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Name: David E.
Status: Other
Age: 40s
Location: N/A
Country: N/A
Date: February 2004

How does NASA send an instruction to one of the Mars Rover crafts? How does this command travel to the Rover and how long before the instruction is received?

David, Very thought provoking question.

Using the speed of light as the speed of the radio transmissions it takes anywhere from 4 to 20 minutes for a one way trip of radio waves to Mars. I am not totally certain about this but I believe that NASA uses Earth based extremely high gain antennas and at relatively high power. Why? Sure, it is all well and good that the communications are "line of sight" between Mars and our planet. However, consider the distance. It takes 1.28 seconds (approximately) for a signal to get to the Moon (ONE WAY TRIP). It takes 188 times that long (at the nearest Mars -- Earth planetary distance) to get a signal to Mars. And Physics tells us that radiation (or radio wave density) usually measured in Watts / m^2 will drop according to the 1/r^2 rule. To put it in simpler terms: Transmit signal A to our Moon and it hits right smack in the middle of the crater Tycho with a radiation power intensity (or signal strength) of lets say 0.0001 W / m^2. Now let us aim that same signal A at Mars. Now let us target the rover. The rover will "see" the signal at a signal strength of about 0.0001 / (188^2) in Watts / meter^2 ... or .... 2.8 x 10 ^ (-9) or 0.0000000028 Watts per square meter. My guess is that NASA, in fact, does use ground based high power transmitters into high gain antennas just to make sure they can get as much radiation density (radio wave strength) onto the Martian surface as possible ... or at least as allowed by the FCC ;)

Good question.
Darin Wagner

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