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[escepticos] [ASTRO] MGS Prepares To Photograph Landing Sites, Cydonia Region
>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.
>
>
>
>
---------------------------------------------
Víctor R. Ruiz rvr en idecnet.com
Agrupación Astronómica de Gran Canaria
Sociedad de Meteoros y Cometas de España
info.astro http://www.astrored.org/infoastro
http://ccdis.dis.ulpgc.es:8086/AAGC/aagc.html
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