[Eric A. Taylor (OP)]

My computer has 3, well I guess you'd call them "off" settings. There is of course, off. I understand what that is, then there is hibernate and standby. I think I sorta get "standby" If placed in standby my computer doesn't need to "boot", it simply comes back on with whatever I had been doing last. The wireless network light and the hard drive lights are off, so I assume my computer is not connected to anything. What I don't understand really is what is the difference between "hibernate" and "off"?

[RD]

... what is the difference between "hibernate" and "off"?

Quicker restart from "hibernate" than from "off" ... http://en.wikipedia.org/wiki/Hibernation_%28computing%29

[Pre-empting your next question ...]

What is the difference between "standby" and "hibernate" ?

"Standby" relies on volatile memory (RAM chips) to store what you were doin',
"hibernate" uses non-volatile disc memory (hard-drive) to store what you were doin’.

http://en.wikipedia.org/wiki/ACPI#Power_States

[Eric A. Taylor (OP)]

Thanks. That answers my question. However NASA was talking about putting the Mars Rovers into hibernation mode rather than letting the batteries run down until the rovers shut down. I got the impression that if this happened they would be unable to restart.

I know NASA, and I suspect ESA designs their space probe without an "off switch" They can not be shut down from the ground at all. Why would the probe not turn itself back one once it got enough energy in it's batteries?

[Geezer]

Thanks. That answers my question. However NASA was talking about putting the Mars Rovers into hibernation mode rather than letting the batteries run down until the rovers shut down. I got the impression that if this happened they would be unable to restart.

I know NASA, and I suspect ESA designs their space probe without an "off switch" They can not be shut down from the ground at all. Why would the probe not turn itself back one once it got enough energy in it's batteries?

I think RD's description is quite correct as the terms apply to PCs. Strictly speaking, hibernate mode in a PC is really more of a power-down with quick restart mode. The quick restart is achieved by putting an image of everything that was in memory back into (volatile) memory prior to the power-down.

Sounds like NASA means something more like a real biological hibernation (which is probably more like standby in PC terms!) where just about everything is shut down to conserve power except for a small kernel that periodically wakes up and listens for incoming commands for a short interval. If it hears nothing, it would go back to minimal power consumption mode. It might even transmit on wake-up, but as that will require a lot more power, I would think it will only listen.

The advantage of this approach is that NASA always maintains some degree of control of the Rover. If it totally shut down until its batteries recharged, NASA would have no telemetry at all. For example, they would not even be able to tell if the batteries were actually charging.

[engrByDayPianstByNight]

NASA's done a real great job with the Mars rovers. They're still functional, more than 10 times the designed life span.

My roommate at grad school (Cornell University) was a student of Steve Squyres, the PI of the Rover project. He lent me a book written by his advisor on the entire history of the project with his first-hand experience (since he was the one who first proposed it to NASA in something like 1990). Pretty fascinating to read.

[LeeE]

When the NASA Mars Rovers are put into hibernation mode certain parts and subsystems need to be kept above a certain temperature to avoid permanent damage due to excessive cold.  Whilst in hibernation mode these parts are kept warm by heaters until the ambient temperature rises again and the probe can come back out of hibernation.