LAUNCH WINDOWS
Source: The Indian Express
Introduction:
- Space missions cannot just take off at any time, they require a precise timeslot for launch to find an efficient way, in terms of time & energy to get to their destination.
- The space agency NASA has to abort the launch of its Artemis-1 mission to the moon due to a problem found during fueling engine tanks.
- NASA said it would not make any further attempts to send the mission during the available launch opportunities till September 6.
- The next set of launch windows -14- are available between September 19 & October 4.
Launch Windows:
- Nothing is stationary in space. The origin(earth) is moving and the destination (moon/other planets/satellites)too.
- One reason for scheduling a launch window is to look for the shortest distance to the destination.
- The other is fuel efficiency, as space has no refuelling opportunity.
- The more fuel the rocket carries, the heavier it becomes, which means more fuel is needed to propel it.
- Depending on which side a satellite is headed in space, the distance it has to travel to get into orbit, the nature of the probe it has to carry & several other factors are also calculated in a precise launch window.
- The launch windows can sometimes be very narrow, even a few minutes.
Mechanism:
This mechanism can be better explained by the following example.
- Let's suppose we have to travel from our home(Point A) to the workplace(Point B).
- If the traffic considerations are removed, any time is good to travel.
- If the office is not stationary, but moving in a circular or elliptical path, point B might still be the shortest distance from home(point A), but the office is now not always located at point B.
- It arrives there periodically.
- If we do not time/schedule our journey to coincide our arrival at point B with that of the office, then it has to wait.
- 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.
- The challenge of charting an efficient journey grows manifold.
- Depending on the trajectories of Point A & B, the shortest distance between the two may be along a different route.
- There would be even fewer opportunities to make the most efficient journey and the journey would have to start at very precise timings.
Objectives:
- The objective of the mission is to land on a particular side of the Moon or go around it in a way that a certain part of the moon is observable.
- There can be hundreds of orbits around a planetary body.
- The mission has to enter one of them for specific needs.
- The choice of the orbit or the location of landing, matters in the launch window.
- The power of the rocket is also a factor.
- One of the reasons why the Artemis-1 has many launch opportunities is the fact that it is being carried by the most powerful rocket ever built.
- Named the Space Launch Vehicle /SLS, this rocket can take the spacecraft directly to lunar orbit.
- The Saturn V rockets used by the Apollo missions could go directly to the moon, making the journey in 3-4 days.
- Most other lunar missions, including India's Chandrayan missions, take a circuitous route because they don't have the energy to make the direct journey.
- The rockets deposit the spacecraft in near-earth orbits, which requires less energy.
- Once there, they periodically fire thrusters to progressively increase the orbit and gain energy to move forward.
- Chandrayan-2 took 12 days to reach the moon's orbit.
- The routes have to be taken into account for launch window calculations.
- The satellites in space, in most cases, derive energy from the sun.
- So, their trajectories have to be designed in a manner that the sun is not entirely blocked from their view for prolonged periods.
Earth's Rotation & Revolution :
- Because of their circular/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 1.1lakh km/hour.
- The nature of the revolving orbit is such that the direction of movement is changing continuously.
- If the mission's destination is in the same direction in which the earth is moving at the time of the launch, 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 the destination, then the rocket has negative momentum.
- It has to change the direction in which a lot of energy is wasted.
- Depending on the location of the mission, particular dates of the year are more favourable for launch than others.
- There is the earth's rotation on its axis 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, 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 for heavier satellites, are located near the equator.
- many factors decide the location of a launch site, including proximity to the sea, and distance from highly populated areas as a security consideration.
Weather :
- A space mission cannot be launched if the temperature is too cold or too hot, or if the wind is blowing at a strong speed.
- Rainfall, Humidity, lightning, cloud, and smoke are all factors that need to be considered.
- Favourable conditions are required not only at the time of the launch, but also during the fuelling of the engines.
