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More Interstellar Travel

This week, researchers announced that they have found seven rocky planets around another sun that may be capable of having water – three of which are in the habitable zone of the solar system. The star, called TRAPPIST-1, is about 39 light years away.

pia21422_-_trappist-1_planet_lineup_figure_1
From NASA/JPL.

This discovery raises the issue, once again, of interstellar travel.  A while ago, I wrote a post that discusses how long it would take to get to our nearest neighbor star using modern technologies. Long story short: it would take a many thousands of years.

Since then, articles have been published that discuss getting to another sun using lasers. In the last post, I talked about using lasers to get to Mars. This time, I will talk about the idea of using lasers to get to another star.

The idea of using light to accelerate things has been discussed for a long time. Basically, light bouncing off of a reflective surface will impart some pressure on that surface.  There are two extremely interesting things about using light to accelerate things: (1) if it is sunlight, it is free, which is the cheapest type of energy; and (2) if it is not sunlight, the energy can be generated somewhere besides on the spacecraft (like on the Earth or the Moon), which means that the spacecraft can be much smaller and won’t need huge engines with gigantic fuel tanks. The big disadvantage of using light to accelerate things is that the efficiency is absolutely horrible, with a huge amount of the energy being completely wasted.

The general idea with using lasers for interstellar travel is exactly the same as using lasers for getting to Mars: bounce a laser off the spacecraft, or a gigantic sail, and accelerate it up to an extremely large speed, moving in the right direction. For a trip to Mars, you could imagine having a similar laser system on Mars, so the spacecraft could be slowed down. On an interplanetary trip, the spacecraft would simply (quickly) pass through the solar system of the other star.

So, why don’t we do this now? Well, there are a bunch of reasons:

  1. We don’t have lasers that are large enough and can operate for long enough to accelerate something (relatively large) up to close to the speed of light. The article above talks about using a laser array that is in orbit that would be about 6 miles across.  That is a pretty big array of lasers.  Basically, you would need a ton of lasers that would all fire for a very short amount of time, but combined, the array would provide a constant stream of energy that would rapidly accelerate the spacecraft.
  2. The article talks about accelerating the spacecraft up to speeds of 1/3 of the speed of light in 10 minutes. That would be an acceleration that is 17,000 times gravity. We don’t build spacecraft that can experience that type of acceleration – even 20-30 times gravity is pretty horrible for a spacecraft!
  3. Even with the huge laser array, the spacecraft that is getting the energy would have to be super tiny.  The article talks about a spacecraft that is something like 1 inch in size.  That is pretty small! Considering that the smallest satellites in orbit around the Earth are CubeSats, which are about 4 inches cubed (and that is REALLY hard!), it is unlikely that we will launch anything even CubeSat size on an interstellar trip.
  4. Since the spacecraft are so small, it is hard to imagine how we would get signals from it. Let’s take the New Horizons mission to Pluto as an example. New Horizons has a dish that has a diameter of 2.1 meters. At Pluto, it had a bandwidth of 4.5 kilo-Bytes/sec. Compared to a standard cable modem, this is about 1000 times slower. Ok, New Horizons is not going to stream Netflix – that is clear.  But, it has taken the mission over six months to stream all of the images that they took in their flyby of Pluto. That is a very slow bandwidth! Bandwidth falls off as the square of the distance between objects.  So, if we launched New Horizons to a solar system 39 light years away (which is about 62,500 times further away), the bandwidth would be 0.0000012 Bytes/second. Yikes!  That is slow!  If the spacecraft were really only an inch in size, then the antenna could only be about that big, which means that the bandwidth would decrease by a factor of 10,000. That is really not good.  So, this is the largest issues with this idea.  In some ways, it is like trying to use your cell phone to call someone on Earth from Pluto. “Can you hear me now?” “Uh. No.”

    new_horizons_transparent
    The New Horizons spacecraft.  The antenna is about 2 meters across. The black thing to the right of the antenna is the power source for the spacecraft.  It is a Radioisotope Thermoelectric Generator (RTG), which is just cool to say.
  5. Since the spacecraft would quickly move away from our own sun, it would not have a power source for the entire trip to the other solar system. Which is bad for two reasons: we wouldn’t be able to communicate with it, since it takes energy to send signals, and it would quickly cool down to the background temperature of the universe, which is -269°C. Not many electronic components will survive those temperatures! So, we would HAVE to have a power source that would last the trip, just to heat up the system. That would be big.
  6. It would take at least 40 years as a bare minimum to get there, and would pass through the system in about 10 minutes. On the first front, you would have to count on the scientific community to keep its eye on the prize for the 40+ years of the trip. And, it should be noted that 40 years is the absolute minimum, assuming that we can accelerate it up to almost the speed of light. If we can get it up to 1/3 of the speed of light, it would take 120 years. That is a fair bit of time.  Next, when it arrives and passes through the system, it will need to have an extremely fast camera, since it will be passing by the planets at >500,000,000 MPH. That is a good camera.  Not a cell phone camera.

Ok, I think that you get the point.  This is a really, really hard mission. We are not really close to having interstellar travel. It is great to think about these things, but they really are science fiction at this point.

But, if we just …..

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Using Lasers to Get Moving

In the chain of crazy ideas of how to get to space and how to get from one planet to another, there is an idea to use lasers. Actually, there are a couple of ideas on how to do this.  This is the first of a two-parter where I talk about this idea.  The first part will cover one project that has actually gotten off the ground (literally) and an idea on getting to Mars, while the second post will look at interstellar travel with lasers.

The first idea on using lasers makes a tiny bit of sense.  It is called Lightcraft (get it – light and craft?).  The general idea with this is that you have an object that has a very specific shape on the bottom side. Then you shoot a laser at it and the shaped bottom focuses the laser so that it superheats the air that is touching the object.  The air then is propelled away from the object, resulting in a net thrust that is towards the top of the object.

Interesting, eh?  They have actually tested this with some very shiny objects that are about the size of a fist and are pretty light. Here is a picture:

lightcraft

That is actually almost real size, too! These little things have flown about 75 meters into the air.  That is not, um, unimpressive, I guess. There are several problems with this technology, since it is hard to keep the Lightcraft pointed in the right direction and keep the laser pointed directly at it, and all sorts of other things. My guess is that they have not had the right public relations people and the large amount of funding that is needed to take a project like this from the tiny prototype stage to anything of real size.

Recently, another team has also been working on using lasers to move things about in the solar system.  This idea with this team is to use very high powered lasers in a similar way as we would use the sun and a solar sail.

A quick aside on solar sails (boy, I really need to write a post about solar sails):  When light hits an object, it actually imparts a super, super small amount of momentum.  When you feel the sun beating down on you on a very bright days, it is totally because it is actually beating down on you. Well, technically it is, but in reality, the amount of force on you by sunlight is less than a paperclip put on you.  Like, way less.  But, if you were out in space, and you had a huge reflective “sail”, the light would shine on it and impart a very small force – something like a pound for a sail that is about 1 km². But, imagine if you could turn the brightness of the sun up by a factor of 100. Or 1,000. Or 1,000,000! Then you could get some real force to act on your spaceship!

So, the general idea with using lasers is that you could have a reflective surface on your ship that you would point a really really really intense laser at.  This would impart a large force on the ship and accelerate it. The beauty of this plan is that the lasers all would need to be powered here on Earth, so we could generate it using a nuclear power plant or hydroelectric or even good old-fashioned coal.  The ship could be very small, since it wouldn’t need a lot of fuel to accelerate it, since that power is coming from Earth.

In the article that I linked to above, the researchers say that they could envision getting to Mars in 3 days using this type of technology. Please excuse me if you heard a cough that subtly masked my slight doubt of this claim.

The first (and most obvious) issue with this is that you would have to have some sort of a laser system that would be on Mars to slow down the ship.  So, you would have to build something like a nuclear power plant on Mars. I am sure that this is not really likely to happen soon, since we are so successful at building them here in the United States (sarcasm). But, there are probably less regulations on Mars, so it should be easier. But then there is the whole getting all of the (highly radioactive) materials to Mars to actually build the plant.  Well, any ways, we will get there eventually!

Ok, so now that we have a laser system on Mars and a laster system on Earth, how much acceleration would we need to get to Mars in 3 days?  Well, we would accelerate for half the distance and then decelerate for the other half of the distance.  If we make a very simple approximation that the acceleration is constant, the problem is easy to solve.  Let’s assume that Mars and Earth are the closest they can be together, which is 0.3 AU, or about 45 million kilometers. We need to accelerate through about half that distance in about 1.5 days. Do a little math and we get that the acceleration needs to be a constant 5.3 m/s², which is about half of the acceleration of Earth on the surface.  This is extremely reasonable!

The problem with this is that the power from the laser falls off as the distance squared. This means that the acceleration that the laser system could supply would have to start off extremely large, then would fall to almost nothing, or that the power that is consumed by the laser would have to start off relatively small, and would have to increase dramatically.

Let’s think about how high-powered of a laser you would have to have in either case. I am going to simplify the problem significantly, since I am a relatively simple person. The sun, for reference, exerts about 4.5667e-6 Newtons of force per meter squared of area. This is an incredibly small force! Like, really, really, really small.  In order to exert that much force, the energy in that light is about 1350 Watts, which is a LOT of energy.  So, this idea is not very efficient at all!

Let’s say that we want to send something to Mars that is a 100 kg, or about 220 lbs. This is an extremely small satellite. If we want to accelerate it at a rate of 5.3 m/s², like the example above, we would have to use 530 Newtons. If we had a sail hooked up to this object that was, say, 100m by 100m (about the size of a football field), how much force would the sun exert on it?  About 0.0457 Newtons.  That is not much! And that is taking about 1350 W, as described above.  So, we would need a laser that is about 11,600 times more powerful than the sun to give us our 530N of force.  That would require a 15.7 Mega-Watt laser.  And this would only accelerate it at the 5.3 m/s² for a little while, since the distance between the laser and the satellite would increase and the received power from the laser would decrease.

Let’s say that the laser delivered the 15.7MW (or 530N of force) at a distance of about 10 Earth radii away from the surface of the Earth (I had to choose a distance, and this was quite arbitrary, but whatever). If you wanted to continue to accelerate the satellite at 5.3 m/s² all the way to the halfway point between Earth and Mars, the power of the laser would have to increase by a factor of about 500,000 times while it was shining on the craft. This means that in order to accelerate it all the way to the halfway point, the laser would have to be a 7,800,000 MW (7.8 Tera-Watt) laser, and would have to fire (ramping up in intensity) for about 36 hours.

Practical?  I don’t know.  This website talks about a 2,000 TW laser that was fired for 1 pico-second (not very close to 36 hours). Another website talks about getting a 10 TW laser that fires for about a femtosecond (that is also pretty short), but fits on a desk.

Where could we get the power? Well, if the sun delivers 1,350W of power per 1m x 1m area, then we would need about 5,800 km² of solar panel area to get that much energy.  Oops, solar cells are not perfectly efficient (more like 25% efficient), so we would need about 23,000 km² of area, which is about 150 km by 150 km of solar cells. This is about the size of New Jersey.

Anyways, the idea is that power on Earth is very cheap, while getting that power into space is really painful.  So, it is ok to take a HUGE hit on efficiency to accelerate something up to enormous speeds in space using Earth-based systems, instead of trying to haul some sort of chemical rocket engine up to space. In fact, chemical rockets will never get us to another star, so it is a non-starter. But, that is a conversation for next week (I promise!)