After an aborted attempt on August 29, space agency NASA was to make another bid to launch its Artemis-1 mission, the first in a new generation of Moon probes, on Saturday. Like the previous time, there was a two-hour window to launch the mission, starting 2.17 pm eastern daylight time (11:47 pm IST). However, the mission was aborted for a second time due to a hydrogen leak.
The problem that had arisen during last Sunday’s attempt to launch – one of the four engines of the rocket had not cooled down sufficiently enough — was fixed by Wednesday and a final go-ahead to make another launch attempt was given on Thursday. However, due to the persistent problem its second attempt was also aborted.
Space missions cannot be launched anytime they are ready to go. Very precise timeslots are calculated for their launch. This is because of two main reasons. Nothing in space is stationary. The origin (Earth) is moving, and so is the destination (Moon, or any other planetary body the mission is headed to). One reason for scheduling a launch window is to look for the shortest distance to the destination. The other, related, reason is the need for fuel efficiency. There is no refuelling opportunity in space. And, more the fuel the rocket carries, the heavier it becomes, which again means more fuel is required to propel it.
So depending on which side a satellite is headed in space, the distance it has to travel to get into an orbit, the nature of the probe it has to carry out — surely, several other considerations must also be in play — a precise launch window is calculated. The overall objective is to find the most efficient way — in terms of time and energy — to get to the destination. The launch windows can sometimes be very narrow, may be just a few minutes. At other times, like in the case of Artemis-1 last Sunday and now on Saturday, there is a comfortable two-hour window. The mission can be launched at anytime during this two-hour slot.
So how does it work?
Let’s suppose we have to travel from our house (Point A) to office (Point B). If the traffic considerations are removed, anytime of the day (or year) is good time to travel. The journey is likely to be equally efficient.
But what if the office is not stationary, but moving in a circular or elliptical path? Point B might still be the shortest distance from our home (Point A) but the office is not always at Point B. It arrives there only periodically. If we do not time our journey to coincide our arrival at Point B with that of the office, then we might have to wait. Or, alternatively, we try to get to our office at some point other than Point B, which then is not the shortest route. In either case, the journey is not the most efficient. It takes us longer time to get to office, and we also end up using more fuel. Even in this simple scenario, it is clear that we cannot start our journey anytime and have to wait for suitable time slots.
But what if our home is also not stationary, and is moving around in a completely different orbit? The challenge of charting an efficient journey grows manifold. The points A and B are rendered meaningless now. Depending on the time periods that our home and office takes to complete one revolution in their orbits, the shortest distance between the two may be along a very different route. There would be even lesser opportunities (than in the case where only the office was moving) to make the most efficient journey. And the journey would have to start at very precise timings. Similar to what our space missions have to look for.
Earth’s rotation and revolution
But this is not all. Because of their circular (or elliptical) orbits, it is possible that the two heavenly bodies are closest in distance at a time when they are moving in opposite directions. The Earth goes around the Sun at the speed of about 1.1 lakh kilometres per hour. The nature of the revolving orbit is such that the direction of movement is changing continuously.
If the mission’s destination (say, Moon) is in the same direction in which the Earth is currently moving, then the rocket gets a big boost. It gains from the Earth’s momentum. In the opposite case, if the Earth is moving in a direction that is opposite to where the destination is located, then the rocket has a negative momentum. It has to change direction. A lot of energy is wasted in that.
That is why, depending on where the mission is headed, some times of the year are more favourable for launch than the others.
There is also the Earth’s rotation on its own axis to be taken into account. The Earth rotates from west to east. If the rocket is launched towards the east, it again gains from the Earth’s rotation speed.
Because of its shape (bulge in the centre), the Earth rotates faster at the equator than at the poles. This is also the reason why some of the most prominent launching sites, especially those for heavier satellites, are located near the equator. There are, of course, many other factors that decide the location of a launch site, including proximity to the sea, and distance from heavily populated areas. This is a safety consideration to minimise damages in case of a launch failure.
All these moving parts have to be accounted for while calculating the most suitable launch window.
But there is still weather to contend with. A space mission cannot be launched if the temperature is too cold or too hot, or if the wind is blowing at a very strong speed. Rainfall, humidity, lightning, cloud or smoke, are all factors that need to be considered for a safe and successful launch.
Favourable weather conditions are required not just at the time of the launch, but also during the fuelling of the engines. For the Artemis-1 launch on Saturday night, for example, scientists have predicted at least 60 per cent favourable weather conditions.