Press Release

Wheels In The Sky – NASA’s Mars Exploration Program

By SpaceRef Editor
May 30, 2003
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When Chris Voorhees thinks about wheels, he doesn’t
imagine the rubber hitting the road, but aluminum
crawling across the surface of Mars. In fact, he has
already seen some of his handiwork making its way
across the red planet. One of the first jobs Voorhees
was handed as an intern was stamping out over 1,000
stainless steel cleats for the Sojourner rover on
NASA’s Mars Pathfinder mission. Fast-forward six
years and tack on a 365-pound weight-gain and
mobility specialists are dealing with a whole new
animal –the large twin "robot geologists" known as
the Mars Exploration Rovers, launching in early
summer 2003.

"We started with the Sojourner wheels as a base to
work from," Voorhees said. "Because of many different
engineering demands on the wheels, the wheels for our
new rovers didn’t mature until late in the game."

Mobility engineers were tasked with making the
wheels lightweight, so as not to add any more weight
to an already hefty spacecraft; compact, so that when
the rover is stowed in the lander they would
fit; and capable, so the twin geologists can maneuver
off of the lander safely and climb rocks up to
ten inches high. Basic parameters were set, based on
the weight of the rover and the contact area
on the surface and then the challenge began to make
the wheels deliver on all requirements.

A Design to Keep on Turnin’

The rocker-bogie suspension that was developed for
Sojourner, the first vehicle to rove on another
planet, will be used again in a modified design.
This flexible mobility system allows the wheels to
conform to obstacles like rocks, strengthening their
grip and maximizing their ability to clear any
"road blocks." At 26 centimeters in diameter (a
little over ten inches), these aluminum wheels are
twice the size of those on Sojourner and are missing
the recognizable sharp cleats.

"A big challenge is to be able to get enough traction
to get through soil and over rocks but also to be
benign enough to get off of the lander without getting
entangled in the deflated airbags," Voorhees said. The
design is "basically like a paddlewheel that is machined
onto the outside of the wheel, providing both safety
and capability."

Each wheel has its own drive and steering actuators,
which control movement and direction. The internal
volume that each wheel can hold was increased to house
both systems within the wheel’s crown-shaped design. When
steered, the wheel’s unique shape bears the load
continuously from inside to outside and prevents it from
riding up on its outside edge.

Hubcaps to Minimize the Shock

Inadvertently adding to the rovers’ panache are the
spiral flectures. The futuristic-looking "hubcaps" were
chosen over dozens of other flecture and spoke options
and are designed to absorb shock and to protect the
rest of the vehicle during driving. Next Intent, a
company in San Luis Obispo, California that
specializes in machining complex shapes,
manufactured the wheels. The overall wheel design
allowed them to machine each wheel from one piece (or
billet) of aluminum. Being able to use just one piece of
aluminum minimizes what’s called scar mass, or
useless leftover material where parts would join and
makes the wheel stronger, Voorhees noted.

The outside of the wheels are anodized, or covered
with a black coating, to provide additional strength.
This smooth surface also minimizes the threat of the
wheels getting caught up in the deflated airbags.

The "orange filling" between the spaces in the spiral
flecture is an open-cell foam called Solimide. It
was cut into crescent shapes and bonded to the wheel.

"The idea came from a concern that because the wheel
has an open geometry design to the drive and steering
actuators, it could pick up rocks and debris and cause
a problem," Voorhees said. "We needed to fill the gaps
but still be flexible – we couldn’t use a solid for
shock absorption. Solimide maintains its flexibility
even at very low temperatures so it’s ideal for
conditions on Mars."

Test Tracks: A Race against Time

Planning such a complex mission is, as Voorhees said, a
race against time. Designs are fluid and subject to
intense testing and subsequent change. While nothing
can substitute for being on Mars, the next best thing
is to run trials in simulated martian environments at
the JPL’s test beds. An obstacle course dubbed the "rock
gauntlet" challenged test wheels to scale everything from
small rocks to concrete blocks. Engineers also conducted
airbag interaction tests in which they drove the
wheels into the deflated airbags again and again until
they had enough information to proceed with wheel design
changes. The mobility team and the assembly test and
launch operations team gathered to conduct ramp tests
with the flight rovers to make sure the rover brains were
communicating effectively with its legs and wheels.

Preparing for the Rover’s First "Steps"

Preparing a robot to perform to exact specifications on a
harsh planet 460 million kilometers (286 million miles)
away is no easy task. Still the excitement of sending a
spacecraft to another planet has not waned. While engineers
are anxious to see Mars through the eyes of a rover again, they
know that the deployment process will be slow and precise
once the rovers land on Mars in January, 2004. Once the
lander petals open and the rover "wakes up," it may take up
to five days for it to drive off the lander.

"It’s hard to explain the minutiae – everything has to work
exactly as you plan," Voorhees said. "After every command
sequence we give the rover, we have to wait to make sure
everything is working properly before we proceed. And due
to the delay in sending and receiving signals from
Earth to Mars and back, it’s like taking 20 minutes just to
talk to yourself!"

When ground controllers confirm that all systems are working
as they should, they will tackle the decision of which
direction to go. Nearby obstacles like rocks or deflated
airbags will determine the safest route to leave the
tetrahedron-shaped lander. As it emerges from the lander, its
interplanetary cocoon, the rover will not be breaking any
speed records to conduct its research. Top speed for the
rovers is five centimeters (two inches) per second.
However, as many scientists and engineers are quick to
point out, the goal is not to travel as far and fast as
possible, but to uncover the most interesting science
wherever it presents itself. And as long as the wheels do
their job, Voorhees and the mobility team can live without

During Mars Exploration Rover hardware development, Chris Voorhees was
one of two cognizant engineers for the rover’s mobility system. In preparation
for the launches, he is currently serving as the Assembly Test and Launch
Operations integration engineer for the MER-2 rover at Kennedy Space Center
in Florida.

SpaceRef staff editor.