Sky and Telescope - November 2018 - 27

u HOKUSAI During a flyby of Mercury in 2008, Messenger recorded
this prominent 114-km-wide crater and its bright splash pattern. The
impact that created Hokusai might have delivered water that migrated to
the planet's poles and formed ice deposits on the floors of permanently
shadowed craters there.

while aurorae appear high in the atmospheres of Earth and
Venus, there could be aurora-like emissions at ground level on
Mercury, at least at X-ray wavelengths.
Mercury's churning molten outer core generates a global
magnetic ﬁeld, albeit one that's only 1% as strong as Earth's
at the surface. A weird (and still unexplained) aspect of
Mercury's magnetic ﬁeld is its offset toward the north pole
by roughly 500 km, some 20% of the planet's radius. The
planet's weak magnetosphere can hold off solar radiation -
sometimes. But when coronal mass ejections blast toward
Mercury, the incoming plasma can compress the ﬁeld drastically enough to let solar radiation smash directly into Mercury's surface. The south pole gets bombarded more because
of the northward offset of the ﬁeld.
When solar wind particles slam into Mercury's surface,
they knock atoms from surface rocks and into Mercury's
tenuous exosphere. It's not a proper atmosphere - individual
atoms rarely encounter each other, so it has no wind or
weather - and spacecraft like Messenger and BepiColombo
can ﬂy directly through it unimpeded, tasting atoms that
until recently were part of the planet's surface.
Solar wind, intrinsic magnetic ﬁeld, and neutral and
charged particles are all interlinked and dynamic on very
short time scales. For example, when the solar wind's magnetic ﬁeld lines connect with a planet's dayside magnetic
ﬁeld, the latter's ﬁeld lines peel back around to the planet's
nightside, where they reconnect. On Earth, this process takes
about an hour; on Mercury, magnetic ﬁeld lines can shift
from one side of the planet to the other in a matter of minutes. It's so dynamic that it's difﬁcult to understand all the
interactions with only one spacecraft in one location at one
time, especially because many of the relevant measurements
are performed in situ, with the spacecraft directly measuring ﬁeld strength or ion composition. With two spacecraft
in different orbits, BepiColombo will be able to measure the

magnetic ﬁeld and particle environment at two points simultaneously. MPO will operate at a distance where the planet's
inﬂuence is stronger, while MMO will explore a region where
the solar wind takes over. Together, they'll watch how the
whole system responds to solar storms.

BepiColombo's Odyssey
To get MPO and MMO to Mercury, ESA built a third spacecraft, the Mercury Transfer Module (MTM). The MTM has
huge solar arrays to power ion thrusters that will accomplish
the hard work of setting up BepiColombo's rendezvous with
Mercury. This is the ﬁrst time that ESA will use solar electric
propulsion on an interplanetary mission. MTM employs the
full power-generating capability of its solar panels only when
it's farther from the Sun than Venus's orbit; once it passes
inside Venus's distance, it has to tilt its solar panels at an
angle to the Sun to avoid overheating and also to limit damage from solar energetic particles.

Day
0

Day 176
2 orbits
3 rotations

Day
132

N ASA / JHU A PL / CA R NEGIE INSTIT U TION FOR SCIENCE (2); ORBIT
A ND DAY DIAG R A M: SUE LEE / THE NE W SOL AR SYSTEM, SOURCE:
ROBERT STRO M (UNIV. OF A RIZON A)

0.47 AU

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\PMUQ[[QWV¼[\PZMMÆaJa[

Day 88
1 orbit
1½ rotations

0.31 AU

Sun

Day
44

p A DAY ON MERCURY The innermost planet takes 58 Earth days to
turn once around its axis, and 88 Earth days to orbit the Sun. But as
seen from the planet's surface, the Sun takes far longer - 176 Earth
days - to return to the same point in Mercury's sky. Thus the planet
experiences one "day" every two "years."

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Sky and Telescope - November 2018

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