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If all goes according to plan, the Mars Science Laboratory, nicknamed Curiosity, will land gently on Mars at 10:31 PDT Sunday night. The rover's entry, descent and landing will last for a total of seven minutes. During that time, the rover must slow down from 13,000 mph to a dead-stop touchdown on the surface of Mars.
When you've spent $2.5 billion on a mission, you probably want to check in on how it's doing once in a while. So how will scientists be communicating with a spacecraft that's 154 million miles away?
Richard Kornfeld, a senior engineer on the Mars landing team, says there are two ways to keep in touch: One is a radio system that can transmit a signal directly from the spacecraft to Earth; another requires the radio signal to be relayed through satellites that are orbiting Mars.
Kornfeld says there are three of those satellites: the Mars Reconnaissance Orbiter, NASA's Mars Odyssey and the European Space Agency's Mars Express spacecraft.
Mars Odyssey can take a signal from the rover and send it directly back to Earth. The other two satellites have to record the rover's transmissions and send them to Earth at a later time.
Kornfeld says the direct-to-Earth signal can't provide much information during the landing — what the engineers at the Jet Propulsion Laboratory affectionately call the "seven minutes of terror." All it can do is send tones.
"There are a set of 256 tones, and each tone means something specific," he says. Things like — the heat shield is warming up or the parachute has deployed.
There's one other problem during the landing.
"Unfortunately, Earth sets when the rover is on the parachute," Kornfeld says.
Once Earth goes below the horizon of Mars and the rover can no longer see it, the direct-to-Earth signal no longer works. So the plan is to rely on a relayed signal from Mars Odyssey to let NASA engineers know whether the landing was a success.
Kornfeld says the other thing that you have to keep in mind is that radio signals take a while to go from Mars to Earth, even traveling at the speed of light.
"The signal on landing day takes almost 14 minutes to travel," he says.
That's why the rover is on its own for the landing. Even if the team wanted to help the rover, it would be 28 minutes round-trip for the radio signal — and the whole thing last seven minutes.
"In fact, by the time we will hear the first signals from the rover as it hits the atmosphere, the landing will have been over already by seven minutes," Kornfeld says.
To break it down: the landing takes seven minutes, but the signal takes 14 minutes to travel, so by the time Earth gets the first signals, "that show is over at Mars already," he says.
Not to be too Earth-centric about it, but the truth is there's no one on Mars to watch. So the show won't be over until 10:31 PDT here on Earth.