Friday, June 20, 2008

Communicating with Mars

In a landmark discovery this week, NASA's Phoenix lander dug a trench in the Martian soil and watched as small chunks of water ice slowly evaporated. Scientists have known about the existence of water ice in the polar cap of Mars for a long time, but it was only in 2002 that the Odyssey spacecraft found evidence of much more ice just below the surface. This is what motivated the experiment on Phoenix that directly confirmed this vast reservoir of frozen sub-surface water. It's not easy to control an experiment from a hundred million miles away. Have you ever wondered how NASA actually does it?

Even when the Earth and Mars are as close as they can come to each other in their orbits they are separated by more than 30 million miles, and sometimes they are nearly 250 million miles apart. How do we communicate across such enormous distances? It's similar to how we do it on Earth: we encode our signals into radio waves and send them from one antenna to another. Radio waves travel at the speed of light, more than 186000 miles or eight times around the Earth in a single second. This is very practical for communicating across the planet, but it's fairly slow to communicate between planets. When it is closest, a radio signal to the Mars lander takes several minutes to get there -- and its response takes several minutes to return to Earth. More typically, the trip takes 10 minutes each way. Imagine playing a video game where any move you make with the controller doesn't show up on the screen for 20 minutes!

With such slow communication, scientists do not control the Phoenix lander the same way you might play a video game. Many of the functions of the experiment need to be automated and programmed ahead of time. So they simply tell the lander to "dig a trench" then "take a picture" and wait for the image to be sent back. This led to the discovery earlier this week of some "white stuff" beneath the soil. To test whether it might be water ice they waited for a day, asked the lander to take another picture, and compared the second picture to the first one. This showed that some of the crumbs from the digging actually disappeared in the second picture -- suggesting that they were made from ice and had evaporated.

If you think playing a video game with a 20 minute lag time might be frustrating, imagine what it would be like with people on both ends of the line. This is one of the many challenges of sending astronauts to Mars, but it's something that most people don't think about.

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