Sky & Telescope - USA (2019-08)

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skyandtelescope.com • AUGUST 2019 9

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 — espe-

cially if they’re outside the galactic

disk — they’re probably part of a stellar

stream. To fi nd these stellar groupings,

Ibata’s team used a computer algorithm

called Streamfi nder to pick out more

than a dozen new streams.

One of these, dubbed Fimbulthul

(for one of the 11 rivers that coursed

Omega Centauri

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.

through the primordial void in Norse

mythology), contains 309 stars

across 18° on the sky. The research-

ers calculated the stellar orbits, which

take the stars as close as 5,000 light-

years 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 under-

stand the Fimbulthul stream’s origins.

■ MONICA YOUNG

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 se-

ries of orbital boosts that elongated its

orbit for capture by the Moon’s gravity

on April 4th. While the fi rst 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 Reconnais-

sance 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 mis-

sion’s development.

■ DAVID DICKINSON

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.

But are the faults these scarps are

associated with still active today? The

researchers used a new computer algo-

rithm to better estimate the locations of

the quakes’ epicenters, and their analy-

sis 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, result-

ing in the tremors observed.

Brigitte Knapmeyer-Endrun (Univer-

sity of Cologne, Germany), who was not

involved with the current study, agrees

the result is statistically signifi cant. 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 explora-

tion, such as avoiding high-risk areas

when planning permanent structures.

■ JAVIER BARBUZANO