Sky & Telescope - September 2022 - 19

ing up at least 10 times as much. Digging into Bennu was
more like dipping a hand in water than penetrating solid rock.
" This material just doesn't seem to dissipate energy, " explains
Statler. " It's friction-free. It's strength-free. It doesn't cohere. "
This should perhaps have been less surprising than it was.
When a 2-kilogram copper impactor from
JAXA's Hayabusa 2 crashed into asteroid
Ryugu at 2 km/s more than a year earlier,
over 8 minutes passed before the crater finished
forming. " You would think this would
all just settle down and stop within a few seconds,
because you surely lose a lot of energy
just in the friction, " explains Statler. " But
instead, these little bits of rock kept pushing
on each other little piece even when it got down to less than
walking speed motion - it's just totally counterintuitive. "
Both Bennu and Ryugu are rubble-pile asteroids consist15%
ing
of a hodge-podge of large rocks and gravel, held together
by the asteroid's weak gravity rather than material strength.
If Dimorphos turns out to be similar, reconstructing DART's
collision becomes a challenge for theorists. Researchers traditionally
base impact simulations on known shock physics and
verify them through laboratory-based experiments here on
Earth. The codes are not built to handle this liquid-like, friction-free
material moving in slow motion, Michel explains.
As a result, experts have been developing simulations that
incorporate granular physics, accounting for the strange,
cohesionless material encountered on Bennu and Ryugu. " It
becomes very complex code, and therefore we need to make
sure we do it correctly, which is why DART and Hera are so
important, " he says.
With the real-world data that DART and Hera will offer,
physicists should finally have everything they need to confirm
the impact's beta value and reproduce exactly what happened
on Dimorphos. And if they can simulate the DART impact
accurately, then they can apply the same physics in simulations
of other asteroids and impacts, taking a lot of the guesswork
out of planning a real deflection mission.
What If . . . ?
A concern on many people's minds is that Dimorphos will
follow Bennu and Ryugu's lead and throw a curveball. What
if DART's crash produces a beta value models cannot explain?
In this case, asteroid impact science will need to go back to
the drawing board. " As a scientist, I would
like to be surprised by DART, " says Statler. " As
a planetary defender, I really don't. "
Another worry is that the DART impact
Fraction of nearEarth
asteroids
that are binaries
simply won't provide enough data. Crashing
into Dimorphos represents just one realworld
data point to verify a vast range of
impact scenarios. How can one test validate
an entire field of research?
In reality, it can't. All DART and Hera can really do is tell
us whether physicists' current understanding and simulations
are heading in the right direction. We'll need more missions
to other asteroids before any planetary defender can say with
confidence that we could successfully deflect a dangerous
asteroid headed for Earth.
Some people might see DART and Hera, and planetary
defense more generally, as a frivolous waste of money when
the chances of a large asteroid impacting Earth in the near
future are vanishingly small. But recent world events have
taught us that low-probability situations can and do have
huge, global consequences - and preparedness is key to
mitigating their impact. Michel feels it would be foolish to
put off planning for the worst: " We have the means and time
to prepare for what's necessary to offer security for future
generations, " he says. As the proverb goes: " Failing to plan is
planning to fail. "
ยข BENJAMIN SKUSE is a science writer based in Somerset,
United Kingdom.
Explore Sentry's list of potential hazardous objects: cneos.jpl.
nasa.gov/sentry. Find a list of observable asteroids and comets
(hazardous or not) at https://is.gd/ssd_observe.
LICIACube
r i g i n al o r b i t
N e w o r
HERA
3. A few minutes later, LICIACube flies safely by, observing the wreckage.
The smack robs Dimorphos of orbital energy, slightly shrinking its circuit
around Didymos (size change and elongation exaggerated for clarity).
4. In 2026, Hera and its two tagalong CubeSats arrive to investigate.
sk yand tele scope .o r g * SEPTEMBER 2022 19
O
b i t
http://www.cneos.jpl.nasa.gov/sentry http://www.cneos.jpl.nasa.gov/sentry https://www.is.gd/ssd_observe http://skyandtelescope.org

Sky & Telescope - September 2022

Table of Contents for the Digital Edition of Sky & Telescope - September 2022

Contents
Sky & Telescope - September 2022 - Cover1
Sky & Telescope - September 2022 - Cover2
Sky & Telescope - September 2022 - 1
Sky & Telescope - September 2022 - Contents
Sky & Telescope - September 2022 - 3
Sky & Telescope - September 2022 - 4
Sky & Telescope - September 2022 - 5
Sky & Telescope - September 2022 - 6
Sky & Telescope - September 2022 - 7
Sky & Telescope - September 2022 - 8
Sky & Telescope - September 2022 - 9
Sky & Telescope - September 2022 - 10
Sky & Telescope - September 2022 - 11
Sky & Telescope - September 2022 - 12
Sky & Telescope - September 2022 - 13
Sky & Telescope - September 2022 - 14
Sky & Telescope - September 2022 - 15
Sky & Telescope - September 2022 - 16
Sky & Telescope - September 2022 - 17
Sky & Telescope - September 2022 - 18
Sky & Telescope - September 2022 - 19
Sky & Telescope - September 2022 - 20
Sky & Telescope - September 2022 - 21
Sky & Telescope - September 2022 - 22
Sky & Telescope - September 2022 - 23
Sky & Telescope - September 2022 - 24
Sky & Telescope - September 2022 - 25
Sky & Telescope - September 2022 - 26
Sky & Telescope - September 2022 - 27
Sky & Telescope - September 2022 - 28
Sky & Telescope - September 2022 - 29
Sky & Telescope - September 2022 - 30
Sky & Telescope - September 2022 - 31
Sky & Telescope - September 2022 - 32
Sky & Telescope - September 2022 - 33
Sky & Telescope - September 2022 - 34
Sky & Telescope - September 2022 - 35
Sky & Telescope - September 2022 - 36
Sky & Telescope - September 2022 - 37
Sky & Telescope - September 2022 - 38
Sky & Telescope - September 2022 - 39
Sky & Telescope - September 2022 - 40
Sky & Telescope - September 2022 - 41
Sky & Telescope - September 2022 - 42
Sky & Telescope - September 2022 - 43
Sky & Telescope - September 2022 - 44
Sky & Telescope - September 2022 - 45
Sky & Telescope - September 2022 - 46
Sky & Telescope - September 2022 - 47
Sky & Telescope - September 2022 - 48
Sky & Telescope - September 2022 - 49
Sky & Telescope - September 2022 - 50
Sky & Telescope - September 2022 - 51
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Sky & Telescope - September 2022 - 54
Sky & Telescope - September 2022 - 55
Sky & Telescope - September 2022 - 56
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Sky & Telescope - September 2022 - 60
Sky & Telescope - September 2022 - 61
Sky & Telescope - September 2022 - 62
Sky & Telescope - September 2022 - 63
Sky & Telescope - September 2022 - 64
Sky & Telescope - September 2022 - 65
Sky & Telescope - September 2022 - 66
Sky & Telescope - September 2022 - 67
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Sky & Telescope - September 2022 - 70
Sky & Telescope - September 2022 - 71
Sky & Telescope - September 2022 - 72
Sky & Telescope - September 2022 - 73
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Sky & Telescope - September 2022 - 76
Sky & Telescope - September 2022 - 77
Sky & Telescope - September 2022 - 78
Sky & Telescope - September 2022 - 79
Sky & Telescope - September 2022 - 80
Sky & Telescope - September 2022 - 81
Sky & Telescope - September 2022 - 82
Sky & Telescope - September 2022 - 83
Sky & Telescope - September 2022 - 84
Sky & Telescope - September 2022 - Cover3
Sky & Telescope - September 2022 - Cover4
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