[meteorite-list] Intense Testing Paved Phoenix Road to Mars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 9 May 2008 18:20:48 -0700 (PDT)
Message-ID: <200805100120.SAA00621_at_zagami.jpl.nasa.gov>

http://mars.jpl.nasa.gov/spotlight/20080509_PHX.html

Intense Testing Paved Phoenix Road to Mars
Jet Propulsion Laboratory
May 09, 2008

When NASA's Phoenix Mars Lander descends to the surface of the Red
Planet on May 25, few will be watching as closely as the men and women
who have spent years planning, analyzing and conducting tests to prepare
for the dramatic and nerve-wracking event known as EDL - Entry, Descent
and Landing. For after all their hard work, they know that landing on
Mars is not a walk in the park. Less than 50 percent of all previous
lander missions have made it safely to the surface.

Like all missions, Phoenix was motivated by the potential science
rewards. With its robotic arm, Phoenix will be the first mission to
reach out and touch water ice in Mars' north polar region. The mission
will study the history of the water in the ice, monitor weather of the
polar region, and investigate whether the subsurface environment in the
far-northern plains of Mars has ever been favorable for sustaining
microbial life.

Much of the Phoenix spacecraft already sat in secure storage when, in
2003, NASA selected it over other proposals to fly to Mars. Phoenix's
main systems were designed and built for launch as the Mars Surveyor
2001 Lander, but that mission was canceled in February 2000, after the
loss of a similar spacecraft, the Mars Polar Lander, during its arrival
at Mars in 1999.

The team that proposed the Phoenix mission, led by Peter Smith of the
University of Arizona, Tucson, developed a plan to bring the spacecraft
out of storage, thoroughly analyze and test it, resolve all known
problems, and add upgrades so it could pursue a new set of science
goals. The spacecraft heritage of the 2001 lander, derived from the
"faster, better, cheaper" era, brought with it opportunities, along with
several challenges.

Phoenix Project Manager Barry Goldstein of NASA's Jet Propulsion
Laboratory, Pasadena, Calif., discussed the team's approach to adapting
a pre-built spacecraft for this mission, instead of developing one from
scratch: "One consequence of having so much of the hardware in place
from the start was that we could focus our resources into testing and
analysis. We evaluated the robustness of the vehicle to perform the
mission we designed, most notably the entry, descent and landing."

The team first focused on correcting all the vulnerabilities identified
by earlier investigations into the loss of the Mars Polar Lander. "That
wasn't enough," Goldstein said. "We eventually identified and mitigated
more than a dozen other potential issues with the spacecraft that could
have had dire consequences." Extensive testing and analysis also
identified concerns that could have affected the lander, solar array
deployment, and its science instruments after arrival on the Martian
surface. However, an acceptable amount of risk still exists--for
example, most hardware is at least 8 to 10 years old, and certain
subsystems have no redundancy during the entry, descent and landing.

Goldstein said, "We've done everything we can to lower the risks of this
mission to acceptable levels, but in no way does that mean we've
eliminated all risk. Planetary exploration is risky by its very nature,
and there are numerous challenges ahead of us, the first of which is
entry, descent and landing."

Here are descriptions of five examples of problematic hardware and
resolutions resulting from the extensive work done by the Phoenix
engineering and science team.

Radar

Phoenix uses a radar system initially designed as an altimeter for
fighter jets. During the final minutes before landing, after the
spacecraft has jettisoned its heat shield, Phoenix will rely on the
radar for information about not just the altitude, but also the descent
velocity and the horizontal velocity. The onboard computer will use that
information several times per second to adjust the firing of 12 descent
thrusters.

Using the radar for this novel purpose required a tremendous amount of
testing, "We did more than 60 hours of flight testing, including 72
different drops at three sites with different geological
characteristics," said David Skulsky, a JPL engineer on the Phoenix
team. That's more radar flight testing than all previous NASA Mars
missions combined."

Radar tests also included custom-developed simulations of performance
under Martian conditions. Running one of those simulator tests just four
months before the spacecraft was due to be delivered to Florida for
launch, Curtis Chen, a JPL radar engineer, noticed some strange
behavior. Analysis confirmed that, under some circumstances, the radar
could be confused by the jettisoned heat shield.

JPL's Dara Sabahi, chief engineer for Phoenix, said, "If this occurred
in flight, the spacecraft would think it was much closer to the ground
than it actually was. It would be a guaranteed failure."

Once the testing had revealed the potential problem, engineers designed
a relatively simple solution using adjustments related to the timing of
radar pulses. However, the schedule was tight, and additional flight
tests were needed to be sure that fixing that issue had not created
others. "We worked all the way to launch on the testing, and even did
more testing after launch to be sure we understand the performance,"
Sabahi said.

In addition, NASA formed a Radar Independent Review Team of key radar
experts to evaluate the activities of the Phoenix team working with the
radar. The review team was chartered to determine if the radar had been
properly characterized, if the important risks associated with its
performance have been identified, mitigated, and that unmitigated
residual radar risks represented a low risk to the mission. The Phoenix
team followed all recommendations from the Independent Review Team. The
review team endorsed the approach taken by the project to resolve all
anomalies. They concluded that the probability for a successful landing
on Mars under radar guidance was comparable to or better than that of
prior missions.

Parachute

The lander will separate from its parachute about 40 seconds before
reaching the ground. Thrusters will begin firing half a second later and
continue pulsing all the way to the surface, controlling both vertical
and horizontal velocity, plus the spacecraft's orientation.

"We did some analysis that showed there was a three-to-five percent
chance, depending on wind conditions, that the lander would have some
kind of re-contact with the parachute," said Rob Grover, chief of the
Phoenix entry, descent and landing team at JPL. "The worst situation
would be to have the parachute come down right on top of the lander and
prevent deployment of the solar arrays."

Rather than rely on the odds against such an occurrence, engineers
designed a maneuver for the lander to avoid the parachute. Horizontal
motion identified by the radar while the lander is still connected to
the parachute will indicate wind direction and speed. If the wind is
strong, the parachute will blow away on its own. If the wind is weak,
the lander will use its thrusters after separating from the parachute to
push itself upwind, away from the falling parachute.

Motors

The robotic arm on Phoenix uses four electric motors from the same lot
of 211 motors originally purchased for NASA's Mars Exploration Rover
project. Fifty of the motors were sent to Mars on rovers Spirit and
Opportunity. Of the remaining motors, later testing identified two whose
brushes were broken. Motor brushes provide electrical contact between
moving and stationary parts of the motor. The brushes in these motors
are solid pieces of a special mixture of copper, graphite and molybdenum
made for Martian conditions.

The motors installed on the Phoenix spacecraft had been tested and
showed no trouble. In addition, their counterparts on Spirit and
Opportunity have far outperformed their design life under stressful
real-Mars conditions. For the Phoenix team, the issue was how to assess
whether the two broken brushes were enough reason not to rely on the
motors in the robotic arm. Goldstein, the Phoenix project manager, said,
"We did not rest on these motors' excellent track record with Spirit and
Opportunity. We did our own testing."

The Phoenix project put the arm motors through additional testing and
also turned to the NASA Engineering and Safety Center, a resource
created for providing just such assistance with independent analysis of
engineering issues related to risk for NASA projects. The Phoenix team
followed recommendations from a review team formed by the center. These
recommendations included using sensors to monitor any jarring of the
motors during transportation of Phoenix from Denver, where it was built
by Lockheed Martin Space Systems, to Florida for launch.

Scoop

Central to the design of the Phoenix mission is the intent to dig to an
icy layer under the surface and deliver some of the ice-rich soil to a
small laboratory on the deck of the lander. That icy soil will probably
be as hard as concrete.

The original design for the scoop at the end of the arm had three sets
of metal blades for cutting and scraping to loosen enough icy soil to
sample. The Phoenix team ran tests using sample materials as tough as
those expected on Mars.

JPL engineer Lori Shiraishi said, "We found it took four to six hours to
get enough material, but you are also fighting sublimation of the ice.
The ice would be disappearing by the time you are trying to pick it up."

In 2005, the team began working on an alternative design to loosen and
collect an icy sample more quickly. JPL's Gregory Peters came up with
the idea of a motorized rasp to replace one of the sets of blades.
Honeybee Robotics Spacecraft Mechanisms Corp., New York, built and
tested the redesigned scoop. The rasp uses a tile-cutting bit lowered at
an angle through a slot in the bottom of the scoop. Tests indicate the
system can loosen and lift and deliver an icy sample in about half an
hour, which is believed to be quick enough to outrun sublimation of the
exposed ice under Martian atmospheric conditions.

Stowaway carbon

The Phoenix team has tested all of the lander's science instruments
extensively. One that sniffs vapors generated from heating samples of
soil and ice will be checking for organic molecules. Most
carbon-containing chemicals are called organics. Organic chemicals can
be present without life, but they are an essential ingredient for life
as we know it. Testing made clear that this instrument -- the Thermal
and Evolved-Gas Analyzer -- is sensitive enough to detect the trace
amounts of organics that are likely to come from Earth aboard the lander.

"We want to be able to determine whether we're just seeing organics we
brought along with us," said William Boynton of the University of
Arizona, Tucson, lead scientist for this instrument.

The university assembled a meeting of organic chemists from around the
country in 2005 for a discussion of how to prepare for analyzing the
data from Phoenix. From that workshop came a recommendation for Phoenix
to carry "blank" material specially made to be as free of carbon as
possible, for use as an experimental control for comparison with samples
of Martian soil and ice.

The Phoenix team assessed various possibilities for the blank material.
The lander is carrying a block of a custom-made, very-low-carbon ceramic
product from Corning Inc. During operations at the landing site, the
powered rasp will be able to produce shavings from the blank for
analysis. The results will help scientists interpret whether any
organics found during analysis of Martian samples actually came from
those samples.

There are many other examples of how the Phoenix mission has identified
concerns through testing and analysis, and then resolved them.

Goldstein said, "I can't guarantee success. We are in the business of
taking risks, doing things that are very difficult. However, I am
confident that we have a world-class team that has dug as deep as it
could to find any problems."
Received on Fri 09 May 2008 09:20:48 PM PDT