Image of the asteroid Dimorphos, with compass arrows, scale bar, and color key for reference. The north and east compass arrows show the orientation of the image on the sky. Note that the relationship between north and east on the sky (as seen from below) is flipped relative to direction arrows on a map of the ground (as seen from above). Credits: NASA, ESA, David Jewitt (UCLA); Alyssa Pagan (STScI)
The popular 1954 rock song
"Shake, Rattle and Roll," could be the theme music for the Hubble
Space Telescope's latest discovery about what is happening to the asteroid
Dimorphos in the aftermath of NASA's DART (Double Asteroid Redirection Test)
experiment. DART intentionally impacted Dimorphos on September 26, 2022, slightly
changing the trajectory of its orbit around the larger asteroid Didymos.
Astronomers using Hubble's
extraordinary sensitivity have discovered a swarm of boulders that were
possibly shaken off the asteroid when NASA deliberately slammed the half-ton
DART impactor spacecraft into Dimorphos at approximately 14,000 miles per hour.
The 37 free-flung boulders range in
size from three feet to 22 feet across, based on Hubble photometry. They are
drifting away from the asteroid at little more than a half-mile per hour –
roughly the walking speed of a giant tortoise. The total mass in these detected
boulders is about 0.1% the mass of Dimorphos.
"This is a spectacular
observation – much better than I expected. We see a cloud of boulders carrying
mass and energy away from the impact target. The numbers, sizes, and shapes of
the boulders are consistent with them having been knocked off the surface of
Dimorphos by the impact," said David Jewitt of the University of
California at Los Angeles, a planetary scientist who has been using Hubble to
track changes in the asteroid during and after the DART impact. "This
tells us for the first time what happens when you hit an asteroid and see
material coming out up to the largest sizes. The boulders are some of the
faintest things ever imaged inside our solar system."
Jewitt says that this opens up a
new dimension for studying the aftermath of the DART experiment using the
European Space Agency's upcoming Hera spacecraft, which will arrive at the binary
asteroid in late 2026. Hera will perform a detailed post-impact survey of the
targeted asteroid. "The boulder cloud will still be dispersing when Hera
arrives," said Jewitt. "It's like a very slowly expanding swarm of
bees that eventually will spread along the binary pair's orbit around the
Sun."
The boulders are most likely not shattered pieces of the diminutive asteroid caused by the impact. They were already scattered across the asteroid's surface, as evident in the last close-up picture taken by the DART spacecraft just two seconds before collision, when it was only seven miles above the surface.
This is the last complete image of the asteroid Dimorphos, as seen by NASA's DART (Double Asteroid Redirection Test) impactor spacecraft two seconds before impact. The Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO) imager aboard captured a 100-foot-wide patch of the asteroid. The DART spacecraft streamed these images from its DRACO camera back to Earth in real time as it approached the asteroid. DART successfully impacted its target on September 26, 2022. Credits: NASA, APL
Jewitt estimates that the impact
shook off two percent of the boulders on the asteroid's surface. He says the
boulder observations by Hubble also give an estimate for the size of the DART
impact crater. "The boulders could have been excavated from a circle of
about 160 feet across (the width of a football field) on the surface of
Dimorphos," he said. Hera will eventually determine the actual crater
size.
Long ago, Dimorphos may have formed
from material shed into space by the larger asteroid Didymos. The parent body
may have spun up too quickly or could have lost material from a glancing
collision with another object, among other scenarios. The ejected material
formed a ring that gravitationally coalesced to form Dimorphos. This would make
it a flying rubble pile of rocky debris loosely held together by a relatively
weak pull of gravity. Therefore, the interior is probably not solid, but has a
structure more like a bunch of grapes.
It's not clear how the boulders
were lifted off the asteroid's surface. They could be part of an ejecta plume
that was photographed by Hubble and other observatories. Or a seismic wave from
the impact may have rattled through the asteroid – like hitting a bell with a
hammer – shaking lose the surface rubble.
"If we follow the boulders in
future Hubble observations, then we may have enough data to pin down the
boulders' precise trajectories. And then we’ll see in which directions they
were launched from the surface," said Jewitt.
The DART and LICIACube (Light
Italian CubeSat for Imaging of Asteroids) teams have also been studying
boulders detected in images taken by LICIACube’s LUKE (LICIACube Unit Key
Explorer) camera in the minutes immediately following DART's kinetic impact.
The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble and Webb science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.
Source: Hubble Sees Boulders Escaping from Asteroid Dimorphos | NASA
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