ISRO Chandrayaan-2 launch: The GSLV MkIII vehicle stands at Satish Dhawan Space Center in Sriharikota, on, July 15, 2019. (AP Photo: Manish Swarup) ISRO Chandrayaan-2 launch: The GSLV MkIII vehicle stands at Satish Dhawan Space Center in Sriharikota, on, July 15, 2019. (AP Photo: Manish Swarup)

A 20-hour countdown to the launch of Chandrayaan-2 at 2.43 pm on Monday (1443 hrs on July 22, 2019) is rapidly approaching its end. These are tense hours and minutes for scientists at the Indian Space Research Organisation (ISRO) because the window of opportunity for the launch of India’s first lander mission to the Moon is extremely small — barely a few minutes — and the scientists have very little flexibility in the final minutes before lift-off.

ISRO called off its first attempt at sending Chandrayaan-2 to the Moon less than an hour before the scheduled lift-off in the early hours of July 15, after a sudden drop in pressure was observed in a chamber filled with helium gas in the launch vehicle, the GLSV-Mk III heavy-lift rocket.

Chandrayaan-2: What is this ‘launch window’ that ISRO is targeting?

Launching a spacecraft or a satellite is not as simple as an aircraft taking off from one airport for another. A ‘launch window’ is the time slot within which a particular mission must be launched in order to ensure that it reaches its destination — which could be a rendezvous with another spacecraft (or the International Space Station), a heavenly body (a planet like Mars or, as is the case with Chandrayaan, the Moon), or another point in space (such as a satellite targeting a specific orbit around the Earth, or a spacecraft seeking to enter into orbit around, say, Mars).

Follow LIVE UPDATES on the Chandrayaan-2 launch

Should bad weather or a technical snag (as it happened in the case of Chandrayaan-2 on July 15) result in a spacecraft missing that launch window, scientists have no option but to postpone the launch until another window opens up. Such a window has arrived now for Chandrayaan-2, but it is much smaller than the window originally identified for the launch a week ago.

Since everything in space is in motion, and because there are no straight lines in space flight, a successful launch is a matter of detailed and complex mathematical calculations on the basis of which scientists ensure that the intended orbits overlap at the desired point in time in the future.

Imagine you are sitting in a merry-go-round or carousel, in which your seat is spinning rapidly even as the carousel itself is revolving. Some distance away is another rapidly moving carousel. Now imagine you have a ball in your hand, which you are trying to throw into a particular seat, also spinning, of the second carousel. You can imagine that it’ll be a lot more difficult for you to take proper aim and hit your target than it might have been if your own carousel and your seat in it, as well as the other carousel and its seats, were all stationary!

For an idea of these motions, recall that the Earth is racing around the Sun at 1,07,000 km/hr while also spinning eastward on its axis at a speed that is 460 metres per second (or 1,656 km/hr) at the Equator (and slower by various degrees elsewhere). The Moon is traveling slower, at 3,683 km/hr around the Earth — and also spinning slower about its axis than the Earth.

The Moon needs about 27 days to complete a full rotation (which is about the same time that it takes to go once around the Earth), compared to the 23 hours, 56 minutes, and 4 seconds that the Earth takes to rotate once about its axis. Add to this the fact that our Solar System itself — including the Earth, Moon, planets, etc. — is whirling around the centre of our galaxy, the Milky Way, at a dizzying 7.89 lakh kilometres per hour!

Launch windows are defined for two separate time intervals. There is a “daily window” of a few hours in a given 24-hour period, and there is a “monthly window” of a few days during a given month (or, in the case of Chandrayaan, lunar cycle). A monthly launch window is not continuous.

Obviously, it is desirable for reasons of operational flexibility to have as large a daily and monthly launch window as is possible. A daily launch window covers for possible delays (or “holds”) during the countdown. A mission does not have to be postponed to another day if the daily launch window is bigger than the sum of all the delays or holds.

A top ISRO scientist told The Indian Express that the most suitable launch window for Chandrayaan-2 this month was between July 9 and 16. On these days, the daily window during which the mission could be launched extended to about an hour or more. But today (July 22), this window is barely a couple of minutes. That is why all operations have to be completed with extreme precision; there is no room for a hold.

Similar narrow time slots are available on a few other days of this month too, but after that, a suitable widow will be available only in September.

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