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20030813 The Moons of Mars
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From the Everett and Seattle Astronomical Societies, this is IT’S OVER
YOUR HEAD for the week of August 13, 2003, a look at what’s up in the sky over
Puget Sound.
The Mars watch continues this week as the Red Planet approaches
opposition on August 27th when it will be closer to the Earth than it has been in
50,000 years. Such a close approach will make for great viewing if the weather
cooperates. Tonight it rises around 10:00pm and will appear as a magnitude -2.5
object in the constellation of Aquarius. If you’ve never seen the Martion polar ice
caps, now is the time to do it as the Southern pole is clearly visible in even small
telescopes. This is also a good time to view Mars’ two small moons, Phobos and
Deimos. That’s right, Mars has two tiny moons that can be viewed from the
Earth but you’ll need at least an 8 inch telescope to do so.
Planets may have acquired their natural satellites in at least three different
ways. Jupiter and its large moons look like a miniature solar system. In much the
same way that the planets formed from a nebulous disk of material revolving
around the infant Sun, so these moons seemed to have formed from matter
encircling the young gas giant planets.
Somewhat surprisingly perhaps, the Earth's Moon does not fall into this
"co-formation" category, though for a long time many assumed that it did.
Evidently, our own Moon formed from debris expelled when the Earth received a
grazing impact from another planet-sized body early in its history. Calculations
show that this impacting body must have been at least the size of Mars, and
probably much larger.
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20030813 The Moons of Mars
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And speaking again of Mars, that brings us to the third category of natural
satellites: namely small interplanetary bodies, such as asteroids, that were
subsequently captured by a planet. Such is the case for the Red Planet's two
little moons, Phobos and Deimos. Meaning "fear" and "panic", Phobos and
Deimos were sons of Ares, the Greek god of war, whom the Romans knew as
Mars.
Asaph Hall, an American Astronomer at the U.S. Naval Observatory,
discovered Deimos on August 11, 1877. Less than a week later, on August 17th,
Hall also discovered Phobos. But Jonathan Swift's novel Gulliver's Travels,
published in 1720, made reference to two moons around Mars more than a
century and a half before Hall's actual discoveries. How could Swift have known
about these tiny satellites?
The answer is, he didn't. In the century prior to the publication of Gulliver's
Travels, Johannes Kepler, famous for discovering the laws of planetary motion,
predicted that Mars should have two moons. Kepler based that prediction on his
superstitious belief in numerology. Kepler knew of only the four largest of
Jupiter's moons. And the Earth had only a single moon. So Kepler believed that
since Mars lies between the Earth and Jupiter, it must have two moons, in order
to maintain the simple numerical doubling sequence of one, two, and four.
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20030813 The Moons of Mars
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With diameters of 16 miles and 10 miles, respectively, Phobos and Deimos
are among the very smallest moons in our solar system. And these two natural
satellites of Mars also hold the record for the smallest orbits. Deimos orbits at
under 12,500 miles, and Phobos careens around Mars at a distance of less than
4,000 miles! For comparison, the average distance between the Earth and Moon
is about 240,000 miles, more than 60 times the distance between Mars and
Phobos.
Since they lie so close to Mars, Deimos and Phobos might one day serve
as useful staging areas for manned missions to Mars. This situation won't last
forever, though. Tidal forces between themselves and the planets they circle
effect all moons. Those with "prograde" orbits – meaning that they orbit in the
same direction as their planets rotate – tend to slowly move farther and farther
away. The distance between the Earth and our own Moon is slowly increasing by
this very mechanism.
"Retrograde" moons – those that orbit counter to the direction their planets
rotate – tend to be pulled in closer and closer. Eventually they either impact the
planet, or are torn asunder by the ever-increasing tidal forces. While Phobos is a
prograde moon, it is nonetheless still being drawn closer to Mars all the time.
This is because it is closer to Mars than the so-called "synchronicity" limit –
where the satellite's orbital period exactly matches the planet's rotational period.
Many man-made satellites orbit Earth at the "geo-synchronous" point, remaining
stationary in the sky above a fixed point on the surface. A moon that is closer
than the synchronicity limit is drawn inward regardless of the direction of its orbit.
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20030813 The Moons of Mars
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So Phobos will one day either impact the surface of Mars, or break apart as
it spirals in. We probably don't have to worry about that anytime soon, though.
The process is expected to take somewhere in the neighborhood of 50 million
years. So we'll have to wait a long time to find out if an impact by Phobos
eventually "mars" the surface of the Red Planet (groan)!
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20030813 The Moons of Mars
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This month also includes two great astronomy events. First ["Mars Day
Celebration and Star Party" – change this last paragraph however you want, I
just wanted to supply the format for you to save you work.] Then, coming up on
the 28th through the 31st is the Oregon Star Party. Held annually at Indian Trail
Spring in the Ochoco Mountains, it’s an astronomy adventure for everyone. In
the evenings, a forest of telescopes grow on the high prairie. Large telescopes
accompanied by sophisticated computer and tracking systems share space with
small dime store scopes, binoculars and naked eye observers. And for those
who want to find out what the view is like through someone else’s equipment,
almost everyone welcomes a polite request to share a telescopic view. To find
out more about this great even check it out online at www.oregonstarparty.org.
For more on this week's topic, set your browser to “ItsOverYourHead.org".
Our broadcast was written by Ted Vosk and engineered by Greg Donohue. Until
next time, this is Ted Vosk and Greg Donohue wishing you clear skies and good
viewing!
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20030813 The Moons of Mars
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REFERENCES:
1. Martin Ratcliffe Et AL., The Sky This Month, Astronomy, August 2003,
at 60.
2. Oregon Star Party Association http://www.oregonstarparty.org
© Celestial North, Inc. All rights reserved.