Conquest of Mars: but wouldn't it have been better to descend to Phobos first?
The planet Mars is already mysterious, always at the center of convoluted
historical and observational events and conjectures bordering on science fiction...
Such a planet, could it have normal moons with a normal history? No, obviously not!
And in fact, the red planet is found - not enough the rest - with the smallest, strange,
and talked about moons of the solar system. Talked about even before their discovery,
since their existence and their almost exact distance from Mars was already predicted by
the Irish writer Johnatan Swift in 1726, that is 150 years before their actual discovery,
which occurred in 1877 thanks to the observational tenacity of the American astronomer Asaph Hall.
Mysterious moons, because even today it is not possible to understand where they came from,
since they look like two small asteroids, but they follow perfectly regular orbits,
as they were big moons like Ganymede or Titan.
And now, thanks to their enviable position, they are also involved in the plans for
the conquest and exploration of Mars. In the immediate future, there are in
fact plans to visit them and bring home some soil to examine, and in the near future, there is talk
of using them as space stations permanently facing the mysteries of the red planet.
But before we get into the details, it will help to know all the features of
these two fantastic mini-worlds. What do you say? Shall we begin?
Dimensions. Contrary to our Moon, the Martian satellites are small and their size does not
exceed tens of kilometers. Impossible to speak of diameter because both have a very irregular shape
so you need three values to define the proportions.
Phobos measures 27x22x19 km and its surface is literally pockmarked by impact craters,
the largest of which has a diameter of 10 km: more than a third of the entire satellite! It was
given the name Stickney, in honor of Hall's young wife, who encouraged her husband to continue the
research despite the first disappointing results. The event that formed Stickney must have been
of such violence as to endanger the very existence of Phobos. And a series of long steps,
which are like giant nails depart from the crater, is there to witness the brutality of the impact.
They were first observed in the 1970s by the Mariner and Viking missions, and
their origin has been the subject of much debate. Some have suggested that violent impacts suffered
by Mars have poured on the small moon a shower of debris capable of scratching the surface, those
who think that the Martian gravity has created structural cracks on the satellite, and those who
finally believe there is some kind of connection with the impact that generated Stickney.
And apparently, a recent simulation has given reason to the third hypothesis, showing that
following the devastating impact that has formed Stickney, the tons of rock raised (accomplices
the reduced gravity and the small size of Phobos) have continued to roll making "the
full circle" of the moon, returning in some cases to the starting point. A continuous bouncing on
the surface has left those incredible scars that make Phobos an immediately recognizable object.
Deimos is also an approximately triaxial object. Its dimensions are 15x12x11 km, and its surface
appears much smoother, although the density of craters is similar to that of the surface of
Phobos and the lunar plateaus. This is due to the presence of a thick layer of regolith that almost
completely fills some craters. It is also believed that the formation of a concavity of about 10 km
in diameter, present in the southern hemisphere, would be responsible for the degradation of the
surface. The seismic energy produced by the impact, in fact, would have propagated
through the entire object in the form of waves, causing the collapse of smaller structures.
Mass and Density. Due to their very small mass, Phobos and Deimos obviously have a
very weak gravity: less than one-thousandth of the Earth's gravity... Just think that a
70 kg person would weigh on Phobos just 40 grams, and to get away from the surface and
get lost in space would be enough to reach an escape velocity of just 41 km per hour!
The density of the two satellites is quite low, 1.9 grams per cubic centimeter for Phobos
and 1.5 grams for Deimos, but in line with that of most asteroids of carbonaceous type.
For comparison, the Moon has a density of 3.3, Earth 5.5, and Mars 4.4.
With such a density it is almost certain that the two moons cannot
have a completely solid structure. Their interior could be made of a set of very
fine particles aggregated to large rocky blocks, with large gaps in the interstices.
Orbit. Strangely enough, the two moons travel in almost circular orbits,
only slightly inclined to the equatorial plane of Mars. Orbits that, as we will see
later, we would expect from moons of great mass, and not from two wandering asteroids captured by
chance by the gravitational field of Mars. Phobos orbits at a distance of 9380 km,
while Deimos is a little further away, 23 460 km from the center of the planet.
Phobos makes a complete revolution in 7 hours and 39 minutes, which means that every Martian day
(just over 24 hours) completes more than three revolutions of the planet.
But the most unusual thing is that because of its period of revolution, much lower than the
rotation period of Mars, this small moon appears to move in the sky with retrograde motion. Seen
therefore from the surface of Mars, Phobos appears as an object that, rising in the west,
crosses the sky very quickly, setting in the east after only 3 hours and 10 minutes from its rising.
Deimos, on the contrary, is much slower; it normally rises in the east and sets in the west,
completing a complete revolution in 30 hours and 18 minutes.
The rotation of the two moons is synchronous: both always show the same face to Mars - with the major
axis of their figure pointing to the planet - exactly as the Moon always shows the same face
to the Earth. In addition, being very close to the planet, Phobos and Deimos are not visible from all
Martian regions: the polar regions are excluded. To see Phobos an observer must be in locations
between 69° North and 69° South latitude, and between 82° North and 82° South to see Deimos.
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Origin and Formation. The origin of the moons of Mars has always puzzled astronomers:
their irregular shape and their tiny size have always suggested that they were two
asteroids captured by the planet ... But their orbital parameters,
orbits almost perfectly circular on the equatorial plane of Mars and a rotation synchronous with that
of the planet have led scientists to believe more likely that they had originated from the
impact of Mars with a celestial body of a thousand kilometers in diameter.
However, a problem remains: why did that impact end with the formation of two small moons
and not a single large moon, like ours, or a ring of orbiting debris?
Recent computer simulations have shown that the innermost part of the ring of debris
initially formed one or more large moons, but under the action of tidal forces caused
by the nearby intense gravitational field of Mars, would then disintegrate. Their remains
would have finally fallen on the planet. The same fate would have happened to the
smaller satellites and will happen in the future to Phobos, whose orbit - already
dangerously close to the point of disintegration - narrows every year by about two centimeters.
Nothing is forever. And in fact, whatever their origin, we know anyway that the two
satellites will not have a long life. Almost at the limit of the synchronous orbit, Deimos is
in fact moving away very slowly from the surface of the red planet: a calm and quiet end, in stark
contrast to the more dramatic one that awaits Phobos. The largest of the Martian moons is in
fact destined to fall on Mars. Slowly, but surely, the tidal forces are tightening on the unfortunate
satellite, which in a period between 20 and 40 million years will end up exceeding the so-called
Roche limit, equal for Mars to about 5470 km away from the surface. Reached this point of no return,
the difference between the tidal forces that are exerted on the face closer to the planet, and the
farthest, will exceed its internal cohesion and the satellite will explode into a thousand pieces.
Although some large fragments will fall on Mars as many meteorites,
most of the debris will probably contribute to the formation of a ring around the planet.
Space exploration. Long before their disappearance, the two small moons will however
have plenty of time to give us a hand in the exploration of Mars.
And not in a thousand years. Already for 2024, in fact, is scheduled start the Japanese mission
Martian Moons eXploration, or MMX, responsible for bringing back to Earth a sample of the
material that covers the surface of Phobos. MMX will enter Mars' orbit a year after launch.
It will gradually get closer to Phobos, entering what mission planners call a quasi-orbit:
while it may look like MMX is orbiting Phobos, it is actually just orbiting Mars
in a way that lets it circle Phobos at the same time. This will be no easy challenge
since Phobos is deep inside Mars’ gravity well and just 6,000 kilometers above the surface.
MMX will spend a year studying Phobos with its remote science instruments while searching for
a safe and scientifically interesting place to land. A wide-angle camera and laser mapping system
will map the moon in 3D, while a narrow-angle camera provides a closer look at specific areas.
Three spectrometers will determine the mineral and elemental makeup of Phobos’ surface.
A dust sensor will also look for orbiting particles knocked off Mars and Phobos by
meteor impacts. The spacecraft also includes a pair of ultra-high-resolution video cameras
that will provide stunning views of Mars and its moons. Some images will be transmitted to
Earth during the mission, with the rest stored in the sample return capsule for later playback.
As MMX descends to Phobos for landing, it will deploy a small German and French-built
rover based on the MASCOT, which Hayabusa2 dropped to tumble around asteroid Ryugu.
The rover will travel around Phobos for at least 100 days analyzing the surface.
We don’t know exactly what Phobos’ surface is like: is it rocky, fluffy, or sandy?
MMX is prepared for a variety of possibilities with two robotic-arm-mounted drills
that can bore 10 centimeters beneath the surface. Once MMX collects its samples, it will lift off
from Phobos and begin spiraling out of Martian orbit, passing Deimos several times on its way
out to analyze and map the smaller moon. MMX will leave Mars in 2028 and return to Earth in 2029.
The spacecraft will jettison a small capsule containing the samples,
which will plummet through Earth’s atmosphere and use a parachute to land safely for collection.
Future space stations? If all goes well, the results of the MMX mission will be
decisive for the choice that awaits us in the near future, that is, whether or not to
use the two satellites as permanent bases for remote exploration of the red planet.
Phobos presents potential advantages over Moon and Mars as a human-operated outpost,
at least in the early stages of real-time Mars exploration and future development.
What is most important, Phobos offers an excellent position versus Mars,
from which to conduct the foregoing studies and many kinds of mission operations.
Reaching Phobos does not require the costly development of a major propulsive or
aerobraking lander, nor does it require the delivery of that lander to lunar or Mars orbit.
Phobos affords a practicable waypoint at which to locate an astrobiology laboratory that can safely
conduct the examination of Mars samples with reduced danger of back-contamination to the Earth.
The topography of Phobos, its orbit close to the Martian surface,
and rotation with one side always facing Mars provide advantages for radiation shielding.
As a trade-off for not needing a big, expensive lander,
the near-microgravity of Phobos will demand solutions and technologies to attach
or adhere all elements of the base to the Phobos' surface
The Phobos mission will entail exposing its human crew to space radiation outside the
friendly confines of the Earth's magnetic field and unmitigated by any significant atmosphere.
One potential mitigation might derive from the shielding afforded by Stickney crater,
the planet Mars overhead (~25% of the visible hemisphere above a point on Phobos' surface),
and the mass of Phobos itself. The most important thing about having an outpost
so close to Mars, however, would be that from Phobos we could control in real-time the movement
of the Martian rovers, something that at the moment is prevented by the distance Mars-Earth,
which leads to a delay in communication of up to 40 minutes between the return of the signal.
A decade ago it was even thought that even the human descent on Mars would be
much easier starting from a base on Phobos, rather than with a direct trip from Earth.
But now the games are done and we will try the way of direct landing. But there is a strong bet on
the fact that soon after we will set foot on the Martian moons, which from that moment will become
our window constantly open on the red planet. And in this regard, we can not fail to quote
at this point a passage of Carl Sagan taken from his famous book "Cosmic
Connection" that does not cease to make us dream: "Sooner or later, it is only a matter of decades,
on the surface of Phobos there will be instruments; and then men who will observe
with terror and wonder the immense red planet filling the sky, from the horizon to the zenith."