Explained: Why It Took Chandrayaan-2 So Long To Get Into Moon’s Orbit And What Happens Next
Chandrayaan-2 has successfully entered the Moon’s orbit. While this travel time may appear absolutely normal, given the distance between the Earth and the Moon, other missions have covered the distance in much lesser time.
What were the constraints preventing ISRO from getting the module into the Moon’s orbit much earlier? How did they overcome it?
On 22 July, Chandrayaan-2’s orbiter and lander module 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 30 days and several orbital manoeuvres later, Chandrayaan-2 has successfully entered the Moon’s orbit.
Chandrayaan-2’s lander and rover will reach the surface of the Moon only on 7 September, nearly a month and a half after leaving the Earth.
While this travel time may appear absolutely normal, given the distance between the Earth and the Moon, other missions have covered the distance in lesser time.
China’s Chang’e 2, launched in 2010, took just four days days covering the distance between the Earth and the Moon. Chang’e 3, Beijing’s follow-up mission launched in 2013, reached the celestial body in only four days and 12 hours.
The Soviet Union’s Luna-1, the first unmanned mission to reach close to the Moon, took as little as 36 hours to make the journey. Even the first manned missions to the Moon, Apollo-11, reached the lunar surface in nearly four days.
So Why Did Chandrayaan-2 Take Nearly A Month?
The simple answer is because ISRO does not have a rocket powerful enough to put Chandrayaan-2 on a direct path to the Moon.
GSLV Mark-III, equipped with a cryogenic third stage engine, is the most powerful rocket ISRO has ever built. The three stages of this 640 tonnes rocket produce just enough thrust (the push a rocket engine provides to the rocket) to lift about four tonnes into the Geosynchronous Transfer Orbit.
In comparison, the mammoth Saturn V rocket, which lifted Apollo-11 into space 50 years ago, was designed to send nearly 41 tonnes to the Moon.
Given its limited thrust, GSLV Mark-III can’t carry the Chandrayaan-2’s lunar and orbiter module directly into the lunar-transfer trajectory (LTT).
In Apollo-11’s case, the J-2 engine part of the third stage of the Saturn-V rocket had burnt for over five minutes, performing lunar transfer in one go. This single manoeuvre sent the command, service and lunar modules of Apollo-11, much more heavier than Chandrayaan-2’s module, towards the Moon.
ISRO Used Earth’s Gravity To Workaround This Disadvantage
After the third stage of GSLV Mark-III, called C25, delivered the Chandrayaan-2 module into a highly elliptical orbit around the Earth, its orbit was raised progressively with five burns of the onboard engines. These burns are called orbit-raising manoeuvres.
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 farthest from the Earth and at its slowest speed.
The module’s speed varies across different points in the elliptical 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. Therefore, the module is at its highest speed when it passes through the perigee.
When 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 engine, the module will keep spiralling outwards in increasingly elongated ellipses.
Chandrayaan-2 is not the only craft to have used this method. Of course, ISRO’s Chandrayaan-1 took the same circuitous route to the Moon. Israeli company SpaceIL’s Beresheet lunar lander, which crash-landed on the Moon earlier this year, had also taken this route. And so did China’s Chang'e 1 mission.
Eventually, the Chandrayaan-2’s speed reached the escape velocity (around 11 kilometre per second), required to escape Earth’s gravity, and its orbit was elongated enough for it to set a course to the Moon. This orbit is noting but the LTT (lunar transfer trajectory).
Thus, instead of being sent towards the Moon with a single engine burn, the Chandrayaan-2 module reached LTT with five burns and help from Earth’s gravity.
While Apollo-11 was already on its way to the Moon just hours after Saturn-V lifted-off from the Kennedy Space Center, Chandrayaan-2 had to spent nearly 23 days around the Earth before it could start travelling towards the Moon.
While this is not much of a disadvantage when it comes to unmanned missions to the lunar surface, spending a month in space would not be an option when it comes to undertaking manned missions to the Moon.
ISRO Is Working On A Solution
GSLV Mark-III’s second stage uses liquid fuel while most heavy-lift vehicles, including the Saturn-V, use cryogenic or semi-cryogenic fuel for the second stage.
A cryogenic or semi-cryogenic stage is most sought after as it provides more thrust per kilogram of propellants. (Read more about the cryogenic stage here.)
ISRO is currently developing a semi-cryogenic stage, SC200, and plans to replace liquid-fuelled stage of GSLV Mark-III with it.
Unlike the cryogenic third stage of GSLV Mark-III, which uses liquid hydrogen, SC200 stage will be fuelled by a highly refined form of kerosene (called RP-1).
The advantage of using RP-1 is that the same volume of kerolox (RP-1 and liquid oxygen) will generate more thrust than the same volume of hydrolox (hydrogen and liquid oxygen) because the latter is 10 times denser.
ISRO is also working on increasing fuel loading of the existing cryogenic stage C25 from 20 tonnes to 30 tonnes to improve its performance. With these changes, ISRO says, GSLV Mark-III’s lift capability will go up to 6 tonnes.
Now, back to Chandrayaan-2.
Earlier today, Chandrayaan-2’s orbiter and lander module reached the end of the LTT and was captured into lunar orbit by the Moon’s gravity.
What Happens Next?
At present, the orbiter and lander module is 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.
After the module reaches the 100 km orbit, the lander will separate from the orbiter, which will continue to revolve around the Moon. A separate entity now, the lander will de-boost with the firing of its four breaking engines.
This manoeuvre will bring it to a periapsis of around 18 km. (Periapsis is the point in the path of an orbiting body at which it is nearest to the body that it orbits.)
When the lander reaches this point, that is, at the height of 18 km, the onboard position-detection camera and hazard-avoidance sensor will study the landing site for accuracy.
Using the data obtained, the lander will autonomously determine the trajectory it will have to take to get to its pre-determined landing site and steer itself to a location 100 metres above the site.
Here, the lander will hover and allow the hazard-avoidance sensor to determine the safest landing point. The lander will then be guided to this point and hover at the height of 2 metres above the location.
At this point the thrust will be cut off and the lander will go into a free fall to the impact point with the landing legs attached to it absorbing the impact shock.
The lunar module will land between two craters, Manzinus C and Simpelius N, about 70 degrees south of the lunar equator. If it manages to soft-land at this point sometime around 7 September, it would have reached closer to the moon’s south pole (around 600 km away) than any previous mission.
Once on the ground, the lander will deploy the six-wheel rover to the lunar surface using a ramp. Like NASA’s Mars rovers Spirit and Opportunity, the Pragyan (which translates to 'wisdom' in Sanskrit) rover sports six independently motorised wheels, but unlike the former two, its corner wheels do not steer.
The rover will operate for 14-15 earth days, or one lunar day, on the surface of the Moon in a semi-autonomous manner with ISRO exercising partial control from Earth. The orbiter, meanwhile, will continue to revolve around the Moon and do so for a year.
Also Read: Why ISRO Is Going To The Moon’s South Pole
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