Sky & Telescope - March 2021 - 49

Mutual Event Season is
Underway
EVERY SIX YEARS the orbital plane of

V ESTA: N ASA / JPL- CA LTECH / UCL A / MPS / DLR /
IDA; M U T UA L E V EN TS: WINJUPOS

S ASTEROID INSPECTED The Dawn spacecraft captured this image of Vesta as it studied
the asteroid from July 2011 to September
2012. The conspicuous mountain at the south
pole (bottom) is the central peak of Rheasilvia,
a 505-kilometer-wide crater excavated about a
billion years ago.

NASA's Dawn spacecraft parked
itself in orbit around Vesta from July
2011 to September 2012 to map and
characterize the asteroid with a suite of
instruments. Signiﬁcantly, it found that
the object is a differentiated protoplanet
with an iron-rich core, silicate mantle,
and basaltic crust. So far, Vesta is the
only conﬁrmed cased of a differentiated
protoplanet - bodies like it were either
incorporated into the newly formed
planets or destroyed in collisions during the solar system's infancy. Vesta
squeaked through intact, but barely.
Two enormous impacts decimated its
south polar region, excavating the
505-km-wide crater Rheasilvia and its
older cousin, 395-km-wide Veneneia.
Fragments from the dual cataclysms
are the likely source of both the Vesta
family of asteroids and a subgroup of
meteorites called howardites, eucrites,
and diogenites - collectively known as
the HED clan. Scientists had suspected
a Vesta-HED connection for years
based on spectroscopic similarities, but
Dawn's in situ measurements clinched
the relationship.
Incredibly, you can purchase a piece
of an HED meteorite online. Imagine
having one of those in hand as you spy
Vesta in your telescope!

occultations can be partial to nearJupiter's four Galilean moons is edge-on total. The greater the brightness drop,
with the Sun and Earth, and we enter
the more dramatic the event. During
mutual event season, when Jupiter's
a deep eclipse, you can watch a moon
Galilean moons occult and eclipse one
nearly fade away. In central occultaanother. The current season began
tions, one moon approaches another
on January 3rd when Europa partially
and the two brieﬂy merge into a single
eclipsed Io and will wrap up on Novemobject before parting again. I always get
ber 16th with a Ganymede-Io occula kick out of these temporary arrangetation. Jupiter's low
ments because they
altitude and proximity
allow me to see the
to the Sun precluded
movement of the Jovian
viewing the ﬁrst part
moons in near-real
of this celestial hidetime.
and-seek show, but by
Easiest to observe
mid-March the planet is
are events in which
high enough in the early
the brightness of the
dawn for good views.
occulted or eclipsed
An eclipse occurs
moon dips by 0.3 magwhen the shadow of
nitude or more. The
Io occults Europa:
March 17
one moon lands on
table below includes
the surface of another.
the best ones visible
An occultation happens
from the Americas
when the disk of one
from mid-March
moon covers the disk of
through mid-April.
another. Eclipses and
(Note that some of
these events may occur
in daylight from your
X MUTUAL EVENTS These
location.)
two simulations depict the
circumstances of an eclipse
For a complete list
and an occultation visible in
of mutual events, or to
the Americas. Both produce
generate predictions for
brightness changes great
Ganymede eclipses Europa:
your area, visit https://
April 15
enough to be detected in
amateur telescopes.
is.gd/mutualevents.

Selected Jupiter Mutual Satellite Events
Date

Time (UT)

Event

Mag change

March 17

13:04 - 13:08

Io occults Europa

0.6

March 29

12:06 - 12:14

Io occults Ganymede

0.4

March 30

9:30 - 9:35

Europa eclipses Io

0.5

April 1

13:17 - 13:23

Ganymede occults Io

0.6

April 6

11:46 - 11:51

Europa eclipses Io

0.3

April 11

10:01 - 10:11

Io eclipses Callisto

0.4

April 12

11:51 - 12:13

Io eclipses Callisto

0.5

April 15

10:08 - 10:16

Ganymede eclipses Europa

0.5

sk yandtelescope.org * MARCH 2 021

https://is.gd/mutualevents https://is.gd/mutualevents http://www.skyandtelescope.org

Sky & Telescope - March 2021

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