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GUY RAZ, HOST:

At this precise moment, a spacecraft is speeding toward Mars. If all goes to plan, the Mars Science Laboratory, nicknamed Curiosity, will land gently on Mars at 10:31 Pacific Time tonight.

Now, if you're hoping to see live pictures of the landing, forget about it. It'll only send radio signals indicating whether it was successful and only after it's all over. And why? Sounds like a question for NPR's Joe Palca who is tracking that story from the Jet Propulsion Laboratory in Pasadena.

JOE PALCA, BYLINE: When you've spent $2.5 billion dollars on a mission to Mars, you kind of want to hear from the spacecraft once in a while

RICHARD KORNFELD: We have two ways to communicate to and from the rover.

PALCA: That's Richard Kornfeld. He's a senior engineer on the Mars landing team. The rover's entry, descent and landing lasts for a total of seven minutes. During that time, the rover must slow down from 13,000 miles an hour to a dead stop when it touches down on the surface of Mars. The two ways of communicating Kornfeld is talking about include one radio system that can transmit a signal directly from the spacecraft to Earth and another that requires the radio signal to be relayed through satellites that are orbiting Mars.

KORNFELD: We currently have three orbiters around Mars. One is the Mars Reconnaissance Orbiter.

PALCA: There's also NASA's Mars Odyssey. The third is 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 JPL affectionately call the seven minutes of terror. All it can do is send tones.

KORNFELD: There are a set of about 256 tones, and each tone means something specific.

PALCA: Things like the heat shield is warming up or the parachute has deployed. And there's one other problem during the landing.

KORNFELD: Unfortunately, Earth sets when the rover is on the parachute.

PALCA: 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 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.

KORNFELD: The signal on landing day takes almost 14 minutes to travel.

PALCA: That's why the rover is on its own for the landing. Even if you wanted to help the rover, I mean, it would be 28 minutes round-trip for the radio signal, so - and the whole thing last seven minutes.

KORNFELD: That is correct. In fact, by the time we'll hear the first signals from the rover as it hits the atmosphere, the landing will have been over already by seven minutes.

PALCA: That's a little hard to wrap your mind around.

KORNFELD: The whole landing takes seven minutes. There is 14 minutes of the signal it takes to travel down to Earth, so by the time we get the first signals, that show is actually over at Mars already.

PALCA: Not to be too Earth-centric about it, the truth is there's no one on Mars to watch. So the show won't be over until 10:31 Pacific Time here on Earth. Joe Palca, NPR News.

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