Ion drive

This is the kind of thruster that powered the spacecraft Dawn to the asteroids Vesta and Ceres.  It's beautiful!  It creates a beam of xenon ions.  These ions blast into space at 40 kilometers per second - 90,000 miles per hour! - pushing the ship forward. 

Dawn is solar powered.  It sucks up 10 kilowatts of solar power and uses this to run the ion thruster.  It started out with 275 kilograms of the noble gas xenon.  It takes atoms of this gas and strips off some the electrons, leaving the atoms positively charged.  These are called ions.  

It accelerates these ions with an electric field, and they shoot out of thousands of tiny holes - which I think you can see here.  Each hole acts as a lens that electrically focuses the ions.

Because the ion thruster puts out positive ions, an equal amount of negative charge must be expelled to keep the spacecraft from getting a huge electric charge.  So, a small gadget called the neutralizer shoots out electrons.

The force produced by Dawn's thrusters is tiny: just 40 millinewtons.  A newton is the force it takes to accelerate one kilogram one meter per second each second.  Dawn's thrusters push as hard as a sheet of paper pushes down on your hand!  

So, this spacecraft takes four days to accelerate from 0 to 100 kilometers per hour, while a good car can do it in 3-6 seconds.  The advantage of Dawn is that it can keep up this acceleration for years without running out of propellant.  This is what made it the first spacecraft able to slow down and orbit one body in our Solar System, then take off and go to another, then slow down and orbit that!

Puzzle 1:  Why does Dawn use xenon?  Is it just because this gas has a really cool-sounding name?

Puzzle 2: What bad things might happen if Dawn built up a big electric charge?

Puzzle 3: Why does the ion beam glow?  Why is it blue?
 
Puzzle 4: Another elegant form of propulsion is a solar sail, where sunlight pushes a spacecraft directly.  Why isn't this better than converting sunlight to energy and using that to run an ion drive?  After all, converting energy from one form to another tends to create waste heat.

#dawn