debatable whether its inclusion could be justified by the single technology of establishing communication link between two moving objects that it helped validate which is no big deal at the cost of skewing the satellites mass budget significantly.

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