NASA’s Hubble Space Telescope Imaged Phobos Orbiting Mars May 12, 2016

Summary: NASA’s Hubble Space Telescope Imaged Phobos orbiting Mars May 12, 2016, in 13 separate exposures taken over 22 minutes.

Composite image of Phobos orbiting Mars comprises 13 separate exposures obtained over the course of 22 minutes on Thursday, May 12, 2016, by NASA’s Hubble Space Telescope; NASA ID PIA21837; image credit NASA/ESA/STScI; image addition date 2017-07-20: May be used for any purpose without prior permission, via NASA JPL Photojournal

NASA’s Hubble Space Telescope imaged Phobos orbiting Mars May 12, 2016, in 13 separate exposures, taken over the course of 22 minutes, that show a portion of the natural satellite’s orbital trek.

Hubble’s photoshoot took place while Mars was at a distance of 50 million miles from Earth, according to Space Telescope Science Institute’s (STScI) Ann Jenkins, Ray Villard and Zolt Levay in their July 20, 2017, NASA feature, “NASA’s Hubble Sees Martian Moon Orbiting the Red Planet.” The authors note: “This was just a few days before the planet passed closer to Earth in its orbit than it had in the past 11 years.”

Phobos traces an almost circular, equatorial orbit around its parent planet, according to the European Space Agency’s (ESA) Science & Technology website. Phobos completes its revolution around Mars every 7 hours 39 minutes. Phobos makes three complete orbital treks around Mars during the Red Planet’s day of 24 hours 40 minutes (24.6 hours). Jenkins et al. note: “It is the only natural satellite in the solar system that circles its planet in a time shorter than the parent planet’s day.”

The Phobosian speedy orbit occasions the moon’s rising in the west and setting in the east, as viewed from Mars. Contrastingly, the second Martian moon, Deimos, which requires 30 hours 12 minutes for orbital completion, rises in the east and sets in the west.

At the moon’s orbital closest point, periapsis (Ancient Greek: περί, perí, “about, around” + ἁψίς, hapsís, “arch, vault”), Phobos is distanced at 9,234.42 kilometers (5,738.00 miles), according to Universe Today’s Matt Williams. Farthest point, apoapsis (Ancient Greek: ἀπό, apó, “from, away from” + ἁψίς, hapsís, “arch, vault”) measures a center-to-center distance of 9,517.58 kilometers (5,913.95 miles) between Phobos and Mars.

Phobos claims the honor of closest-positioned satellite to its parent planet in the solar system. Mars has an approximate equatorial radius of approximately 3,396 kilometers (2,110.17 miles). At periapsis, Phobos hugs the Martian surface at a distance of approximately 5,838 kilometers (3,627-plus miles). At apoapsis, Phobos orbits approximately 6,121 kilometers (3,803-plus miles) above the Martian surface.

For observers on Mars, Phobos appears at about one-third of the full moon’s size in Earth’s sky. “Conversely, someone standing on Phobos would see Mars dominating the horizon, enveloping a quarter of the sky,” Jenkins et al. observe.

The Martian polar regions, however, are not afforded views of Phobos. The moon’s equatorial orbit and orbital proximity to the Martian surface keep Phobos hidden below the horizon in the Red Planet’s highest latitudes.

Phobos has an irregular, football- or potato-like shape. Crater chains and grooves scar the moon’s heavily cratered, lumpy surface.

Phobos is the innermost and larger of the two Martian moons. NASA’s Solar System Exploration website details the Phobosian diameter as 17 by 14 by 11 miles (27 by 22 by 18 kilometers). Its equatorial radius measures 11.1 kilometers. Its equatorial circumference spans 69.7 kilometers. Earth is 574.0 times larger than Phobos.

Contrastingly, outer and smaller moon Deimos is sized at only 9 by 7 by 6.8 miles (15 by 12 by 11 kilometers). The lumpy, heavily cratered Deimos has an equatorial radius of 6.2 kilometers and an equatorial circumference of 39.0 kilometers. Earth is 1,027.6 times larger than Deimos.

American astronomer Asaph Hall III (Oct. 15, 1829-Nov. 22, 1907) is credited with discovering both Deimos and Phobos in 1877 at the U.S. Naval Observatory in Washington, DC. Asaph Hall, in turn, acknowledged the persuasive influence of his wife, mathematician Chloe Angeline Stickney Hall (Nov. 1, 1830-July 3, 1892) as a critical factor in the success of his lunar discoveries. He explained that “. . . the chance of finding a satellite appeared to be very slight, so that I might have abandoned the search had it not been for the encouragement of my wife” (page 5). Hall discovered Deimos on Sunday, Aug. 12, and Phobos six days later, on Saturday, Aug. 18.

The takeaway for NASA’s Hubble Space Telescope images of Phobos orbiting Mars May 12, 2016, is that a composite image of 13 separate exposures, taken over the course of 22 minutes, shows a portion of close, equatorial orbit of the Red Planet’s innermost, larger moon.

Graphic depicting relative shapes and distances from Mars for the planet’s two natural satellites (Deimos, Phobos) and five active orbiter missions (European Space Agency's Mars Express; India’s MOM [Mars Orbiter Mission]; NASA's Mars Odyssey, MRO [Mars Reconnaissance Orbiter] and MAVEN [Mars Atmosphere and Volatile Evolution]) illustrates the potential for orbital intersections and collisions; NASA ID PIA19396; image credit NASA/JPL-Caltech; image addition date 2015-05-04: May be used for any purpose without prior permission, via NASA JPL Photojournal

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

Composite image of Phobos orbiting Mars comprises 13 separate exposures obtained over the course of 22 minutes on Thursday, May 12, 2016, by NASA’s Hubble Space Telescope; NASA ID PIA21837; image credit NASA/ESA/STScI; image addition date 2017-07-20: May be used for any purpose without prior permission, via NASA JPL Photojournal @ https://photojournal.jpl.nasa.gov/catalog/PIA21837;
Generally not subject to copyright in the United States; may use this material for educational or informational purposes, including photo collections, textbooks, public exhibits, computer graphical simulations, and Internet Web pages; general permission extends to personal Web pages, via NASA Image and Video Library @ https://images.nasa.gov/details-PIA21837

Graphic depicting relative shapes and distances from Mars for the planet’s two natural satellites (Deimos, Phobos) and five active orbiter missions (European Space Agency's Mars Express; India’s MOM [Mars Orbiter Mission]; NASA's Mars Odyssey, MRO [Mars Reconnaissance Orbiter] and MAVEN [Mars Atmosphere and Volatile Evolution]) illustrates the potential for orbital intersections and collisions; NASA ID PIA19396; image credit NASA/JPL-Caltech; image addition date 2015-05-04: May be used for any purpose without prior permission, via NASA JPL Photojournal @ https://photojournal.jpl.nasa.gov/catalog/PIA19396;
Generally not subject to copyright in the United States; may use this material for educational or informational purposes, including photo collections, textbooks, public exhibits, computer graphical simulations, and Internet Web pages; general permission extends to personal Web pages, via NASA Image and Video Library @ https://images.nasa.gov/details-PIA19396

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