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to take about an hour. Then the lander petals will open. No matter which of the four
petals is on the bottom when the folded-up lander stops rolling, the petal-opening
action will set all four face up, with the rover's base petal in the center.
Mars Surface Operations
Opening of the four-sided lander will uncover the rover tucked snugly inside. Each
rover's first action will be to unfold its solar-array panels. Then, still in a crouch, it will
take images of the immediate surroundings with four hazard-identification cameras
mounted below the plane of the solar panels.
Since the rovers rely on sunlight to generate electrical power, their operations on the
surface will run on a schedule timed to the length of the martian day. A martian day, or
"sol," lasts 24 hours, 39 minutes and 35 seconds.
Each rover will need to spend several sols completing housekeeping tasks before
moving off its lander. Before the first martian night, each rover may deploy its main
antenna and the mast on which its panoramic camera and navigation camera are
mounted. The navigation camera will take the first panorama of the landing site. Once
transmitted to Earth during the following sol, the panorama and initial imaging by the
rover's hazard-identification cameras will help mission engineers identify the safest
route for the rover's later departure from the lander.
The rover will rise up from its crouching position and stand up at its full height while
still on the lander base petal. From this height, it will take a 360-degree high-resolution,
stereo, color panorama with its panoramic camera and a matching 360-degree panora-
ma with its miniature thermal infrared spectrometer. Scientists will rely heavily on
those images to decide which rocks the rover should go examine.
Unlike Mars Pathfinder, when each Mars Exploration Rover rolls off its lander, the lan-
der's role in the mission will have ended. A new chapter in Mars exploration will begin.
In the next few sols after roll-off, the rover will finish checking and calibrating its sci-
ence instruments and move to whichever nearby rock or patch of soil the science team
has selected as the first target by analyzing the panoramic and infrared images taken
earlier. The rover will examine each target up close, then begin moving on the follow-
ing sol toward its next target. It may travel as much as about 40 meters (44 yards) in a
sol, but is likely to cover less than that on most travel days as it maneuvers itself to
avoid hazards on the way.
To coordinate their work with the rovers, flight team engineers and scientists operating
the rovers from NASA's Jet Propulsion Laboratory in Pasadena, Calif., will be living on
a martian schedule, too. The 40-minute difference from Earth's day length means that,
by about two weeks after the rovers land on Mars, team members' wake-up times and
meal times will have shifted by about 9 hours. After the second rover reaches Mars, its
team will be working on a different martian schedule that the first rover's team because
the two chosen landing sites are about halfway around Mars from each other. When
it's noon at Meridiani, it's midnight at Gusev. Each rover will typically transmit each
sol's accumulation of data in the martian afternoon. The flight team will analyze that
data, refine plans for the next sol's rover activity, and send updated commands to the
rover the next martian morning.
Each rover has a prime-mission goal of operating for at least 90 martian sols (92 Earth
days) after landing, though environmental conditions such as dust storms could cut the
mission shorter.
Mars' distance from the Sun varies much more than Earth's does, and Mars will have
passed the closest point to the Sun in its 23-month elliptical orbit about 5 months
before the rovers arrive. The distance between Mars and the Sun will therefore
increase by about 7 percent between mid-January and mid-April 2004, resulting in two
principal consequences for how long the rovers can keep working. The rovers land at
the end of summer in Mars’ southern hemisphere, and with the onset of autumn the
decreasing intensity of solar radiation reaching their solar panels will lessen the
amount of electrical power produced. Also, colder nights will increase the need for
electrically powered heating to keep the batteries warm enough to work. On top of
those factors, a less predictable but possibly most important element limiting the
rovers' lifetime will be the accumulation of dust on their solar panels.
Like all of NASA's interplanetary missions, the Mars Exploration Rover project will rely
on the agency's Deep Space Network to track and communicate with both spacecraft.
During the critical minutes of arrival at Mars, the two rovers will communicate essential
spacecraft-status information throughout their atmospheric entry, descent and landing.
On the surface of Mars, the rovers will be capable of communicating either directly with
Earth or through Mars orbiters acting as relays. The distance between Earth and Mars
will increase by about 65 percent between mid-January and mid-April 2004, reducing
the rate at which data can be transmitted across space.
The Deep Space Network, which will be 40 years old on December 24, 2003, transmits
and receives radio signals through large dish antennas at three sites spaced approxi-
mately one-third of the way around the world from each other. This configuration
ensures that spacecraft remain in view of one antenna complex or another as Earth
rotates. The antenna complexes are at Goldstone in California's Mojave Desert; near
Madrid, Spain; and near Canberra, Australia. Each complex is equipped with one
antenna 70 meters (230 feet) in diameter, at least two antennas 34 meters (112 feet) in
diameter, and smaller antennas. All three complexes communicate directly with the
control hub at NASA's Jet Propulsion Laboratory, Pasadena, Calif. The network
served more than 25 spacecraft in 2002.
The network has been preparing to deal with an extraordinary level of demand for
interplanetary communications in late 2003 and early 2004. Several missions besides
the Mars Exploration Rovers will be conducting critical events. Among others, the
European Space Agency's Mars Express will enter Mars orbit after dropping the Beagle
2 lander to the surface; Japan's Nozomi orbiter will be arriving at Mars; NASA's
Stardust spacecraft will fly by a comet; and NASA's Cassini spacecraft will be nearing
its mid-2004 arrival at Saturn. The Deep Space Network is upgrading antenna capabil-
ities at all three complexes and is completing the construction of a new 34-meter
antenna at the Madrid complex. That new antenna alone will add about 70 hours of
spacecraft-tracking time per week during the periods when Mars is in view of Madrid.
During each Mars Exploration Rover mission's early cruise phase, a low-gain antenna
mounted on the cruise stage will provide the communications link with Earth. A low-
gain antenna does not need to be pointed as precisely as a higher-gain antenna.
During early cruise it would be difficult to keep an antenna pointed at Earth and the
solar panels oriented toward the Sun, due to the Sun-Earth angle at that stage of the
mission. Later in the cruise toward Mars, the angle between the Sun and Earth will
shrink, making it possible for the spacecraft to switch to a more directional medium-
gain antenna, also mounted on the cruise stage.
Data transmission is most difficult during the critical sequence of atmospheric entry,
descent and landing activities, but communication from the spacecraft is required dur-
ing this period in order to diagnose any potential problems that may occur.
Minutes before the spacecraft turns to point its heat shield forward in preparation for
entering Mars' atmosphere, the cruise stage's low-gain antenna will take over again,
which will reduce the data transmission rate to 10 bits per second, less than 2 percent
of the mid-gain antenna's rate. Through this antenna and later through other low-gain