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Comment by joaorj | Hacker News Reader

this article is wrong on many things.

one being that the rover can drive during night.

the small power output of the "nuclear battery" is not used to drive the rover. it used to recharge batteries during day and night so that the rover can operate on the batteries during the day.

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That's a good point... while in theory is probably could drive at night, it probably isn't a good idea to have a rover on a different planet driving around in the dark.

Is radio communication with the rover even possible at night? We only have line of sight to mars during the martian daytime.

Our line of sight to Mars is independent from Mars seeing the sun.

Not really. If the rover is on the side of Mars facing away from the Sun, it's almost always facing away from the Earth, too, since the Earth is deeper inside the solar system.

Facing the Earth and facing the sun are highly correlated. Seeing one xor the other is possible, but unusual.

He's still right in a very technical, strict sense. I'm too lazy to look up numbers and do the math, but I expect there's a window near the Martian sunrise/sunset when the sun isn't visible over the horizon and we have line of sight.

I expect that window is small to the point where no one actually cares. It would be an interesting applied-math problem for a grade-schooler, though.

The geometry of viewing Earth in the Martian sky is the same as viewing Venus in our own Terran sky. The interior planet periodically approaches a certain maximum angular separation from the Sun. Just as we see Venus but not the Sun sometimes in the early morning or evening twilight, Martians (and rovers on Mars) could see Earth but not the Sun at corresponding intervals of Mars' rotation. It's a significant amount of time, up to several hours per day at maximum separation.

The angular separation between the Sun and the Earth as seen from a space probe is significant as far out as Cassini at Saturn. The probe can receive commands from Earth without the signal being overwhelmed by solar radiation, except for a few days each (Earth) year when Earth is too close to the Sun as seen from the spacecraft. (Earth doesn't literally go behind the sun often, thanks to inclination of the planetary orbits plus Cassini's own inclined orbit at Saturn.)

> Our line of sight to Mars is independent from Mars seeing the sun.

Our line of sight to Mars as an entire body is independent, but our line of sight to a particular point on Mars is indeed correlated with that point facing the Sun.

Do you happen to have illustrations and/or numbers? I was planning to actually look everything up and work it out when I got home from work, but it'd be nice to have confirmation of my results, too.

It's also not possible to directly-drive the Rover anyway, with latency varying between 4 and 20 minutes depending on Mars/Earth orbit.

Curiosity drives itself.

Not sure about this specific mission but I think we have had satellites around mars in the past that have helped relay messages when line of sight isn't available.

Yup. The 3 satellites will be used for primary communication along with an X-band transmitter on the rover itself to talk to earth.

Edit: Link (communications): http://en.wikipedia.org/wiki/Curiosity_rover#Specifications

But the rover can drive at night, right? It just doesn't for safety reasons.

Correct. If the writer had only watched the NASA broadcast he would have heard it right from the horse's mouth. Because of the low temps at night, the power requirements to heat up the rover and devices would not make it worthwhile. In addition, the total output of the generator and batteries on a particular day is not enough for it to run continuously for the entire day.

Another interesting point brought up was that the rover was tested to 3x life but not to failure. It's certainly possible for it to run for many years longer than the stated life.

They talked about this during a press conference today. Theoretically it could operate at night, but the power draw to heat the motors and joints would be excessive since it's so cold after sunset. So they don't and allow the MMRTG to charge the battery (and keep the heater for the electronics) while most of the machine is idle.

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