Also, an asteroid is made of reaction mass. An asteroid is made of physical matter, yes, but it's not made out of the noble gas xenon, which NEXT used. Skimmin

Also, an asteroid is made of reaction mass. An asteroid is made of physical matter, yes, but it's not made out of the noble gas xenon, which NEXT used. Skimming the electric propulsion doesn't reveal any high-isp thrusters designed to use powdered asteroid. Don't forget Newton's 2nd law says that if you even touch it with your little finger, it'll keep going somewhere else forever. You don't need a lot of force to move large distances in space. This is... true... but your intuitions as to the forces involved are badly off. Let's run the numbers for 16 Psyche real quick. Assume you're putting together an asteroid tug that has an array of a thousand NEXT thrusters, and the 10 megawatt solar array you need to power the thrusters. Assume they're making a solid 50km/s exhaust velocity, the top end for ion propulsion. And, just for fun, include 10,000,000,000,000 metric tons of xenon. This is a pretty big spacecraft, but it's a thousandth the size of 16 Psyche. (2.27×10^19 kg) Light off the thrusters, and run them until they use up all the xenon. It'll take a while. 63,218,390,804 years, in fact. (870 tonnes per 5.5 years) But, good news! After that burn, you've added 0.05km/s to 16 Psyche's orbital velocity. Delta V between the belt and Earth is around 5km/s, so you'll need to do that about a hundred more times. There are better candidates for asteroid capture than 16 Psyche.