Ion Rocket to Map Moon with X-Rays

jralls writes "The Guardian is reporting that a European ion-rocket has taken the last year to reach the moon and is about to enter lunar orbit. Once it slows and gets into a very low orbit, it will probe the surface with x-rays in an effort to solve the long standing puzzle of the moon's origin."

Comments

Re:Where do you think the Moon came from?

[MrWim]

It was a spaceship!

AKA "Thats no moon"

Re:A year?!

[Garion Maki]

unlike in the 60s and 70s, they are using ion engines for this mission, which can run of solar power.
they give less trust/second, but they can keep burning for allot longer, since the sun gives a constant supply of fuel (in the form of electricity from solar panels).
so you've got a smaller probe, which means easier to get into orbit from where it can fly on it's own power, so even tho it takes longer to get where you want, it will be cheaper to get it into orbit.

btw, they are planning on bigger engines in the future, so hopefully they will go faster someday.

Re:A year to reach the moon?

[SpryGuy]

The ultimate speed of ion propulsion is higher than that of chemical propulsion.

But the mass being expelled at high speeds (the ions) is so low, that accelleration is VERY slow. So it takes a long time to get up to speed, but the maximum speed you can theoretically reach is much greater than that of chemical rockets.

Possible Resolution to US moon landing hoax theory

[jeoin]

Will these guys be able to snag some good shots of the trash we left on the moon? Exluding the flag of course, which can't be trash cuz its on a stick.

Re:Visibile from Earth?

[NetKraft]

Ion rockets can't generate very high accelerations. They can, however, keep going for a long time.

Re:A year?!

[GileadGreene]

Ion engines still use propellant (typically Xenon, but I haven't looked at what this particular mission is doing), they simply accelerate the propellant using electrostatic fields (in the case of ion engines) instead of chemical combustion. The key to ion engines isn't so much the solar power, as the fact that they have a much greater specific impulse (a rocket engineering term that relates to the efficiency with which propellant is used). Where a chemical engine may top out at ~400 s of specific impulse, ion engines have hit around 4000 s, or an order of magnitude greater propellant efficiency. That translates into a much smaller amount of propellant to do the same mission.

The tradeoff is betwen the extra time it takes to get to the destination (due to the low thrust of an ion engine), and the reduced cost created by being able to launch a much smaller amount of mass into space in order to do the mission.

btw, they are planning on bigger engines in the future, so hopefully they will go faster someday.

The issue with ion enginer thrust is not so much size, as it is power. The thrust you get is directly proportional to the amount of power you can generate. If you're using solar arrays, then you're limited to something between 15-20 kW (the Boeing 702 has solar arrays that produce ~15 kW at end-of-life).

Re:A year to reach the moon?

[Brett Buck]

> The ultimate speed of ion propulsion is higher than that of
> chemical propulsion.

Depending of course on the fixed mass of the spacecraft, vs it's propellant mass, of course. You get more momentum change from given amount of propellant, but if you only had a teaspoon full of propellant, or the spacecraft was exceptionally massive, you wouldn't get more velocity.

> But the mass being expelled at high speeds (the ions) is so
> low, that accelleration is VERY slow. So it takes a long
> time to get up to speed, but the maximum speed you can
> theoretically reach is much greater than that of chemical
> rockets.

To expand, the measure of efficiency of a rocket engine is the specific impulse or ISP. It's how much momentum change you get per unit of propellant mass, and the usual unit is seconds (lb-sec/lb). The highest actually-achievable ISP from a chemical rocket is somewhere in the 475 seconds. The Saturn 5 first stage was more like about 350, and monopropellant thrusters used for many satellite propulsion systems is more like 150-180! That means that if you want to change the velocity a lot, you need a whole lot of propellant.

I'm not sure which engine this particular program uses, but the ISP of the typical Xenon ion thruster is something like 1800. So you have to carry fantastically less propellant for a given velocity change, meaning it can weight less at liftoff, meaning you can use a weaker/cheaper booster.

The downside is that you don't get something for nothing. It takes, not surprisingly, a whole lot of electrical power to make it go. So you put in 4000-5000 watts of power, and it only generates .04 lb of thrust - .64 of an ounce, pushing a spacecraft weighing thousands of pounds on the ground. So the acceleration is very small, meaning takes a long time to get going. The other downside is that the Xenon ions, although chemically pretty neutral, shoot out at such high speeds that anything that gets in the exhaust gets eaten away. This may or may not be an issue depending on there you put it relative to the rest of the spacecraft.

Brett

The ion drive is the real story

[SimURL]

Ion drive technology allows you to explore space in ways that chemical rockets simply can't.

Quoting from the article,
"We have shown that even a small ion engine like Smart's can get us across space. Now we are planning to build space telescopes and robot probes to planets such as Mercury, using bigger and more powerful ion engines. These will take years off space-travel times. Instead of decades-long missions, we will take only a couple of years to cross space for future projects."

But,
"Ion engines need electricity and only solar panels can provide enough at present. So ion engine missions will be restricted to planets and moons near the Sun."

So the solution to deep space exploration is nuclear-powered ion-drives and NASA is working on it.