How NASA’s Phoenix Will Land on Mars
Phoenix streaks through the martian atmosphere protected by its head shield as it uses friction to decrease its velocity.
Credit: NASA/JPL-Caltech/University of Arizona

NASA's Phoenix Mars Lander aims to not flame out when it descends to the arctic surface of the red planet in less than two weeks.

The new Martian probe will try to avoid the fate of its crashed predecessor, NASA?s Mars Polar Lander, when deploying a parachute and braking rockets to slow its plunge and make a successful three-point landing.

"This is not a trip to grandma's house,? said Ed Weiler, associate administrator of NASA?s Science Mission Directorate at the agency?s headquarters in Washington, D.C. ?Putting a spacecraft safely on Mars is hard and risky.?

Phoenix managers refer to the probe?s descent as ?seven minutes of terror? that will define the future of the spacecraft?s $420-million mission. The robotic arm-equipped spacecraft is due to land near the Martian north pole on May 25 to study nearby water ice and determine if the region was once habitable for primitive life.

"Hopefully the outcome will be different from the Mars Polar Lander outcome," said Rob Grover, NASA engineer at the Jet Propulsion Laboratory in Pasadena, Calif.

Mars Polar Lander (MPL) entered the Martian atmosphere near the planet?s south pole in 1999, but a software glitch caused a premature shutdown of the spacecraft's engines. It crashed while falling at 50 mph (80 kph) instead of making a soft landing. NASA has worked since then to ensure Phoenix doesn?t suffer the same fate.

"The number one cause was the faulty indicator on touchdown sensor," Grover told SPACE.com, adding that the sensor falsely told the MPL that it had already landed.

Engineers have since corrected the software issue and made the overall system more robust to avoid future errors.

?We feel like we have adequately tested this vehicle,? Phoenix project manager Barry Goldstein said in a Tuesday mission briefing, but added that there is always room for the unexpected. ?We fire 26 pyrotechnic events in the last 14 minutes of this vehicle, and every one of those has to go off as planned?We?re very hopeful for success on the 25th.?

Phoenix User?s Guide for Mars Arrival

The exact fate of the lost MPL remains somewhat uncertain because that probe had no way of communicating with Earth once it entered the Martian atmosphere. That won't be the case for Phoenix, which has a small crowd of three Mars orbiters to watch and relay information from the spacecraft throughout landing.

For Goldstein, it is the three-second communications gap between Phoenix?s departure from its cruise stage and the first signals to its relay network that gives him the shivers. If Phoenix fails to land successfully, any signals just before landing will prove vital in learning its fate, he said.

?Getting that communications down is the important thing,? Goldstein said. ?That will be the three seconds that I?m really biting my nails over.?

A wraparound antenna sits on Phoenix's back-shell, capable of transmitting an ultra-high frequency signal to Earth via NASA?s Mars Reconnaissance Orbiter (MRO) or Mars Odyssey spacecraft Europe?s Mars Express orbiter is also on call in case of an emergency, mission managers said.

"This is the first time for any Mars landing having orbital relay communications for both landing and being on the surface," Grover said.

Phoenix will descend and land much the same way that MPL was meant to, plunging into the Martian atmosphere at about 13,000 mph (21,000 kph). That's similar to respective 2004 descents of NASA?s Spirit and Opportunity rovers, though Phoenix?s arrival would mark first powered landing on Mars since NASA?s Viking missions of the 1970s

The probe combines new technology with proven methods for landing, including an Apollo-era Earth entry software algorithm to guide the spacecraft's early descent into the Martian atmosphere.

A Viking-era parachute is designed to open once Phoenix falls within 7.8 miles (12.6 km) above Mars, creating drag to slow the spacecraft as it screams through the atmosphere at supersonic speed. The probe?s landing radar should begin giving altitude and velocity of descent as Phoenix nears the surface, so that the onboard computer can make any necessary landing adjustments.

"By the time you get the parachute opening, there can be significant errors in positioning on order of kilometers," Grover said. "So that's where radar is critical, because it turns on and gets fresh knowledge of altitude."

Vertical Martian lander

Two minutes after the parachute deployment, Phoenix will have descended to approximately 0.6 miles (1 km) above the surface. The lander should then jettison its backshell and freefall for half a second before lighting up its engines.

Nine of the twelve engines will pulse furiously 10 times per second — an effect Grover likened to "coming down on a jackhammer." The three non-pulsing engines should fire steadily to help ensure added stability.

?Just before touchdown, we actually pirouette the vehicle,? Goldstein said, adding that the move will aid Phoenix?s vital solar arrays. ?We actually turn it so we maximize solar exposure.?

Navigators at JPL can upload fresh orders to Phoenix?s guidance computer up to three hours before landing, in case course adjustments are required. However, Grover and other NASA engineers will only be able to stand by and trust in their spacecraft technology once the Mars lander begins its descent.

"We've done all that's humanly possible," Grover said.

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