Sky & Telescope - April 2021 - 40

Oddball Satellites

POCKMARKED The crater Stickney is the largest scar on
Phobos, seen here from a distance of less than 200 km by
the European Mars Express orbiter. Linear fractures also
mar the moon.

APRIL 2 021 * SK Y & TELESCOPE

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Phobos

MOI-2
Second maneuver for
orbit plane change and
periapsis height increase

Orbit of Phobos

Deimos

Mars
MOI-3
Third maneuver for
Phobos co-orbit insertion

Orbit
of Deimo
s

MOI-1
First maneuver for
Mars elliptical orbit insertion
(periapsis height: 500 km)

S PLANNED TRAJECTORY Once Japan's MMX spacecraft enters the
Mars system, it will execute a series of maneuvers to insert itself into a
quasistationary orbit (QSO) around Phobos. A QSO is similar to a geostationary orbit, except that, instead of hovering over the same patch of
surface, the satellite moves in a figure eight.

spacecraft. While riddled with technical problems, the former
managed to return some grains of material from near-Earth
asteroid 25143 Itokawa. The follow-up mission, Hayabusa 2,
had a smoother ride, successfully retrieving samples from
asteroid Ryugu that safely dropped to Earth in December
2020 (see page 11).
Those who investigate the moons hope that MMX will
deliver as much information about the Martian system as the
Apollo program did for Earth and its Moon.
" Phobos and Deimos were kind of ignored for a long time,
but they're the only other moons in the inner solar system, "
Minton says. " We've learned a lot about how our solar system
formed by understanding how our Moon formed and what
history it went through. So I think the existence of these two
little moons of Mars is telling us something about the early
solar system and how planets form. "
The results will also enhance our understanding of a
process that might occur time and again across the universe
- and might be key to the evolution of life. " It's very important to understand if moons around terrestrial planets are
commonplace, because our moon has played and still continues to play a very important role for the apparition and the
maintaining of life, " Rosenblatt says. " It has an impact on
our future understanding and the exploration of the extraterrestrial planets. "
But ﬁrst and foremost, MMX will help solve a riddle that
has puzzled astronomers for decades. " We're all really looking forward to the Japanese mission and keeping our ﬁngers
crossed, " Rivkin says.

JAVIER BARBUZANO is a freelance science writer based in
Spain. While researching and writing this feature, he welcomed
his second daughter, Clara, into the world. He proposed
several astronomy-related names for her, but his wife wouldn't
budge.

STICK NE Y: ESA / DLR / G. NEUK U M (FU BERLIN), CC BY-SA 3.0
IGO; TR A JECTORY OF M M X: G REGG DINDER M A N / S&T

to Japan's Martian Moons Exploration (MMX) mission.
Expected to launch in 2024, it will send a spacecraft to study
both moons from orbit for about three years. It will also
deliver a microwave-sized rover that will explore Phobos from
the surface. Finally, the spacecraft itself will land on Phobos
to retrieve samples and bring them to Earth.
MMX aims to unravel the origin of the Martian moons
and to investigate the evolution of the Martian system. " With
these samples, " Rosenblatt says, " we will be able to have a
very reliable measurement of the age of Phobos, and we will
see if David Minton is right or if I am right. "
Or, the captured-asteroid scenario might still win out.
Whatever the mission ﬁnds, MMX will provide invaluable
insight into the evolution of Mars, our own planet, and
the early solar system. If the impact scenario is correct, the
moons should contain minerals from Mars from a time when
it might have been habitable. " Comparing this satellite time
capsule with present-day Mars would provide insight into
how a habitable environment developed and then died, " says
Tomohiro Usui (JAXA), project scientist on the MMX team.
Conversely, if the moons are captured asteroids, it means they
originated elsewhere in the solar system. " This would make
the moons examples of celestial delivery packages that were
scattered between the outer and inner solar system, potentially delivering water and even primitive organics to the
terrestrial worlds. "
The highlight of the mission is the sample return. JAXA
is building on its experience with the Hayabusa 1 and 2

Sky & Telescope - April 2021

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