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Mars Pathfinder Kevin Barron 
Sunrise on Mars
Mars Pathfinder mission
The Mars Pathfinder mission is primarilt an engineering
demonstration of key technologies and concepts for employing scientific landers to Mars.
Pathfinder also delivers science instruments and a
microrover to the surface of Mars, to investigate the structure of the martian atmosphere,
surface meteorology, geology, and the elemental composition of martian rocks and soil.
Mars Pathfinder is a lander spacecraft carrying a
surface rover.
It was launched on a Delta II-7925 launch vehicle on 4
Dec. 1996.
After a 7 months cruise in space, Pathfinder arrived at
Mars on 4 July 1997. The lander was safely landed on a rocky plain in an area called Ares
Vallis.
Entry and landing strategy
Upon arriving at Mars, this spacecraft was scheduled to enter the Martian atmosphere without going into orbit around the planet and land on Mars with the aid of parachutes, rockets and airbags.
The Pathfinder lander was packaged inside a space
capsule-shaped entry vehile.When it arrived Mars and hit the thin upper atmosphere at
about 27,000 km per hour, the entry vehicle's heat shield slows down the craft speed to
1,450 km per hour in about 2 minutes.
An onboard computer sensed the slow-down in speed and then
deployed a large parachute. The parachute slows the lander down to about 250 km per hours
in the rarified atmosphere of Mars.
The distance to the ground was monitored by an onboard radar altimeter inside the lander. At about 100 meters above the surface, the computer inflated the air bags.
Seconds later, 3 solid rockets motors inside the entry vehicle above the lander were fired, and in 2 seconds time, the lander was brought to a stop at a level of about 12 meters above the Martian ground.
Then the parachute was released, and the lander which
was nestled inside its protective air bag cocoon,
started to fall to the ground, bouncing and rolling until
it stopped.
Within about an hour, the air bags were deflated and
partially retracted.
Pathfinder then opened its 3 metallic petals and stood
itself up.
The microrover which was attached to the inside of one of the petals, was exposed to the Martian terrain for the first time.
The camera on the lander was released and began to search for the Sun. The high-gain communications antenna was deployed and pointed toward Earth.
After the positional
photographs taken by the lander camera were transmitted to Earth, the NASA engineers
started to instruct the rover to drive off and run on the Martian surface - The first time for a robotic roving vehicle placed on another planet and
being remote controlled from Earth. 
Comparison of Apollo LRV (Lunar Roving
Vehicle) used on the Moon and today's
Mars Sojourner (to scale).
Remote control of the rover on Mars from Earth
Sojourner
on Mars
Microrover - Sojourner
When you watch the movie of this little rover moving on
Mars, you might wonder how could the remote control on Earth reach the rover through a
distance of about 200 million km ?
This microrover has a special name: Sojourner.
It is very small, about the size of a child's small wagon
- 63 cm ling, 48 cm wide and 28cm high. Its weight is 11 kg on Earth. (about 4 kg on
Mars).
However this small wagon costs $25 million.
Sojourner has 6 wheels and can move at speeds up to 0.6
meters per minute.
Its wheels and suspension use a rocker-bogie system that
is unique in that it does not use springs. It was designed to provide the greatest degree
of stability for traversing rocky, uneven surfaces.
The wheels are made of aluminum. Stainless steel treads
and cleats on the wheels provide traction. Each wheel can move up and down independently
of all others.
When Sojourner climbed over a rock, one side of it could
tip as much as 45 degrees without tipping over. In addition, 3 motion sensors along
Sojourner's frame can detect excessive tilt and stop the rover before it gets close to
tipping over.
Earth
- Mars, Remote Control System
Telecommunication System
The remote control and communications between Sojourner
and Earth is via a telecommunication system
- a two-way wireless UHF (Ultra High Frequency) radio link
is used to send commands from Earth to the rover and receive images and data from the
rover.
Becasue the radio has a signal range similar to a
walkie-talkie, we cannot communicate directly to the rover from Earth. All rover
communacations are done with the aid of the lander communications interface.
There are 4 components of the telecommunications
system:
The UHF Radio Modem of the rover
The LMRE UHF Radio Modem of the lander
The UHF antenna of the rover
The LMRE UHF antenna of the lander.
These radios send and receive data in the form of
digital symbols. The radio modem transmit short bursts of data symbols, termed
"packets".
The data packets transfer rover camera images and
engineering telenetry from Mars, as well as transfer commands from Earth. The radio modem
can either talk or listen at any given time.
The direction of the flow of information between the
rover and the lander being controlled by the rover radio modem using a "half-duplex
operation" protocol. The rover is the master of the link between lander and
rover. It controls the initiation of all communication sessions
These session types are:
Heartbeat
Time Request
Command Request
Telemetry
The lander's UHF radio modem has an extra LMRE(Lander
Mounted Rover Equipment) eletronics board attached to it, as it is meant to be powered
using a +28 volt source.
The function of the lander radio is to upload fairly small
command sequences to the rover and receive data and telemetry downloads from it.
The function of the antenna is to aid in the effective
transmission of radio waves through space. These are
"momopole" antennas and working very much like
the antennas on walkie-talkie or on car radios.
Radio signals travel time
The radio waves travel at the speed of light, 2.9979245 x
100,000 km per second.
Durinmg the 4 July landing Mars and Earth were 192 million
km apart - the radio signal took 10 minutes and 39 seconds to arrive.
As of 4 September mars is 249 million km from Earth - it
took 13 minutes and 50 seconds for the signal travel.
On 22 June 1998, Mars and Earth will be their farthest
apart at a distance of 2.52 AU ( 1 AU = 1.4956 x 100,000,000 km ) - the signal will take
20 minutes and 57 seconds to travel in one direction.
These time delays make it so that the remote control of
the rover from Earth cannot be in real time.
That means that goal locations or move commands must be
sent to the rover ahead of time. Then Sojourner traverses to these locations on her own.
The microwave data link from the lander to Earth is
handled by the Deep Space Network (DSN).
DSN consists of a collection of 34 and 70 meter radio
telescopes equipped with cryogenically cooled low-noise amplifiers. They operate in two
microwave frequency bands : S-band (2.3 GHz) and X-band (7.2 GHz uplink, 8.4 GHz downlink
).
Mars Pathfinder uses a 12 Watt RF output X-band SSPA
(Solid State Power Amplifier) transmitter to downlink data back to Earth.
The lander has two ways to communicate with Earth. One way
is with its High Gain Antenna and another is with its Low Gain Antenna. Both antennas are
capable of receiving the 7.2 GHz uplink signal and transmitting the 8.4 GHz downlink
signal.
The noise levels received are fairly high. Only with
the carefully designed front end of the DSN Block V receiver and the extremely high gain
of the large dish antenna, a carrier-to-noise ratio of about 20 - 40 dB-Hz (ball park)
could be obtained.
Uplinks to the lander are sent via high power transmitters
(up to 20 KW) through the same high gain 34 or 70 m antennas.
The radio signal transmitted from Mars to Earth is so weak
by the time it gets here it is only 7.9 x 0.0000000000000000001 watts.
Hence it is impossible for the home satellite dish and
receiver to detect it.
Sojourner mission
The rover is primarily a technology experiment itself.
However Sojourner has to do a lot of experiments.
There are three main mission objectives:
Technology Experiments:
Mars terrain geometry
reconstruction from imagery.
Mars basic soil mechanics.
Mars dead reckoning sensor
performance and path reconstruction/recovery.
Sinkage in each Martian
soil type.
Logging/trending of vehicle
performance data.
Rover thermal
characterization.
Rover imaging sensor
performance.
UHF link effectiveness.
Material abrasion.
Material adherence.
Rover Science Experiments:
Alpha Proton X-Ray
Spectrometer.
APXS deployment mechanism.
Imaging.
Mission Experiments:
Lander imaging.
Damage assesment.
Rover
deploying APXS
This little vehicle has completed the scheduled 7
sols operation extremely well. She has moved around the lander within a 10 meters area.
The extended mission was planned for the rover for up to
30 sols, and for the lander for up to one Martian year. ( A Mars year is about 687 earth
days. A Mars day last for about 24 hours and 37 minutes. It is called a sol.)
So far, the rover is still functioning. Hopefully it will
function as long as the lander does. Once the lander falls silent then nobody knows the
situation of the rover.
Scientific Investigations
Beside the beautiful sunrise and sunset scenes from Mars
and the panorama views of the Red Planet,
what have we learned from the Mars pathfinder mission?
Scientific Instruments
Mars Pathfinder carries three scientific instruments:
Imager for Mars Pathfinder (IMP) - This is a stereo imaging system with color capability provided by a set of selectable filters for each of the two camera channels. A magnetic properties investigation and the observation of wind direction are included as part of the IMP investigation.
Alpha Proton X-Ray Spectrometer (APXS) - This instrument can determine elemental chemistry of surface materials.
The analytical process is based on 3 interactions of alpha
particles with matter:
Elastic scattering of alpha particles by nuclei.
Alpha-proton nuclear reactions with certain light
elements.
Excitation of the atomic structure of atoms by alpha
particles.
By exposing material to a radioactive source that produces
alpha particles with a known energy, and to acquire energy spectra of the alpha particles,
protons and X-Rays returned from the sample, the APXS can identify and determine the
amounts of most chemical elements.
The APXS sensor head is mounted external to the rover chassis on a deployment mechanism. This mechanism places the APXS in contact with rock and soil surfaces .
Atmospheric Structure Instrument /
Meterology Package (ASI / MET) - This is an
engineering subsystem which acquires atmospheric information during the descent of the
lander through the atmosphere and during the entire landed mission. The profiles of
atmospheric density, temperature and pressure from altitudes in excess of 100 kn to the
Martian surface can be collected.
Scientific Results
Pathfinder
APXS analysis of soils (yellow dots) extend the range of Viking soil analysis.
APXS
analysis of Martian soils are compared with Viking soil analysis. The yellow boxes
representing Viking data. There is some variability and a few significant differences.
Specifically, soils at the Pathfinder site generally have higher aluminum and magnesium,
and lower iron, chlorine, and sulfur.
This
X-Ray data shows chemical differences between terrestrial rocks and meteorites inferred to
have been derived from Mars. The Martian meteorites all plot to the left of tthe fields
for Earth rocks. Pathfinder APXS analysis of rocks (stars) and soils(yellow dots)
appear to plot in the gap between these two fields.
Pathfinder
APXS chemical analysis of Barnacle Bill and Yogi. They are both possibly consist of
orthopyroxene(magnesium-iron silicate), feldspars (aluminum silicate of potassium, sodium,
and calcium), quartz (silicon dioxide), and other minerals that include magnetite,
ilmenite, iron sulfide and calcium phosphate.
Diversity in Rover deployment area. The
surface near the rover's egress from the lander contains bright red drift, dark gray
rocks, soil intermediate in colour to the rocks and drift. And dark red soil on and around
the rock Lamb.
Globally, Mars is characterized by similar colour
variations. The spectra provide evidence for the mineralogy of the unweathered rocks and
highly weathered red soils.
Spectral
Mapping of Site. The top image showns the region southeast of the lander in true colour.
The bottom image is shown in false colour, blue rocks are the least weathered, red soils
are most weathered, and green soils and rocks show an intermediate state of weathering.
These data reveal at least 5 kinds of rocks and soil in the immediate vicinity of the
lander.
Mars Color Variation. The first color
panorama returned by IMP. Rocks and soils on the surface are thought to be composed of
minerals similar to those found on Earth's surface. The most important coloring materials
in the Martian surface are iron minerals. The right image shows a Stripe Rock, a bright
vertical stripe appears on the center of the rock face. It was thought that this stripe
might be an intruded vein of material of different composition than the surrounding rock.A
detailed examination suggests that the stripe is actually an accumulation of soil
deposited in a crack in the rock face.
Unusual
Rock Named Ginger. One of the most unusual rocks at the site. Part of it has the
reddest colour, where as its rounded lobes are gray and relatively unweathered. The origin
of this rock is uncertain. Ginger will be a target of future super-resolution studies.
Flat
Top Rock Colour Imaging
The Evidence of Layering in Martian soil.
In this investigation, the rover wheels are spun while simultaneous measurements of motor
currents, vehicle kinematic configuration, and temperature are recorded. These tractive
coefficients are used to characterize the soil structures.
Mars Pathfinder Landing
Site. The inferred wind direction is from the northeast blowing toward the southwest (the
red arrows).
Mars
interior is simply modeled as a core and mantle with a thin crust, similar to Earth. The
combination of Pathfinder Doppler data with the Viking landers' data has determined the
parameter of the moment of inertia, through measurement of Mars' precession rate. However
the completion of this interior model needs another future measurement.
If the core is as dense as possible (i.e. 100% iron) and
the mantle is similar to Earth's, then the minimum core radius is about 1300 km.
If the core is made of less-dense material (i.e. iron plus
sulfur), then the core radius could be <2000 km.
Beautiful
Scene of Sunset on Mars
So far, until on 8 Oct. 1997 (sol
92), the lander and the rover are both still operating on Mars.
However the rover has to sleep during the night, since
her non-rechargeable battery had gone.
In the morning, she awakes to the song "Old
Time Rock-N-Roll" played by the rover engineer on Earth. And the Sun gives her new
energy in her GaAs solar panel.
Let us hope that she will be happily living on Mars as
long as possible.
In the next century, perhaps one day, a manned flight
to Mars will pick her up and bring her back to Earth.
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