[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?

[mcaplinger]

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?

How many more flyby spacecraft do we expect to see? For an orbiter in a roughly-circular orbit, the instruments are all more or less nadir-pointed all the time anyway. Pointing the spacecraft is done with momentum wheels and costs little additional fuel.

Scan platforms have always had poor pointing accuracy relative to spacecraft, they're heavy and mechanically complex, and they complicate the cable design between the instruments and the spacecraft a lot. Most needed pointing/scanning can be done more effectively within an instrument.

[djellison]

the cable design between the instruments and the spacecraft

That's the bit of the MER design that keeps Squyres most worried I think.

Doug