Sky and Telescope - November 2017 - 8

NEWS NOTES
PRO-AM

Amateurs Find Asteroid's Moon

Amalthea's satellite
(based on occultation)

p These lines represent the projected paths of a 10th-magnitude star
recorded by observers on March 14, 2017, as the star passed behind the
asteroid Amalthea. Sam Insana in Gila Bend, Arizona, measured the red
track, and the white line below it is actually two tracks close together.
The yellow oval marks the asteroid's approximate shape; the small yellow circle just below it corresponds to the location of the potential moon.

A TEAM OF AMATEUR OBSERVERS, some armed with

just 3-inch telescopes, found that the main-belt asteroid
113 Amalthea probably has a small companion.
Paul Maley, a retired NASA staffer and a key member of the
International Occultation Timing Association, enlisted amateur observers in Texas to observe the March 14th occultation
of a 10th-magnitude star by the 13th-magnitude asteroid.
As detailed in the Central Bureau for Astronomical
Telegrams no. 4413, issued on July 12th, a "fence" of 10
observing sites spread across the occultation's predicted path
yielded seven positive occultations and three "misses." One
of those misses (red track in diagram), by Sam Insana in Gila
Bend, Arizona, fell between ﬁve positive occultation tracks
to his north and two to his south. The two southern ones,
recorded by Dave Eisfeldt and Dick Campbell (both with the
Central Texas Astronomical Society), appear to correspond to
interruptions of the star's light by a moon. Other members
of the team were Sam Deen, Wayne Thomas, Paul Facuna,
Don Boyd, and Ted Blank (who set up four robotic cameras
for the event).
Not to be confused with Jupiter's small moon of the same
name, Amalthea orbits the Sun every 3.66 years in the inner
part of the main asteroid belt, not far outside the orbit of
Mars. It circles the Sun at an average of 2.37 astronomical
units (355 million km) with an inclination of 5°. Discovered
in 1871, it's about 50 km (30 mi) across and has a rocky
"S-type" surface spectrum typical of bodies populating the
inner main belt. Based on the lengths of each occultation
record and how they line up in the plot, Amalthea must have
a distinctly elongated shape.
The size of the satellite isn't known, though typically such
companions are much smaller than their hosts. According to
8

N O V E M B E R 2 0 17 * S K Y & T E L E S C O P E

■ J. KELLY BEATTY

SOLAR SYSTEM

Huge Storm Rages on Neptune
ASTRONOMERS using the Keck Observatory have spotted a

large storm complex on Neptune (shown), spanning at least
30° in both latitude and longitude and centered near the
planet's equator. Some intrepid amateurs also detected the
storm in the weeks preceding the Keck observations.
"Historically, very bright clouds have occasionally been
seen on Neptune, but usually at latitudes closer to the
poles," says Imke de Pater (University of California, Berkeley). "Never before has a cloud been seen at or so close to
the equator, nor has one ever been this bright."
The cloud cover might form
when methane condenses above
a vortex anchored deep in
Neptune's atmosphere. However, wind speeds on Neptune
vary drastically with latitude, so
it's unclear why such a vortex
doesn't quickly break apart.
■ SEAN WALKER

A M A LTHE A: LE A H TISCIONE / S&T, SOURCE: IOTA / BR A D TIMERSON; NEPT UNE: N. M OLTER / I. DE PATER (UNIV. OF
CA LIFOR NIA , BERK ELE Y ) / C. A LVA RE Z ( W. K ECK OBSERVATORY )

Amalthea
(shape based
on occultation)

the deﬁnitive compilation by W. Robert Johnston, the census
of asteroids with moons now includes 145 main-belt asteroids (8 with two each), 23 Mars-crossing asteroids (1 with
two), and 63 near-Earth asteroids (2 with two each). Looking
farther outward, Johnston's list includes 4 of Jupiter's Trojans
and 82 trans-Neptunian objects.
Nor is it clear what kind of orbit the newfound companion might have. According to Daniel Green (Harvard
University), small satellites around large main-belt asteroids
typically have orbits roughly ﬁve times larger than their
primary's diameter, which here would correspond to about
250 km. But the observed gap was far smaller, so perhaps
the companion's orbital plane was seen nearly edge-on with
the satellite near one of its conjunction points (rather than
being far off to one side).
The whole claim to a satellite discovery rests on Insana's
negative observation - take that away, and the two brief outages recorded by Eisfeldt and Campbell could simply be due
to a large bump on Amalthea's surface. However, Maley says,
"The Insana video has been checked over and over, as was his
location, and the only conclusion that we can come to is that
the miss is completely clean."
Conﬁrmation of the discovery might not have to wait
long. Although no one had previously observed a stellar
occultation by Amalthea, four are predicted for next year -
including an April 14th event with a track that crosses the
northeastern U.S.

Sky and Telescope - November 2017

Table of Contents for the Digital Edition of Sky and Telescope - November 2017