Delta-V of ion thrusters So I recently started a podcast along with a cohost. It's all about spaceflight. We ran into a question we couldn't quite resolve due to conflicting information on the internet.

Put simply: Can the Dawn spacecraft actually achieve a delta-V of 10 km/s with its ion thrusters alone? 

My cohost seems to think that's far too much change in velocity for xenon ion thrust. I myself am not sure what to conclude.
If by some chance you have a more conclusive answer we'd love to hear it.
Our podcast can be found at our website along all other info, just in case you're interested:  www.theorbitalmechanics.com
 A: What matters for the performance of rocket engines is a figure of merit called specific impulse: http://en.wikipedia.org/wiki/Specific_impulse. In case of the Dawn engines that's 3100s. A typical chemical rocket engine can achieve somewhere between 250s-400s, so one could say that the Dawn engines are performing about 10 times better than typical chemical engines. 
One way to interpret specific impulse is by using it to express the time the rocket could hover over the launch pad by thrusting, assuming that it would always weigh the same as its original propellant mass weighs. An ideal chemical rocket can therefor hover approx. 5-6 minutes before the propellant runs out. Dawns engines (if they could produce enough thrust) could hover for 52 minutes! Since during this time gravity accelerates the craft with approx. 10m/s^2, the Dawn engines can make up for over 30km/s of delta v for their own propellant mass! 
The spacecraft has 425kg of Xenon propellant and a total wet mass of 1240kg, i.e. over one third of the craft is propellant. Given the theoretical max. delta v and the spacecraft mass the 10km/s effective spacecraft delta v is in the right ballpark. Including the mass decrease during thrusting we can get a little more. This page has a delta v calculator: http://www.strout.net/info/science/delta-v/
If I enter a wet mass of 1240kg, a dry mass of 815kg and a specific impulse of 3100s I get a delta v of 12.758km/s. 
A: To answer your question: yes, it's perfectly possible to achieve a sizable $\Delta V$ with an Electric Propulsion device as long as you take enough time and have enough propellant. EP is in general low thrust but high $I_{sp}$. I assume they have enough time and/or planned a low-thrust transfer rather than a more traditional method so we wont consider that. Instead let's make sure the propellant cost isn't ridiculous. From the Dawn wikipedia page the following values are taken:
Thruster Isp: 3100s.
Spacecraft wet mass: 1240kg. 
So we just use the Tsiolskovsky equation with $\Delta V = 10$km/s to find the mass of propellant required:
$m_f = m_{wet}e^{-\Delta V/(gI_{sp})} \approx 1240kg e^{-10000/31000}=898kg$
which is a propellant mass of just about 341kg. This is reasonable for a large s/c on a major mission. There is a claim on the wiki however that the s/c exceeded a previous record of 4.3 km/s using less than 67 kg. I am not sure whether or not they mean to claim that Dawn used less than 67 kg as well or if they are claiming that Dawn's EP has more Isp. The former claim would be highly suspicious given this calculation, so I assume they mean the latter.
