What’s the first thing that comes to your mind when you hear the word ‘home’?
The dictionary may define it as a place where one lives, but to me home isn’t really just a physical place, but more like a feeling. Philosophical much?! It’s funny how, in addition to helping me pay my rent and ensuring my sustenance, satellites teach me important life lessons once in a while.
The home I grew up in, would always hold a special significance for me. But I have been away from it for more than half of my life now. In this spirit of ‘keep going, keep growing’, I have had the pleasure of calling many places ‘home’ and each has been special in it’s own way. Presently I write this article from my husband’s childhood home, which is also one of my many homes now.
Anyway, thinking from a satellite’s perspective, what would constitute as home for it? The earthly residence where it grows up, from a mere concept in a meeting room to a fully integrated spacecraft in the clean room? Or up there in space, the orbit it’s bound to spend the rest of it’s life in? I don’t know, maybe both. Hence the title. But how does a satellite really make it’s way ‘from home to home’? Without getting into a lot of complex physics (after all it IS rocket science), I’d take help from a simple experiment about projectiles.
Imagine you are standing in a field with a ball in your hands. If you drop the ball, it would vertically fall on the ground because of something our friend Newton accidentally discovered while sitting under a tree a few centuries ago. You get a bit adventurous and decide to add some horizontal velocity while throwing the ball instead of just dropping it. You’d notice that after traversing a short horizontal distance, the ball now takes a curved trajectory and falls a few steps away from you. Next, you decide to increase the velocity with which you throw the ball horizontally. It still falls after following a different curved trajectory, and this time manages to travel a bit farther as compared to before. I guess you get the point now.
On the extreme end of the spectrum, suppose you throw the ball horizontally with infinite force, such that it never falls, since the gravitational pull would be negligible as compared to the tangential push your superhumanly powerful throw gave it, thereby forcing it to travel in a straight line, forever. But who wants that, right? (Note: satellites not wanting to orbit the Earth and aiming to leave it’s sphere of influence would need that kind of velocity, but we talk about that some other day).
Like everything in life, moderation and balance is the key! So imagine if you threw the ball imparting just enough horizontal velocity, with the tangential force balancing the pull of gravity, so that the ball no longer literally falls on the ground, but symbolically continues falling, thereby following an infinite curved trajectory.
Did you notice how I conveniently put the onus of imparting a sufficiently large enough tangential velocity on you? Well, on our atmosphere-surrounded home-ball called Earth, air resistance or drag would also work against you. What I mean is that, in addition to gravity pulling the ball down, it is also subjected to air drag that’s pulling it back and basically reducing it’s tangential velocity that you imparted by swinging your arms to the maximum extent you physically could. How do you solve that? Simple, rise above!
I know it is difficult to visualise this, but I propose a sub-experiment to make my point. Imagine hypothetically running in water. Seems like a lot of effort, right? Don’t worry, let’s make it a bit easier. Air is about a thousand times less dense than water. Picture running against air, with much less resistance, feels like a walk in the park now, doesn’t it? And, now imagine outer space (say 100 km above), where, for all practical purposes, no air exists, or it is about a trillion or so times less dense than here on the Earth’s surface. With lesser density the air drag acting on the spacecraft is much lower but we can talk about that some other day and for now just naively assume that space equals no air equals no air drag.
And that is how powerful rockets or launch vehicles take satellites from one home to another.
For more, you may check out this link (Credit: NASA/JPL-Caltech).
A typical Low Earth Orbit (LEO) satellite is injected into it’s final operational orbit by a launch vehicle. They are multi-stage rockets that take satellites to space by burning propellant (solid, liquid or cryogenic) to generate huge amounts of thrust required to counter the Earth’s gravitational pull. The multi-stage design helps them be more efficient, since spent stages are simply discarded (or recovered when you’re SpaceX) and need not be carried ahead as dead mass. The last stage injects the satellite into its operational orbit or into a transfer orbit in case of satellites that are destined to reach much higher orbits than LEO, for example the geostationary orbit (GEO) 36000 km away from Earth’s surface or those headed to other planets millions of km away. Most satellites are equipped with their own propulsion systems (thrusters or mini-rockets relying on chemical or electrical means to generate thrust) to help make their way through space. Many satellites which are on their way to far-off planets also efficiently rely on flyby manoeuvres and gravitational slingshots to get a somewhat “free” (or propellant-wise inexpensive) push in the right direction.
At this juncture, I think it would also be useful to understand the difference between orbit (position and velocity) and attitude (orientation and rotation rate) control. A satellite’s propulsion system helps in acquiring, maintaining or changing it’s orbit and also serves essential purpose during contingencies like attitude loss and re-acquisition. Most of the satellites I have worked with rely on chemical propulsion. Some have mono-propellant systems with a liquid propellant in a single tank and some have bi-propellant systems with fuel and oxidizer in separate tanks. There are also satellites relying on nuclear propulsion, ion propulsion and electric propulsion. But for nominal attitude control during the operational phase, satellites primarily rely on reaction wheels or momentum wheels which work based on conservation of angular momentum by spinning up or down depending on the sensed deviation from reference orientation or rate. They offer finer control without spending life-limiting propellant.
Some satellites meant to orbit in LEO, reach their final orbit in a matter of minutes (thanks to powerful launch vehicles). Those going to GEO could take a few days to reach their new home. Others are travelling so far away that they take months and sometimes years to reach their final destination.
I don’t know why I am obsessed with them, but I think the Voyagers are my favourite man-made objects in space, so I keep referring to them (like in my first ever post here). They have been in space for over four decades and are presently beyond the limits of our solar system in the interstellar space. They are still on their way, to wherever home is. Being explorers, wanderers and voyagers in the truest sense, for them home is everywhere and nowhere!
I couldn’t end this post without mentioning the much awaited launch of James Webb Space Telescope (JWST) from Europe’s spaceport in Kourou, French Guyana scheduled for tomorrow. It reached there a few months ago after undertaking a long sea journey from California. It has been around 25 years in the making so the platform, the telescope and all the parts that make up JWST would have spent a lot of time at their various homes being developed, tested, assembled, integrated and re-tested. It would take another 30 days for JWST to reach its final orbit at Sun-Earth Lagrange point L2 which is about 1.5 million kilometres away.
Well, JWST deserves a post of its own, so I will write on it very soon!
I hope you enjoyed reading this post. Thanks for flying by!
If you’d like to read the other space posts I have written, you can find them here:
Until next time!
the drifting satellite 