58 AUSTRALIAN SKY & TELESCOPE JULY 2016
Hunt the moons of Mars
With Mars now big and close, try for Phobos and Deimos.
T
he closest planetary moons to Earth, after our
own familiar Luna, are Phobos and Deimos
orbiting Mars. Most amateurs have never
seen them. With Mars having just reached its close
approach to Earth, now’s a good time to seek them out
before the planet and its moons get too far away.
The problem with seeing Phobos and Deimos
through a telescope isn’t that they’re too dim. You
might think they’d be fairly easy, since they shone at
magnitudes 11.4 and 12.5, respectively, in the week
or two around Mars’ May 30 closest approach. (The
planet and its moons are a half magnitude fainter by
the beginning of July.) The problem is that they orbit
so close to Mars amid its dazzling glare. Mars blazes
230,000 and 600,000 times brighter than Phobos and
Deimos, respectively.
I’ve been able to see Deimos quite distinctly with
my 32-cm reflector at good oppositions in the past. But
I’ve never definitely succeeded with Phobos, which
never gets more than about a third as far from Mars as
Deimos does.
You’ll need to use some tricks. The first is to choose
the right time to look. You’ll want to observe when one
of the satellites is at one of its greatest elongations: at
its farthest Mars. The elongations currently happen
to the planet’s northwest or southeast, as shown
in the orbit diagram on the top of the facing page.
Find a convenient elongation time athttp://is.gd/
phobosdeimos2016, where timetables are reproduced
from the 2016 Astronomical Almanac.PhobosPhobosDeimosJames McGaha took these images of
Mars with tiny Deimos and Phobos on
August 29, 2003, using a webcam on a
1-metre reflector atop Kitt Peak. To pick
up the faint moonlets, McGaha had
to vastly overexpose Mars (left), so he
also made a composite using a short
exposure of Mars (right) to show all
three objects well.completes a full orbit every 7.6 hours, Deimos every
30.2 hours.
Or, with a good sky-mapping program, you can
lock on Mars, zoom way in on it, and then run the
time forward and backward to see when Phobos and
Deimos reach their elongations.
Next: make sure your eyepieces are as clean as can
be. Dust on your main mirror matters less because
it’s further from your eye (meaning scattered light
from the dust has more places to go that aren’t into
your pupil). But if you’ve been waiting for an excuse
to clean your mirrors (gently!), this
might be it.
To your highest-power eyepiece,
add a temporary occulting bar
across the centre of the field so20 ”Orbits of
Phobos and
DeimosNorthPhobosDeimosMarsSouthEastWestDeimos never gets more than three Mars diameters from Mars’
limb. Phobos always remains within just one. Try for them
when they’re at their northwest and southeast elongations.ALAN M.
MACROBERT
became an
amateur
astronomer in
1965 and has
been writing
about the subject
since 1982.Satellite sightings