The moon of Mars – Deimos (II) – was discovered by A. Hall
(1829-1907) on August 10, 1877 two days later he discovered Phobos.
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
eleven but separated only by 86'' 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 one of the sons 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
panic.
The moon by this name is in a circular prograde orbit (→eccentricity
e = 0.0002) with a →semimajor axis a = 23,460 km. At pericenter (closest to the planet) Deimos is separated from the Mars a distance of q = 23,455 km. At apocenter (furthest from the planet) this moon is separated from the Mars a distance of Q = 23,465 km.
The mass of Deimos ~1.4687×1015 kg
is diminutive – over five times less than
a mount like Giewont
!
THE ROTATING MAP OF DEIMOS
Shape
model
,
the
texture
Unlike Phobos, Deimos revolves a bit slower than Mars is spinning on its axis. Its quite in teresting however that for such a small moon it still has its revolution completely
synchronized with its orbital motion! (like most of large moons)
If people will ever land on Mars they will have the opportunity to follow its motion and changes in brightness. More details could however not be resolved by the naked eye. They could also be treated with occulations of Deimos by Phobos and transits of one or both moons in the Sun's disk! Their angular size is to small to cause a true eclipse like the ones seen from our planet...
Another in teresting fact about Deimos is that although it revolves 2.5 times further than Phobos it is still not observable from Mars' poles! Since this moon's orbit is not very inclined to the equator and is only 20,100 kilometers above the surface it is not visibile further than about 82 south or north
latitude.
You can read more about Deimos 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
12.5m
(therefore 398 times fainter than faintest stars visible to the naked eye!). For comparison Mars is about
–2.0m at an average opposition, so Deimos is fainter by about
14.5m. This corresponds to a difference in brightness by the factor of
631 thousand!
I have given some crucial data of Deimos 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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