Dec 07 2018

Real Ion Drives in Mercury Probe

Most Star Wars fans know that the name of the iconic TIE fighters is an acronym standing for “Twin ion engines.” Most space fans know that ion engines are real, but are nothing like what we see in the movies.

There is a disappointing disconnect between science fiction and science reality when it comes to space travel. The unfortunate reality is that space travel is really hard, so most science fiction simply makes up super-advanced spaceships with highly unrealistic capabilities. There is artificial gravity, impervious shielding, faster-than-light travel, and seemingly inexhaustible fuel. Only hard science fiction, like the recent show Expanse (which I highly recommend) deals with the reality of even future space travel.

It’s tempting to think that, yeah but this is future or at least very advanced technology, so it’s not unrealistic for that tech. That is the disappointing part – when you realize that, yeah, it is. We won’t be zipping around the galaxy in 200 years. It will still be a challenge to zip around the solar system.

There is some basic physics in the way. First, the human body can only take so much acceleration for so long. To get up to really fast speeds quickly, however, you need acceleration that will challenge human physiology. Optimally, ships will accelerate at a comfortable 1g. This will also solve the lack of gravity thing.

But this gets us to the second problem – maintaining that acceleration requires a massive amount of fuel. There is also something known as the fuel equation, because you need fuel to carry the fuel to carry the fuel, etc. So we have a few options.

First, we can have really energy-dense fuel. This means converting as much of the fuel into usable energy as possible. Right now we rely on chemical energy, which is massively inefficient, with only a tiny fraction of the mass turned into energy. Even nuclear power is not very efficient – fission coverts 0.08% of mass to energy, and fusion 0.7%.

This is where matter-antimatter comes in. When matter and antimatter annihilate each other, 100% of the mass is converted into energy. However, this might not all be usable, and roughly half the energy flies away as neutrinos. But even a 50% efficiency would be great.

The other problem, however, is propellant. You have to throw something out the back of your rocket to accelerate it forward. Chemical rockets combine the fuel and the propellant – the burned fuel is the propellant.

So – one potential solution to the limitations of spaceships is to use efficient energy production. This would ideally be matter-antimatter, but even fusion would be a huge improvement. But we also need to use propellant efficiently, and that means accelerating the propellant as much as possible – optimally near the speed of light. The faster the propellant, the more acceleration you get out of it.

This is exactly where ion drives come in. They accelerate ions to very high speeds, so that we get the most we can out of the propellant on board. If we combine a matter-antimatter engine with a particle accelerator that can achieve near relativistic speeds, that is probably the best we can do with onboard fuel and propellant.

Of course the other solutions involve not carrying your own fuel or propellant. Briefly, this could involve a solar or light sail, which uses either the solar wind, or pressure from photons from the sun or a laser to push the craft. We could also scoop up hydrogen from space and use that as propellant or even fuel.

You could also get around the propellant problem if you use fancier physics, like warping space (hence the “warp drive”). But the energies required for this are likely prohibitive.

But let’s get back to the ion drive, because this is a real technology currently in use. The acceleration, however, is nothing like the TIE fighters. BepiColombo is a European-Japanese mission to Mercury which uses a craft with four ion engines. It successfully launched in October, and has now tested all four of its engines and is on its way to Mercury.

The acceleration of each of its engines, however, is the equivalent of the weight of a AAA battery at sea level. This is tiny, but when they fire for months at a time the acceleration adds up. The trip will take 7 years and 5.6 billion miles (9 billion kilometers). It will also require multiple flybys of the Earth, Venus, and even Mercury itself.

That may seem strange – to flyby Mercury on your way to Mercury, but the goal of this path is to insert the probe into Mercury’s orbit. So, the craft doesn’t just fly to Mercury, it has to match Mercury’s orbit around the sun. This requires it to slow down significantly, and that is the purpose of the long ion engine burns and the multiple gravity assists.

Gravity assists, by-the-way, are another way to accelerate a craft without onboard fuel or propellant. It is time-consuming, however, and of course you need large bodies to do this.

Once in orbit the problem will have at least a one year mission observing Mercury. This might be extended to two years if funding is available, and the craft survives that long in good shape.

The ion engines are also the latest technology for spacecraft. They are electric engines that first strip the electrons off Xenon gas, then propel the ions at 50,000 meters per second. This is 15 times faster than chemical rocket propellant. Because of the rocket fuel equation, this leads to a much greater than 15 times efficiency in terms of fuel carried to acceleration achieved (the less fuel you need, the less fuel you have to carry for the fuel, etc.).

The trade-off, though, is the slowness. The engines are great for probes, where we don’t care if it takes years to get to our destination. Sure, it would be nice to get their more quickly, but if we have to wait 7 years to get to Mercury that’s fine. This would not work for human space travel, however. Right now we can do fast short burns or slow long burns, but we cannot do fast long burns (say 1g for weeks or months).

If  we think of getting humans to Mars, for example, it seems like for the foreseeable future we have a few options. We can use conventional rockets, with fast short burns, and then coast to Mars. That’s actually our only choice right now. But what technology might we have in 50 or 100 years? Could we make ion propulsion so much more powerful that we can get significant acceleration out of it? Maybe. That is at least one plausible path.

The other, perhaps more likely, is to use light sails pushed by lasers. This way you don’t have to carry any fuel or propellant, except perhaps some maneuvering rockets. Maybe this will also be supplemented by a highly efficient bank of ion drives. This is the reality of space travel, perhaps forever. Exotic physics is necessary to break out of this reality, and it’s unknown if any such physics will ever be discovered and harnessed – which is very disappointing this at least this science-fiction fan.

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