Sunday, April 2, 2023

India’s Lunar Mission Chandrayaan-1

India’s lunar spacecraft Chandrayaan has already started releasing images of the moon surface. Let’s review its journey, payloads and mission objectives -- Dr S.S. Verma

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[stextbox id=”info” caption=”Chandrayaan beams back 40,000 images in 75 days”]

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.

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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.

 [stextbox id=”info”]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.[/stextbox]

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.

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.



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