Figure 1. Deimos, as photographed by the Mars Reconnaisance Orbiter (MRO) on 23 July, 2011.



Sol Sun

MercuryMercury ____________

VenusVenus crescent

Earth &
Earth's Moon

Mars Mars: Spirit 
       Sol 30 ____________

Asteroids Vesta

Jupiter Jupiter storm 
       north of Great Red Spot ____________

Saturn Saturn's Rings

Uranus Uranus' Rings

Neptune Neptune

Pluto & Pluto Kuiper Belt

Comets &
Oort Cloud

Comments & Submissions ____________

Figure 1. Deimos, as photographed by the Mars Reconnaisance Orbiter (MRO) on 23 July, 2011. Image credit: NASA/JPL

The Night Sky on Mars - Looking at Deimos

November 15, 2011


While researching my article on Phobos last month, I began to think a similar article on Deimos might be worthwhile. Deimos is a much smaller moon than Phobos, but also of great scientific interest. It is unknown if Phobos and Deimos are captured asteroids, comets, or "natural" moons that formed along with Mars, in the earliest days of the Solar system.

Mars is an amateur astronomer's dream. The nights are almost always cloudless, and clear of dust storms. The atmosphere is much thinner than Earth's, so the stars shine brighter. Perhaps best of all for someone interested in the Solar system, Mars is much closer to the asteroid belt. Many asteroids would be much easier to see in Mars' night sky, than from Earth. The only problems are that it is rather cold, the air is too thin to breath, and there is no Oxygen.

1997 photo by Mars Pathfinder
Figure 2. 1997 photo by Mars Pathfinder of Deimos, on Sol 4, Pathfinder's 4th day on the planet. This is the first picture of Deimos, taken from the surface of Mars. "Mars' outermost natural satellite, Deimos, is seen from the planet's surface in this Pathfinder image taken at night on Sol 4. This picture was acquired by the Imager for Mars Pathfinder (IMP) camera. Using IMP images of Deimos and its companion moon Phobos, the spectral characteristics of the satellites and properties of the Martian atmosphere are determined. Mars Pathfinder was the second in NASA's Discovery program of low-cost spacecraft." Image Credit: NASA/JPL

It is possible to stargaze from Mars, by combing through the Mars Expedition Rovers, Spirit and Opportunity's, pictures. On several occasions, between 2004 and 2011, the Panorama Cameras of the rovers were turned up to look at the night sky. Many of these pictures were listed in the NASA - JPL Photojournal, and given captions.

Mars Pathfinder, in 1997, also looked upward at Deimos.

Pathfinder and Sojourner a.
PathfinderSol1-1c b. PathfinderSol3-1 c.
Figure 3. a. Mars Pathfinder and Sojourner, undergoing testing at JPL prior to spaceflight and landing on Mars. b. Color photo from Sol 1 of the surface of Mars, with rover Sojourner still on lander, folded up. c. Sojourner unfolded and exploring its first rock, "Barnacle Bill," Sol 3. Image Credit: NASA/JPL

One of the fun things about stargazing on Mars, is seeing both moons crossing the sky at the same time. Because Phobos orbits fast and close, it rises in the West and sets in the East. Deimos, which is above geostationary orbit, rises in the East and sets in the West, just like Earth's Moon.

Deimos Solar eclipse
Figure 4. Deimos (and Phobos) Solar eclipse. Unlike Earth's Moon, Deimos is too small to block the Sun completely during an eclipse. Instead it appears as a spot on the surface of the Sun. The photo of Deimos was shot by Mars Exploration Rover (MER) Opportunity on Sol --- , March 4, 2004. A photo of Phobos in partial eclipse, 3 days later, is also shown in the figure. Image Credit: NASA/JPL

While the jury is out until it has actually been visited, Deimos looks very much like a captured asteroid, of the type known as Carbonaceous Chondrite. Deimos orbits Mars in a nearly circular orbit, at about 23,460 km altitude. This is much higher than Mars's other moon, Phobos. Deimos orbits Mars every 30.30 hours. Since Mars's rotational period is 24.7 hours, this is slightly lees than geostationary, and Deimos appears to move slowly backward with respect to the horizon or zenith, when it is in the sky, much like Earth's Moon. Deimos's diameter is only 15 kilometers (9 miles), making it one of the smallest known moons in the Solar system, but huge compared to man-made geostationary satelites. It is easily visible from the surface of Mars.

Figure 5. Phobos and Deimos, with the constellation Saggitarius. Click on image to see without text overlay. Image Credit: NASA/JPL

PIA06337PhobosDeimosa. PIA06338PhobosDeimosb.
Figure 6. Phobos and Deimos timelapse shots by MER Spirit on (a) Sol 590, 2005/08/30, and (b) Sol 594, 2005/09/04. The camera tracked nearly with Deimos in the picture, so the image of Aldeberan, one of the brightest stars in the sky, appears multiple times. There are also streaks and several dots that are probably cosmic rays striking the camera. Image creddit: NASA-JPL

The closest study of Deimos's surface has been done by the Mars Reconnaisance Orbiter (MRO). Figure 1, at the top of this page, is a color composite of several images taken by MRO. Here, for the record, is one of the first picture of Deimos taken by MRO, as it approached Mars.

Figure 5. Optical Navigation Demonstration Near Mars. MRO's "Optical Navigation Camera was used in February and March 2006 to demonstrate the use of pictures from a small camera for calculating precise location of a Mars-bound spacecraft by comparing the observed positions of Mars' two moons to their predicted positions relative to background stars. While this technique was not necessary for the Mars Reconnaissance Orbiter's own navigation, the demonstration prepares the way for relying on it for navigating precise arrivals for future missions that land on Mars. " March 6, 2006 Image Credit: NASA/JPL-Caltech

Because of the huge fuel requirements for manned missions to Mars, innovative solutions have been proposed. Rather than shipping all of the necessary fuel for the mission from Earth, current thinking is that it would be better to manufacture the fuel on, or in the neighborhood of Mars. Phobos and Deimos are considered to be prime candidates as sources for fuel, and as locations for fuel depots.

A probe or an expedition can use Mars's atmosphere for braking. After launch from Earth, the probe can perform a slingshot maneuver around Earth's Moon to accellerate it on its way to Mars. Because of these tricks, the fuel requirements for a 1-way trip to Mars are essentiually the same as for a trip from Earth to the Moon. The main difference is that the time required is months, instead of days, for the trip, so a much larger capsule is required for the journey.

Section 4: Conclusions


With the recent failure of the Phobos-Grunt mission, exploration of the Martian satellites has been set back by several years. One can hope that the Russians will not be discouraged, and will take their prototype chassis and instruments, and prepare a replacement for Phobos-Grunt, to launch in about 2 years.

If the Russians do not launch an improved version of Phobos-Grunt soon, then NASA and the ESA should prepare their own lander to visit Mars's satellites, in 4 to 6 years. There is much to be learned at Phobos and Deimos, including whether using either satellite as a source for fuel, may be practical.

By F. E. Harris


1. "Phobos as seen from Mars and Orbit," Solar System Science, October 19, 2011.
2. "Deimos (moon)," Wikipedia, 11 October 2011.
3. "Mars Microrover Photo Gallery," Mars Pathfinder mission pages, JPL, September 3, 1998.
4. "MarsŐ Moon Deimos," Space Photos, March 12, 2009
5. "PIA02699: Optical Navigation Demonstration Near Mars ," JPL Photojournal, 2006-03-10.
6. "PIA06338: The Two Moons of Mars As Seen from "Husband Hill" ," JPL Photojournal, 2005-09-11.
7. "PIA06337: The Night Sky on Mars," JPL Photojournal, 2005-09-11.
8. "Martian Eclipses: Deimos and Phobos ," JPL Photojournal, 2004-03-08.
9. "PIA00996: Deimos," JPL Photojournal, 1997-11-04.
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