The cover of the author’s paperback copy of Cosmic Connection by Carl Sagan.
During my teens, I became a voracious reader of books on spaceflight, astronomy and (eventually) science in general. For whatever reason, there are certain stories I read way back then that have stuck with me to this day. Among these can be found in a chapter in The Cosmic Connection by Carl Sagan (1934-1996) I first read around 1976 which recounted the story of the first closeup images of the Martian moon, Phobos, taken by NASA’s Mariner 9 while orbiting Mars.
Phobos and its smaller sibling, Deimos, were discovered on August 18, 1877 (as reckoned by modern astronomical convention) by American astronomer Asaph Hall (1829-1907) while working at the US Naval Observatory. Phobos was found to be in an orbit with a semimajor axis of 9,376 kilometers with an orbital period of only 7 hours and 39 minutes while the dimmer Deimos was in a wider 23,463 kilometer orbit with a period of 32.2 hours. Since these tiny bodies appeared as mere points of light in Earth-bound telescopes, the diameters of Phobos and Deimos were estimated by the mid-20th century to be around 16 and 8 kilometers respectively, assuming a Moon-like albedo – among the smallest known natural satellites at the time.
When NASA was launching its first robotic explorers to flyby Mars in the 1960s, Phobos and Deimos were considered hazards to be avoided instead of targets for exploration. It was only by luck that NASA’s Mariner 7 spacecraft captured an image of Phobos at a range of 140,000 kilometers during its last far encounter imaging session in August 1969. Phobos was silhouetted against the disk of Mars revealing an oval object about 22.5 by 17.5 kilometers wide. This was larger than expected indicating that Phobos was a darker object than previously expected.
An enlargement of far encounter image 7F91 acquired in August 1969 by Mariner 7 at a range of 140,000 kilometers showing Phobos silhouetted against the disk of Mars. Phobos was shown to be a 22.5 by 17.5 kilometer object – noticeably larger than expected. (NASA/JPL)
Despite the fortuitous findings of Mariner 7, there was little interest in targeting Phobos or Deimos as part of the Mariner 9 mission to orbit and map Mars. Observations of small bodies had yet to gain much traction in the space exploration community at the time and there was almost no interest in the management at NASA headquarters to modify the Mars-centered mission plans of Mariner 9 to make moon observations. It was not until about four months after the launch of Mariner 9 on May 30, 1971 that a former student of Sagan’s, Dr. James Pollack (1938-1994) who was already part of Mariner’s science team, was formally allowed to form the mission’s Satellite Astronomy Task Group to plan and analyze observations of the Martian moons.
Diagram showing the coverage of Mariner 9 far encounter imaging in search of undiscovered moons orbiting Mars. Click on image to enlarge. (NASA/JPL)
During Mariner’s approach to the Red Planet in November 1971, Pollack and his team were able to image the vicinity of Mars looking for new Martian moons as well as observe Phobos and Deimos against background stars to help refine our knowledge of their orbits. Based on five distant observations of Phobos, it was found that the mean longitude of Phobos’ orbit had to be corrected by 3° (amounting to a change of 600 kilometers in the predicted position) and its orbital inclination changed by 0.3° compared to the quantity derived from Earth-based observations. Unfortunately for scientists interested in observing Mars, the Red Planet was engulfed in a planet-wide dust storm which obscured any surface features save for a dim view of the bright Martian southern polar cap and four enigmatic dark spots in the Tharsis region (which would later prove to be the summits of four huge volcanoes).
As Mariner 9 approached Mars, its surface was obscured by a global dust storm as seen in this image from November 11, 1971 taken at a range of 716,000 kilometers. (NASA/JPL)
On November 14, 1971, Mariner 9 fired its main engine for 916 seconds to enter an initial 1,398 by 17,916 kilometer orbit with an inclination of 64.5° and period of 12 hours and 34 minutes – the first spacecraft to orbit the Red Planet beating its Soviet competitors by 13 days (see “The Mars Orbiter That Almost Was Not”). The following day, Mariner 9 trimmed its orbit to 1,394 by 17,144 kilometers with a slightly shorter period of 11 hours and 57 minutes. But with the Martian surface still hidden by the global dust storm, Pollack and Sagan saw an opportunity to schedule observations of Phobos and Deimos. Any encounters with these moons would have to be at ranges greater than several thousand kilometers, not so much for the safety of the spacecraft, but to ensure that the targets would fall inside the field of view of the camera despite the remaining uncertainties in the new satellite ephemerides which amounted to ±100 kilometers, in the case of Phobos.
Diagram showing the major components of the Mariner 9 spacecraft including its instruments. Click on image to enlarge. (NASA/JPL)
Mariner 9 carried a pair of vidicon-based cameras similar in design to those used in the highly successful Mariner 6 and 7 spacecraft which flew past Mars in the summer of 1969. Mariner’s wide angle camera employed a 50 mm f/4.0 lens to yield images with a field of view of 11° by 14°. It was fitted with a filter wheel containing eight color and polarization filters to observe the atmosphere and surface of Mars. The narrow angle camera, which would be used to observe Phobos and Deimos, employed a 508 mm f/2.3 telephoto lens with a 1.1° by 1.4° field of view. This camera was fitted with a fixed “minus-blue” filter to help reduce the effects of atmospheric scattering while observing the Martian surface. Both cameras scanned their images into 700 lines consisting of 832 pixels each digitized to 9 bits. The narrow angle camera images covered an area of about 190 by 240 kilometers with a pixel footprint of approximately 300 meters from a range of 10,000 kilometers. Up to 31 images could be digitally stored onboard, using a magnetic tape recorder, for later playback to Earth.
This diagram shows the major components of the Mariner 9 narrow angle camera which was used to observe Phobos. Click on image to enlarge. (NASA/JPL)
Mariner 9’s first closeup image of Phobos was acquired on November 29, 1971 during Rev 31 at a range of 14,440 kilometers and a phase angle (i.e. the angle between the Sun-Phobos and spacecraft-Phobos lines) of about 78°. The 350 by 275 kilometer image (with a pixel footprint of around 400 meters) easily captured Phobos in the narrow angle camera’s field of view with the raw image initially showing just a smudge. Later processing of the image revealed a cratered, irregularly shaped body that was about 21 by 25 kilometers in size. The prominent nine-kilometer crater visible in upper left of image would be officially named “Stickney” in 1973 after Hall’s supportive wife, Chloe Angeline Stickney Hall.
A processed and enlarged version of the first closeup image of Phobos taken by Mariner 9 during Rev 31 on November 29, 1971 at a range of 14,440 kilometers. (NASA/JPL)
With this success under their belts, Pollack and his team scheduled a second observation two days later during Rev 34 at a much closer range. On December 1, Mariner 9’s narrow angle camera once again caught Phobos this time at a range of 5,720 kilometers and a phase angle of 59°. The more oblique lighting and a finer image scale of about 160 meters per pixel showed much more detail on the moon’s surface. Clearly visible was a 5-kilometer crater (which was officially named after Asaph Hall in 1973) with a clear “notch” in the upper left corner caused by the 9-kilometer Stickney Crater on the moon’s limb.
A processed and enlarged version of the second closeup image of Phobos taken by Mariner 9 during Rev 34 on December 1, 1971 at a range of 5,720 kilometers. (NASA/JPL)
During the following months, Mariner 9 would continue to make observations of Phobos and Deimos even as the dust cleared from the Martian atmosphere. These observations allowed much of Phobos’ surface to be mapped and permitted a more definitive characterization of its shape and size – approximately a triaxial ellipsoid about 20 by 23 by 28 kilometers in size with the long axis pointing towards Mars. With the improvements made to the ephemerides used to predict the positions of these moons thanks to the Mariner 9 observations, the stage was set for much higher resolution images to be acquired starting 4½ years later by NASA’s Viking orbiters as well as the missions to follow in the decades to come.
A montage of the 16 best views of Phobos acquired by Mariner 9 between December 1, 1971 and February 2, 1972. Click on image to view the original source. (NASA/JPL/Emily Lakdawalla)
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Related Reading
“Meridiani Planum in 1969: Our First Closeup View of Opportunity’s 2004 Landing Site”, Drew Ex Machina, August 8, 2020 [Post]
“The Mars Orbiter That Almost Was Not”, Drew Ex Machina, May 22, 2014 [Post]
“A Brief History of Planetary Orbit Insertion Failures”, Drew Ex Machina, February 13, 2020, [Post]
General References
William K. Hartmann and Odell Raper, The New Mars – The Discoveries of Mariner 9, NASA SP-337, 1974
Harold Masursky et al., “Mariner 9 Television Reconnaissance of Mars and Its Satellites: Preliminary Results”, Science, Vol. 175, No. 4019, pp. 294-305, January 21, 1972
Michael M. Mirabito, The Exploration of Outer Space with Cameras, McFarland, 1983
Carl Sagan, The Cosmic Connection – An Extraterrestrial Perspective, Dell Publishing Co., 1973
Andrew Wilson, Solar System Log, Jane’s Publishing, 1987