NASA Mars Rover Team’s Tilted Winter Strategy Works

NASA’s senior Mars rover, Opportunity, has
just passed the
shortest-daylight weeks of the long Martian year with its
panels in encouragingly clean condition for entering a
potential dust-storm
season in 2018.

Before dust season will come the 14th
Earth-year anniversaries
of Mars landings by the twin rovers Spirit and
Opportunity in
January 2004. Their missions were scheduled to last 90 Martian
days, or sols, equivalent to about three months.

“I didn’t start working on this project
until about Sol 300,
and I was told not to get too settled in because Spirit
Opportunity probably wouldn’t make it through that first
winter,” recalls Jennifer Herman, power subsystem
operations team lead for
Opportunity at NASA’s Jet Propulsion
Laboratory in Pasadena, California.
“Now, Opportunity has made
it through the worst part of its eighth Martian

The minimum-sunlight period for southern Mars
this year was in
October and November. Mars takes 1.88 Earth years to orbit the
Sun and, like Earth, it has a tilted axis, so it gets seasons
resembling Earth’s but nearly twice as long.

Both Opportunity and Spirit are in Mars’
southern hemisphere,
where the Sun appears in the northern sky during fall and
winter, so solar-array output is enhanced by tilting the rover
Spirit could not maintain enough energy to survive
through its fourth Martian
winter, in 2009, after losing use
of two wheels, long past their planned
lifetime. It became
unable to maneuver out of a sand trap to the favorable
northward tilt.

Opportunity’s current exploration of
fluid-carved “Perseverance
” positioned it well for working productively through
late fall and
early winter this year. The rover has used stops
at energy-favorable locations
to inspect local rocks, examine
the valley’s shape and image the surroundings
from inside the

The valley runs downhill eastward on the inner
slope of the
western rim of Endurance Crater, which is 14 miles (22
in diameter. Since entering the top of
the valley
five months ago, Opportunity’s stops between
drives have been at
north-facing sites, on the south edge of
the channel. The rover team calls the
sites “lily pads” and
plans routes from each one safely to the next,
like a frog
hopping from lily pad to lily pad.

Herman’s role includes advising others on the
team how much
energy is available each sol for activities such as science
observations and driving. “Relying on solar energy for
Opportunity keeps
us constantly aware of the season on Mars
and the terrain that the rover is on,
more than for
Curiosity,” she said. She performs the same role for NASA’s
younger Mars rover, Curiosity, which gets its electrical energy
from a
radioisotope thermoelectric
instead of solar
Wintertime conditions affect use of electrical heaters
and batteries on both
rovers, but influence Opportunity’s
activities much more than Curiosity’s.

Opportunity has not always been on such
suitable terrain for
winter operations. In its fifth winter, in 2011-2012, it
19 weeks at one
because no other places with favorable tilt were
within acceptable
driving distance. In contrast, it kept busy
its first winter in the southern
half of a stadium-size
, where all of the ground faced north.

Besides tilt and daylight length, other
factors in
Opportunity’s power status include how much dust is on the
array and in the sky. Wind can clean some dust off the
array, but can also stir
up dust storms that block sunlight
and then drop dust onto the rover. Southern-hemisphere
and winter tend to have clear skies over Opportunity, but the
amount of
dust on the solar array going into autumn has varied
year-to-year, and this
year the array was dustier than in all
but one of the preceding autumns.

“We were worried that the dust
accumulation this winter would
be similar to some of the worst winters we’ve
had, and that we
might come out of the winter with a very dusty array, but
we’ve had some recent dust cleaning that was nice to see,”
Herman said.
“Now I’m more optimistic. If Opportunity’s solar
arrays keep getting
cleaned as they have recently, she’ll be
in a good position to survive a major
dust storm. It’s been
more than 10 Earth years since the last one and we need
to be

Planet-encircling dust storms are most likely
in southern
spring and summer on Mars, though these storms don’t happen
Martian year. The latest such storm, in 2007, sharply
reduced available
sunlight for Spirit and Opportunity,
prompting emergency
in operations and communications to save energy.
Some atmospheric
scientists anticipate that Mars may get its
next planet-encircling dust storm
in 2018.

In coming months, scientists and engineers
plan to continue
using Opportunity to investigate how Perseverance Valley was
cut into the crater rim. “We have not been seeing anything
diagnostic, in the valley itself, about how much
water was involved in the
flow,” said Opportunity Project
Scientist Matt Golombek, of JPL. “We
may get good diagnostic
clues from the deposits at the bottom of the valley,
but we
don’t want to be there yet, because that’s level ground with no
lily pads.”

For more information about Opportunity, visit:

News Media Contact

Guy Webster
Jet Propulsion Laboratory, Pasadena, Calif.

Laurie Cantillo / Dwayne Brown
NASA Headquarters, Washington
202-358-1077 / 202-358-1726 /