CHANDRAYAAN 2: VIKRAM’S SENSATIONAL DESCENT TO THE MOON

Science should not be looked for the results, Science should be for the Experiment, moreover, Experiment leads to the Result”

Chandrayaan-2 Orbiter and Lander configuration during flight. The Rover is inside the Lander.
Credits – Department of Space, Government of India. Source. License – Government Open Data License – India (GODL).

Introduction

As already said by Dr. K. Sivan, the last 15 minutes of descent on the moon are the “moments of terror” due to its complex operational procedure and the automated nature of the lander. According to the official sources, the Chandrayaan 2 is near 95% success.

But the last phase of a smooth landing has not happened as planned. Here we are going to discuss what happened with the lander which terminated the successful soft landing on the moon’s surface even after utmost precaution and testing.

Credit – ISRO.

Some basic facts about Chandrayaan 2’s ‘Vikram’ lander

Let us look at a few basic information about Chandrayaan 2’s “Vikram” lander.

Weight

It had a dry mass of 626kg, and the mass after loading the fuel was 1471kg.

Power Supply

It operated on a 650W power supply. Li-ion battery powers the lander during the descent phase and eclipse.

Propulsion

The lander’s propulsion system consisted of five 800N liquid main engines derived from the ISRO’s 440N Liquid Apogee Motor and eight 50N thrusters for attitude control. ISRO initially planned for only 4 liquid main engines, but the fifth engine was added later to handle the new requirements of having to orbit the moon before landing. Vikram can safely land on slopes up to 120 degrees.

Structure

The lander architecture is a truncated pyramid around a cylinder that houses the propellant tank and interface for the separation mechanism from the orbiter. The lander legs provide stability upon landing on different terrains.

Guidance, Navigation, and Control

The sensors are configured for inertial navigation from separation to the end of rough braking and the absolute sensors determine the position and velocity with respect to the landing site to guide the lander beyond the rough braking phase to the landing site. The Guidance, Navigation, and Control system will be automated from the orbiter separation till landing.

Communication System

The communication between the lander and Indian Deep Space Network (ISDN) is in S-band and the payload data is transmitted by a high torque dual gimbal antenna. The Lander has a TM-TC data handling system with inbuilt storage.

Scientific Instruments Onboard

The scientific instruments on board the Vikram lander are; RAMBHA (Radio Anatomy of Moon Bound Hypersensitive ionosphere and Atmosphere) which will measure the surface plasma density as it changes with time and ChaSTE (Chandra’s Surface Thermo Physical Experiment) which will measure the thermal properties of lunar regolith near the polar region and the ILSA (Instrument for Lunar Seismic Activity) which will measure the seismic activity around the landing site.

‘Pragyan’ Rover

The Chandrayaan-2 Rover is stowed in the lander during launch and upon landing the ramps are deployed and Rover starts its journey on the lunar surface.

The ‘Pragyan’ Rover emerges out of the ‘Vikram’ lander, through a ramp.
Credit – ISRO. Source. License – Government Open Data License – India (GODL).

Vikram landing sequence

After the successful insertion of the Chandrayaan 2 probe into lunar orbit (127km  X 119km), the lander  ‘Vikram’ separated from the orbiter. It performed its first deorbit burn for 4 seconds on 3 Sep 2019 at 03:20 UTC and was inserted into (128km X 104km) orbit. The second deorbit burn was carried on for 9 seconds on 3 Sep 2019 at 22:12 UTC which put it in an elliptical orbit of (101km X 35km). From the periselene of this orbit, the powered descent started on 6 Sep 2019 at 20:08 UTC to the designated landing site. It deorbited and followed a Hohmann Transfer Trajectory. The descent happened in four phases; Rough braking phase, Coasting phase, Fine braking phase, and Terminal phase.

Rough Braking Phase

When the rough braking phase was initiated, the velocity of the lander in the orbit was 1.64 km/s. In the rough braking phase, the lander fired its engines for 10 minutes and brought the speed down to nearly 200 m/s horizontally and 70 m/s vertically.

It was nearly 11 km from the intended landing site. The throttleable engines of the lander worked flawlessly, accurately following the planned trajectory. The altitude of the lander above the surface of the moon was 7km at the end of the rough braking phase.

Coasting Phase

This lasted for 38 seconds. The four throttleable engines were continuously on. The vertical velocity at the end was the phase was 86m/s and 4.4 km downrange from the intended landing site. Hazard Avoidance sensor and other sensors calculated the position and velocity of the lander with respect to the landing site which was already predetermined.

Fine braking phase

This phase started at 5km altitude and was expected to continue for 96 seconds and bring the lander to 400m altitudes. At 2.1km altitude, communication with Earth was lost. The last telemetry showed that the lander was moving with the speed of 48m/s horizontally and 60m/s vertically.

Credit – ISRO.

We originally wrote this article in 2019, the day after Vikram lander lost communication with ISRO Deep Space Network. At that time, analysts and the people in the media were thinking out the probable reasons for the communication loss. They included;

  1. Thruster failure caused a loss of attitude control and orientation.
  2. One or more of the four-engine failed or lacked synchronicity with the other engines.
  3. Software error that may have sent a false signal to the propulsion system. (like what happened with Israel’s Beresheet lander).
  4. Altimeter/ Hazard detection system failure.
  5. Disorientation of high-gain antenna.
  6. Fuel exhaustion.

What were the reasons for the sudden loss of contact of Chandrayaan 2 lander so close to Touchdown?

ISRO Chairman Dr. K. Sivan tasked senior scientist Prem Shanker Goel to head the Failure Analysis Committee to find out the causes of the loss of Vikram Lander. The Failure Analysis Committee concluded that a software glitch caused the Vikram Lander to hard-land on the lunar surface. They found out that Phase One of the descent had proceeded normally. But, during Phase Two of the descent, the velocity was more than expected.

This deviation in velocity could not be handled by the onboard software, as it was beyond its design parameters. This caused Vikram Lander to hard-land on the moon. The lander’s point of impact is found to be at (70.8810°S, 22.7840°E).

This was found by ISRO after corroborating input given by Shanmuga Subramanian, a citizen scientist volunteer from Chennai, India, who located spacecraft debris in pictures released by NASA. The lander was broken into pieces upon impact, with some pieces found a few kilometers away. ISRO will launch the Chandrayaan 3 moon mission, which will consist of only the lander and rover.


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