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Chandrayaan-2 set to begin final descent — indigenous mission hours away from history

Chandrayaan-2 orbiter is ready, Vikram lander has separated, all eyes are on Pragyan rover now. ThePrint details what the mission is expecting on the moon.

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Bengaluru: India’s second lunar mission, Chandrayaan-2, is all set to touch down upon the moon’s surface in a few hours from now. The orbiter is already in place, whipping around the moon in circles, going over its poles. The Vikram lander has separated and is making its way to lower altitudes over the moon, preparing to perform a precision firing of its engines and touch down on our celestial neighbour.

Once it comes to a rest and Indian Space Research Organisation (ISRO) engineers determine that everything is stable, the lander will pull open and release a ramp on to which will descend India’s first extra-terrestrial moving vehicle, the Pragyan rover.

A successful landing on the moon will make India only the fourth country to land on the moon after the US, former USSR and China. India will also become the fifth country to land a spacecraft on another body, discounting the European Union (EU). Japan has landed their spacecraft on asteroids, and EU’s European Space Agency (ESA) has landed on comets as well as Mars and Saturn’s moon Titan. Russia has also landed on Venus, while the US, USSR, and EU have touched down on Mars.

But landing on the moon isn’t all that easy — despite the fact that it has low gravity and the feat has been achieved before.

Landing missions to the moon have had a success rate of only 52 per cent so far. Just in April this year, a private company from Israel sent a lander, Beresheet, to the moon but lost communication with it mid-way. It crashed on the lunar surface merely three months after China successfully planted the Chang’e-4 lander on the far side of the moon.

We never see the far side of the moon as the body is tidally locked to earth. This means its rate of rotation on its axis is equal to its rate of rotation around the Earth. Thus, the moon-equivalent of Earth’s 24 hours is 28 days: the time it takes to go around the earth. The moon sees 14 Earth-days of sunlight followed by 14 Earth-days of darkness with a temperature variation of 130 ºC to -180 ºC.

Because of this long duration of lack of sunlight, the Vikram lander and Pragyan rover’s solar panels can’t work beyond 14 days. The two vehicles will go to sleep in the cold darkness and most likely never wake up again. But ISRO will try, and the situation will clear up only 28 days after the landing.

What the mission has on-board — and the science behind it

As part of the Chandrayaan-2 mission, there are eight payloads on the orbiter, four on the lander and two on the rover.

Aboard India’s first space mission, Chandrayaan-1, over half the payloads were from other countries. But the new mission is almost fully indigenous. It carries only one passive payload from the US space agency, National Aeronautics and Space Administration (NASA), on the lander.

On the orbiter are two cameras. The Orbiter High Resolution Camera (OHRC) snapped pictures of the lunar surface as soon as it got near the moon. It imaged the same location twice on different orbits and from different angles, thus creating a digital elevated model of the landing site. This helped ISRO engineers ensure there aren’t craters or boulders where the lander would go, before they release it.

The other camera is a Terrain Mapping Camera (TMC2), a smaller version of the TMC on Chandrayaan-1. Its objective is to map the lunar surface and help prepare 3D maps of it.

There are two payloads dealing with solar x-rays: the Solar X-ray Monitor (XSM) observes X-rays from the sun’s corona and works in conjunction with the Chandrayaan-2 Large Area Soft X-ray spectrometer (CLASS). This instrument will determine how much of elements like silicon, calcium, aluminium, iron, sodium, and magnesium are present in the lunar soil — called ‘regolith’ — by measuring the x-rays they emit when they’re hit with sun’s rays. The XSM calibrates the data for CLASS.

Two other payloads work on studying water: the Imaging IR Spectrometer (IIRS) will not only perform a full mineralogical mapping of the moon, but also study water and hydroxyl. Additionally, it will measure solar radiation that’s reflected off the moon’s surface. The Dual Frequency Synthetic Aperture Radar (DFSAR) will also perform lunar mapping, while also determining the thickness of the regolith and of course, estimate the quantity of water ice.

The last two instruments aboard the orbiter will study the wispy lunar atmosphere, more accurately called exosphere. The Chandrayaan-2 Atmospheric Compositional Explorer 2 (CHACE 2) will continue the work of CHACE 1 which was present on the first mission’s impactor and studied the composition and variation in the lunar exosphere. The Dual Frequency Radio Science (DFRS) experiment will study the evolution of electrons in the ionosphere of the moon by transmitting radio signals.

Of the four payloads on the Vikram module, one complements a radio experiment. The Radio Anatomy of Moon Bound Hypersensitive ionosphere and Atmosphere (RAMBHA) instrument will measure electron density near the surface using radio signals.

The other payloads perform physical work.

Also read: As we await Chandrayaan-2 landing, here’s how Vikram Sarabhai hailed Apollo-11 fifty yrs ago

The Chandra’s Surface Thermo-physical Experiment (ChaSTE) is a digger. A thermal probe will dig and insert itself 10cm into the regolith to understand temperature variation and conductivity in the soil.

The Instrument for Lunar Seismic Activity (ILSA) is perhaps one of the most interesting payloads on board. The moon, much like earth, generates quakes. These moonquakes were detected in the data from the first four seismometers that US astronauts Neil Armstrong and Buzz Aldrin — first humans to walk on the moon — left behind, but their formation is entirely different as plate tectonics doesn’t apply to the body.

ILSA will be able to measure ground displacement, velocity, and acceleration caused by quakes using two sensors. The findings would be important if and when humans decide to build a settlement in the southern polar region some day.

The last of the Vikram instruments belongs to NASA and consists of reflectors. The Laser Retroreflector Array (LRA) will reflect lasers shone on it from the Earth, just like other retroreflector arrays on the moon already do. This enables a more accurate calculation of the earth-moon distance and orbital dynamics.

The Pragyan rover payloads focus on the composition of the regolith.

The Laser Induced Breakdown Spectroscope (LIBS) is a futuristic laser-blasting payload that will identify elements on the regolith by firing high-powered laser pulses and observing the radiation emitted. The Alpha Particle X-ray Spectrometer (APXS) will perform a similar function but by observing x-rays.

The lunar South Pole

The landing spot for Vikram is in between two craters near the South Pole of the moon, called Manzinus C and Simpelius N at 70.9°S latitude, 22.7°E longitude. These are ‘satellite craters’: a large main crater carries a name, and craters around it have the same name followed by capital letters.

The landing spot will be a highland, but there is also a backup at 67.7 °S latitude and 18.4 °W longitude.

Both these locations will still be the southern-most locations visited by any spacecraft.
All previous missions, both by NASA as well as USSR, have stuck to the equatorial belt of the moon. Because the moon’s equator is at an angle just slightly inclined to the Earth’s equatorial plane, flying towards it is comparatively easier and consumes less fuel. To land near a pole, the previous missions would have had to burn more fuel to then change the direction of their spacecraft. All Apollo missions by the US flew directly towards the moon — without performing any of ISRO’s trademark orbit-raising manoeuvres, and carried only limited quantities of fuel.

All lunar samples we have today are from the equatorial region — upon their return, astronauts Neil Armstrong and Buzz Aldrin famously declared the moon rocks they brought home at customs.

The lunar samples available aren’t fully representative of the geology and evolution of the moon, and the ones that can be collected from other parts of the moon could reveal newer information. Understanding the moon’s evolution could also help us understand better the evolution of other rocky bodies in our solar system by extension.

But the main pull of the South Pole is ice.

On astronomical bodies, ice is common and is made of many a frozen gas. But water ice is special, because water is special. India’s Chandrayaan-1 mission over a decade ago had confirmed the presence of water ice on the southern pole of the moon.

Water, specifically in liquid form, immediately signifies a huge spike in the habitability of a body. In the case of the moon, it’s a great resource to utilise if humans build settlements there. It can be used for consumption and split into hydrogen and oxygen for fuel and respiration, respectively. Ice is also a great shield against radiation — underground structures beneath ice offer great protection to life.

Also read: India’s Chandrayaan-2 matters, but don’t dream about moon picnics with friends just as yet


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