Sky & Telescope - July 2023 - 9

EJECTA PLUME: SCIENCE: NASA / ESA / STSCI, JIAN-YANG LI (PSI); IMAGE PROCESSING:
JOSEPH DEPASQUALE (STSCI); ARTIST'S CONCEPT OF DISK AROUND V883 ORIONIS: ALMA
(ESO / NAOJ / NRAO), B. SAXTON (NRAO / AUI / NSF)
SOLAR SYSTEM
Measuring the Aftermath
of an Asteroid Collision
A SET OF STUDIES published March
1st in Nature recount the aftermath of
the September 26, 2022, intentional
collision between NASA's Double Asteroid
Redirection Test (DART) spacecraft
and Dimorphos, the moon of the nearEarth
asteroid 65803 Didymos.
A team led by Terik Daly (Johns
Hopkins Applied Physics Laboratory)
employed ground-based radar observations
and images from DART's onboard
camera to make the first estimate of
Dimorphos's density: 2,100 to 2,700 kg
per cubic meter. That's roughly half the
density of Earth, indicating the moonlet's
rubble-pile nature.
A team led by Christine Thomas
(Northern Arizona University) used
visible-light and radar data to narrow
down the change in Dimorphos's orbital
period to 33 minutes, give or take 1
minute. (This result is more precise
than previous measurements, which
had an uncertainty of 2 minutes.)
COSMOCHEMISTRY
Some Water on Earth Might Predate the Solar System
ASTRONOMERS HAVE DISCOVERED
that some of Earth's water could predate
the Sun.
Observing a planet-forming disk
around the infant star V883 Orionis,
John Tobin (National Radio Astronomy
Observatory) and colleagues report in
the March 9th Nature the chemical
composition of its water vapor.
Water is arguably Earth's most distinctive
feature, but where and how it
formed is still up for debate (S&T: Mar.
2023, p. 34). The team's observations
- made with the Atacama Large Millimeter/submillimeter
Array in Chile
- provide a missing link, showing that
habitable planets can inherit a portion
of their water chemically unchanged
from the interstellar medium.
Most water molecules are a marriage
of two hydrogen atoms and one
oxygen atom. Yet sometimes one of the
hydrogen atoms is replaced by an atom
of deuterium - an isotope of hydrogen
that contains a neutron in its nucleus.
The ratio of the two varieties of water
depends on the conditions under which
they formed.
The ratio measured in the disk of
V883 Orionis is roughly the same as
in star-forming gas clouds and only
slightly higher than in solar system
comets. The result suggests water molecules
remain largely unaltered as they
pass from the interstellar medium and
into planetary bodies.
What this means for Earth's water
is still unclear - our planet's oceans
have a much lower deuterium/hydrogen
ratio, so comets may only have contributed
a small fraction of the water.
Tobin's team was able to make the
measurements of V883 Orionis because,
while the water in most planet-forming
pThis cutaway diagram shows an artist's concept
of the disk encircling the young star V883
Orionis (water vapor is blue).
disks is frozen out as ice, the young
star is undergoing a dramatic outburst,
heating the disk and turning its water
from ice to gas.
The next step for Tobin's team is
to look at other young systems using
next-gen facilities such as the Extremely
Large Telescope to better understand
planet-forming disks.
¢COLIN STUART
sk yand tele scope .o r g * JULY 2023
9
These three panels from the Hubble Space
Telescope capture the post-impact breakup of
asteroidal moon Dimorphos.
These characterizations of Dimorphos
enabled Andrew Cheng (also at
Johns Hopkins APL) and colleagues to
show that the material knocked from
Dimorphos's surface during the impact
ended up transferring more momentum
to Dimorphos than the DART spacecraft
did by itself. From images of the debris
plume from the accompanying LICIACube
satellite and other telescopes, they
find that the ejected material bolstered
DART's effect by a factor of 3.61.
Observers at large telescopes weren't
the only ones watching the DART
impact. Ariel Graykowski (SETI Institute)
led a study of Dimorphos's light
curve before, during, and after the
impact, which was made possible by the
30 citizen scientists listed as coauthors
on the paper. These observers, located
across five continents, submitted data
from their Unistellar eVscopes. From
the light curve, Graykowski's team
inferred the mass, speed, and energy of
the ejecta, confirming Cheng's results.
Didymos-Dimorphos System
Didymos-Dimorphos System
Didymos-Dimorphos System
Sep 27 01:06:21
Sep 27 01:06:21
Sep 27 01:06:21
Ejecta cone
Ejecta cone
Ejecta cone
Sep 28 17:06:51
Sep 28 17:06:51
Sep 28 17:06:51
Curved ejecta stream
Curved ejecta stream
Curved ejecta stream
Oct 08 19:62:10
Oct 08 19:62:10
Oct 08 19:62:10
Double tail formation
Double tail formation
Double tail formation
They estimate the ejecta carried only
0.3% to 0.5% of Dimorphos's total
mass, leaving the moonlet altered but
still intact.
¢LAUREN SGRO
http://skyandtelescope.org
Sky & Telescope - July 2023

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