I've been searching the online documents at the NTRS for precursors to the Dawn mission and found something that while not directly precursory to Dawn, was certainly of interest.

In the early 1970's NASA had commissioned several studies into uses for solar-electric (eg. ion) propulsion. North American/Rockwell came up with a proposal for a single spacecraft to be launched on a three (+) year particles & fields survey of the asteroid belt.

The spacecraft was not going to be targeted on a specific asteroid, but it was expected that it would pass through the debris trails of the comets, Denning, Kulin & Shajn-Schaldach where data would be obtained on particle density and size.

The primary science instrument was an optical detector (not a camera) called Sisyphus designed to measure the size and velocity of particles that passed it's field of view.

Solar electric propulsion asteroid belt mission study. Final Report. Vol. 1, Summary Report

Solar electric propulsion asteroid belt mission study. Final Report. Vol. 2, Technical Report

Solar electric propulsion asteroid belt mission study. Final Report. Vol. 3, Program Development Plan

Very interesting!

Thanks for posting it.

I'll certainly agree with that assessment, if things had gone differently, this would have been Deep-Space One (Though NASA would doubtless have called it something else (Mariner 11?)). The mission was proposed as being complimentary to the Pioneer 10 & 11 missions. The documents in question are one of the few cases I've found that have both a specific mission & hardware. Most of the other documents on this subject either deal with possible missions or with proposed hardware, but I'm still looking through it.

Sisyphus was the same optical particle detector mounted on Pioneer 10 and 11

That was one of the reasons it was selected for this mission. However it would have been much larger than the one's fitted to the Pioneer's. The planned aperture was 67cm. I've attached a scan of the proposed experiments.