June 19, 1998, Update:
First Two Weeks of Science Phasing Orbit II Observations
Mars Global Surveyor resumed photographing the Red Planet on June 1, 1998, after being off for a month while Mars moved behind the sun as seen from the Earth. At the beginning of June, the orbit of MGS was crossing the equator at approximately 9:30 a.m. -- this means that the local time on the ground underneath the spacecraft was about 9:30 in the morning.
As the closest point to the planet slowly moves north under the influence of subtle variations of the gravity field of Mars, the latitude range over which the camera can photograph also migrates. During the early part of the mission, the camera could observe mostly in the southern hemisphere; now, the camera mostly views the northern hemisphere. The southernmost latitude also varies now from orbit to orbit as the pointing of the spacecraft and the time when pictures can be taken are changed to accommodate the observing requests of other experiments aboard the spacecraft. Unfortunately, most of the remaining areas accessible for imaging by the Mars Orbiter Camera (MOC) have been found to be socked-in by morning clouds, fog, and haze.
Taking pictures during the second Science Phasing Orbit period (SPO-2) has become a considerable challenge for the MOC science team. Not only must each picture be carefully planned and targeted by one or more Mars scientists, but commanding and retrieving the observations from the spacecraft now takes several days because MGS is being tracked from Earth only 8-10 hours per day (between September and May, the spacecraft was tracked 24 hr/day). Targeting takes place a few days before the data are acquired, often using position predictions several days old, and the results of the targeting aren't known until several days after the images are taken. Thus, variations on Mars that occur on timescales of less than a few days, such as changes in weather, cannot be anticipated in planning.
For example, between May 30 and June 9, a total of 22 orbits were targeted-- Orbits 336 through 357. The first images from these orbits were received on June 2 (from Orbit 338, as the data planned for orbits 336 and 337 were never received). The last of these images--Orbit 357--were received on June 13th. The time elapsed between targeting an orbit until the images are received on Earth varied from 3 to 5 days (6 to 10 orbits). During these 3 to 5 day intervals, additional orbits are targeted with no knowledge of the success or failure of recently-targeted pictures.
The first two weeks of SPO-2 had a spotty record for MOC high resolution images. On any given orbit, the MOC team might select as many as 5 or as few as 2 places to take a high resolution picture. Under SPO-2 orbital conditions, these images vary from about 3 meters (10 feet) to 12 meters (39 feet) per pixel. For Orbits 336 to 357, about 30% of the high resolution images were never received back from Mars owing to factors beyond the control of the camera team and 20% were completely cloudy. This means that about 50% of the targeted high resolution pictures for Orbits 336-357 were essentially "lost" to science. Another 25% of the images were hazy--owing to the clouds--and thus were of reduced resolution and contrast.
The good news is that about 25% of the images received from Orbits 336 to 357 were not hazy. A fair number of these good images were acquired during the last three orbits of this period--Orbits 355, 356, and 357--the first orbits targeted after the team had received sufficient images to tell just how cloudy Mars had become. By changing the targeting strategy to observe higher elevations (such as the Tharsis volcanoes and the heavily-cratered Arabia Terra) rather than lower elevations (such as the canyons of the Valles Marineris), and favoring southernly areas over those farther to the north, it is possible to target images that avoid the clouds.
The MOC team now has a complete set of wide angle (low resolution) images that cover the northern hemisphere as it appeared during the first two weeks of June. The team uses these images to find cloud-free areas to target new high resolution images for the orbits occurring since Orbit 357. (Examples of these wide angle images were released June 12, 1998: (A) Northern Tharsis and (B) Tempe Terra/Kasei Vallis).
June 19, 1998, Image Releases
Every MOC image is targeted by one or more Mars scientists. Each orbit that is targeted takes 2 to 5 hours to plan. Mars Global Surveyor conducts two orbits per day. This means approximately 28 to 70 hours per week are spent planning these observations. This week's MOC image release shows two pictures--one that is cloudy (to show how much of the data presently appears), and one that is not cloudy.
Release #MOC2-53: Cloudy Image of Cerberus Rupes Dark Lineation
The first,
"Cloudy Image of Cerberus Rupes Dark Lineation,"
shows the difficulty that clouds have posed for the MOC team.
The image was targeted on June 5, 1998. The image was obtained
by MOC during orbit 350 on June 7, 1998, and it was received
and processed at Malin Space Science Systems on June 9, 1998.
Release #MOC2-54: Eroded Crater Adjacent to Huygens Impact Basin
The second,
"Eroded Crater Adjacent to Huygens Impact Basin," shows a
successful image acquisition. This picture was targeted on June 9,
1998, hours after the images from orbit 350 were received. The
image was obtained by MOC during orbit 357 on June 10, 1998, and it
was received and processed at Malin Space Science Systems on June 13,
1998.
Malin Space Science Systems and the California Institute of Technology built the MOC using spare hardware from the Mars Observer mission. MSSS operates the camera from its facilities in San Diego, CA. The Jet Propulsion Laboratory's Mars Surveyor Operations Project operates the Mars Global Surveyor spacecraft with its industrial partner, Lockheed Martin Astronautics, from facilities in Pasadena, CA and Denver, CO.
Contact: info@msss.com
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