NASA images unlock complex history of two near-Earth asteroids
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[July 31, 2024]
By Will Dunham
WASHINGTON (Reuters) - In the moments before NASA's DART spacecraft
slammed into the asteroid Dimorphos in a landmark planetary defense test
in 2022, it took high-resolution images of this small celestial object
and its larger companion Didymos.
These images have enabled scientists to unravel the complicated history
of these two rocky bodies located in the vicinity of Earth and gain
insight into the formation of what are called binary asteroid systems -
a primary asteroid with a secondary moonlet orbiting it.
An analysis of the craters and surface strength on Didymos indicated it
formed about 12.5 million years ago. A similar analysis indicated
Dimorphos formed about 300,000 years ago. Didymos probably formed in our
solar system's main asteroid belt, between the planets Mars and Jupiter,
and then was knocked into the inner solar system, the researchers said.
An examination of the largest boulders on Didymos and Dimorphos gave
clues about the origins of the two asteroids.
"Both asteroids are aggregates of rocky fragments formed from the
catastrophic destruction of a parent asteroid," said astronomer Maurizio
Pajola of the National Institute for Astrophysics (INAF) in Italy, lead
author of one of five studies on the asteroids published on Tuesday in
the journal Nature Communications.
"These large boulders could not have formed from impacts on the surfaces
of Didymos and Dimorphos themselves, as such impacts would have
disintegrated these bodies," Pajola added.
Didymos, which has a diameter of about a half mile (780 meters), is
classified as a near-Earth asteroid. Dimorphos is roughly 560 feet (170
meters) wide. Both are "rubble pile" asteroids, comprised of pieces of
rocky debris that coalesced through the influence of gravity.
"Their surface is covered with boulders. The largest on Dimorphos is the
size of the school bus, while the largest on Didymos is big as soccer
field," said Olivier Barnouin, a planetary geologist and geophysicist at
the Johns Hopkins University Applied Physics Laboratory in Maryland and
lead author of another of the studies.
"There are cracks on the surface and the rocks of Dimorphos, while
Didymos may have finer-grained soils at the equator, although it is
difficult to be sure with the images we have. The surfaces of both
asteroids are weak, much weaker than loose sand," Barnouin added.
The researchers concluded that Dimorphos is composed of material that
flew off the equatorial region of Didymos due to the speed at which it
was spinning.
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Asteroid moonlet Dimorphos as seen by the DART spacecraft 11 seconds
before impact in this image taken by DART’s on board DRACO imager
from a distance of 68 kilometers, and released September 26, 2022.
NASA/Johns Hopkins APL/Handout via REUTERS/File Photo
"In the case of Didymos, it is thought that in the past, it rotated
faster around its axis due to the YORP effect (spin acceleration
driven by the effect of sunlight on its uneven surface), and thus
ejected the boulders from its equatorial region, forming Dimorphos,"
Pajola added.
Didymos currently spins at a rate of once every 2-1/4 hours.
Few boulders were observed at the equatorial region of Didymos.
"Its equator is much smoother, while mid-latitudes up to the poles
are much rougher, with big boulders sitting on the surface," Pajola
said.
The U.S. space agency's DART (Double Asteroid Redirection Test)
carried out a proof-of-principle mission, demonstrating that a
spacecraft could apply kinetic force to change the path of a space
object that otherwise might be on a collision course with Earth.
Didymos and Dimorphos do not pose an actual threat to Earth.
DART struck Dimorphos on Sept. 26, 2022, at about 14,000 miles per
hour (22,530 kph) at a distance of roughly 6.8 million miles (11
million km) from Earth, and succeeded in modestly changing its path.
The collision also slightly changed the shape of Dimorphos.
The DART data has improved the understanding of binary asteroid
systems.
"Binary asteroid systems represent about 10-15% of the total number
of asteroids that are in near-Earth space," Barnouin said. "More
generally, with every new observation of an asteroid or asteroid
system, we learn more about how asteroids form and evolve. They are
complex systems, but have some key similarities, especially when we
consider the smaller - less than a kilometer (0.62 mile) -
asteroids."
(Reporting by Will Dunham; Editing by Daniel Wallis)
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