After more
than four months on the arctic plains of the red planet, NASA's Phoenix Mars
Lander's days are finally numbered.As the sun begins to set for the frigid
Martian winter, the spacecraft will lose its energy supply, freeze and
eventually fall into a mechanical coma from which it will likely never wake up.
Phoenix's mission has been to dig up
samples of Martian dirt and the subsurface layer of rock-hard water ice at its
landing site in Mars' Vastitas Borealis plains. The lander has been scanning
the samples for signs of the region's past potential
for habitability.
Phoenix landed
on Mars on May 25, late spring in the Martian northern hemisphere. The
mission was originally slated to last three months, to the end of August, but
was extended twice; first to the end of September and recently through the end
of December.
But whether
or not Phoenix will survive that long is uncertain and depends on how the
spacecraft's systems handle its ever-dwindling energy supply and the harsh
conditions of the Martian winter.
"We're
at the mercy of Mars," said Phoenix project manger Barry Goldstein, of
NASA's Jet Propulsion Laboratory in Pasadena, Calif.
Winter
sets in
As winter
descends on the Martian arctic, two important things will happen: The sun will
sink below the horizon, and "it's going to get cold," said Phoenix
meteorological team member Peter Taylor of York University in Toronto, Canada.
Of course,
Mars is never warm by Earth standards (it is further from the sun and lacks our
planet's thick, heat-trapping
atmosphere), but summer above the Martian arctic circle is downright balmy
compared to the winter.
Midday
temperatures at Phoenix's landing site hit about -4 degrees Fahrenheit (-20
degrees Celsius) in the summer (as measured by the lander's meteorological mast
thermometer). Nighttime temperatures then still dropped to -112 F (-80 C).
Currently, those daytime temperatures have started dipping down to -22 F (-30
C), with nighttime temperatures hitting about -130 F (-90 C).
By
mid-November, those night temperatures are expected to plummet to -184 F (-120
C).
The reason
of course, is that setting sun.
The sun is
constantly above the horizon during the arctic summer, just as it is on Earth.
Come fall, it starts to dip below the horizon more and more each day until
winter, when it sets for good and doesn't rise again until the spring.
The sun has
already
begun to sink below the horizon for part of the day at Phoenix's location,
Goldstein said. Phoenix's landing site is at a latitude similar to northern Alaska on Earth.
Dwindling
energy
The colder
temperatures and setting sun combined will diminish the energy available to Phoenix for its science operations.
During the
summer, there is plenty of sunlight hitting Phoenix's wing-like solar arrays,
its sole source of power on the planet. But once the sun is gone, so is its
energy supply.
"The
sun is going down, so there's less and less energy being fed into the batteries
through the solar panels, and that really is the biggest problem" facing
the mission, Taylor said.
Specifically,
the orientation of Phoenix's solar arrays limits how much sunlight it can take
in as the sun changes its position in the sky.
"The
problem is that the solar panels are horizontal, and we can't tilt them, so as
the sun gets lower and lower on the horizon, there's less and less power being
generated," Taylor explained.
At the
beginning of the mission, Goldstein said, Phoenix's solar arrays were generating
about 3300 Watt-hours per sol, or Martian day (about 24 hours, 39 minutes)
that's enough energy to light a 60-Watt light bulb for 55 hours.
On Sept.
13, or 109th sol since landing, energy generation had already dipped down to 2400
Watt-hours per sol.
"And
we're steadily decreasing," Goldstein said.
The energy
cut-off point for the lander will come around the time the arrays can only
generate about 1000 Watt-hours, which is "the absolute minimum amount of
energy that it takes for the spacecraft to wake up in the morning,"
Goldstein told SPACE.com.
"Not
surprisingly, we kind of hit that number at about the middle to end of
November" according to model projections, Goldstein added. Those models
are somewhat conservative in their estimates he said, so it's possible the
lander could hold on for a few extra days, "but it's not going to be
much," he said.
Mission leaders are planning to have all
their science operations, such as gathering
and analyzing samples, completed by that cut-off date.
Mission scientists had planned to gather
all the remaining samples (for the one unused cell in the wet chemistry lab and
the four unused ovens in the Thermal and Evolved-Gas Analyzer, both of which
analyze the composition of samples) by the end of September. Since that didn't
happen, they are now aiming to complete sample gathering by mid-October.
Goldstein
said he changed the strategy of sample analysis to focus on gathering all the
remaining samples and then analyzing them instead of processing them one-by-one.
This shift was made because moving the robotic arm and scraping up the dirt and
ice is still a tricky process that involves using an unknown amount energy,
whereas sample analysis involves a known, discrete amount of energy and will be
easier to budget as energy supplies diminish. (Another complication is the
energy that must be used to keep the instruments warm enough to function, which
takes a chunk out of the remaining energy supplies.)
"So
the sooner we get as many of the samples into the cells, the better,"
Goldstein said.
After October,
the lander will essentially transition into a weather station, observing the transition
into winter for as long as it can hold on.
"We'll
keep measuring temperatures and pressures as long as we can," Taylor said. If possible the team will also try to use the lander's lidar to take
measurements of the clouds that have been gathering overhead as the
atmosphere has cooled and its camera to take images of the frost that has
already started forming on the ground.
Frost
forming
The frost
that has already begun to accumulate in the area around Phoenix's landing site,
as well as the sheer cold temperatures will also affect the lander, though
their impact will mostly come after Phoenix ceases operations.
Images
taken recently with the lander's Surface Stereo Imager have shown pockets of
frost forming on the ground, especially in the trenches that Phoenix has
been digging, Taylor said.
So far, the
frost hasn't formed on the lander except for on the small mirror used to view
the wind telltale at the top of the meteorological mast because Phoenix stays
warmer than the ground around it.
"In
general the lander itself is designed to absorb as much solar radiation as it
can, and to emit relatively little radiation in the infrared. So the lander
deck has been much hotter than the surrounding ground surface, for
instance," Taylor explained. "It's a bit like the top of a relatively
warm computer, if you like."
The lander
will likely stay warmer than its surroundings for awhile after Phoenix loses
the energy it needs to operate, "so it'll be pretty late on when frost
actually starts to form on the lander," Taylor said. So Phoenix isn't
likely to get any pictures of itself coated in frost.
Right now
the frost that is forming is all water ice because it is not yet cold enough at
Phoenix's latitude for carbon dioxide ice to form, though it eventually will.
Whether the frost will come as a thin coating or a thick sheet, like Mars'
polar ice caps, isn't known.
"We're
not sure how much CO2 will deposit at this latitude most of it is on the
polar cap," Taylor said.
Taylor tends to think the frost won't
build up as much as at higher latitudes. "We'll see little flakes of ice,
we'll see ice crystals and frost, but it won't be [like] the ice that freezes
on your windscreen on a winter's morning," he said.
After Phoenix shuts down, the only way to observe the mounting frost will be through NASA's Mars
Reconnaissance Orbiter's High Resolution Imaging Science Experiment (HiRISE)
camera. Scientists aren't sure just how much HiRISE will be able to see, but
the team is hoping it will be able to shed some light on Phoenix fate, since
communication with the lander will be lost.
"We
won't be able to talk, but we'd sure like to watch," Goldstein said.
Resurrection?
The big
question will be what happens when Phoenix
emerges on the other side of winter: Can it live up to its name and come back
to life when spring brings back the sun?
Not likely,
Goldstein and Taylor said.
Phoenix
does have a built in reboot program that its designers call a "Lazarus
mode," "where when energy comes back into the vehicle from the solar
arrays if and when energy comes it'll automatically try to reboot and try
to communicate," Goldstein explained. But he doubts that will happen.
"I
would be overjoyed to hear something come back from Phoenix; I'm extremely ... I
find it very, very unlikely," Goldstein said.
The reason
Goldstein, and others on the Phoenix team, think it unlikely that Phoenix will make a comeback is simple: The lander has entered conditions on the surface
beyond what it was built and tested to withstand.
"We
passed the warranty a long time ago," Goldstein said.
The
build-up of frost is part of the problem. While the amount of ice reaching down
to Phoenix's latitude may only be a thin layer, it could also be enough to
encase the lander in ice. Phoenix's engineering team tested the lander's
survivability under many scenarios, but "that was a test I refused to do
during development, survivability if encased in CO2 ice," Goldstein said.
("I was going to call that the Ted Williams test," he joked,
referring to the legendary Boston Red Sox player who had his body cryogenically
frozen after he died.)
But even
without being entombed in carbon dioxide ice, Phoenix likely won't survive the
harsh winter because even in the summer, the lander needs heaters to keep its
electronics warm enough to function.
Phoenix's circuit boards and wiring are
generally regulated to about -40 degrees Fahrenheit (-40 degrees Celsius) for
optimum performance.
"So
will they survive past that? Yeah they will, but at some point they're going to
get so cold that they won't survive," Goldstein said.
Most
electronics can only last down to about -148 or -193 F (-100 or -125 C), after
which some of the materials that make them up go below their glassification
temperature.
Goldstein
explains glassification this way: "Think about a rubbery substance or a
plastic substance becoming brittle like glass, and once that happens, it starts
to crack," Goldstein said.
If Phoenix's electronic components crack, it's unlikely the lander will be able to resurrect
itself even when sunlight returns to the northern hemisphere in the spring.
"The
kind of temperatures we're talking about with no energy to keep the vehicle
warm, it's pretty difficult to imagine," Goldstein said.
Come spring
on Mars (summer on Earth, as the
Martian year is longer), when sunlight has been streaming down long enough to
potentially re-awaken the spacecraft, NASA will likely listen for any beeps
coming from Phoenix, though Goldstein doesn't think they'll hear anything. So
once Phoenix dips below its energy threshold around the end of November, that
will likely be all she wrote for the mission.
"It's
a fun project but we're getting near the end," Goldstein said.
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