[X] My Assistant

[monitorlizard]

It used to be that nearly all U.S. planetary spacecraft had their remote sensing instruments mounted on a scan platform. For the past fifteen years or so, all instruments have been body-mounted. I've been wondering if this is a permanent change in planetary spacecraft design. Scan platforms have the advantage of a faster slew rate than moving an entire spacecraft, so more targets can be acquired in a given amount of time. Scan platforms also mean no attitude control gas is used (except to stabilize the spacecraft), although this advantage is nullified if reaction control wheels are used instead. An additional advantage is that using a scan platform means all its instruments can be used at once, whereas body-mounting can mean the spacecraft blocks the view of some instruments when others are able to see the target.

Body-mounting instruments is advantageous only in that it saves money in the overall design of the spacecraft. I don't know of any other advantage. The last spacecraft that would have used a scan platform was Cassini, but the project switched to body-mounting in a cost-cutting descope. Only JIMO would have had a scan platform (or two) because the spacecraft was so monstrous there was no practical way to slew it quickly to change targets.

So, will we ever see a planetary spacecraft with a scan platform again? Is there some engineering reason why scan platforms shouldn't be used again? Or is it all just to save money, sacrificing some science observations to have an affordable spacecraft?

[dvandorn]

I always thought that the ultimate in scan platforms was Galileo, which, IIRC, was a spinning spacecraft with a central bus that was entirely despun. All of the control cabling, instrumentation data flow and power flow between the spinning portion and the despun bus had to connect along continually moving surfaces.

I'm still amazed it worked.

-the other Doug

[djellison]

I'm still amazed it worked.

Most of the time.

That spun/despun interface caused something like 20 safe modes - 6 in the second half of 1993 alone- and having not occured for nearly 5 years, it occured three times inside of 20 hours in July '98 and three times in 10 hours in August '99.

It was a million miles from perfect.

Doug