Sky & Telescope - August 2019 - 9

p Apollo 17 astronauts Eugene Cernan and
Harrison Schmitt zigzagged their rover up and
over the Lee-Lincoln fault scarp (arrows) that
cuts across the Taurus-Littrow Valley.

M OON: N ASA / GSFC / A RIZON A STATE UNIV.; O MEG A CEN TAURI: ESO / IN A F-VST / O MEG ACA M
/ ACK NOWLEDG EMEN T TO A . G R A DO, L. LIM ATOL A / IN A F- CA PODIM ON TE OBSERVATORY

when NASA's Lunar Reconnaissance
Orbiter started systematically mapping
the surface at high resolution. Thomas
Watters (Smithsonian Institution) and
colleagues estimate that the scarps are
younger than 50 million years old.

torn apart by our galaxy's gravity, most
of its stars would have streamed away,
looping around the galaxy. Rodrigo
Ibata (University of Strasbourg,
France) and colleagues reported new
evidence for this theory April 22nd
in Nature Astronomy: the long-sought
detection of a stellar stream belonging
to Omega Centauri.
The European Space Agency's Gaia
satellite provides precise distances to
and movements of more than a billion
stars. When stars that are near one
another also move together - especially if they're outside the galactic
disk - they're probably part of a stellar
stream. To ﬁnd these stellar groupings,
Ibata's team used a computer algorithm
called Streamﬁnder to pick out more
than a dozen new streams.
One of these, dubbed Fimbulthul
(for one of the 11 rivers that coursed

But are the faults these scarps are
associated with still active today? The
researchers used a new computer algorithm to better estimate the locations of
the quakes' epicenters, and their analysis revealed that eight of the quakes
were centered within 30 kilometers
(19 miles) of a fault scarp. Moreover, six
of these quakes occurred during lunar
apogee, when the Moon's elliptical orbit
takes it farthest from Earth. (In fact, 18
of the 28 shallow shakes occurred close
to apogee.) The connection with apogee
is important, because that's when
Earth's tidal pull per lunar surface area
is largest. This gravitational stress is
what can cause the faults to slip, resulting in the tremors observed.
Brigitte Knapmeyer-Endrun (University of Cologne, Germany), who was not
involved with the current study, agrees
the result is statistically signiﬁcant. If
the results pan out, they'd point to a
Moon that is still tectonically active
more than 4 billion years after its
formation. The results could also have
practical implications for lunar exploration, such as avoiding high-risk areas
when planning permanent structures.
■ JAVIER BARBUZANO

through the primordial void in Norse
mythology), contains 309 stars
across 18° on the sky. The researchers calculated the stellar orbits, which
take the stars as close as 5,000 lightyears to the galactic center and as far
as 21,300 light-years - remarkably
similar to Omega Centauri's orbit.
Moreover, the range of heavy-element
abundances detected in Fimbulthul's
stars is consistent with stars in the
globular. Computer simulations show
that Fimbulthul could be the trailing
arm of stars that our galaxy's gravity
has pulled from Omega Centauri.
The researchers acknowledge that
their computer simulations are still
simple and not a perfect match to the
data. Future plans include working on
more realistic models to better understand the Fimbulthul stream's origins.

IN BRIEF
Beresheet Crash-lands
on the Moon
Israeli company SpaceIL attempted
to land Beresheet (Hebrew for "in the
beginning") on the Moon on Thursday,
April 11th. Launched on February 22,
2019, the Beresheet mission took six
weeks to reach the Moon, using a series of orbital boosts that elongated its
orbit for capture by the Moon's gravity
on April 4th. While the first phases of
the descent went off without a hitch,
the lander began having trouble with
its main engine. Although the team was
able to re-establish contact and restart
the engine at an altitude of about 150
meters (490 feet), it was too late. Final
telemetry showed that the lander was
still going 1,080 m/s (2,400 mph) when
it slammed at a low angle into the
lunar surface. The Lunar Reconnaissance Orbiter later imaged the crash
site at the edge of Mare Serenitatis.
Beresheet cost only $100 million
to build and was funded mostly by
private donors. Beresheet 2.0 is now
under way, and Peter Diamandis of
the XPRIZE foundation has already
pledged $1 million toward the new mission's development.
■ DAVID DICKINSON

Omega Centauri

■ MONICA YOUNG
sk yandtele scope.com * AUGU S T 2 019

http://www.skyandtelescope.com

Sky & Telescope - August 2019

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