FEBRUARY 2009: India’s first unmanned moon mission from Sriharikota (Nellore district in Andhra Pradesh) with orbiting spacecraft Chandrayaan-1 (moon craft) taking the moon impact probe (MIP) has been launched successfully. The MIP having Indian tricolour painted on all its sides landed on the moon surface after having flown 386,000 km from the earth, making India the fourth country on the earth to land a probe on the moon. Indian scientists involved in particular and all the public in general are very proud of this scientific and technological endeavor.

According to Dr G. Madhavan Nair, chief of Indian Space Research Organisation (ISRO), “We have given moon to India.” Every concerned Indian has traveled all the way to the moon virtually along the MIP. The MIP has already sent images with high resolution of the moon for analysis.

Fig. 1: Launch of Chandrayaan-1
Fig. 1: Launch of Chandrayaan-1

The United States, the former Soviet Union and the European Space Agency comprising 17 countries already have their flags on the moon and Chandrayaan-1 is the world’s 68th mission. Chandrayaan-1 was assembled to have eleven payloads. The spacecraft being readied at another building was moved
to the vehicle building to couple Chandrayaan-1 with the launch vehicle and with all the payloads it was then moved to the launch pad. The successful flight of MIP is supposed to prepare for the second
Indian moon mission, Chandrayaan-2, which will carry a Russian rover and a lander slated for lift-off between 2010 and 2012.

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Let’s review Chandrayaan’s journey, payloads and mission objectives.

Space probe
A space probe is a scientific space exploration mission in which a robotic spacecraft leaves the gravity well of the earth and approaches the moon or enters interplanetary or interstellar space; approximately twenty are currently existant. The space agencies of the USSR (now Russia and Ukraine), the United States, the European Union, Japan and China have in the aggregate launched probes to several planets and moons of the solar system as well as to a number of asteroids and comets.

A space probe destined for a planet or other astronomical body can be classified as a ‘flyby,’ ‘impactor,’ ‘orbiter’ or ‘lander’ mission. Historically, flyby missions proved easiest to accomplish, as these did not require the precise navigation needed for an impact, nor the additional propulsion to conduct a manoeuvre to enter the orbit. Upon landing some landers have released ‘rovers,’ which travel across the surface of the astronomical body upon which they have landed.

Once a probe has left the vicinity of the earth, its trajectory is likely to take it along an orbit around the sun similar to the earth’s orbit. To reach another planet, the conceptually simplest means is to execute a Hohmann transfer orbit manoeuvre.

More complex techniques, such as gravitational slingshots, can be more efficient, though these may require the probe to spend more time in transit. A technique using very little propulsion, but possibly requiring a considerable amount of time, is to follow a trajectory on the interplanetary transport network.

Fig. 2: The mission at a glance
Fig. 2: The mission at a glance

Launch vehicle journey
The polar satellite launch vehicle, usually known by its abbreviation ‘PSLV,’ is an expendable launch system operated by the Indian Space Research Organisation (ISRO). It was developed to allow India to launch its Indian remote sensing (IRS) satellites into sun synchronous orbits—a service that was, until the advent of the PSLV, commercially viable only from Russia. PSLV can also launch small-size satellites into the geostationary transfer orbit. The reliability and versatility of the PSLV is proven by the fact that it has launched 30 spacecrafts (14 Indian and 16 from other countries) into a variety of orbits so far. In April 2008, it successfully launched ten satellites in one go, breaking the world record held by Russia.

Chandrayaan beams back 40,000 images in 75 days

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Chandrayaan-1 has transmitted more than 40,000 images of different types since its launch on October 22, 2008, which many in ISRO believe is quite a record compared to the lunar flights of other nations. ISRO officials estimated that if more than 40,000 images have been transmitted by Chandrayaan’s cameras in 75 days, it worked out to nearly 535 images being sent daily. They are first transmitted to Indian Deep Space Network at Byalalu near Bengaluru, from where they are flashed to ISRO’s telemetry, tracking and command network at Bengaluru.

Some of these images have a resolution of up to five metres providing a sharp and clear picture of the moon’s surface. In comparison, many images sent by some of the other missions had a 100-metre resolution.

On November 26, The indigenous terrain mapping camera, which was first activated on october 29, 2008, took shots of peaks along with craters. This came as a surprise to ISRO officials because the moon consists mostly of craters.

The first Indian moon mission was proposed to be a lunar polar orbiter at an altitude of about 100 km from the lunar surface.  Considering the maturity of PSLV demonstrated through PSLV-C4/KALPANA-1 mission, PSLV was chosen for the first lunar mission.

The upgraded version of PSLV, viz, PSLV-C11, which has a liftoff weight of 316 tonnes and is 44.4m tall, was used to inject a 1304kg mass spacecraft at 240×24,000km orbit. The corresponding spacecraft mass was 590 kg when the target lunar orbit of 100 km was achieved.

 

The Chandrayaan-1 mission is aimed at high-resolution remote sensing of the moon in visible, near-infrared, low-energy X-ray and high-energy X-ray regions.

The PSLVC 11, also called PSLV-XL because of the increased weight of the six strap-on motors, soared into the sky from the second launch pad at the Satish Dhawan Space Centre, Sriharikota. It traveled to the vicinity of the moon by following the lunar transfer trajectory (LTT).

At first, Chandrayaan-1 reached a highly elliptical orbit. After encircling the earth for a while, the spacecraft was taken into two more elliptical orbits whose apogees were still higher at 37,000 km and 73,000 km, respectively. This all was done at a very precise moment by firing the spacecraft’s liquid apogee motor (LAM) when the spacecraft was near perigee. Subsequently, the LAM was fired to take the spacecraft to a high orbit whose apogee lied at about 387,000 km. When the Chandrayaan-1 reached the vicinity of the moon, the spacecraft was oriented in a particular way and its LAM was fired again to slow down the spacecraft sufficiently to enable the gravity of the moon to acquire it into an elliptical orbit.

About 20 days from the date of launch, Chandrayaan-1 was in the required moon orbit. When the orbital height of Chandrayaan-1 was lowered to its intended 100km height from the lunar surface, the MIP was ejected from Chandrayaan-1 at the earliest on to the lunar surface in a chosen area.

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The spacecraft
The spacecraft for lunar mission is cuboid in shape with each side approximately 1.50 metres. It accommodates eleven science payloads. The 3-axis stabilised spacecraft uses two star sensors, gyros and four reaction wheels.

A canted single-sided solar array will provide the required power during all phases of the mission. This deployable solar array consisting of a single panel generates 700W of peak power. The solar array along with yoke was stowed on the south deck of the spacecraft in the launch phase. During eclipse, the spacecraft was powered by lithium-ion (Li-ion) batteries. After deployment, the solar panel plane was canted by 30º to the spacecraft pitch axis.

The spacecraft employs an X-band, 0.7m diameter parabolic antenna for payload data transmission. The antenna employs a dual gimbal mechanism to track the earth station when the spacecraft is in lunar orbit.

The spacecraft uses a bipropellant integrated propulsion system to reach the lunar orbit as well as orbit and attitude maintenance while orbiting the moon. The propulsion system carries the required propellant for a mission life of two years, with adequate margin.

The telemetry, tracking and command communication is in S-band frequency. The scientific payload data transmission is in X-band frequency.

The spacecraft has three solidstate recorders (SSRs) on board to record data from various payloads. SSR-1 will store science payload data and has capability of storing 32GB data. SSR-2 will store science payload data along with spacecraft attitude information (gyro and star sensor), satellite housekeeping and other auxiliary data. The storage capacity of SSR-2 is 8 GB. M3 (moon mineralogy mapper) payload has an independent SSR with 10GB capacity.

Scientific payloads
The indigenously developed payloads are:
1. Terrain-mapping stereo camera in the panchromatic band, having 5m spatial resolution and 20km swath
2. Hyper-spectral imaging camera operating in the 400-950nm band with a spectral resolution of 15 nm and spatial resolution of 80 metres with a swath of 20 km
3. Lunar laser-ranging instrument with height resolution of about 10 metres
4. High-energy X-ray spectrometer using cadmium-zinc-telluride (CdZnTe) detector in the 30-250 keV energy region with spatial resolution of 40 km
5. Moon impact probe as piggyback on the main orbiter of the Chandrayaan-1 spacecraft which impacted on the surface of the moon

The imported payloads on-board Chandrayaan-1 are:
1. Chandrayaan-1 X-ray spectrometer through ESA—a collaboration between Rutherford Appleton Laboratory, the UK and ISRO Satellite Centre, ISRO. Part of this payload was redesigned by the ISRO to suit scientific objectives of Chandrayaan-1
2. Near-infrared spectrometer (SIR-2) from Max Plank Institute, Lindau, Germany, through ESA
3. Sub-keV atom-reflecting analyser through ESA, from Swedish Institute of Space Physics, Sweden, and Space Physics Laboratory, Vikram Sarabhai Space Centre, ISRO. The data processing unit of this payload was designed and developed by the ISRO, while Swedish Institute of Space Physics developed the payload
4. Radiation-dose monitor from Bulgarian Academy of Sciences
5. Miniature synthetic aperture radar (MiniSAR) from Applied Physics Laboratory, Johns Hopkins University, and Naval Air Warfare Centre, USA, through NASA
6. Moon mineralogy mapper from Brown University and Jet Propulsion Laboratory, USA, through NASA

Moon impact probe
The moon impact probe is slightly smaller than a TV cabinet and weighs a bit more than a school-going kid, but there’s a lot tucked away inside it. The MIP detached from the Chandrayaan a 100 km above the moon. The 29kg moon impact probe—a payload developed by the Vikram Sarabhai Space Centre at Thiruvananthapuram—will help identify future landing sites on the moon and also aid in scientific exploration of the lunar surface. The dimensions of the impact probe are 375×375×470 mm3.

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The three major payloads in the MIP are:
1. Radar altimeter to measure the altitude of the moon impact probe above the lunar surface and qualify technologies for future landing missions. The operating frequency band is 4.3 GHz±100 MHz
2. Video imaging system to acquire images of the surface of the moon from the descending probe. The video imaging system consists of an analogue CCD camera along with a video decoder
3. A state-of-the-art quadrupole mass spectrometer with a mass resolution of 0.5 amu. It is sensitive to partial pressure of the order of 10-15 torr for measuring the constituents of tenuous lunar atmosphere during descent

In addition to the instruments, the separation system, the de-boost spin and de-spin motors, it comprises the avionics system and thermal control system. The avionics system supports the payloads and provides communication link between the moon impact probe and the main orbiter, from separation to impact, and provides a database useful for future soft landing. The mission envisages collection of all the instrument data during descent and transmission to the main orbiter, which, in turn, will transmit it to the ground station during visible phases.

Fig. 3: The Chandrayaan-1 spacecraft
Fig. 3: The Chandrayaan-1 spacecraft

Mission objectives
1. To harness the science payloads, lunar craft and the launch vehicle with suitable ground support systems including deep space station
2. To realise the integration and testing, launch into the lunar polar orbit of about 100 km, in-orbit operation of experiments, communication/telecommand, telemetry data reception, quick-look analysis of data and archival for scientific utilisation by an identified group of scientists

Scientific objectives
The Chandrayaan-1 mission is aimed at high-resolution remote sensing of the moon in visible, near-infrared, low-energy X-ray and high-energy X-ray regions. Specifically, the objectives are:
1. To prepare a three-dimensional atlas (with a high spatial and altitude resolution of 5-10 metres) of both near and far sides of the moon
2. To conduct chemical and mineralogical mapping of the entire lunar surface for distribution of elements such as magnesium, aluminium, silicon, calcium, iron and titanium with a spatial resolution of about 25 km and high-atomic-number elements such as radon, uranium and thorium with a spatial resolution of about 20 km

Simultaneous photo geological and chemical mapping will enable identification of different geological units, which will test the early evolutionary history of the moon and help in determining the nature and stratigraphy of the lunar crust.

Next in line: Chandrayaan-2
ISRO is also planning a second version of Chandrayaan named Chandrayaan- 2. According to ISRO chairman G. Madhavan Nair, “The ISRO hopes to land a motorised rover on the Moon in 2012, as a part of its second Chandrayaan mission. The rover will be designed to move on wheels on the lunar surface, pick up samples of soil or rocks, do on-site chemical analysis and send the data to the mother-spacecraft Chandrayaan-2, which will be orbiting above. Chandrayaan-2 will transmit the data to the earth.”
The author is from Department of Physics, S.L.I.E.T. (deemed to be university), Longowal, Distict Sangrur, Punjab

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