As ISRO prepares for the launch of Chandrayaan-3, there are many questions in the minds of people about the mission. Here are some questions and their answers.
What is Chandrayaan-3?
Chandrayaan-3 is the name of the spacecraft to be sent to the moon by India’s space agency, ISRO.
We come across many terms such as LVM-3, propulsion module, lander, rover, and instruments like RHAMBHA. It is a bit confusing. What are all these, and which of these is called Chandrayaan-3?
LVM-3 is the the rocket that will take Chandrayaan-3 up and drop it at a certain point above the earth. With that LVM-3’s job would end.
From that point, Chandrayaan-3 will journey towards the moon.
The spacecraft consists of two parts — the propulsion and the Lander-rover modules. The propulsion module’s main job is to take the lander-rover payloads to the moon. You can think of the propulsion module as a truck and the lander-rover payload as the cargo.
After reaching the vicinity of the moon, the lander-rover payload will detach itself from the propulsion module and falling on to the moon. The lander has engines that will slow down the fall, so that it descends gently onto the moon rather than crash-landing on it.
The rover is a tiny, trolley kind of device with wheels. Once the lander lands on the moon, the rover will slide out of the lander’s belly and crawl over the moon’s surface.
Both the lander and the rover have instruments for experiments, such as analysing the moon soil, checking how the moon’s surface conducts heat, and how quake waves move through the moon surface.
Why does Chandrayaan-3, like Chandrayaan-1 and 2, take a month to reach the moon, when fifty years ago, the United States’ Apollo spacecraft reached the moon in four days?
We can also shoot off a rocket straight to the moon. Only, the rocket will have to be extremely big. To travel the distance of 384,400 km, the rocket will have to carry enormous amounts of fuel. The fuel adds to the weight of rocket, so it would need to be more powerful. The Saturn V rocket that took Apollo 11 to the moon in 1969 stood 363-feet tall. The LVM-3 is 142-feet tall. Big rockets are very expensive. Besides, there is no urgency, there is no need for the Chandrayaan-3 to reach the moon fast. That is why it takes a route that makes use of the gravity of the earth to sling itself towards the moon.
The Chandrayaan-3 circles the earth many times before it reaches the moon and then circles the moon many times before the lander detaches itself from the propulsion module and descends onto the moon surface? Why this peculiar route?
Kepler’s second law of planetary motion states that the imaginary line that connects a planet and its satellite sweeps equal areas in equal intervals of time. This means that the satellite travels faster as it approaches the planet and slows down as it moves away, while moving in an elliptical orbit. The law also means that the farther an object approaches the planet from, the higher the velocity it acquires as it comes closer to the planet. We want to make use of this property to get Chandrayaan-3 enough velocity to shoot off towards the moon.
So, after the LVM-3 puts it above the earth, Chandrayaan-3 will start circling the earth, on its own, in an elliptical orbit. When it reaches the farthest point, engineers on the ground will nudge it slightly to change the direction a little so that its next loop is bigger than the first. So, when the spacecraft approaches the earth on its second loop, it will acquire a higher velocity. Again, when it reaches the farthest point, called apogee, the engineers will once again change the direction a little, so that on the third loop, the spacecraft acquires an even higher velocity. On completing 5-6 such loops, the spacecraft will have acquired enough velocity to sling itself towards the moon.
Once it reaches the moon, the reverse will happen. Loop-by-loop the spacecraft will get closer to the moon. When it is about 100 km from the moon’s surface, the lander will detach itself and begin its descent onto the moon.
How does the lander descend onto the moon?
The lander actually ‘falls’ on the moon. But it has four thrusters — or engines — which will provide it an upward thrust and slow down its descent. It is calculated that just before the touchdown, it will be traveling at a speed of 2 meters per second.
We have seen spacecraft sent to Mars, like Curiosity and Perseverance, slowly parachute down, but there is no parachute in our moon missions. Isn’t parachuting down simpler and cheaper than using engines to slow down the descent?
That is because Mars has an atmosphere, while the moon doesn’t. Yes, the Martian atmosphere is thin. The average atmospheric pressure is about 1 per cent of Earth’s. But still there is an atmosphere, which is, by the way, made of carbon dioxide. You need some air to be put under the parachute — to provide what is called ‘drag’. Mars has some, the moon has none.
What happens after the lander touches down on the moon surface?
After the lander has soft-landed, it will make sure everything is okay. Then, figuratively speaking, a sort of a trap door under the lander will open and guiderails will slip out of it. The rover will slide down the rails to the moon surface.
What is the rover and what does it do?
Equipped with wheels, the rover will crawl around the moon’s surface like a cockroach, pick up soil and do experiments, punch a probe a foot down the surface to check thermal conductivity. Instruments on the lander will also do experiments. Basically, these instruments check-out the moon, to know more about it.
Do the lander and rover return to earth?
No. The propulsion module, the lander, the rover are all up there forever. Unless some day an astronaut lands on the moon and decides to bring them back as souvenirs.
The lander and rover do experiments and analysis. How do we, on earth, get the information?
They digitize the data and transmit it in the form of electromagnetic waves, to a receiver on the propulsion module, which is still circulating the moon. For back-up, we still have the orbiter module of Chandrayaan-2, the previous moon mission, which also has a receiver. The propulsion module or the orbiter will transmit the data to the earth.
Is the method of sending information like how radio stations broadcast, say, a running commentary?
No. Broadcasts are done through audio waves, which need a medium — the air — to propagate. Signals through space are sent in the form of electromagnetic waves — such as radio waves or microwaves — which are progressions of energy. They don’t need a medium to travel.
Howlong will the lander and rover function?
The lander and rover will be alive for 14 earth days, which corresponds to one moon day. When the moon rotates one full round on its axis, the earth would have completed 29.5 days. A moon day is about 14 earth days, as is a moon night. Since the solar panels that provide electricity to the lander and rover need sunlight, they will be alive for one moon day, which is 14 earth days.
What is the cost of the Chandrayaan-3 mission?
The Chandrayaan-3 is estimated to cost around Rs 615 crore.
What is the significance of Chandrayaan-3? Why go to the moon at all?
For decades after the US Apollo missions, mankind ignored the moon. But now, after the presence of ice in the southern polar region of the moon has been conclusively established, there is renewed interest. Ice means water, water could be split into hydrogen and oxygen, both of which are rocket fuels. This means, in future, rockets could be built on the moon and powered by locally produced fuels, for other space missions. It is easier and cheaper to launch deep space missions from the moon, because of its low gravity, but this wouldn’t have been economically feasible if you have to carry rocket fuel from the earth to the moon.