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Astronomy Now Find Martian moons Phobos and Deimos at the Red Planet’s closest approach

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Ade Ashford


Do you like extreme observing challenges? If you own a quality telescope of 25-cm (10-inch) aperture or larger, have excellent seeing and sky transparency, then you might just glimpse Phobos and Deimos, the diminutive moons of Mars around the time that the Red Planet is closest to Earth on 31 July. The proximity of magnitude -2.8 Mars means that you need to catch the moons near elongation, since Phobos (magnitude +11) and Deimos (magnitude +12) never get farther than 21½ and 72 arcseconds, respectively, from the limb of their parent planet. See the diagram at the foot of this page for other elongation events. AN graphic by Ade Ashford.
After a long 15-year wait for devotees of close Martian oppositions, the Red Planet is fast approaching the point of least separation from Earth. At 8:45am BST on Tuesday, 31 July the distance between the centres of both planets will shrink to 0.385 astronomical units, or 57.6 million kilometres (35.8 million miles).

At the end of this month the Red Planet attains a maximum angular size of 24.3 arcseconds, when a telescope magnifying just 80× will enlarge it to the same size as the full Moon appears to the unaided eye. Note that Mars’ disc is 20 arcseconds or wider until 6 September this year. You can learn more about observing Mars by reading this article.


A colour image of Mars’ innermost and largest moon Phobos, dimensions 27 × 22 × 18 kilometres, imaged by the Mars Reconnaissance Orbiter’s HiRISE instrument on 23 March 2008. Image credit: NASA/JPL-Caltech/University of Arizona.
Mars presently looms large and bright for observers worldwide, but the ongoing dust storm has robbed many of the detailed views they were expecting. However, the storm is clearing and contrast is returning to the prominent surface features highlighted in our interactive online Mars Mapper. But there’s one other observing challenge you can attempt when the Red Planet is this close – looking for its two diminutive moons, Phobos and Deimos.

‘Fear’ and ‘terror’
Named after the mythological twin sons of the Greek god of war, Ares, Phobos (meaning fear) and Deimos (terror) were discovered by American Astronomer Asaph Hall just before the favourable Mars opposition of 1877 using the 26-inch refractor at the US Naval Observatory (USNO) in Washington, DC. He sighted outermost moon Deimos on 11 August and Phobos a week later.

We now know that potato-shaped Phobos is the larger of the two moons with dimensions of 27 × 22 × 18 kilometres, orbiting just 6,000 kilometres above the surface of Mars — closer to its parent planet than any other known moon — once every 7⅔ hours. Deimos has dimensions of 15 × 12 × 11 kilometres, orbiting Mars at an average distance of 23,460 kilometres every 30.3 hours.

Finding Phobos and Deimos
Given their diminutive stature, the Martian moons are only accessible to large backyard telescopes when the Red Planet is particularly close. Around the end of July and early August 2018, Phobos glimmers at magnitude +11, while Deimos is a magnitude fainter at +12. Were it not for the searing glare and proximity of Mars — Phobos never gets further than 21½ arcseconds from the limb of Mars and 71.8 arcseconds for Deimos — the two moons would be comfortable targets in 15-cm (6-inch) aperture telescopes. At the last close opposition of Mars in 2003, several observers succeeded in glimpsing a moon in 20-cm (8-inch) instruments. At this opposition, you also have to factor in the glow from Earth’s waning gibbous Moon.

Since the disc of Mars is at least 800,000 times the brilliance of its faintest moon Deimos, the extreme challenge lies in spotting (or imaging) the faint specks of light amid the glare of their parent planet. Ideally, an unobstructed aperture telescope is your instrument of choice, since you wish to minimise diffraction (it’s easy to lose sight of a moon if its image falls on the diffraction spike of a Newtonian with a traditional secondary mirror support, for example), plus you need clean and collimated optics with quality eyepieces that are multi-coated to reduce light scatter. When looking for a moon at elongation, you may find it helpful to nudge the disc of Mars just outside the field of view.


AN graphic by Ade Ashford based on data provided by NASA’s Planetary Data System (PDS) / SETI / Martian Moon Tracker 2.6 / Mark Showalter. Click the graphic for a full-size version suitable for printing.
Interpreting the orbits and elongations diagram
In the accompanying illustration, time flows from the top down, with dates and universal times of events on the left. The central grey column represents the disc of Mars, while the helical black and red lines are the paths of Phobos and Deimos, respectively.

The graphic is orientated with north up and east to the left as Mars appears in the sky, hence observers with Newtonian/Dobsonian telescopes should rotate the illustration through 180° to match their eyepiece view, while users of refractors and catadioptrics (Schmidt- and Maksutov-Cassegrains) with a star diagonal need to mirror this graphic left-right to replicate what they see through the eyepiece.

For the benefit of UK-based observers (more on this in a moment), the horizontal green lines represent the dates and times that Mars transits as seen from the heart of the British Isles (longitude 2.5° west). For example, consulting the diagram for 1 August around 1:40am BST, you can see that Phobos lies close to eastern elongation and Deimos near western elongation (this is the event illustrated at the top of the page).

For observers on the east coast of North America, favourable elongations of Phobos and Deimos occur near 5h UT, while events happening near 8h UT favour the west coast. For Australasian observers, elongations of the moons occurring between 12h and 16h UT are optimal. You can consult our interactive online Almanac to find the UT dates and times that Mars transits at your location.

The view from the British Isles
Given that Mars currently struggles to attain a peak altitude of 14 degrees around 1am BST as seen from the southern counties of England, observers in the UK are almost inevitably subject to sub-standard seeing and about a magnitude of atmospheric extinction (hence Phobos and Deimos would appear around magnitudes +12 and +13, respectively). An atmospheric dispersion filter will certainly help anyone having to view Mars at low altitudes.

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