Australian Sky & Telescope - April 2018

NASA / JHUAPL / SWRI / JAMES TUTTLE KEANE

• ToseewhatwecanexpectfromtheNewHorizonsmissioninthecomingmonths,
  go tohttps://is.gd/newhorizonstimeline.

NEW ANALYSIS FROMthe New

Horizons team suggests that the

spacecraft’snexttarget,aKuiperBelt

object known as 2014 MU 69 ,mighthave

amoon.Theobjectwasalreadythought

tobeabinary,sothisbringstheparty

members to three. Planetary scientists

presented the preliminary results at the

meeting of the American Geophysical

Union in New Orleans.

New Horizons will fly 3,

km from 2014 MU 69 on January 1,

2019. Initial views, to be obtained in
      September 2018, won’t resolve the
      object—itwon’tbeuntilafterthe
      flybythatwe’llbegintoseehigh-
      resolution images of its surface. Data
      will continue trickling in through 2019
      and most of 2020. Meanwhile, New
      Horizons will also be exploring 30
      otherKuiperBeltobjectsfrommuch
      greaterdistancesthrough2021.
      In preparation for the flyby, the team
      deployed observers across the globe to
      monitoroccultationsonJune3,July
      10andJuly17,as2014MU 69 passed

TColored chords mark the path of background

starsasseenfromdifferenttelescopeson

three different days. The blank spaces on those

linesindicatethefewsecondswhen2014MU 69

blockedthelightfromthestar.

20 kilometers

July 10th Satellite

(based on occultation)

July 17th

Center

of mass

2014 MU 69

(shape based

on occultation)

infrontofthreedifferentstars.The

teamcaughtablipduringthesecond

event, then struck gold with the third

occultation, as five of 24 observers

caught the star blinking out of view.

Analysis of the third event showed

that the object could be a binary. But

theplotthickenedwhentheteamtook

anotherlookatthebliprecordedduring

the second occultation. A new analysis,

which includes observations of 2014

MU 69 ’s orbit around the Sun using

data from the Gaia mission, suggests

the blip is evidence for a third member

ofthesystem:amuchsmallermoon

orbiting at a larger distance from the

closer binary.

“This is probably a sign that the

object itself was not a collisional

fragment. We think [2014 MU 69 ]

was made like this,” says Marc Buie

(Southwest Research Institute).

However,hecautionsthatthethree-

bodyexplanationisn’tafinalresult;

further observations could change

the scientists’ conclusions. Regardless

of how many pieces the little world

consists of, it will shed light on the

planet-formingenvironmentofour

system’s earliest years.

■MONICA YOUNG

http://www.skyandtelescope.com.au 9

Meteoroids explode

from the inside

Space rocks breaking up in Earth’s

atmosphere might be their own Trojan

horses. Spurred by an attempt to

understand why the 2013 Chelyabinsk

meteoroid fragmented so easily,

Marshall Tabetah and Jay Melosh (both

at Purdue University) simulated the

rock’s interaction with the surrounding

air as it dove through the atmosphere.

The key was the meteoroid’s porosity,

which hadn’t been included in previous

simulations. Asteroids are more like piles

of coarse gravel than solid boulders. As

one of these rocks shoots through the

atmosphere, a bow shock forms around

it, with piled-up air in front and a vacuum

in the wake. This setup creates something

of a suction effect, with the high-pressure

air in front shooting through the rock’s

fractures to fill the vacuum behind. The

air’s infiltration breaks the meteoroid apart

from the inside out. The team’s paper

appeared December 11 in the journal,

Meteoritics & Planetary Science.

■ CAMILLE M. CARLISLE

Exoplanet loops

over star’s pole

Reporting December 18 in the journal

Nature, Vincent Bourrier (Geneva

University Observatory, Switzerland) and

colleagues have found that the Neptune-

mass exoplanet GJ 436b orbits around its

star’s poles rather than its equator. The

default in planet formation is for worlds to

coalesce from a dusty disk of gas around

a newborn star’s middle, a consequence

of the conservation of angular momentum.

Furthermore, huge planets close to their

parent stars ‘should’ be on circular,

equator-aligned orbits due to tidal

interactions. But GJ 436b is wonky:

The team’s 3D mapping of the planet’s

trajectory, coupled with measurements of

the star’s rotation, reveal that the gaseous

world is following an elongated orbit that

passes over the star’s pole. This setup

suggests that an outer, unseen exoplanet

might have driven GJ 436b inward to its

current location. The migration would also

have exposed the planet’s atmosphere to

more starlight and might have triggered

GJ 436b’s comet-like atmosphere.

■ CAMILLE M. CARLISLE

Next New Horizons target has a moon