Sky and Telescope - March 2018 - 13

SOLAR SYSTEM

Why Isn't Venus
Magnetized?
BASED ON DENSITY alone, Venus must

have an iron-rich core that's at least
partly molten. So why does our likesized neighbor lack the kind of global
magnetic ﬁeld that Earth has? Theorists long suspected that the planet's
slow, 243-day spin was inhibiting the
necessary internal churning needed to
generate a ﬁeld. But recent research says
that's not the cause.
A new idea has emerged that attacks
the problem from a wholly new angle.
As Seth Jacobson (now at Northwestern
University) and four colleagues detail in
the September 15th Earth and Planetary
Science Letters, Earth and Venus might
both have ended up without magnetic
ﬁelds save for one critical difference:
The nearly assembled Earth endured a

catastrophic collision late in its formation - the one that led to the Moon's
creation - and Venus did not.
Jacobson and his team simulated the
assembly of rocky planets early in the
solar system's history. Over time the
hot, molten cores developed several layers of differing compositions. Such an
onion-like core would lack the wholesale circulation necessary for a dynamo,
so there'd be no magnetic ﬁeld. This
might explain the fate of Venus.
On Earth, meanwhile, the Moonforming impact affected our planet
literally to its core, creating turbulent
mixing that disrupted any compositional layering. With the same mix of
elements throughout, the core started
circulating heat into the mantle. The
churning core became the dynamo that
created our planet's global, life-sustaining magnetic ﬁeld.

IN BRIEF

■ J. KELLY BEATTY

help determine how they formed so early

Most Distant Black Hole
Astronomers have discovered a supermassive black hole scarfing down gas just
690 million years after the Big Bang, at a
redshift of 7.54. Eduardo Bañados (Carnegie Institution for Science) and colleagues
announce the source, called J1342+0928,
in Nature. While astronomers have netted
galaxies back to when the universe was a
mere 400 million years old, J1342+0928
contains the earliest supermassive black
hole detected, squeaking into first place
some 50 million years ahead of the
previous record holder. With a mass of
about 800 million Suns, it's a little less
massive than the supplanted contender
(J1120+0641, at 2 billion Suns) and within
the ballpark for other supermassive black
holes found a few hundred million years
later. The discovery of such objects will
in the universe's history. Read more at

SPACE

Reentry of China's
Tiangong 1 Proto-Station

TIA NGONG 1: CHIN A M A NNED SPACE ENGINEERING

CHINA'S FIRST PROTOTYPE space sta-

tion will make an uncontrolled reentry
into Earth's atmosphere in mid-March,
give or take a month, according to a
prediction by the Aerospace Corporation on December 8, 2017. It will be
the largest object to reenter Earth's
atmosphere since January 2012, when
Russia's 13-metric-ton Phobos-Grunt
mission fell to Earth.
The China National Space Administration (CNSA) launched Tiangong 1
("heavenly palace" in Chinese) on September 29, 2011. The school bus-size,
8½-metric-ton space station was used
to test automated rendezvous and docking techniques, and Chinese astronauts
brieﬂy visited the station in 2012 and
2013. Tiangong 1 and the slightly larger
Tiangong 2, launched on September
15, 2016, are precursors to the more
permanent Tiangong 3, expected to be
operational in 2022.
Ground controllers lost contact with
Tiangong 1 in late 2016, and in May
2017 China stated in a letter to the

https://is.gd/J1342quasar.

United Nations that the station's altitude was decaying at a rate of about 160
meters (0.1 mile) per day. Its orbit is
inclined 42.8° to Earth's equator, which
means its reentry could occur anywhere
between latitudes 43°N and 43°S.
"Even shortly before reentry, only a
very large time and geographical window can be estimated," says Holger Krag
(ESA). Variable atmospheric drag makes
predictions uncertain.
The Inter-Agency Space Debris Coordination Committee, a forum of 13
space agencies (including CNSA) that
coordinates activities related to space
debris mitigation and management, will
be monitoring the reentry to improve
prediction accuracy.
■ DAVID DICKINSON

■ CAMILLE M. CARLISLE

First Light for SkySweeping Telescope
The Zwicky Transient Facility has taken
its first image, capturing a chunk of
Orion, including the Orion, Flame, and
Horsehead nebulae, in a single shot that
covers a 47-square-degree field of view
with 24,000 by 24,000 pixels. This image
precedes the facility's science phase, set
to begin in February and be completed
by 2020. The ZTF is installed on the 48inch Samuel Oschin Telescope and is the
sequel to the Palomar Transient Factory
(PTF). ZTF scans the sky 12 times faster
than PTF, recording 3,750 square degrees
per hour at its full speed. It could cover
the entire sky in a single, good night, but
astronomers plan to repeatedly observe
some regions of sky to catch transient
celestial objects that vary on a shorter
cadence, such as supernovae. As a result,
the facility will scan the full sky every
three nights. See the first-light image and
read more about future plans for ZTF at
https://is.gd/ﬁrstlightZTF.

p Artist's concept of Tiangong 1 in orbit

■ MONICA YOUNG
sk yandtelescope.com * M A RCH 2 018

13
https://www.is.gd/J1342quasar https://www.is.gd/firstlightZTF http://www.skyandtelescope.com
Sky and Telescope - March 2018

Table of Contents for the Digital Edition of Sky and Telescope - March 2018
