Phoenix Dazzles Scientists With Mars Arctic Landing

False-color image of the ground and polygonal structure near the Phoenix Mars lander. Credit: NASA/JPL

"Parachute deployment trigger detected," said Kornfeld. "Heat shield trigger detected. Ground relative velocity 90 meters per second... At this point in time standing by for radar on and altitude convergence." Cheers erupted in mission control when telemetry confirmed a good deployment.

In reality, Phoenix was already on the ground. It took over 15 minutes for signals from Odyssey to reach Earth, 170 million miles away. Entry, descent and landing takes only seven minutes, so by the time Phoenix deployed its parachute, in reality the lander was already sitting on the surface of Mars proceeding through all of its post-landing deployments.

In mission control at NASA's Jet Propulsion Laboratory in Pasadena, California, nobody could know for sure that Phoenix was safely down. Instead, the only thing people could do was wait for the time-delayed confirmation of each landing milestone.

Seconds after parachute deployment, when Phoenix had decelerated to about 268 mph, the heat shield was jettisoned to fall free.

Ten seconds later the lander's three legs deployed and locked in place, prompting more cheers from mission control and just a few cases of hyperventilation. ""Landing leg deployment trigger detected. Ground relative velocity 60 meters per second. Standing by for altitude convergence," Kornfeld said as he continued to tick off the sequence of landing events.

The next key event was activation of the lander's radar altimeter, the key piece of equipment that told Phoenix how high it was above the surface, and hence, when to shutdown the landing thrusters for a safe, soft touchdown.

"At this point in time, Phoenix would have nominally reached altitude convergence. Standing by for confirmation by telemetry," Kornfeld said as mission control waited nervously for confirmation of a good radar. "Radar reliable! Altitude 2,000 meters. Altitude convergence detected. Altitude 1,800 meters, 1,700 meters, 1,600 meters."

After falling for another minute and forty seconds and slowing to 125 mph, the lander separated from the aeroshell's "backshell" and parachute and fell free for about 5 seconds before firing the twelve descent thrusters.

As Phoenix descended to the separation altitude of 1 kilometer, Kornfeld kept the status reports coming. "Standing by for lander separation," he said as people in mission control crossed their fingers and collectively held their breath. "Altitude 1,100 meters. Signal may drop out during lander separation. Altitude 1,000 meters."

"Separation detected!" said Kornfeld as the control room burst into cheers and shouts. "We have reacquired the signal, gravity turn detected. Altitude 600 meters... 500 meters."

The twelve thrusters, each providing about 65 pounds of thrust, pulsed on and off rapidly 10 times per second to stabilize Phoenix as it descended toward the ground and also to turn the spacecraft, no longer in space, so that its solar arrays would face east-west when deployed to maximize their ability to collect sunlight in the far arctic region of the planet.

Under the power of its thrusters, Phoenix decelerated to a gentle 5 mph for the first soft touchdown on Mars since NASA's Viking landers accomplished the feat 32 years ago.

80 meters, 60 meters. Constant velocity phase detected. Altitude 40 meters, 30 meters, 27 meters, 20 meters, 15 meters. Standing by for touchdown," said Kornfeld as Phoenix neared its moment in history. Then, after a nerve-wracking entry and descent, the confirmation everyone was waiting to hear. "Touchdown signal detected! Landing init sequence initiated. Helium vent detected!"

Touchdown was as perfect as one could have hoped, or even better. Phoenix landed with a barely noticeable one quarter of a degree tilt and oriented almost precisely in an east-west direction. Seconds after landing, Phoenix opened valves in the thruster propellant pressurization system and vented all remaining helium gas, thereby safing the propulsion system. The onboard hydrazine propellant should freeze overnight and pose no hazard to the lander. That event confirmed the last major landing event before communications with the lander were cut off.

Forty-six seconds after landing, communications with Phoenix terminated, as expected as Odyssey passed out of communications range. "Standing by for normal termination of EDL comm(unications)," said Kornfeld.

"We have nominal termination of EDL comm by Odyssey and direct to Earth! Phoenix... Phoenix has landed! Phoenix has landed! Welcome to the northern plains of Mars!" Kornfeld exclaimed, barely heard above the cheers, shouts, backpatting and tears of joy.

After waiting 15 minutes for dust to settle around the landing site, Phoenix began a series of deployments: the two solar arrays, weather mast, stereo camera and bio-barrier protecting the soil-digging arm. Meanwhile, in mission control controllers erupted in tears, cheers, backslapping and a celebration as emotional as the tension just minutes before.

And they waited. For a nervous hour and fifty minutes, mission control waited for Odyssey to swing back around Mars for the first post-landing communications pass and much anticipated photos from the stereo imager.

"We've passed the hardest part and we're breathing again, but we still need to see that Phoenix has opened its solar arrays and begun generating power," said JPL's Barry Goldstein, the Phoenix project manager.

The first images were to be of the lander itself. Phoenix was programmed to snap photos of the solar arrays, bio-barrier and landing legs to confirm good deployment of each of the critical systems and get a first-look at the landing site.

If there was any apprehension about the landing, the first pictures from the Mars surface put to rest any doubt.

"Seeing these images after a successful landing reaffirmed the thorough work over the past five years by a great team," said Goldstein.

The first photographs sent back by Phoenix showed pretty much what scientists expected to see: a relatively flat landscape with only small rocks strewn about. The ground has numerous trenchlike depressions, signs of the polygonal shaped texture to the ground in the arctic region of Mars.

The polar regions of Earth also exhibit this feature, which is indicative of ice expanding and shrinking just below the surface. For scientists, the photos provided a mouth-watering peek at the icy cold surface of Mars polar region, never before seen up close.

"We see the lack of rocks that we expected, we see the polygons that we saw from space, we don't see ice on the surface, but we think we will see it beneath the surface. It looks great to me," said Peter Smith of the University of Arizona, Tucson, principal investigator for the Phoenix mission.

The photos also showed perfectly deployed solar arrays, just as planned. The pictures also proved the protective bio barrier had retracted and the weather mast, which had been stowed next to the arm, had also deployed. The fact the camera worked proved that it, too, deployed and was working.

Among those in the JPL control room was NASA Administrator Michael Griffin, who noted this was the first successful Mars landing without airbags since Viking 2 in 1976.

"For the first time in 32 years, and only the third time in history, a JPL team has carried out a soft landing on Mars," Griffin said. "I couldn't be happier to be here to witness this incredible achievement."

It will take engineers several days to reconstruct Phoenix's descent trajectory and pinpoint precisely where the landed touched down. However, initial telemetry indicates the probe landed at 68.22 degrees North Latitude and 234.3 Longitude.

The vertical velocity at touchdown was about 5.4 mph, right in line with expectations. Phoenix was moving a scant 0.2 mph horizontally when it touched down, which was better than expected, ideal for the lander's delicate instruments.

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