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MGS Prepares To Photograph Landing Sites, Cydonia Region

http://mars.jpl.nasa.gov/mgs/target/pressrel.html

MARS GLOBAL SURVEYOR COMPLETES FIRST AEROBRAKING PERIOD AND PREPARES TO
PHOTOGRAPH THE MARS PATHFINDER LANDING SITE AND FEATURES IN THE CYDONIA
PLAIN

The Mars Global Surveyor spacecraft is about to resume scientific
observations of the surface of Mars with its first objective to attempt to
photograph the Mars Pathfinder landing site, the features in the Cydonia
region, and the Viking lander sites. Surveyor is coming up on a period
beginning near the end of March and continuing for about a month in which
orbital and lighting conditions will be suitable for these observations.

The opportunities to see these targets from the Surveyor spacecraft will
occur in three clusters of two and a half days each during the next month.
Each target will be visible once in each cluster and the clusters will be
separated by eight days. It will not be possible to predict on which orbits,
and thus, on which days, the spacecraft will come closest to the targets
until after aerobraking has been terminated on Friday, March 27th. Then
several orbits of navigation tracking data have been obtained in order to
pin point Surveyor's new orbital characteristics.

The exact time of the observation opportunities and the schedule and process
for the release of the resulting photographs will be announced in a few
days. Within a few days before the actual observations, a detailed sequence
of the spacecraft's activities will be posted on this webpage, and the
project staff will provide a near real time commentary on the events as they
occur.

Surveyor's science instruments will be turned on again on Friday, March
27th, after having been off since February 20th when the orbital period
became too short for both science and aerobraking operations to be conducted
simultaneously. Now that aerobraking will be on hold for five months,
Surveyor can return to acquiring science data.

                End of Aerobraking and Science Phasing Orbit

                             Transition Timeline

       ( Subject to change, depending on level of drag encountered )
 Date Time   Orbit Event A = apoapsis P = periapsis

  3/23/98     A194 UP ABM to 0.1 N/m2 (0.5 m/s) Took orbit up to an
 23:10 UTC         aerobraking altitude where the dynamic pressure is 0.1 N/m2
                   so that aerobraking is slowed to more easily manage the
                   arrival at the target period of 11.6 hours. THIS EVENT HAS
                   BEEN COMPLETED IN A SATISFACTORY MANNER!

  3/26/98    A201  ABX-1 (Aerobraking Termination Maneuver) (4.43 m/s) A
 21:20 UTC         bi-propellant main engine burn to terminate aerobraking by
                   raising the altitude of periapsis to 170 km establishing
                   the science phasing orbit

  3/27/98    A202  Instrument Turn-On command window opens (shortly after A201
 08:57 UTC         apoapsis which is at 08:52)

  3/28/98    P203  P202 First PERISCAN (Periapsis Science Acquisition) Pass.
 02:24 UTC         This will be the first time in the science phasing orbit
                   that science data will be acquired, and the start of the
                   six orbit period where the navigation baseline for targeted
                   opportunities will be established.

                   Science acquisition will continue until early September
                   when aerobraking will be resumed.



PHOTOGRAPHING THE FEATURES IN THE CYDONIA PLAIN

At the launch of the Mars Global Surveyor mission, NASA announced that it
would re-photograph the Cydonia region of Mars -- an area that contains a
number of features including the famous "Face on Mars" -- when Surveyor was
over that region during its mapping mission. In addition, NASA said it would
announce to the public when these opportunities would occur and when the
resulting pictures would be released. The opportunity to accelerate the
schedule of photographing these areas significantly before the mapping
period has been afforded by the recent modification of Surveyor's mission.
This modification was made to extend aerobraking for a year in order to
compensate for a structural weakness discovered in one of Surveyor’s solar
panels.


TARGETS FOR OBSERVATIONS

Mars Pathfinder landed last July 4th, deployed the Sojourner rover and
captivated world interest as it explored a small region in Aris Vallis. The
two Viking landers that NASA placed on the surface of Mars in 1976 conducted
inconclusive experiments to try to discover life in the Martian soil. The
Cydonia region has become notable from the discovery of an object that looks
much like a human face in several pictures taken by the Viking Orbiter
spacecraft over 20 years ago. Some researchers have proposed arguments that
the "Face" and other objects in its vicinity are artifacts of an extinct
civilization and have pressed NASA for further investigations of the region.




Latitude and Longitude of four targets located in East longitude
  Target            Latitude          Longitude
  Cydonia Region     41.0 North       350.5 East
  Pathfinder         19.01 North      33.52 East
  Viking 1 Lander    22.27 North      312.03 East
  Viking 2 Lander    47.67 North      134.48 East


HOW THESE OBSERVATIONS WILL BE MADE

It is anticipated that Surveyor's ground track will not pass directly over
any of the targets so it will be necessary to rotate the spacecraft to sweep
the field of view of its cameras across the targets as the spacecraft
travels south from over the Martian north pole as the spacecraft gets closer
and closer to the surface Photographs will be taken as long, narrow strips
as the field of view is sweeping across the targets.

The orbital conditions chosen for the next five month period when Surveyor
will not be aerobraking offer a particularly advantageous pattern of near
overflights of these targets. Because of the position of the targets in
longitude around the planet (Viking 2 is 182 degrees to the east of Viking
1, Mars Pathfinder is 14 degrees to the east of Viking 1, and Cydonia is 24
degrees east of Viking 1) the near overflights will occur in clusters of
five orbits every 17 orbits. Surveyor's orbital period of 11.6 hours, which
is slightly less half a Martian day, causes the spacecraft's ground track to
alternate sides of the planet on consecutive revolutions. At every closest
approach to the planet or periapsis, the spacecraft is about 190 degrees to
the east of where is was one orbit ago and about 20 degrees to the east of
where it was two orbits ago.

These observations are termed "targeted" because mission controllers will
take extraordinary steps to try to assure that the selected targets are
within the high resolution camera's field of view. This is a difference
process than has been used in the past or will be used in the future to
collect images of Mars from Global Surveyor. The normal manner of acquiring
images and other science data is to point the instruments straight down at
the surface or to take science data as the instrument fields of view sweep
across the planet as the spacecraft performs maneuvers to accomplish
aerobraking. During the aerobraking hiatus last Fall, the instruments were
pointed straight down at the surface during the few minutes that the
spacecraft was closest to the planet.


During the two years of mapping that will start in March 1999, the
instruments will always point straight down at the planet's surface.


The photographs that have been acquired during the just concluding
aerobraking phase were acquired on each orbit, a few minutes after the
closet approach to the planet's surface and after aerobraking had completed,
as the spacecraft was being rotated from the aerobraking attitude to the
array normal spin attitude used during the rest of each orbit.


WHY ARE THESE OBSERVATIONS BEING MADE NOW?

Surveyor is just completing its first period of aerobraking -- a portion of
the mission in which the spacecraft skims through the top of the Martian
atmosphere at each closest approach to the planet in order to circularize
its orbit. Currently, Surveyor's orbital period has been reduced from its
initial 45 hour duration to under 12 hours. The orbital period will stay at
11.6 hours until early September when aerobraking will resume again for the
final five months of aerobraking to reach the exact orbital conditions
necessary to begin Surveyor's two year long mapping mission. During the
period without aerobraking, Mars will move around the Sun to a position
where the lighting of the Martian surface under Surveyor's flight path will
be optimum for the mapping observations.

The upcoming opportunities appear to be the best of the period because the
periapsis location will be migrating to higher latitudes and going over the
north pole later in the period, and thus, the distance to the targets will
be increasing. In the next few weeks the elevation of the sun will be
between 15 and 20 degrees at the high latitude targets (Cydonia and Viking
2) which will make for good imaging. The sun elevation will be between 40
and 45 degrees for the low latitude sites (Viking 1 and Mars Pathfinder)
which will make for acceptable imaging.



HOW WELL WILL WE BE ABLE TO SEE THE TARGET IN THE IMAGES?

For Example, the field of view of high resolution camera covers a width of 3
km (1.9 miles) when the camera is 400 km (249 miles) from its target. The
length of the image will be several kilometers (several miles). The
resolution, or smallest feature discernible in the image varies with the
distance to the target, but at this distance will be approximately 1.4
meters (4.6 feet). The Mars Pathfinder and Viking landers are about 2 meters
(6.6 feet) in diameter, or very close to the minimum resolution obtainable.
The features in the Cydonia region are on the scale of 1 to 2 km (0.6 to1.2
miles) and should be readily visible and may nearly fill the width of field
of view of the images. Until the exact orbit characteristics are known, we
will not know the exact distance to the targets. It could be further than
the 400 km quoted in the example above and the resolution would be poorer,
or it could be closer.

The Mars Pathfinder and Viking landers are very small targets, at the limit
of resolution of the camera, even at the closest distance. It will be an
extraordinary event if they are recognized in the images. Features in the
Cydonia region, however, being hundreds to thousands of times larger, will
be very easily seen, even at the more distant ranges, and while all features
in this area may not be within the field of view due the expected targeting
errors, there is a high probability that many will be seen with good
resolution. The best known location of the "Face" will be the target point
in Cydonia.



WHAT IS THE PROBABILITY THAT THIS IMAGING WILL BE SUCCESSFUL? OR ARE WE SURE
WE'LL GET THE PICTURES?

The probability that the targets of interest will be within the camera's
field of view varies between 30 and 50 percent. This is because there are a
number of sources of error or uncertainties associated with the targeting
process.

One such error source relates to how good the current maps of Mars are. As
all early explorers on Earth found, early maps contain many inaccuracies.
The data obtained by Surveyor's laser altimeter and cameras in the last few
months have indicated that locations of observed objects on the surface are
displaced 1 to 2 km (0.6 to 1.2 miles) from where the Viking era maps locate
them.

Another source of error is the accuracy with which the spacecraft's
trajectory is predictable. This involves where the ground track of the
flight path lies or will lie on the surface, and the time the spacecraft
will fly over or near the desired targets. The accurate prediction of the
ground track allows the mission controllers to decide how much to rotate the
spacecraft to point the camera, and the timing prediction will be used by
the camera operators to control when to record the image. In preparing the
Surveyor's sequences for these observations, mission controllers will use
the results of orbit computations made as near to the planned observation
time as possible in order to minimize this uncertainty.

In addition, some error is introduced by the planet's rotation translating
downtrack error into crosstrack error.

The last source of error is how accurately the spacecraft can be rotated and
pointed. The design specifications for Global Surveyor call for it to be
pointable with an accuracy of 10 milliradians ( 0.057 degrees), that is,
mission controllers should be able to point the instruments to within 10
milliradians (0.057 degrees) of a target. Experience with the spacecraft
indicates that it actually performs much better, and that a pointing
accuracy of 3 milliradians (0.017 degrees) is possible.

Combining these error sources together in the proper statistical manner with
the distance from the spacecraft to the targets tells us the probability
that the targets will be within the camera's field of view. This probability
varies from about 70% when the targets are 1000 km (621 miles) from the
spacecraft, to about 25% when the targets are 400 km (249 miles) from the
spacecraft.



WHY ARE THESE IMAGES IMPORTANT?

A great deal of scientific controversy rages over the interpretation of the
features seen in the Viking images of the Cydonia Plain. Additional
photographs with the much better resolution that Surveyor's camera will
provide and perhaps different lighting conditions can provide new
information to aid in the understanding of what is seen there.

In addition, the observations of the previous landing sites provide
scientists with important knowledge to tie together the observations made on
the surface from the landers with those made from orbit above the planet.

The Viking 1 Lander site is the first location on Mars where humans were
able to see and touch the Martian surface at a familiar scale. This site,
the following higher latitude Viking 2 Lander site and the Pathfinder site
play a large role in understanding the processes which have operated on the
Martian surface over time and the state of the surface and atmosphere at
present. These sites serve as "ground truth" locations where ideas developed
from orbital observations can be tested, verified and then extended to other
regions of Mars such as those we may wish to visit in the future.

Several examples of this use of the sites for ground truth illustrates their
significance. One of the results of the Viking Orbiter Infrared Thermal
Mapper experiment was a rock abundance map based on the observed change in
surface temperature over time (large rocks cool more slowly than sand or
dust). The only way to verify the results of this rock abundance map was
with the two Viking landing sites where, fortunately, numerous rocks were
present. Rock abundance knowledge helps in understanding the depositional
history of the surface and large rocks represent a landing hazard. Mars
Global Surveyor carries an advanced version of the Viking instrument called
the Thermal Emission Spectrometer (TES) which will be able to map rock
abundance at more than one hundred times higher spatial resolution than
Viking and the TES experimenters will have another site (Pathfinder) to use
to verify their deductions.

The high resolution mode of the Mars Orbiter Camera (MOC) carried by the
Mars Global Surveyor spacecraft is capable of returning images of objects as
small as 1.4 meters across. Some of the largest rocks in the area of the
landing sites may be visible and such rock or boulder fields have been seen
in MOC images at other locations on Mars. The careful surveys which have
been done of the distribution of rocks as a functions of rock size can now
be used with MOC images to estimate rock populations at other locations on
Mars.



THE CURRENT STATUS OF MARS GLOBAL SURVEYOR

The Global Surveyor spacecraft is in excellent health. For the next five
months, Surveyor will be maintained in an 11.6 hour period elliptical orbit
around Mars. Its closest point to the planet's surface will be 170 km (106
miles) and its furthest distance will be 17,864 km (11,100 miles).



WHAT'S NEXT AFTER THESE SPECIAL OBSERVATONS?

The observations described above will occur three times during the month of
April. Surveyor will continue to acquire science data from its other
instruments during the month. Then, during May, Mars, and hence Global
Surveyor will move behind the Sun as seen from Earth. During this period of
solar conjunction, communications with Surveyor will be greatly degraded.
Surveyor will cease science observations and will be put into a special
attitude to assure proper temperatures of the science instruments. For two
out of every eight hours it will point its high gain antenna to Earth to
conduct radio communications propagation experiments, and, for part of the
time, to allow mission controllers to monitor the spacecraft's health. At
the end of May, Surveyor will return to acquiring science data from all its
instruments.