Sky & Telescope - June 2023 - 8

NEWS NOTES
Soon after the DART impact,
SOLAR SYSTEM
Amateurs Use
Occultations to
Monitor Asteroid
Impact
AMATEUR ASTRONOMERS around the
globe have chased down the shadows of
the Didymos-Dimorphos asteroid system
as it passed in front of, or occulted,
distant stars, thereby helping scientists
evaluate the effects of NASA's Double
Asteroid Redirection Test.
The DART mission intentionally ran
into Dimorphos, the moon of nearEarth
asteroid 65803 Didymos, on
September 26, 2022 (S&T: Jan. 2023,
p. 8). The collision was designed to test
planetary-defense strategies by changing
the moonlet's orbit.
The DART team has measured the
change in Dimorphos' orbit around
Didymos using ground- and space-based
telescopes. But stellar occultations
enable measurements not yet possible
by other means.
One of the groups following up on
the impact is a French-Greek collaboration
known as the Asteroid Collaborative
Research via Occultation Systematic
Survey (ACROSS), initiated with the
SOLAR SYSTEM
A Dozen More Moons
for Jupiter
THE BIGGEST PLANET in the solar
system now has the largest family of
moons. The International Astronomical
Union's Minor Planet Center has
published orbits for 12 previously
unreported moons of Jupiter, based
on observations from Scott Sheppard
(Carnegie Institution for Science).
The newest additions bring the list
of Jovian moons to 92, a hefty 15%
increase from the previous tally of 80.
The new finds put Jupiter's lunar family
8
JUNE 2023 * SKY & TELESCOPE
pThis image, taken from 920 km (570 mi) away,
shows the asteroid Didymos (bottom right) and
its moonlet, Dimorphos, about 2.5 minutes before
the impact of NASA's DART spacecraft.
support of the European Space Agency
(ESA). ACROSS ties together the professional
and amateur communities in
the hunt for stellar occultations by the
Didymos system. While professionals
predict when and where to observe, volunteers
across continents conduct most
of the observations. Every time this
asteroid system blocks a background
star, observers gather to watch - sometimes
traveling hundreds of kilometers
to the narrow path of the pair's shadow.
Their data help trace the shadow and
pin down the asteroids' orbits.
Calculating the path along which the
asteroids' shadow falls can be tricky.
Sometimes amateurs see nothing at all.
Bad weather and orbital uncertainties
plagued early attempts, starting in midJune
last year. Such misses can serve to
rework the orbital model.
well ahead of Saturn's 83 confirmed
moons, at least for now.
All the new moons are small and
far out, taking more than 250 days to
orbit Jupiter. Nine of the 12 are among
the 71 outermost Jovian moons. Jupiter
probably captured these moons, as evidenced
by their retrograde orbits.
Three of the new moons are among
13 others that orbit Jupiter in a prograde
direction, between the orbits of
the large, close-in Galilean moons and
the far-out retrograde moons. The new
discoveries add to two known prograde
groups: Two additions go to the Himalia
group between 11 and 12 million km
Didymos began crossing the Milky
Way's galactic plane, which meant it
was passing in front of many brighter
stars. Observers used mobile stations
to successfully record 19 occultations
between October 15, 2022, and January
21, 2023, with multiple detections for
most of these events. In three of these,
observers even detected Dimorphos,
which spans only 160 meters (520 feet),
making it the smallest object observed
during an occultation.
The background stars involved
ranged from magnitude 9 to 13.5,
visible to telescopes between 4.5 and
14 inches in aperture. In each case, the
background star blinked out for less
than half a second; observers used highspeed
cameras and GPS to accurately
time the disappearances.
ACROSS observations have enabled
a measurement of the impact's effect
on the system's orbit around the Sun,
a valuable constraint for ESA's Hera
mission, which will rendezvous with
the asteroid system in 2026. The team
found that the impact changed Didymos'
velocity by 1 to 3 meters per day.
As the first occultation season has
come to an end, the ACROSS team, led
by Paolo Tanga (Observatory of Côte
d'Azur, France) and Kleomenis Tsiganis
(Aristotle University of Thessaloniki,
Greece), is now preparing for the next
one, which starts in April 2024.
¢DAMYA SOUAMI
from Jupiter, and one to the more distant
Carpo group at 17 million km.
But searches for prograde moons outside
these groups turned up nothing. In
the yawning gap between the Galilean
moons and the Himalia group, there's
only one moon, and it's already known:
the 9-kilometer Themisto. " We have
searched very deeply for objects near
Themisto and have found nothing else
to date, " says Sheppard. He says glare
from Jupiter is strong enough to hide
anything smaller than 3 km across.
Stay tuned for more, as Sheppard still
has observations awaiting approval.
¢JEFF HECHT
DIDYMOS ASTEROID SYSTEM: NASA / JOHNS HOPKINS APL
Sky & Telescope - June 2023

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