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How the Mars Rovers Phone Home

I am sure you have seen them, those incredible pictures from Mars. I have posted many of them in this blog, including one yesterday. Did you ever wonder how these pictures were sent back to earth? Well, it turns out that although each rover has a high-gain microwave antenna that can transmit scientific data directly to Earth, they usually use a low-gain UHF antenna to transmit the data to spacecraft in orbit above Mars.

There have been several previous missions to Mars that have left in place orbiting spaceraft around Mars that serve as "relay station" for the rovers. This is a much more efficient method because the orbiters have much bigger solar arrays and bigger antennas, so they're not so much energy- and bandwidth-constrained as the rovers are. The first 1976 mission to Mars, Viking in 1976, had to use brute force and it used a nuclear powered (not inexpensive solar power like the rover) transmitter.

Each spacecraft to Mars builds on previous missions and provides information that helps future missions. The rover's computer has a timetable that determines when the orbiter is going to be overhead. The rover transmits its data at 250 Kbps, and it's received by a spacecraft orbiting 250 miles above the Martian surface. The Martian satellite then transmits the data to Earth, but even at the speed of light, it takes more than 10 minutes for a signal from Mars to arrive at Earth. Giant 230-foot-diameter satellite dishes with super-sensitive, low-noise electronics receive the signal.

Wired March 2, 2004

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