SMART-1 (Part One)
Published on Aug 31, 2006 at 4:12 pm.
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Filed under moon, rockets.
The vast majority of space missions to the Moon have either been launched by the United States or the Soviet Union. However, in recent years, others have been getting in on the act. On September 27, 2003, the European Space Agency launched SMART-1 to the Moon. While most missions to the Moon took days to arrive, SMART-1 took almost 14 months, arriving November 15, 2004. The mission is now over, and SMART-1 is about to crash into the Moon this weekend. So, what is so smart about a space probe that takes so long to get there?
The instruments aboard this spacecraft, though exception, were pretty much the routine sort of things that you’d expect to send to the Moon. The thing that makes SMART-1 different is its propulsion system.  All the other space missions to the Moon have used chemical rocket propulsion. This is the normal way that you do things. Fuel and oxidizer burn, release energy, and hot gas is expelled from the back of the rocket, propelling the rocket itself forward. SMART-1, though, was designed to use electric ion propulsion. This wasn’t the first ion drive space mission — NASA’s Deep Space One has that honor. Like Deep Space One, SMART-1 was a dual purpose mission: it was designed to do science, but also to act as a test platform for new technology.
So, what is ion propulsion, and what is so special about it? To answer this, we need to stop and think about rocket propulsion. What makes a rocket go? Well, gasses are expelled from the rear of the spacecraft. Conservation of momentum tells us that expelling this gas provides a thrust to the spacecraft. The thrust is given by T = Ru, where R is the mass per unit of time of propellants expelled (for example 50 kilograms per second), and u is the exhaust velocity. This is a form of what is called the “First Rocket Equation.†As I said, most space missions have used chemical propulsion. So, the energy used to heat the gasses and expel them from the rocket comes from chemical reactions. The more gasses that you expel, or the faster that you expel them, them more the thrust. For chemical rockets, while you have some control over how fast the gasses are expelled, you are limited by the amount of energy that is released when you burn the fuel. You can burn more fuel to get more energy, but then you have more material to expel, so the gasses leave with the same energy. But, burning more fuel permits R to be bigger, so you get more thrust. So, the limit on the thrust is mainly an engineering problem: how to build a bigger rocket motor.
Ion propulsion though, is different. Electrical energy is used to accelerate the ions. More voltage means faster ions. There is far more control over the exhaust velocity than with chemical rockets. Also, the energy comes from some other source than the propellants, so you are not limited by chemical reactions. Instead, you are limited by the available source of electrical energy: batteries, solar panels, nuclear reactors, etc. SMART-1 uses solar panels to convert solar energy into electrical energy to power the motor. The disadvantage is that ion engines can’t shoot out as much material per second as chemical engines. In that regard, they are quite limited. So, while the exhaust velocity, u, in the rocket equation is absolutely huge, the value of R is quite tiny. This results in a far lower thrust than you would get from a chemical rocket. So, what good is it?
Another measure of rockets is their specific impulse. The specific impulse is given by I = u / g. Where u is the exhaust velocity and g is the acceleration due to gravity on the surface of the Earth. So, the higher the exhaust velocity, the higher the specific impulse. Since the exhaust velocities of the ions in an ion drive are huge, the specific impulse can be staggering compared with chemical rockets. The total impulse is the total change in momentum of the rocket, and it is the product of the total mass of propellant used times the specific impulse. With a huge specific impulse, it doesn’t take much propellant to add up to a large change in momentum, and that is ultimately what you want. The only problem is that R is so small that it takes a very long time to expel that much propellant. But if you are not in a hurry, the ion propulsion can ultimately yield a far larger velocity using less fuel than chemical rockets.
For an unmanned mission, this is just fine. You can fire the engines for a long time and eventually get where you want to go. Since there aren’t really any moving parts, either, there is almost nothing to wear out in an ion drive, so you can keep it running for a very long time. For manned missions, though, you normally need to get going quicker. Also, the thrust provided by an ion drive is quite low, so ion propulsion isn’t sufficient to lift anything off of the surface of a planet. ESA used an Ariane rocket to launch SMART-1 into Earth orbit. Then the ion drive was able to spiral the craft outward to where the Moon’s gravity could pull it in.
Now, on a lighter, but somewhat related note, this discussion reminds me of a friend who has a T-shirt that reads,
“NASA
It isn’t rocket science.
No, wait … it is!â€
Tomorrow, I’ll discuss SMART-1’s fate. It plows into the Moon this weekend!
-Astroprof
(Image Credit:Â ESA)
