Kavya NarayananJul 26, 2019 13:09:26 IST
With the spectacularly successful launch of ISRO's Chandrayaan 2 mission on 22 July, the three-part spacecraft carrying a lunar orbiter, lander, and rover destined for the moon's surface was launched into the Earth's Parking Orbit, a temporary orbit used during the launch of a satellite or spacecraft.
Surely there's a lot going on between the launch and the planned landing on 7 September. So what's next for Chandrayaan 2?
Update: Two successful orbit-raising manoeuvers have been successfully completed in the Earth-bound phase of the mission.
The Earth-bound Phase: 22 July-13 August
Pending a systems check after launch, the Chandrayaan 2 composite will spend the next three weeks making larger and larger elliptical orbits of the moon. From a 170 x 40,400 km (its nearest x farthest distance from the Earth) elliptical orbit, the spacecraft will make five "orbit-raising" shifts to higher orbits — bringing it closer to the moon with each subsequent one. These five manoeuvres will require "Earth burns" of the spacecraft's onboard engines, with each thrust moving it closer to the moon's gravity.
Here's the schedule for what each of these burns is expected to take place:
24 July 2019, 2 - 3.30 pm: First orbit-raising manoeuver to a final orbit of 230 x 45,162 km (closest x farthest distance of its elliptical orbit around the Earth)
26 July 2019, 1 - 2 am: Second orbit-raising manoeuver to a final orbit of 250 x 54,689 km
29 July 2019, 2 - 3.30 pm: Third orbit-raising manoeuver to a final orbit of 268 x 71,558 km
2 August 2019, 2 - 3 pm: Fourth orbit-raising manoeuver to a final orbit of 248 x 90,229 km
6 August 2019, 2.30 - 3.30 pm: Fifth and final orbit-raising manoeuver to a final orbit of 221 x 1,43,585 km
Orbit raises: 24 July-6 August
The spacecraft will make use of the Earth's gravity to amp-up its own velocity using a technique popularly referred to as 'gravity assist', to conserve fuel. The final orbit raising will take Chandrayaan 2 as far as 1,43,585 km from the Earth — halfway between our planet and the moon. This is when the next critical manoeuvre of the mission will need to take place: translunar injection, where the spacecraft's trajectory is adjusted so it reaches the moon.
Translunar injection: 14 August, 2-3 pm IST
For the spacecraft to enter the lunar orbit, it needs to be put on a trajectory to arrive at a constantly-moving moon. The translunar injection, or TLI, is a manoeuvre of the thrusts used to set a spacecraft on this moon-bound path. The Chandrayaan 2 composite will take less than a week to arrive in the proximity of the moon. This manoeuver will place the spacecraft on a 266 x 4,13,623 orbit that will end in the next stage of the mission: lunar transfer. The spacecraft's Liquid Apogee Motor (a.k.a. thruster engines) were retrofitted to make this lunar orbit insertion possible.
Gradually, over days, the spacecraft will be brought closer to the moon with five 'Lunar burns'. This part of the mission is where the bulk of the adjustments were made to make the planned landing date of 7 September despite the one-week delay in the mission's launch.
Lunar transfer: 14-20 August
The Chandrayaan 2 composite will then be put on a 'lunar transfer trajectory'. During this week-long phase, the spacecraft will break away from the Earth's orbit and begins its 3,84,000 kilometre journey to the moon. The last of Chandrayaan 2's Earth-bound orbits will bring it halfway to the moon. Now, the spacecraft will use its thrusters to come under the moon's 'orbit of influence'.
Chandrayaan 2 will have to slow itself down just enough using its thrusters to fall into the moon's reach — gravitationally speaking. This is the main goal of the lunar transfer phase of the mission.
— Tech2 (@tech2eets) July 22, 2019
Lunar orbital insertion: 20/21 August
The location of the moon relative to the Earth is constantly changing. Thus, the intersection of Chandrayaan 2's path with the moon's was predicted and planned well ahead of time. When the moon approaches the apogee of Chandrayaan 2 — its farthest point from the Earth — the on-board thrusters will fire precisely and slow itself down for 'lunar capture'.
This controlled transfer — breaking free from the Earth's orbit of influence and into the moon's — will take place over days. This will require the liquid apogee motors to be fired in the opposite direction to the spacecraft's movement, therefore slowing it down in a manoeuvre called "retrofiring". This will allow the spacecraft to come under the influence of the moon's orbit.
Lunar-bound phase: 21 August-1 September
The lunar-bound phase, which was 28 days-long in the earlier launch schedule has now been shrunk to just 13 days. This is an important part of the orbiter's mission: surveilling its year-long home for the first time, ensuring that no damage was caused to its instruments on the journey thus far, and a thorough examination of the Vikram lander's landing site in the moon's South Polar region.
The lunar orbital insertion ends with the Chandrayaan 2 orbiter-lander composite entering a circular 100 km-altitude-orbit around the moon.
Separation of the Vikram lander & Deboosting: 3 September
Till the 100 km orbit was attained, the spacecraft's three components were travelling together. After a few circular orbits at 100 x 100 km, the lunar lander will separate from the orbiter. The lander Vikram will then be in action, using its propulsion system to lower its orbit to a 100km x 30 km (farthest x nearest distance from the moon's surface). The lander will remain moving in this new orbit for four days, during which it will carry out checks on its systems and the landing system.
Deboosting is another challenging and critical milestone in the mission. The process of deboosting is necessary to slow the velocity of the spacecraft enough to drop/deliver it into the moon's gravitational field. This way, it will achieve a "nominal" orbit around the moon.
Powered descent of the lander: 7 September
On the day of landing, the propulsion system of the lander will perform an important task. It will break the velocity of the lander in a controlled way, where the engines are shut off well before the spacecraft touches the moon's surface. In the final seconds before landing, the Vikram lander will be assisted only by the moon's gravity and not the spacecraft's thrusters. This move is to prevent a mini-plume of lunar dust from covering the lander — and its solar panels.
From the 30 km orbit the lander will use till the landing is a 15-minute window of "terror". These 15 minutes, during which time the lander will undergo an automated, pre-programmed landing sequence, "will be the most terrifying moment" for ISRO, the agency's Chairman said. This 15-minute flight is something ISRO has never undertaken – it is the most complex mission has ever undertaken.
For the landing site, ISRO has picked a high plain between two craters Manzinus C and Simpelius N near the moon's South Pole. The spacecraft's onboard Navigation, Guidance and Control (NGC) system as well as the propulsion system of the module will need to work in perfect unison for the autonomous landing to be a success.
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