Sky & Telescope - April 2022 - 48

Star magnitudes
ASTEROIDS: ESO / P. VERNAZZA ET AL. / MISTRAL ALGORITHM (ONERA / CNRS) (3)
APRIL 2022 OBSERVING
Celestial Calendar by Bob King
Run With the Asteroids
Three small-scope minor planets highlight the month.
I
f you enjoy chasing asteroids, this is
your month. Three reach opposition
in April and are bright enough to follow
in small telescopes: 8 Flora, 10 Hygiea,
and 15 Eunomia. They share the same
pocket of sky in the Hydra-Virgo-Libra
region, making it easy to catch them all
in a single observing session. Although
in the eyepiece they're indistinguishable
from stars, knowing a little about each
asteroid helps us appreciate their hidden
individuality.
This is a particularly good opposition
for Hygiea (Hi-JEE-uh), a dark,
carbonaceous asteroid compositionally
similar to Ceres and located in
the outer region of the main asteroid
belt. Normally 10th magnitude, Hygiea
reaches magnitude 9.3 at opposition
on the 28th, providing an infrequent
opportunity to glimpse it with binoculars
from a dark-sky location.
Hygiea has a diameter of 434 kilometers
(270 miles), which makes it
the fourth largest asteroid after Ceres,
Vesta, and Pallas. Its impressive size
and nearly spherical shape have some
astronomers considering reclassifying it
as a dwarf planet. Simulations suggest
that more than 2 billion years ago a
collision with an object in the 75-100km
size range may have fully shattered
Hygiea, with the resulting fragments
coalescing and consolidating into the
spherical body we see today.
The spectral signatures of our other
two asteroids indicate they're made of
denser stuff - silicates and iron-nickel.
Flora orbits within the inner region of
the asteroid belt at an average distance
of 2.2 a.u., compared to 3.1 a.u. for
Hygiea and 2.6 a.u. for Eunomia. Flora's
composition implicates it as a possible
source for the LL (low-iron, low metal)
48 APRIL 2022 * SKY & TELESCOPE
chondrite meteorites, an uncommon
stony type that accounts for about
9% of observed meteorite falls. With
a diameter of 146 km, it's large for an
inner asteroid but will only brighten
to magnitude 9.8 during this season's
distant opposition on April 12th. During
more favorable apparitions, Flora
brightens to magnitude 8.0 when it's
closest to Earth.
British astronomer John Russell
Hind (of Hind's Crimson Star fame)
discovered Flora on October 18, 1847,
but it was John Herschel who proposed
naming it after the Roman goddess of
flowers and spring. Appropriately, its
appearance this year puts it nicely in
sync with the season. Puttering westward
across central Virgo, it passes 1.1°
north of 3.4-magnitude Zeta (ζ) Virginis
on the night of April 12-13.
Flora spins once every 12.8 hours
and, like Uranus, rotates tipped on its
side, with an axial tilt of 79° (give or
take 10°). Past impacts have chipped
away at the asteroid to create the Flora
qThe images below, captured by the
SPHERE instrument on the ESO's Very Large
Telescope, reveal each asteroid's general shape
and some surface features. Hygiea could eventually
be reclassified as a dwarf planet because
it satisfies the definition, including possessing a
spherical shape. (For all finder charts, the position
of the asteroids is plotted for 0h UT.)
8 Flora
10 Hygiea
13h 50m
4
3
5
6
7
8
9
+4°
84
78
13h 40m
13h 30m
15 Eunomia
13h 20m
σ
+6°
16
14
12
10
8
4
Apr 2
ζ
-2°
6
0°
VIRGO
18
20
22 24
26 28
30
+2°
P
ath of 8 Fl
ora
Sky & Telescope - April 2022

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