Come July 14, India's lunar mission, Chandrayaan-3, will take off on its journey to the moon. The mission is similar to its predecessor, Chandrayaan-2, but without an orbiter. The propulsion module of the spacecraft will carry the lander and rover configuration to a 100km lunar orbit. It will also behave like a communications relay satellite and carry a payload called Spectro-polarimetry of Habitable Planet Earth (SHAPE) to study the earth from a lunar orbit.
But why would the spacecraft take more than a month to reach the moon?
“The journey to the moon is very difficult. It requires precise calculations, careful planning, and a deep understanding of space physics. The moon's orbit around the earth is elliptical, which means that its distance from the earth varies. At its closest point, the moon is 363,104km from earth. At its farthest point, it is 405,696km away. The average distance between the earth and the moon is 384,400km. Scientists have to take all of this into account when planning trips to the moon. The Chandrayaan-2 mission took about six weeks to reach the moon. It followed a cautious trajectory to ensure a safe landing. The journey involved a series of braking manoeuvres to slow the spacecraft's descent,” explained aerospace and space expert Girish Linganna.
The Chandrayaan-2 mission had used a series of maneuvers to increase its speed and escape earth's gravity. These earth orbit-raising maneuvers were done by firing the spacecraft's engines to increase its velocity. The maneuvers gradually increased the spacecraft's distance from earth until it was able to escape the gravitational pull. After escaping earth's gravity, Chandrayaan-2 entered a lunar orbit. It then began a series of lunar orbit insertion (LOI) maneuvers to descend to the moon's surface. They were done by firing the spacecraft's engines to decrease its velocity. The maneuvers gradually decreased the spacecraft's altitude until it was close enough to the moon's surface to land.
The final maneuver was called the landing burn. This was a short and powerful burn that slowed the spacecraft down enough to allow it to land safely on the moon. The landing burn was successful, and Chandrayaan-2 landed on the moon on September 7, 2019. However, the lander Vikram crash-landed on the moon's surface, and the rover Pragyan was unable to deploy.
“Chandrayaan-3 is expected to reach the moon by the end of August if it launches on July 14. The spacecraft will take a similar approach to the moon as its predecessor, Chandrayaan-2. The journey could take 45-48 days, and the spacecraft could reach the moon by August 23 or 24,” said Linganna.
If one has to go back in history, the Apollo 8 mission was the fastest journey to the moon, taking 69 hours and 8 minutes. Every mission after Apollo 8 took at least 74 hours to reach the moon. The Apollo 17 mission was the last mission to land on the moon, taking 86 hours and 14 minutes. The USSR's Luna-2 craft took just 34 hours to reach the moon in 1959.
To travel long distances in space, you need high speeds and straight trajectories. The Apollo 11 mission used a super heavy-lift launcher called Saturn V to travel at more than 39,000 kilometres per hour. The Saturn V rocket could lift 43 tonnes, including the lunar module, service module, and command module. The third stage of the Saturn V rocket put the modules into a trans-lunar trajectory after orbiting Earth for the second time. The launcher and the lunar craft used powerful engines to reach the moon in just four days, covering 380,000km. The Apollo missions between 1969 and 1971 cost NASA $185 million each, or $1.2 billion in 2016 dollars. Of this, $110 million was spent on building each Saturn V rocket.
“India's rockets are not powerful enough to send spacecraft directly to the Moon. Instead, they use a circuitous route that takes advantage of earth's gravity. The spacecraft is first placed in a geosynchronous transfer orbit by the GSLV Mark3 or LVM 3, which has a 4 ton lifting capacity. Once the spacecraft reaches its apogee (farthest point from earth), it is given a final burst of acceleration that will slingshot it into the Moon's orbit,” added Linganna.
Experts point out that in order to reach the moon, a spacecraft needs to travel at a minimum velocity of 11 km/s. The vehicle provides most of this speed, but the propulsion system provides the remaining 700 m/s. The propulsion system is a small engine, so it is fired in short bursts to maneuver the spacecraft. If the spacecraft had a more powerful engine, like the Saturn V, it could reach the moon in a single shot.
Interestingly ISRO missions are cost effective compared to lunar Missions by NASA. The budget allocated for Chandrayaan 3 is Rs 615 crores, of which launching cost is Rs 75 crores. Chandrayaan 2’s total mission cost was Rs 978 crore.
“Unlike previous missions that prioritised speed, Chandrayaan-3 takes a different approach, focusing on cost optimization and safety considerations while still accomplishing its research objectives. The Apollo missions, which brought humans to the moon, achieved faster travel times but incurred high fuel consumption costs. The specially designed Saturn 5 rockets were used to accomplish this feat, but their construction came with exorbitant expenses. In contrast, ISRO aims to strike a balance between efficiency and affordability,” said Srimathy Kesan, founder and CEO of Space Kidz India.
She explains that in order to navigate this challenge, ISRO scientists have devised a slingshot method, also known as a gravity assist, to propel Chandrayaan-3 towards the moon. “The Lunar Vehicle Module 3 (LVM3) used in the mission is not as powerful as the Saturn 5 rockets, which allows for a more cost-effective approach. Instead of following a direct path, the spacecraft takes an elliptical orbit around the earth, utilising gravitational forces to gain speed and momentum. This strategic trajectory enables Chandrayaan-3 to conserve fuel while still reaching its lunar destination,” added Kesan.