NEWS NOTES
STARS
Astronomers Use Asteroids
to Measure the Stars
ASTRONOMERS USUALLY GLEAN
information about an asteroid’s shape
and size during an occultation, when the
rock briefl y blocks a background star.
Now, astronomers are using occulta-
tions to measure the stars themselves.
The four 12-meter telescopes that
make up the Very Energetic Radia-
tion Imaging Telescope Array System(VERITAS) in Arizona are designed
to watch for the faint blue fl ashes of
Cherenkov radiation, produced when
gamma rays crash into Earth’s atmo-
sphere. But Wystan Benbow (Center for
Astrophysics, Harvard & Smithsonian)
and colleagues instead decided to try
using the telescopes to measure stellar
diameters during asteroid occultations.
They published the results April 15th in
Nature Astronomy.
When an asteroid passes in front of
a star, the edges of the shadow it casts
are lined by so-called diffraction fringes,
where light waves interact to alternately
boost or cancel the signal. So, imme-
diately before and after an asteroid
blocks out a background star entirely,
the star’s brightness will vary in a pre-
dictable way. The VERITAS telescopes’
large collecting areas and incredible
time resolution enable astronomers
to distinguish these small changes in
brightness from the blurring effect of
Earth’s atmosphere. By comparing the
observed fringes to those from a true
point source, Benbow’s team inferred
the diameters of two stars.
First, the asteroid 1165 Imprinetta
passed in front of the 10.2-magnitudeq This artist’s concept exaggerates the diffrac-
tion pattern of a distant star as it’s occulted by
an asteroid.star TYC 5517-227-1. In this proof-of-
principle observation, the astronomers
used VERITAS to snap 300 images a sec-
ond, pegging the star’s angular size at
0.125 milliarcsecond. Given its distance
of 2,700 light-years, that’s equivalent
to between 9 and 12.9 times the Sun’s
diameter. From its size, and follow-up
observations to measure its spectrum,
the researchers conclude the star must
be a red giant.
A second chance for the team came
when the asteroid 201 Penelope swept
in front of 9.9-magnitude star TYC
278-748-1. This time VERITAS captured
2,500 images every second, giving an
angular size of 0.094 milliarcsecond. At
the star’s distance of 700 light-years, it
must have a girth about twice the Sun’s.
A spectrum confi rms the star is in the
Sun’s G class.
The uncertainty in asteroid orbits
makes predicted paths uncertain, too,
so observing asteroid occultations with
large, unportable telescopes is diffi cult.
Nevertheless, the researchers fi gure, any
telescope capable of observing a 10th-
magnitude star should see on average
about fi ve occultations per year.
■ DAVID DICKINSON12 AUGUST 2019 • SKY & TELESCOPE
IN BRIEF
First Marsquake Detected
After two months of science operations, In-
sight’s Seismic Experiment for Interior Struc-
ture (SEIS) picked up its fi rst clear quake.
Insight Principal Investigator Bruce Banerdt
(NASA JPL) announced the detection on April
23rd at the annual meeting of the Seismo-
logical Society of America. The trembling
may have originated from heat loss from the
Martian interior, or it could have come from a
meteorite impact that reverberated inside the
planet. The signal lasted around 10 minutes
and had a magnitude of 2 to 2.5, a shaking
so slight that humans wouldn’t have felt it if it
had happened on Earth. SEIS has recorded
three other signals that could also be mars-
quakes, but researchers are still working to
rule out other possible causes. Depending
on how many of these tentative signals are
confi rmed as real, the data collected so far
suggest that SEIS may see 5 to 18 mars-
quakes per year.
■ JAVIER BARBUZANOChang’e 4 Explores the
Farside of the Moon
Since China’s Chang’e 4 spacecraft landed in
Von Kármán Crater on January 3rd (S&T: Apr.
2019, p. 9), its Yutu 2 rover has been measur-
ing the spectra of sunlight refl ecting off the
crater fl oor and the huge South Pole-Aitkin
basin in which the crater resides. In the May
16th Nature, Chunlai Li (Chinese Academy of
Sciences) and colleagues report the spectral
fi ngerprints of olivine and low-calcium pyrox-
ine, two minerals that probably originated in
the lunar mantle. The presence of mantle ma-
terial is expected; scientists fi gured that the
impact that carved out the largest and oldest
basin on the Moon would have penetrated
the crust and excavated the mantle beneath
it. However, this is the fi rst time a mission has
actually sampled mantle material. The fi nd
sheds light on the mantle’s composition —
which may contain the two minerals in equal
parts — and, ultimately, on the nature of the
magma ocean that once enveloped the Moon.
■ MONICA YOUNGThe Origin of Saturn’s
Inner Moons
At least fi ve of the small moons dancing
among Saturn’s rings likely formed as icy ring
material built up around tiny cores. That’s
the scenario favored by Bonnie Buratti (Jet
Propulsion Laboratory) and colleagues March
28th in Science. The scientists came to this
conclusion using images and other data taken
by the Cassini spacecraft as it whizzed close
by the moons Pan, Daphnis, Atlas, Pandora,
and Epimetheus fi ve times between Decem-
ber 2016 and April 2017. The fl ybys were part
of the “Ring-Grazing” phase of the space-
craft’s fi nal orbits. The observations show
that the moons have low density and porous
surfaces. These properties support the idea
that the moons were created by the buildup of
icy ring material, perhaps around the shards
of an earlier moon that broke up near Saturn.
Ridges around the equators of Pan and Atlas,
which give them a ravioli-like appearance, are
also likely accreted ring material.
■ CAMILLE M. CARLISLE DESY
/^ LUCID^BE
RLIN