Australian Sky & Telescope — November-December 2017

8 AUSTRALIAN SKY & TELESCOPE November | December 2017

Astronomers using the Keck Observatory have

spotted a large storm complex on Neptune

(shown), spanning at least 30° in both latitude and

longitude and centred near the planet’s equator.

Some intrepid amateurs also detected the storm in

the weeks preceding the Keck observations.

“Historically, very bright clouds have occa-

sionally 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

Huge new storm rages on Neptune

AMALTHEA: LEAH TISCIONE /

S&T

, SOURCE: IOTA / BRAD TIMERSON; NEPTUNE: N. MOLTER / I. DE PATER (UNIV. OF CALIFORNIA, BERKELEY) / C. ALVAREZ (W. KECK OBSERVAT

ORY)

ATEAMOFAMATEUROBSERVERS,some armed with just

75-mmtelescopes,hasfoundthatthemain-beltasteroid

113Amaltheaprobablyhasasmallcompanion.

PaulMaley,aretiredNASAstafferandakeymemberofthe

International Occultation Timing Association, enlisted amateur

observersinTexastoobservetheMarch14occultationofa

10th-magnitude star by the 13th-magnitude asteroid.

A‘fence’of10observingsitesspreadacrossthe

occultation’spredictedpathyieldedsevenpositive

occultationsandthree‘misses’.Oneofthosemisses(red

trackindiagram),bySamInsanainGilaBend,Arizona,fell

betweenfivepositiveoccultationtrackstohisnorthand

twotohissouth.Thetwosouthernones,recordedbyDave

EisfeldtandDickCampbell(bothwiththeCentralTexas

AstronomicalSociety),appeartocorrespondtointerruptions

ofthestar’slightbyamoon.Othermembersoftheteam

wereSamDeen,WayneThomas,PaulFacuna,DonBoydand

TedBlank(whosetupfourroboticcamerasfortheevent).

NottobeconfusedwithJupiter’ssmallmoonofthesame

name, Amalthea orbits the Sun every 3.66 years in the inner

partofthemainasteroidbelt,notfaroutsidetheorbitof

Mars.ItcirclestheSunatanaverageof2.37astronomical

units (355 million km) with an inclination of 5°. Discovered

in1871,it’sabout50kmacrossandhasarocky‘S-type’

surface spectrum typical of bodies populating the inner main

belt.Basedonthelengthsofeachoccultationrecordand

howtheylineupintheplot,Amaltheamusthaveadistinctly

elongated shape.

Thesizeofthesatelliteisn’tknown,thoughtypicallysuch

companionsaremuchsmallerthantheirhosts.Accordingto

thedefinitivecompilationbyW.RobertJohnston,thecensus

ofasteroidswithmoonsnowincludes145main-beltasteroids

(8withtwoeach),23Mars-crossingasteroids(1withtwo)

and63near-Earthasteroids(2withtwoeach).Looking

fartheroutward,Johnston’slistincludes4ofJupiter’sTrojans

and82trans-Neptunianobjects.

Norisitclearwhatkindoforbitthenewfound

companionmighthave.AccordingtoDanielGreen(Harvard

Amalthea

(shape based

on occultation)

Amalthea’s satellite

(based on occultation)

University), small satellites circling 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.

■ J. KELLY BEATTY

Amateur astronomers discover an asteroid’s moon

SThese 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.

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