Explained: Why Russia's Luna-25 Launched Today May Reach The Moon's Surface Sooner Than Chandrayaan-3

For the first time in nearly half a century, a Russian spacecraft is headed to the moon. On Friday (11 August) morning, at a spaceport located in the remote eastern part of Russia, a Soyuz rocket carried Luna-25, a moderately sized robotic lander, into Earth's orbit.

Approximately 80 minutes after liftoff, Luna-25's course was adjusted to set it on a trajectory towards the moon.

The plan is for it to reach the moon, establish orbit by 16 August, and then make an attempt to land on the lunar surface, possibly as early as 21 August, two days ahead of India's Chandrayaan-3, which is also poised for a landing in the same vicinity — the moon's south polar region.

This has led many in India to question why the Russian moon lander, launched nearly four weeks after the Chandrayaan-3 mission, might potentially reach the lunar surface days before it.

Chandrayaan-3 began its journey as Indian Space Research Organisation’s (ISRO’s) heaviest rocket, Launch Vehicle Mark-3, lifted off from Sriharikota with its two solid strap-on boosters breathing out orange plume, guiding it through its brief but majestic vertical ascent.

Over 28 days after the launch and several orbital manoeuvres later, the propulsion module has been placed in an orbit around the Moon.

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

For instance, 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.

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

Luna-25 would not be the first Russian lunar mission to take the short route to the Moon. The Soviet Union’s Luna-1, the first unmanned mission to reach close to the Moon, took just 36 hours to make the journey.

Why Chandrayaan-3 Is Taking Longer

The simple answer is because ISRO, unlike the Russians, the Chinese and the Americans, 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 NASA's Apollo missions to reach the lunar surface relatively quickly, within a few days.

Russia's Luna-25 would also follow a somewhat similar approach.

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

The mission used series of Earth orbits and engine burns to gradually increase the spacecraft's speed and position it for a lunar insertion.

Following its launch on 14 July, the spacecraft first entered an initial Earth orbit and then performed engine burns at specific times to transfer to a trajectory that intersects with the Moon's orbit. Finally, another engine burn was 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 relatively less powerful launch vehicles.

While LVM-3 is a capable launch vehicle, it does not have the same power and payload capacity as the Saturn V used in the Apollo missions or the Soyuz rockets used by the Soviets and now the Russians.

As a result, a more gradual trajectory was chosen by ISRO to optimise the mission within the constraints of the launch vehicle.

Clever Use Of Gravity

ISRO has used Earth and Moon's gravity to workaround the constraints.

While orbiting the Earth in an elliptical orbit, a spacecraft is 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 spacecraft is 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 spacecraft is to the Earth, the more the gravitational pull, and the greater the speed.

Each time the spacecraft reaches the perigee, or the point of highest speed, the onboard engines are fired, 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 spacecraft keeps spiralling outwards in increasingly elongated ellipses.

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

This is the approach ISRO has taken in case of the Chandrayaan missions.

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

Once the module reached this point, a precise manoeuvre was 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 then pulled the module into a stable lunar orbit. This successful lunar insertion marked the completion of the crucial phase of placing the spacecraft in orbit around the moon.

Having escaped Earth’s gravity and entered lunar orbit, the module is currently revolving around the moon.

ISRO is currently in the process of gradually lowering the orbit of the module around the Moon to bring it closer to the surface. When it is in a 100 km circular orbit around the Moon, the propulsion module will separate from the lander, which will continue its journey towards the lunar surface.

Chandrayaan-3 entered lunar orbit on 5 August and is set to land on 23 August, requiring over 18 days to traverse from its initial lunar orbit to the cratered surface.

In contrast, Luna-25 is scheduled to enter lunar orbit on 16 August and make a landing attempt on 21 August, accomplishing the journey from orbit to the Moon's surface in five days.

Similar to its journey from Earth's orbit to lunar orbit, the Russian spacecraft will follow a more direct trajectory from lunar orbit to the lunar surface. On the other hand, ISRO will execute orbit reduction manoeuvres to gradually lower the module's orbit and position it for landing.

If it successfully lands on 21 August, the Luna-25 mission would beat Chandrayaan-3 in the race to land near the lunar south pole.