Mars gets a new visitor from Earth on Monday. If all goes well, a NASA probe named InSight will land near the Martian equator shortly before 3 p.m. EST. Once it lands on Mars, it will stay put. InSight isn't a rover. Its mission is to stick to one place after it lands — and study the interior of Mars from the planet's surface.
But before it can carry out that mission, it has to land safely. That means slowing down from 12,300 mph as it enters the top of the Martian atmosphere — to a complete stop on the ground six and a half minutes later.
"The landing is all completely automatic and autonomous," says Rob Grover, leader of the Entry, Descent and Landing team at the Jet Propulsion Laboratory in Pasadena, Calif. "We have no ability to actually, kind of, fly the lander to the surface," Grover says.
The reason real-time control isn't possible is because it takes a radio signal approximately eight minutes to travel from Earth to Mars. Because the entire landing sequence only takes six and a half minutes, the lander would already be on the ground by the time a signal from Earth arrived.
Here's what's supposed to happen: About 90 minutes before atmosphere entry, mission managers will send the latest tracking information to the probe, so it will know where it is and how fast it is traveling. They will also send last-minute tweaks to the software on the spacecraft that controls the landing, based on estimates of the amount of dust in the Martian atmosphere.
Then, the probe is to get 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.
"We eject that from the vehicle seven minutes before we're going to hit the top of the atmosphere," Grover says.
After that, the spacecraft turns, so its heat shield is pointing in the right direction.
As the probe enters the atmosphere, the air molecules that make up the Martian atmosphere strike the heat shield, causing the shield to heat up and the craft to slow down.
"Believe it or not, 99 percent of the energy that we have coming in from space is actually bled off by the atmosphere," Grover says.
The heat shield does its thing for about three and a half minutes.
"The next big event is parachute deploy," says Grover. At this point, the probe is still traveling faster than the speed of sound, so InSight has a special parachute designed for supersonic speeds.
Once the probe is about 4 miles above the surface, the radar comes on. It will help inform how the onboard navigation steers the craft, once the probe cuts loose the parachute and lands using rockets.
"The whole descent under rocket power takes about 40 seconds or so," Grover says. "We have 12 small descent engines grouped around the bottom of the lander that are providing the thrust to slow us down the final kilometer."
Even though they can't do anything to help InSight as it descends, mission managers should be able to watch its progress.
"It'll be sending back data in real time," Grover says, "and the MarCO spacecraft will be helping with that — relaying the data."
MarCO is a small spacecraft that's flying to Mars with InSight.
"It's about the size of a cereal box, explains Anne Marinan, a systems engineer on the team that's in charge of MarCO. It is one of a new generation of really small satellites called cubesats.
"Cubesats were originally developed as a way to easily give students, essentially, access to space," 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 telemetry data from InSight 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.
"To start out with we're behind," Marinan says, "but by the time InSight lands we will be beyond Mars." Unlike InSight, the MarCO spacecraft keep on going past Mars into space. Once they finish relaying InSight's landing telemetry, their mission is over.
If the MarCOs fail to relay the telemetry, the data will also be stored by other NASA satellites in orbit around Mars, and sent to Earth after the landing.
InSight is landing in what seems to be a very boring part of Mars, known as Elysium Planitia. Mission managers wanted a boring spot — they want the probe to sit quietly. Its two primary instruments, a sensitive seismometer and an underground temperature probe will be measuring tiny fluctuations in the planet's interior.
To make accurate measurements, those instruments shouldn't be disturbed. If they work properly, scientists should get a better sense of the interior structure of the red planet.
The mission is expected to last about two Earth-years.
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