European Moon Mission Ready for Launch

merryprankster writes "Europe's first mission to the Moon is set for blast off from Kourou in French Guiana just after midnight, local time, on Sunday. SMART 1 will study the composition of lunar rock through X-ray observations. The probe uses a new solar electric propulsion system which converts solar energy its panels into motion via the expulsion of ions. Details at the ESA mission site."

Comments

A bit on ion propulsion

[Unknown Kadath]

The reason ion engines are a good thing is because they are so efficient. But they also have their share of problems.

The figure of merit for rocket propulsion is specific impulse (Isp). It is a measure of unit thrust per unit mass of fuel consumed per unit time. Conventional (chemical) propulsion, such as solid rocket boosters, have an Isp in the 200 - 300 range. But they generate many many thousands of kilonewtons of thrust. That's why we use them for launching things out of gravity wells.

Ion engines, on the other hand, have Isps from 2000 - 3500 (though the higher end of that range is only test-stand stuff right now). They, however, produce only millinewtons of thrust, and cannot be used for fast orbit transfers or launches. But they can be made small. Very, very small, with correspondingly small amounts of fuel, which is pure joy for aerospace engineers trying to design robotic missions.

Unfortunately, they are also power-hungry little buggers. A single ion engine can use a kilowatt of power while running...and they must be running all the time to generate enough delta-v to have an effect on the course of a spacecraft. (Delta-v is the measurement of how much of a change in a velocity vector is necessary to effect the desired change in course, and mission designers begrudge every cm/s...every maneuver burns propellant, and there are no gas stations in space.) There are only two ways to get power in space right now: solar cells, and some form of nuclear decay. Only solar cells have a good enough power/mass ratio to run ion engines, and as missions proceed farther out from the Sun, array area must be bigger, which adds mass. It's a tricky balancing act.

For this mission, however, the craft will always be close enough to the sun to generate the power it needs fairly easily. (Except when it's in shadow, but that's why we have storage batteries.)

Ion propulsion is an old technology, incidentally. It's been around in some form or another since the 60's. It's only recently that it became economical, though.

I could go on for pages, but I'm unconvinced anyone wants to see that. ;) I did my senior thesis on a solar electric propulsion Mars mission, and I find it to be far more interesting than most people seem to.

-Carolyn