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Hunting Phobos and Deimos: Spotting these diminutive moons is a daunting challenge for dedicated observers.

This year, a three-month-long window from mid-June to mid-September will present an opportunity to glimpse the tiny moons of Mars, which rank among the most difficult targets for amateur telescopes. Your chances of success will increase the closer you observe to July 31, the date of the closest approach of Mars to Earth this apparition. Deimos will then be a 12.0-magnitude object, while Phobos will shine at magnitude 10.9.

Both satellites would be easy prey for a 6-inch telescope if they weren't so close to their parent planet, which is hundreds of thousands of times brighter. They were only detected in 1877 by American astronomer Asaph Hall using the recently commissioned 26-inch Clark refractor at the U.S. Naval Observatory, decades after considerably fainter satellites of Saturn, Uranus, and Neptune were discovered. In 1971, NASA's Mariner 9 spacecraft revealed both Phobos and Deimos as irregularly shaped, heavily cratered shards of cosmic debris that are among the least reflective objects in the entire solar system.

In 1939, astronomers Harry Edward Burton and Bevan Sharpless of the U.S. Naval Observatory began to systematically study the orbital behavior of the Martian satellites. Drawing on observations made between 1879 and 1941, Sharpless published a paper in 1945 in the Astronomical Journal entitled "Secular Accelerations in the Longitudes of the Satellites of Mars" that presented the first evidence that the orbit of Phobos was decaying and would ultimately result in the satellite's destruction.

Sharpless's work attracted little interest until the late 1950s, when Soviet astrophysicist Iosif Shklovsky alleged that tidal forces and the frictional drag imparted by the tenuous Martian atmosphere were causing Phobos to spiral toward the surface of the Red Planet at a much faster rate than was possible for a solid body. Shklovsky speculated that Phobos and Deimos might be hollow artificial satellites. This suggestion was taken seriously by several luminaries, including Raymond Wilson, Chief of Applied Mathematics at NASA, and Carl Sagan, a rising star among American planetary scientists. However, President Eisenhower's national science adviser Fred Singer cautioned:
   The big 'if lies in the astronomical
   observations; they may well be in
   error. Since they are based on several
   independent sets of measurements taken
   decades apart by different observers
   with different instruments, systematic
   errors may have influenced them.

Singer was prudent--Shklovsky's calculations were based on flawed data. Phobos is indeed spiraling inward but at a rate of only 1.8 meters (about 6 feet) per century. The doomed satellite will not collide with Mars or be torn to pieces and scattered as a ring around the planet for another 40 million years.

While Shklovsky regarded the moons as the relics of a long-dead race of Martians, to Felix Zigel, of the USSR Academy of Sciences, they were evidence of an existing civilization on Mars. In 1960 Zigel wrote that he was puzzled by the fact that during the favorable Mars opposition in 1862, Phobos and Deimos had eluded experienced observers equipped with much larger telescopes than the 26-inch refractor employed by Hall in 1877, namely William Lassell's 48-inch reflector on the island of Malta and Lord Rosse's 72-inch "Leviathan of Parsontown" at Birr Castle in Ireland, the largest telescope in the world at the time. Despite the mediocre reflectivity of their metal speculum mirrors, these reflectors were vastly superior in light-gathering power. Zigel reasoned that the failure to detect the presence of Phobos and Deimos was because they had been lofted into orbit around Mars sometime between 1862 and 1877.

Zigel failed to take into account that Hall succeeded where his predecessors failed because he was an unusually astute observer who carefully scrutinized the sky surrounding Mars using a novel technique. Hall recounted:
   I began to examine the region close
   to the planet and within the glare of
   light surrounding it. This was done by
   keeping the planet just outside the field
   of view and turning the eyepiece so as to
   pass completely around the planet.

Several weeks after its discovery, Hall and two of his colleagues glimpsed Deimos through the Naval Observatory's smaller 9.6-inch refractor using this "right way of looking."

In 2001, renowned planetary observer Stephen James O'Meara devised an elegant method of heeding Hall's advice to "get rid of the dazzling light of the planet." Instead of positioning Mars just outside the field of view, he cut a semicircle from a Wratten gelatin filter and mounted it against the field stop at the focal plane of his eyepiece, where it bisected the field of view.

Rather than affixing the filter to the field stop with a dollop of glue or a sliver of tape, a rubber O-ring from the local hardware store makes an ideal temporary retainer on some oculars that is easy to install and remove. Short-focal-length eyepieces that provide high powers generally have small lenses that make the installation of a filter mask more challenging. The combination of a low- or medium-power eyepiece and a Barlow lens to boost the magnification is recommended.

Although a neutral-density filter will give excellent results, O'Meara prefers to use a blue (Wratten #47) filter that selectively attenuates the planet's ruddy glow. "Since Mars will shine dimly through this mask," he advises, "you can more easily judge the distance and direction from the planet where you should look for each moon." It's always best to attempt to detect these tiny wisps of light near the time of their eastern or western elongations, when their apparent distance from Mars is greatest; rotate the eyepiece so that the edge of the filter mask is aligned on the planet's north-south line.

Deimos, the fainter and more distant satellite, circles Mars once every 30 hours 18 minutes, appearing about twoand-a half Mars diameters from the planet's brilliant limb at greatest elongation. Roger Venable of ALPO notes that for 2 1/2 hours before and after each time of maximum apparent elongation, its separation is still greater than 85% of maximum. Outside that interval it is quite hard to see.

Although Phobos is considerably larger and brighter, its closer proximity to Mars makes it a far more challenging quarry. Orbiting a scant 6,000 kilometers (3,700 miles) above the Martian surface and circling Mars in only 7 hours 39 minutes, Phobos never appears more than 16 arcseconds from the planet's limb (only 2/b the apparent diameter of the Martian disk at the end of July), and it moves fast. For 30 minutes before and after the time of maximum apparent separation, it's still greater than 86% of maximum.

Using the highest magnification the seeing will bear and clean, well-collimated optics are essential ingredients for success. Diffraction from the vanes of a Newtonian reflector's secondary mirror support can be a troublesome source of scattered light, so refractors and catadioptrics are the preferred weapons of choice. Experienced observers with superior visual acuity have bagged Phobos using 8-inch telescopes, while Deimos has been glimpsed with only 6 inches of aperture. Good luck and good hunting!

Contributing Editor THOMAS A. DOBBINS has been observing Mars since 1965, the year that NASA's Mariner 4 spacecraft flew by the enigmatic world.

Caption: The tiny moons of Mars do not stray far from the Red Planet. Phobos at its farthest extends less than one Mars diameter from the planet's limb, while Deimos reaches about three. See for a list of elongation times of both moons in the months closest to opposition.

Caption: Instead of making an occulting bar to help him see the moons of Mars, long-time observer Stephen James O'Meara fashioned a semi-circular filter using a Wratten #47 gelatin filter affixed in front of the field stop on a medium-power eyepiece using a 11/a-inch outer-diameter O-ring.

Caption: Because the moonlets are vastly fainter than Mars, amateur John Boudreau overexposed the planet to capture both Phobos and Deimos as they were best positioned on the night of December 26, 2007 using a 51/2-inch Astro-Physics refractor and Imaging Source video camera. He then composited a properly exposed image of the planet into the final image.
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Title Annotation:JULY 2018 OBSERVING: Exploring the Solar System
Author:Dobbins, Thomas A.
Publication:Sky & Telescope
Date:Jul 1, 2018
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