The moon of Mars – Phobos (I) – was discovered by A. Hall
(1829-1907) on August 12, 1877 two days after he discovered Deimos.
It seems the hardest moon to observe of all the major moons in the Solar System! It is not particularly faint though. During the Great Opposition of 2003 it was magnitude ten but separated only by 35" from the planet which was at a dazzling
–2.9m! And that's why it has eluded astronomers for so many years...
Asaph Hall himself had long been unsuccessful in finding this moon! He was running out of time
– the Great Opposition of September 5, 1877 was coming and he still had got nothing. He had repeatedly observed Mars and sometimes even put a tiny disk in to the telescope to screen of its glare.but nothing seemed to help... At one moment he probably thought that if there were any moons at all he should have discovered them already or this is all just a product of his imagination...
Asaph was beginning to loose faith he would discover anything. However his wife
Angelina. was encouraging him to keep observing and never give up. And actually it was her stubbornness which made him finally find both moons! To honour her astronomers have given her name to a crater on Phobos discovered by space probes. It is called
Stickney after her original family name.
The name of the moon itself is of mythological origin. According to some sources Phobos was a son or a comrade of Ares. However most of them say it was the name of a horse put to the cart of the god of war. It is also a Greek word for fear (that's were the word phobia comes from - an obsessive fear of some objects or arrangements etc.).
The moon by this name is in a slightly elliptical prograde orbit (→eccentricity
e = 0.0151) with the SMALLEST semimajor axis of all satellites in the Solar System
a →semimajor axis a = 9,380 km. At pericenter (closest to the planet) Phobos is separated from the Mars a distance of q = 9,238 km. At apocenter (furthest from the planet) this moon is separated from the Mars a distance of Q = 9,522 km. The mass of Phobos is not significant ~1.0727×1016 kg – comparable to less than about 1.5
a mountain like Giewont
!
THE ROTATING MAP OF PHOBOS
the
texture
Names of the main regions/craters:
Working
Group for Planetary System Nomenclature
and
R. Greeley, R. Batson, The NASA Atlas of the Solar
System, (Polish edition) Warsaw 1999
An in teresting feature of this moon is that it is moving much faster in its orbit than its planet spins around its axis (about 3.4 times faster which gives 1128° deg per day!). Although it is not a large moon its revolution is completely
synchronized with its orbital motion! (like most of large moons).
If people will ever land on Mars they will be able to follow the strange behaviour of Phobos in Martian skies. If will rise in the west and move swiftly in the sky quickly changing phases and brightness and then it will set in the east just like an artificial satellite in low Earth orbit!
Let me just add a small remark about possible artificial origin of Phobos. Many (pseudo)scientists have been putting forward a theory that this moon is the work of Martians since it has been observed to be slowly falling onto the surface and thus might be hollow! However unlike artificial satellites the drag on Phobos in its orbital motion is caused by tidal forces and photos sent by space probes have proven it to look like a rather typical asteroid probably captured from the nearby asteroid
belt.
Since we have already moved a bit towards sci fi in our description I have a small tip for future sci fi writers. I once read a story (the author of which I do not recall) where the main character is located at a polar cap and is observing Phobos! Since this moon's orbit is not very inclined to the equator and is only 6000 kilometers above the surface it is
NOT visibile further than about 68 south or north latitude.
You can read more about Phobos on the Web pages from the links listed below the
tables...
The main obstacle in discovering this moon earlier was the already mentioned low separation from Mars and its brightness being usually 11.4m
(therefore 144 times fainter than faintest stars visible to the naked eye!). For comparison Mars is about
–2.0m at an average opposition, so Phobos is fainter by about
13.4m. This corresponds to a difference in brightness by the factor of 229
thousand!
I have given some crucial data of Phobos below. Table one contains the basic information whereas table
two gives more detailed parametrs of its orbit (calculated using the following
formulae).
Translated by Karol Pankowski
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