Explained: Why Chandrayaan-3 Needs Over 40 Days To Reach The Moon When NASA's Apollo 11 Took Only Four

In a little over 15 minutes after launch, the Chandrayaan-3 propulsion module, carrying the lander, was put into an elliptical orbit around the Earth by the Indian Space Research Organisation’s (ISRO’s) heaviest rocket — the Geosynchronous Satellite Launch Vehicle (GSLV) Mark-III.

Over the next two weeks, ISRO will conduct between five to six orbit raising maneuvers using the onboard propulsion system.

With every burn of the onboard propulsion system, the module will keep spiralling outwards in increasingly elongated ellipses.

The speed of the propulsion module will steadily increase until it reaches the escape velocity necessary to break free from Earth's gravity, enabling it to enter a Lunar Transfer Trajectory (LTT) and set a course towards the moon.

Translunar injection, which is when Chandrayaan-3 will leave Earth orbit for the Moon, will take place on 1 August. The landing is projected to take place sometime around 23 August, over three weeks later.

Over 40 days after the launch and several orbital manoeuvres later, the lander will reach the surface of the moon and deploy the rover.

Although the travel duration may seem reasonable given the distance between the Earth and the Moon, it is important to note that previous missions have completed this journey in a shorter timeframe.

China’s Chang’e 2, launched in 2010, also took just four days covering the distance between the Earth and the Moon, and so did its follow up mission to the lunar surface, Chang’e 3.

The Soviet Union’s Luna-1, the first unmanned mission to reach close to the Moon, took just 36 hours to make the journey.

Even Apollo-11's command module, Columbia, carrying three astronauts, reached the Moon in just a little over four days.

Why, then, is Chandrayaan-3 taking weeks to reach the cratered sphere?

The simple answer is because ISRO does not have a rocket powerful enough to put Chandrayaan-3 on a direct path to the Moon.

In the case of Apollo missions, including Apollo 11, a direct trajectory called Translunar Injection (TLI) was used. The Saturn V launch vehicle propelled the Apollo spacecraft into Earth orbit first.

From there, a powerful engine burn was executed to send the spacecraft on a trajectory directly towards the Moon. The spacecraft was directed to the LTT through a single six-minute-long burn of the Saturn rocket's third stage, akin to a slingshot effect. 

This direct path allowed the Apollo missions to reach the Moon relatively quickly, within a few days.

Chandrayaan-3, however, is following a very different trajectory.

As explained earlier, the mission will use a series of Earth orbits and engine burns to gradually increase the spacecraft's speed and position it for a lunar insertion.

The spacecraft will first enter an initial Earth orbit and then perform engine burns at specific times to transfer to a trajectory that intersects with the Moon's orbit. Finally, another engine burn will be conducted to insert the spacecraft into lunar orbit.

This multi-step approach used by the ISRO for the Chandrayaan and Mangalyaan missions requires more time but allows for the use of a relatively less powerful launch vehicles.

While the GSLV Mk-III is a capable launch vehicle, it does not have the same power and payload capacity as the Saturn V used in the Apollo missions. As a result, a more gradual trajectory was chosen to optimize the mission within the constraints of the launch vehicle.

More Manoeuvres And Some Clever Use Of Gravity

ISRO will use Earth and Moon's gravity to workaround the constraints.

While orbiting the Earth in an elliptical orbit, the module will be at its highest speed when it passes through the point in that orbit closest to the planet. This point is called the perigee.

Exactly opposite to this point in the orbit is the apogee, where the module will be the furthest from the Earth and at its slowest speed. The speed varies across different points in the orbit due to the variation in the Earth’s gravitational pull.

The closer the module is to the Earth, the more the gravitational pull, and the greater the speed. Each time the module reaches the perigee, or the point of highest speed, the onboard engine fires, increasing its speed even more, pushing it into a higher, more elongated orbit as a result.

With every burn of the onboard propulsion system, the module will keep spiralling outwards in increasingly elongated ellipses.

Eventually, as the module continues its journey, it reaches the escape velocity necessary to break free from Earth's gravity. At this point, the module's orbit will elongate, allowing it to set a course towards the moon.

The entry of the Chandrayaan-3 module into the LTT is carefully timed to align with the moon's position in its own orbit. This strategic timing ensures that the module reaches proximity to the moon's orbit precisely when the moon is located in that region.

Once the module reaches this point, a precise manoeuvre is executed using the onboard propulsion system. This manoeuvre, known as lunar orbit insertion, is designed to reduce the module's velocity.

The gravitational field of the moon can then pull the module into a stable lunar orbit. This successful lunar insertion completes the crucial phase of placing the spacecraft in orbit around the moon.

Having escaped Earth’s gravity and entered lunar orbit, the module will start revolving around the moon in an elliptical orbit.

A series of manoeuvres will be used to progressively lower the altitude of the module and place it in a 100 km circular orbit around the moon. It is at this point that the propulsion module will separate from the lander, which will continue its journey towards the lunar surface.

If all proceeds as planned, sometime around 23 August, Chandrayaan-3 will accomplish a groundbreaking feat as the first mission ever to successfully soft-land in the vicinity of the lunar south pole.

Also Read:

Landing On The Lunar Surface: How Chandrayaan-3 Will Travel From Earth To Moon 

Chandrayaan-3: How ISRO Built The Cryogenic Engine That Will Put Lunar Module In Orbit Around Earth