Sky & Telescope - July 2024 - 9

SIMULATION FRAME OF DART IMPACT: SABINA RADUCAN; NEW MOON OF URANUS: SCOTT SHEPPARD
SOLAR SYSTEM
DART Impact Reshaped Asteroid Dimorphos
WHEN NASA'S Double Asteroid Redirection
Test (DART) spacecraft whacked
the asteroid moon Dimorphos (companion
to the larger asteroid 65803
Didymos) on September 26, 2022, the
impact was powerful enough to excavate
about a tenth of the moonlet's
material. That conclusion shakes up our
understanding of rubble-pile asteroids.
NASA built DART to study what
would be necessary to redirect wayward
asteroids, by impacting the smaller
asteroid of the pair and watching the
subsequent change in orbital period
(S&T: Feb. 2023, p. 9). Dimorphos is a
" rubble pile, " a jumble of objects ranging
in size from boulders to fine grains
and, based on Hayabusa 2's much
smaller hit on rubble-pile 162173 Ryugu
(S&T: May 2020, p. 14), researchers had
expected the DART collision to produce
a crater, says Sabina Raducan (University
of Bern, Switzerland). Raducan led
a new analysis published on February
26th in Nature Astronomy.
SOLAR SYSTEM
New Moons for Uranus
and Neptune
DOZENS OF MOONS have been found
around Jupiter and Saturn over the past
two decades. However, since 2003, no
new moons had been discovered orbiting
Uranus, and only one new moon
was officially added for Neptune. Now,
three new finds, one for Uranus and
two for Neptune, bring their tallies to
28 and 16, respectively.
The International Astronomical
Union's Minor Planet Center
announced the new finds on February
23rd. They are the faintest moons
yet that ground-based telescopes have
found orbiting the ice giants. Team
lead Scott Sheppard (Carnegie Science)
acknowledges, " These objects are at the
edge of detectability. "
The one orbiting Uranus, provisionally
designated S/2023 U1, is only 8
kilometers (5 miles) across, probably
Yet, with little holding the rubble
pile together, the impact instead blasted
material all over. Images from the
CubeSat LICIACube, which was released
prior to DART's impact and photographed
the first few minutes afterward,
showed what looked like " a big ball of
sand, with very little binding between
the grains, " Raducan noted.
The team simulated what happened
in the following hour based on LICIACube's
initial observations as well as on
physical models. The simulation calculated
the outflying rubble's paths based
on the physics of shock propagation.
The team found that DART's hit, at
6 kilometers per second (more than
13,000 mph), blasted 500 million
kilograms from the surface - of which
50 million kg or so was wholly ejected
from the system.
Other models had suggested rubble
piles might contain denser insides,
perhaps with clusters of big boulders at
their cores, but the simulation instead
the planet's smallest known moon. It
has an orbit of 680 days.
The two new moons orbiting Neptune
had both been spotted earlier but
had not been observed long enough to
calculate their orbits. New observations
provided a firmer grasp on their
trajectories. Provisionally designated
S/2002 N5, the bigger and brighter
moon measures 23 kilometers across
and takes nine years to circle Neptune.
The smaller one, provisionally designated
S/2021 N1, is about 14 km across
and takes almost 27 years
to orbit Neptune - the
longest orbit known for any
planetary moon.
To find these three
moons, Sheppard had to
stack five-minute exposures,
which enabled him
to image faint objects while
keeping their motion from
blurring. These exposures
were repeated over a period
pThis frame comes from a simulation of the
DART impact's effects on Dimorphos.
suggests Dimorphos has a loose interior.
" They nailed it, " says Kevin Walsh
(Southwest Research Institute), who was
not involved in the study. " It's rubble all
the way down. "
The next step will be the October
launch of the European Space Agency's
Hera mission to the Didymos system.
Laden with cameras, other sensors, and
two CubeSats, Hera will study Didymos
and Dimorphos in detail when it arrives
in late 2026.
¢JEFF HECHT
of up to four or five hours over multiple
nights using some of the world's biggest
telescopes, such as the Magellan and
Very Large telescopes in Chile as well as
the Subaru and Gemini North telescopes
in Hawai'i.
Each of the three new moons has
an orbit that groups it with the orbits
of two larger moons. These groupings
probably indicate families formed by the
breakup of a larger, captured object.
Planetary scientists suspect that the
pThe new Uranian moon
S/2023 U1 appears unblurred
in an image from
the Magellan Telescope.
outer solar system may hold the key to
understanding the planets'
formation and early
evolution. Because of that,
the National Academy of
Sciences has put a high priority
on a Uranus Orbiter
and Probe concept. Such
a mission would also offer
close-ups of moons beyond
the reach of Earth's biggest
and best telescopes.
¢JEFF HECHT
sk yand tele scope .o r g * JULY 2024
9
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Sky & Telescope - July 2024

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