美国国家公共电台 NPR NASA Probe Lands Safely On Martian Surface（在线收听）

MICHEL MARTIN, HOST:

Mars is about to get a new visitor from Earth tomorrow. If all goes well, a NASA probe called InSight will land near the Martian equator. Its mission is to study the interior of the planet. But, before it can carry out its mission, it has to land safely. That means slowing down from 12,300 mph as it enters the Martian atmosphere to a complete stop on the ground 6.5 minutes later. NPR science correspondent Joe Palca explains the steps involved in making that happen.

JOE PALCA, BYLINE: The person responsible for getting InSight safely to the Martian surface is Rob Grover. He's in charge of the team that designed the landing scheme.

ROB GROVER: The landing is all completely automatic and autonomous. We have no ability to actually kind of fly the lander to the surface.

PALCA: That's because it's a 6.5-minute ride to the surface, but it takes a radio signal longer than that to reach Mars, so real-time control is out of the question. So here's what's supposed to happen. First, the probe gets rid of what's called the cruise stage. That's a part of the spacecraft only needed while InSight is traveling from Earth to Mars.

GROVER: We eject that from the vehicle seven minutes before we're going to hit the top of the atmosphere.

PALCA: Next, the spacecraft turns so its heat shield is pointing in the right direction. As the probe drops towards the surface, air molecules in the Martian atmosphere strike the heat shield, causing it to heat up and the craft to slow down.

GROVER: Believe it or not, 99 percent of the energy that we have coming in from space is actually bled off by the atmosphere.

PALCA: The heat shield does its thing for about 3.5 minutes.

GROVER: The next big event is parachute deploy.

PALCA: Grover says, at this point, the probe is still traveling faster than the speed of sound, so they have to use a special parachute designed for supersonic speeds. When the probe is about four miles above the surface, the radar comes on. It will help inform the onboard navigation how to steer the craft once it cuts loose the parachute and lands using rockets.

GROVER: The whole descent under rocket power takes about 40 seconds or so. We have 12 small descent engines grouped around the bottom of the lander. They're providing the thrust the slowest down the final kilometer.

PALCA: Even though they can't do anything to help InSight as it descends, mission managers should be able to watch its progress.

GROVER: It'll be sending back data in real time, and the MarCO spacecraft will be helping with that, relaying the data.

PALCA: MarCO is a small spacecraft that's flying to Mars with InSight.

ANNE MARINAN: It's about the size of a cereal box

PALCA: A family-size cereal box, but still. Anne Marinan is systems engineer on MarCO. She says MarCO is one of a new generation of really small satellites called cubesats.

MARINAN: Cubesats were originally developed as a way to easily give students, essentially, access to space.

PALCA: But Marinan says cubesats are now being used at NASA to test new technologies. In MarCO's case, the new technology is communications equipment that will relay InSight's telemetry data back to Earth. There are two nearly identical MarCO spacecraft that launched with InSight last May and have been trailing the probe on its flight to Mars.

MARINAN: To start out with, we're behind. But by the time InSight lands, we'll be beyond Mars.

PALCA: And you're not stopping

MARINAN: No.

PALCA: Marinan says the two MarCO spacecraft keep on going past Mars and into space. Their communications relay mission is over. With luck, InSight's mission on the Martian surface will just be beginning.

Joe Palca, NPR News.