[meteorite-list] The Opportunity Rover Has Its Eye on Martian Bedrock

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:19 2004
Message-ID: <200402021559.HAA09068_at_zagami.jpl.nasa.gov>

http://www.aviationnow.com/avnow/news/channel_awst_story.jsp?id=news/02024top.xml

The Opportunity Rover Has Its Eye on Martian Bedrock
By Craig Covault
Aviation Week & Space Technology
February 1, 2004

Sobered by a near-death experience with Spirit and still
reeling from the landing of Opportunity at a location with scientific
potential "beyond their wildest dreams," Mars Exploration Rover controllers
are this week preparing to restart science operations with the first rover,
while 6,000 mi. away they plan to drive Opportunity onto the Martian surface
for a first taste of the bizarre terrain and soil at its Meridiani Planum
landing site.

Rover drivers here will then steer Opportunity onto Martian
bedrock--possibly 3-4 billion years old--that researchers rate as one of the
best finds ever made by the storied Jet Propulsion Laboratory planetary
exploration team.

The chance landing of a scientifically potent rover within 25 ft. of exposed
Martian bedrock ranks as significant in the annuals of JPL as the 1979
Voyager 1 discovery of volcanoes on Jupiter's moon Io, said Larry Soderblom
of the U.S. Geological Survey, one of the world's most experienced planetary
scientists.

This is because bedrock, normally deeply buried but here remarkably exposed
by meteorite impact, is a fundamental planetary building block formed
directly where it is found. The rover's ability to sample it will allow
scientists to make fundamental judgments about Martian processes, rather
than infer these processes from material deposited perhaps hundreds of miles
from where it was formed.

The Rover's powerful Panoramic Camera already shows the rocks have intricate
layers as if formed by sedimentary deposits in water, a possibility given
the strong potential for past water at Meridiani. But volcanic ash also
remains a viable hypothesis.

The Mini-TES infrared spectrometer will begin to provide compositional data
on the outcrop this week, and during the next month the rover will park over
the site to use its Alliance Spacesystems Inc. arm carrying spectrometers
and a microscopic imager to decipher the nature of the bedrock better than a
lesser equipped human geologist on Earth could. "We are going to beat on it
with everything we have got," said Steve Squyres, MER principal investigator
from Cornell University.

Rock layers are like the pages in a history book, and Opportunity has now
found that book on Mars, said Andrew Knoll, a science team member from
Harvard University.

In the span of just a month, human exploration of the solar system has
changed fundamentally. With two new surface rovers and the highly capable
European Mars Express orbiting Mars along with two other U.S. orbiters,
suddenly there are several hundred people--both U.S. and European--literally
working "on Mars."

And that is especially evident here at JPL where the MER science and
operations team is split into distinct groups, about half working on the
Spirit rover on Gusev Crater time, the other half working with a 12-hr.
time-zone difference for Meridiani. In the meantime, hundreds of support
personnel for both missions are working on Pasadena time. It can be 1 p.m.
PST in Pasadena, 5 p.m. Gusev time and 5 a.m. Meridiani time.

People passing in the congenial central JPL plaza will extend both "good
morning" and "good evening" greetings simultaneously, when it's neither here
at JPL.

THE TWO SURFACE teams are working out of different floors of the Building
264 Surface Mission Support Area (SMSA) here. Controllers for both rovers
and the Gusev science team operate out of the fourth floor, colored red,
while the Opportunity/Meridiani science team is on the fifth floor, colored
blue, so managers and scientists can remember which Martian time zone they
are in--critical for rover decision timing.

This editor visited the SMSA on Jan. 29 as it was linked to Spirit at Gusev
just as the team reinitiated Pancam science operations. I then observed the
Spirit anomaly team led by Jennifer Trosper meeting in an adjoining room as
they decided that the tactical science operations process for Spirit could
be reinitiated on Feb. 1, its 29th day on Mars.

It is a remarkable comeback from a week ago when Ed Weiler, head of Space
Science for NASA in Washington, said he feared he was returning to JPL "for
a funeral" for Spirit (see story p. 36).

Meanwhile at Meridiani, Opportunity was being put through multiple
milestones related to standup and wheel deployment and alignment.

But that was being done cautiously. "We are trying to make sure we are not
accidentally duplicating what caused Spirit to have its problems," said Jim
Erickson, Opportunity mission manager. Opportunity had been planned to roll
off its forward ramp directly facing the bedrock as early as Jan. 31, its
eighth Martian day (Sol) on the surface.

The Opportunity team was working one rover problem unrelated to standup and
roll-off. A thermostat in the elbow joint of its manipulator arm was
continuously cycling heater power at that location on and off. A periodic
drain of up to 15 watts, which if it were to persist for 3-4 months from
now, could limit mission life, depending upon solar array dust buildup.

Of the two rover landing sites, Opportunity's Meridiani site on the equator
about 0 deg. Long. was the highest priority. This is because of the
detection there by Mars Global Surveyor (MGS) of a formation the size of
Oklahoma of gray hematite, an iron oxide mineral that normally indicates the
past presence of water. Since the overall MER missions are to study the
permanency of water as habitats for ancient life forms, this was the highest
priority target.

To get to the 46 X 4-mi. landing ellipse required extremely precise work by
the MER Navigation Team led by Louis D'Amario. Launched on July 7, nearly a
month after Spirit, Opportunity flew a shorter 280-million-mi. circuit to
Mars compared with Spirit's 300-million-mi. trajectory. But Opportunity
intercepted the planet at about 124 million mi. from earth--about 20 million
mi. farther away than when Spirit landed.

This resulted in a one-way light time signal travel time to Earth of 11 min.
for Opportunity during landing compared with 9.5 min. for Spirit's descent.

The effect the extra difference had on signal strength and viewing angle for
the Deep Space Network antennas at Goldstone, Calif., and Canberra,
Australia, indicated that maintaining signal lock at landing would be more
difficult than for Spirit. But this proved not to be the case.

On Jan. 17, JPL commanded a third and final cruise stage trajectory
correction burn of only 0.25 mph. using 120 grams of propellant. This was
just the maximum amount of energy contained in your average hamburger, said
Erickson. But it had the effect of changing the surface arrival location by
380 km. (224 mi.) and adjusting arrival time by 36 sec.

The maneuver targeted Opportunity about one third of the way down the
46-mi.-long landing ellipse with a touchdown planned slightly north of
center, based on intensive overhead imaging of the site.

A 10-15-km. downrange error was also likely, given atmospheric
characteristics at Meridiani, a forecast that proved close to accurate.

WEATHER MONITORING of Meridiani was absolutely critical to final entry,
descent and landing (EDL) updates, said Rob Manning, EDL manager. This
monitoring was done by JPL and Goddard Space Flight Center teams using MGS
and other data.

A critical parameter was the dust level in the atmosphere over the landing
site, given that a dust storm had swept through the area in December. The
dust affected atmospheric heating and therefore reduced atmospheric
density--a critical factor for calculating performance of the 47-ft.-dia.
nylon Pioneer parachute. This was especially critical at Meridiani, the
highest Martian landing site ever attempted--about 1,500 ft. higher than
Gusev. The dust, in effect, forced the chute to function for a time in a
higher, thinner atmosphere than at Gusev.

These calculations resulted in updating Opportunity's computer to open the
chute 2 sec. earlier than planned--a 1-2% difference in chute performance
that would equate to 5% extra margin at the deceleration rocket firing and
airbag-release altitude, Manning said. In the days before landing, these
calculations were verified by thousands of computer simulations using
expected atmospheric density and various dispersions.

The plunge to the surface by Opportunity, encased in its aeroshell, took it
at night directly over Valles Marineris, the solar system's largest canyon,
and just abeam of Olympus Mons, the solar system's largest volcano.

The vehicle heat shield pointed into the direction of flight slammed into
the atmosphere at 12,187 mph. and 73 mi. altitude about 440 mi. from the
landing site to begin its 6-min. descent.

The chute deployed as planned with the vehicle still supersonic above 25,000
ft.

At 5.4-km. altitude, with the lander now hanging below the chute on a
bridle, the critical landing radar altimeter locked up on the surface,
enabling the vehicle to compute retrorocket ignition at about 121 meters
(397 ft.) altitude.

At about 2 km. the first of three images was taken by the Descent Image
Motion Estimation Subsystem (Dimes) camera as input for any lateral velocity
corrections needed.

Unlike on Spirit, none was needed. Initial data showed that as the lander
hung on its 65-ft. bridle under the parachute, it had a drift of 10
meters/sec. north combined with a 2.7 meter/sec. component to the west. The
three 1,124-lb. thrust Alliant retrorockets fired for 2.8 sec., providing 25
meter/sec. deceleration.

This also provided 7.7 meters/sec. of southern velocity that tended to
cancel out the other two values as the bridle holding the airbags was
separated at about 12-meter altitude. The lander experienced only about 3g
at touchdown and is designed for up to 40. The vehicle bounced, all the
while transmitting data to Earth in the western sky.

It was during these bounces that the lander plopped fortuitously into a
shallow crater where it dribbled like a basketball, then rolled like a
marble in a soup bowl, coming to rest in the center of the 20-ft. wide
depression. That was extremely good luck because the object that created the
crater had also excavated the bedrock. The airbag's bouncing also proved to
be an excellent first science experiment, leaving such detailed impressions
that the airbag stitches sewn into the bag by the staff at ILC in Dover,
Del., have now been sharply imprinted on Mars.

The vehicle came to rest 24 km. downrange from its formal target area, but
in a more scientifically rich spot than had it been on target. The lander
settled on a side petal, which also was fortunate, because unlike with
Spirit, it allowed the airbags on the preferred forward drive-off direction
to be fully reeled in before the rover was flipped onto its base petal.

So far the crater in which the spacecraft landed shows signs of what
geologists expected--that if hematite is there, it is on top of a
200-300-meter stack of sedimentary deposits, visible when excavated by
erosion or meteorite impact. The question is whether the hematite was formed
by water or volcanic-related processes.

As expected, the site, with a reflectance ratio of only 10-15%, has the
darkest material ever seen on Mars--totally unlike the two Viking,
Pathfinder or Spirit sites (see front cover).

A FIRST ORDER of business this week is to use the instruments on the arm to
look for hematite in the soil. But Squyres believes the rover will have to
drive out of the crater to more likely find it on the slightly higher
terrain. That will come in a month or so when this second rover in effect
makes its second landing on Mars by driving out of the crater and looking
around at what again will be a whole new world not clearly visible from
inside the 3-ft.-deep depression.

At JPL I also visited their In Situ Instrument Laboratory where an
engineering rover was climbing a dirt slope similar to the crater where
Opportunity landed as a technician carefully monitored rover angles.

The soil, as first indicated by the airbag impact, is an extra surprise that
will keep the rover almost as busy as the bedrock. Soderblom noted that when
disturbed by the airbags it looks as if it has become a nicely sculptured
"Japanese rock garden." "The soil and physical properties group is going
wild. . . . Everybody is having a wonderful time arguing about it," Squyres
said.

"When you look at this soil and you try and interpret what the heck you are
looking at, there are two schools of thought," he said. "One, that we have
soil with two distinct components. That there are coarse grayish grains and
much finer reddish stuff. And when the airbags hit it and push the coarse
gray grains into the red stuff, then all you see is red stuff. The other
idea is that we have aggregates of grains that appear grayish but when
impacted they somehow turn red."

But researchers are also beginning to see pebbles with multiple colors. What
they may be, nobody has figured out yet.

And there are some innovative ideas on what could cause such a unique
powdery soil that changes character when compressed. One idea is a "flash
water" phenomenon where liquid water may have existed briefly on the
surface--perhaps just a second or two in some wide area event, which altered
electrical charges in the soil.
Received on Mon 02 Feb 2004 10:59:36 AM PST