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DART Impact Aftermath: Hubble Sees Boulders Escaping From Asteroid Dimorphos
TOPICS:AsteroidDARTHubble Space TelescopeNASANASA Goddard Space Flight
CenterSpace Telescope Science Institute
By SPACE TELESCOPE SCIENCE INSTITUTE JULY 23, 2023

Boulders Flung Off Asteroid Dimorphos
This Hubble Space Telescope image of the asteroid Dimorphos was taken on
December 19, 2022, nearly four months after the asteroid was impacted by
NASA’s DART mission (Double Asteroid Redirection Test). Hubble’s
sensitivity reveals a few dozen boulders knocked off the asteroid by the
force of the collision. These are among the faintest objects Hubble has
ever photographed inside the solar system. The 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 a little more than a
half-mile per hour. The discovery yields invaluable insights into the
behavior of a small asteroid when it is hit by a projectile for the
purpose of altering its trajectory. Credit: NASA, ESA, David Jewitt
(UCLA), Alyssa Pagan (STScI)

The 2022 DART mission impact rattled the asteroid’s surface
Sorry Chicken Little, the sky is not falling — at least not yet.

Wayward asteroids present a real collision hazard to Earth. Scientists
estimate that an asteroid measuring several miles across smashed into
Earth 65 million years ago and wiped out the dinosaurs, among other
forms of life, in a mass extinction. Unlike the dinosaurs, humanity can
avoid this fate if we begin practicing how to knock an Earth-approaching
asteroid off course.

This is trickier than how it has been depicted in science fiction movies
like Deep Impact. Planetary scientists first need to know how asteroids
were assembled. Are they flying rubble piles of loosely agglomerated
rocks, or something more substantial? This information would help
provide strategies on how to successfully deflect a menacing asteroid.

As a first step, NASA did an experiment to smash into an asteroid to see
how it is perturbed. The DART (Double Asteroid Redirection Test)
spacecraft impact on asteroid Dimorphos happened on September 26, 2022.
Astronomers using the Hubble Space Telescope continue following the
aftermath of the cosmic collision. A surprise is the discovery of
several dozen boulders lifted off the asteroid after the smashup. In
Hubble pictures, they look like a swarm of bees very slowly moving away
from the asteroid. This might mean that smacking an Earth-approaching
asteroid might result in a cluster of threatening boulders heading in
our direction.

Hubble Photographs Boulders Flung Off Asteroid Dimorphos
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).
The bright white object at lower left is Dimorphos. It has a bluish dust
tail extending diagonally to the upper right. A cluster of blue dots
(marked by white circles) surrounds the asteroid. These are boulders
that were knocked off the asteroid when, on September 26, 2022, NASA
deliberately slammed the half-ton DART impactor spacecraft into the
asteroid as a test of what it would take to deflect some future asteroid
from hitting Earth. Hubble photographed the slow-moving boulders using
the Wide Field Camera 3 in December 2022. The color results from
assigning a blue hue to the monochromatic (grayscale) image.
Credit: NASA, ESA, David Jewitt (UCLA), Alyssa Pagan (STScI)

Hubble Space Telescope Sees Boulders Escaping from Asteroid Dimorphos
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.

Surface Image of Asteroid Dimorphos
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.
Credit: NASA, APL

“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.”

NASA DART Spacecraft Prior to Impact
This illustration depicts NASA’s Double Asteroid Redirection Test (DART)
spacecraft prior to impact at the Didymos binary asteroid system.
Credit: NASA/Johns Hopkins APL/Steve Gribben

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.

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.