Setting Sail in the Space

It is the age of space exploration!! The scientists are creating everything we will need to achieve this feat, which will help us... perhaps in growing into a multi-planetary intelligent species. Or at least we will learn a lesson or two from it... and then keep trying till we succeed... I really don't think this is going to stop in any way. It is just a matter of time, and enough knowledge... We really have a lot of problems to solve.

If we fail in our quest living on other planets (or moons) in our solar system, at least we will have a lot of resources to consume and grow. We can't consume much from earth any more because that is impacting us badly, with green house, and climate change, and hurricanes, and tsunamis, and the unending pollution and ever growing population (I'm an Indian, I know we have a huge problem).

One of the most important priorities in our to-do list is speeding up our journeys in the space. If we are exploring, we have to do it fast, and for that we need the most efficient means of transport. We might use nuclear fuel, or methane based fuel, or any other resources, but we would still be exploiting earth. And luckily, since the physicists and scientists are looking for solutions, they came up with something really cool for us, which I'd love to tell you today.

You see, when we are travelling in space, we don't have any friction to deal with. It's very different than travelling on land or water or in air. Let's see how.

On land we use a motor which makes rotates and causes the wheels to push the ground beneath in backward direction, thus causing us to move forward instead... just like we walk, using friction. In water, we use fans, and pedals to push water behind the boat, causing the boat to move forward. In air, it's a little more complicated since we need something like a bird or a dragonfly (I mean an aeroplane or a helicopter) for a proper balance, and we need fuel which we throw backwards at high speeds and then move ahead.

While travelling in space, the friction we can use is zero. So every amount of speed we can achieve, is only by these two means..

We throw some mass at high speed behind us. This increases our speed by some extent, or else we travel at a constant speed.
Or we put our rocket get into the gravitational speed of something massive, like jupiter, and revolve around it till we gather some momentum, and then use it to move ahead in a specific direction at an increased, but still, a constant speed. Just like a slingshot.

Basically, all the methods we used only got us a lot of fuel consumed, or a lot of time consumed in rotating around a heavy mass to gather momentum, and we gained only a temporary acceleration. But our search for a means of constant acceleration helped in the invention of the Sail. It just like a boat being pushed ahead by the wind. We just apply this concept in the space.

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Now, let me first clarify that this concept is too new, and isn't yet properly tested in space, but till now, nobody has any doubt about its functioning. I'll tell you the concept of it, and let's see if you have any questions!!

First : It is powered by... The SUN.

The sun throws out a lot of light and a lot of solar wind. And using this light or the charged particles that make up the solar wind, is the next best thing for us. Let's see the process in a bit detail.

Using The Solar Wind:

The solar wind is nothing but a stream of charged particles, which get emitted from the sun. By charged particles, I mean electrons and protons. These electrons and protons are the source of energy for the two types of sail designs that are yet proposed... the electric sail (E-sail) and the magnetic sail (magsail). There also is a solar sail which uses the momentum of photons emitted by the sun.

How does the sail use these particles to gain speed?

Remember our two ways of gaining speed? Scroll up a bit if you wish. The sail of any kind utilises the first method to gain speed.

The Electric Sail :

Although there are various designs proposed for the electric sail, all the designs function the same way.

Suppose we have a positively charged plate (or disk) in the space. It will obviously repel and hence deflect the positively charged particles (protons) coming from the sun. And this action would push our sail away from the direction in which the charged particles are approaching, thus giving it momentum. But there is a trick here.

There are negatively charged particles (electrons) as well in the solar wind, won't these get attracted to our positively charged plate and strike it and neutralize it? Well this problem is solved as well.

The solution is an electron gun which fires the electrons in the opposite direction, further adding to the acceleration, while maintaining the positive charge on the plate.

To see this in terms of momentum we gain, we can talk about the mass of the particles and the speed with which we throw them away, or deflect them. The mass, we know that, the speed at which we push them back depends on the amount of charge and the area of the plate or disk or any form of sail would cover, so the answer is... it depends.

We are using positive charge because protons are many times heavier than electrons and pushing them backwards would give us more momentum as compared to pushing same number of electrons from the electron gun. Luckily the charged particles are mostly electrons, so we have a huge amount of them to gather a little momentum, which is much better than a constant speed, or momentum at the expense of fuel from earth's resources, or than spending a lot of time gathering it by revolving around Jupiter.

And the almost constant acceleration we might be able to achieve is... 1 mm/s². Looks so slow doesn't it?

Well, acceleration by definition means it adds up speed over time. This 1 mm/s² means that 1mm/s of speed gets added to the spacecraft every second. So suppose a spacecraft starts at any constant speed, the sail would keep adding 1mm/s to that every second.

Which means, every 1000 seconds (almost 16.5 minutes), the speed increases by 1m/s. And with every million seconds (almost in just 11 and a half days) it increases by 1 km/s. Now, considering the scale of distances in the scale of 1 AU, which is around 149.59 million km, "relativity" does change some calculations.

The faster the sail's speed is, the slower it will increase further. An object can only travel at most at the speed of light, on the condition that it has zero mass, like photons do. As a consequence of this, the electric sail mission planned for Uranus, can achieve 20 km/s when it gets there, and it would take six years to reach. Which is quite an accomplishment really, since we don't have any other efficient options to get there.

This acceleration also decreases by the the decrease in relative force as well. You see, when we are in a train, we feel the backward push only when it is starting or accelerating quickly or the forward push while stopping, or decelerating quickly. During the rest of the journey, although the speed of train changes a bit, we feel quite comfortable.

So this means, higher the speed of the sail, lower the relative speed at which the charged particles travel towards the sail, lower their relative momentum, and hence, lower the acceleration we gain from them. Also, the acceleration decreases with the mass of the payload. Since it is about conservation of momentum.

Good news still is the length contraction. The higher the speed we attain, the more the distance decreases by a factor of
L = L₀[1 - v²/c²]^1/2, where v is the speed of sail and c is the speed of light. L₀ is the distance we see from earth, and L is the distance it actually is for the sail. Ignore the math, just remember that travelling faster decreases the distance we need to travel as well.

Which is a great because owing to this, we can get out of out solar system which is about 121 AU, in just 15 years. The last mission by NASA's Voyager spacecrafts, that achieved this, took 35 years.

One disadvantage of electric sail is that it cannot be used for braking. When we arrive near the destination, we obviously need to stop, right? Hence, for this purpose we have to use our natural resources, and/or the magnetic sail, if the planet we arrive at has a magnetic field. I'll discuss the magnetic sail in my next post on space.

I bet you have a question - If the sail pushes itself away from the sun, how will we use it to return to earth from a journey?

Well, I don't know for sure, but those who did the math say , it is possible if we simply align the sail properly, if you have heard of tacking in ships... here is an image of their trajectory when sailing against the wind...
Tacking_Intervals.svg.png
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We do the same for our electric sail against the solar wind.

More applications,we can use the electric sail just for passing by the planets we want to study, just for flyby or just get pass the atmosphere in a controlled way. Or we can just drop some robots in the gravitational field, which could then make their way to the surface on their own.

NASA is building an electric sail along with Pekka Janhunen, the creator of electric sail. This mission is named HERTS or Heliopause Electrostatic Rapid Transit System. Heliopause is the farthest boundary of our solar system that we know of. This mission started last year itself.

The new version will be using thin strands of wire instead of surfaces as Janhunen had originally proposed since collisions with micro meteorites could cause damage. The plus point here is that even if one of the wire gets damaged, the rest of the sail can function uninterrupted. More benefits are their light weight and low volume. These wires will be made to continuously rotate in order to gather more momentum from the solar wind particles. Plus they will have lengths up to 20 km each!!

See you soon, take care.

References:

Setting Sail in the Space - Part 1