P H O B O S   &   D E I M O S  are the two main small, but oddly shaped moons, orbiting Mars. They are the mythological Roman names of his two sons, whose respective meanings are fear, and either dread or terror. Deimos was the first Martian moon to be discovered on 12th August 1877 by Asaph Hall at the U.S. Naval Observatory, while Phobos was discovered six days later by the same observer and telescope. Both moons also share an odd history. In 1727, Jonathan Swift remarkably wrote in his “Guillver’s Travels” fictitious details about these moons almost exactly 150 years before their actual discovery. The quote appears in Chapter 3 as follows;

“[Astronomers] have likewise discovered two lesser stars, or satellites, which revolve about Mars; whereof the innermost is distant from the centre of the primary planet exactly three of his diameters, and the outermost, five; the former revolves in the space of ten hours, and the latter in twenty-one and a half; so that the squares of their periodical times are very near in the same proportion with the cubes of their distance from the centre of Mars; which evidently shows them to be governed by the same law of gravitation that influences the other heavenly bodies.

* Note: The distances of the orbits Phobos and Deimos are about two-thirds longer than this.

PHOBOS

P H O B U S orbits very close to Mars in a near circular orbit. Mean distance from the core of the planet is 9,378 kilometres, placing Phobos merely 6,000 km. above the reddish Martian surface. It orbits Mars very rapidly in merely 0.31891 days (07h 39m 13.8s). From Earth, the apparent magnitude at opposition reaches 11.8, however, it requires moderately large telescopes to see against the overwhelming brightness of Mars itself. Observations can only be made for both moons are at maximum east or west elongation, and this requires an accurate ephemeris for these specific times. Visually seeing any transits and occultations from Earth are nearly impossible, due to their relative small sizes.

Phobos is the largest of these two moons, being an irregularly dark shaped body that is rough potato shaped and measures 27×21.6×18.8 km in size. Its physical appearance seems rocky, but the calculated lower density suggest it is composed of lighter icy-like materials below the surface.

Phobos to the naked eye would be very bright at −7 to −9 magnitude whose phases could be seen with the naked-eye in daylight. However, if an astronaut was on the surface of Mars and close to the Martian equator to the mid-latitudes, they would be able to see both Phobos and Deimos move rapidly across the sky though at high latitudes, both moons can never seen as they remain permanently below the horizon. As Phobos orbits the planet just over three times per Martian day, the moon would be seen to rise and set from any given location seven times. Phobos may also sometimes transit the Sun, whose observation would be dependant on the location on the surface of Mars and the time of year. These events are not very regular, as the orbital inclination is 1.1° to the Martian ecliptic. If observed under good conditions, Phobos would appear as large irregular shape covering the solar disk. However, the whose duration would last merely seconds. Orbiting satellites around Mars have observed and imaged them from space, like the dark umbra of Phobos crossing over the Martian surface. The ground based Opportunity rover had also frequently observed these transits.

Phobos in its own orbit is predicted to end the moon is several tens of millions of years from now as the orbit continues to decay. Astronomers have speculate that its end might not be by crashing into the planet. Instead Phobos will finish as a ring around Mars caused by gravity tearing apart the body within the so-called Roche Limit.

Perhaps in the future, we should expect many unmanned spacecraft to journey to Mars, even expecting it to provide sample the surface and return it to the Earth for chemical analysis.

DEIMOS

D E I M O S is roughly about half the size of Phobos. Its size is irregular and much less rounded than Phobos, being 15×12×10.4 kilometres across and is approximately one-seventh the mass. It is a slightly darker moon than Phobos, but whose density suggest the underlying surface comprises icy materials. Deimos has also been shown to be partial composed of carbonaceous material similar to geology of C-type asteroids.

Deimos has a near circular orbit, some 23,460 kilometres from the centre of Mars or averaging about 20,000 km. above the surface in a period of 1.26244 days or 01d 06h 17m 55s. Apparent magnitude observed at opposition on Earth is 12.9, require a moderately large telescopes to see against the overwhelming brightness of Mars

Observing Deimos from the Martian surface would not show its physical form or phases but would be starlike, as the apparent size is roughly 2 arcmin — requiring optical aid to see. It would only appear stellar at −3 to −5 magnitude — similar in brightness to Venus in the Earth skies. A solar transit by Opportunity during March 2004 has been observed from the surface of Mars, followed two weeks later by the other rover, Spirit.

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Last Update : 21st July 2018

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