Sky & Telescope - September 2022 - 17

DART will have two witnesses to this manufactured
cosmic event: the impactor's own highresolution
imager, named Didymos Reconnaissance
and Asteroid Camera for Optical navigation
(DRACO), and a small Italian CubeSat called
the Light Italian CubeSat for Imaging of Asteroids
(LICIACube) that will piggyback the DART
spacecraft.
DRACO performs two roles, supporting the
autonomous guidance system to ensure DART
hits its target and providing essential data for
the analysis and interpretation of the results.
It will snap images of Didymos and Dimorphos
on approach to measure their size and shape, as
well as images of Dimorphos' surface in order to
characterize the impact site just before the probe's
annihilation.
Meanwhile, LICIACube will have detached from DART
10 days prior to impact and altered its trajectory to lag
just under 3 minutes behind the main spacecraft, so as to
fly safely past the target. As the CubeSat sweeps past the
Didymos system, just 55 km away at its closest approach, its
two optical cameras - dubbed LICIACube Unit Key Explorer
(LUKE) and LICIACube Explorer Imaging for Asteroid (LEIA)
- will witness the impact itself, the initial ejecta plume and
beginnings of the impact crater, and finally the backside of
both Didymos and Dimorphos as it speeds away. With no
way to slow down, the sole survivor of the DART impact will
continue travelling through the celestial abyss, doomed to
purposelessly circle the Sun.
But back on Earth, the DART team's excitement will only
now be reaching its zenith. Some of the world's most powerful
telescopes will be trained on the Didymos system before
and after impact. They will be measuring how the system's
brightness changes with time, plotted in a diagram called a
DRACO
DART The spacecraft
has a solar wingspan of
nearly 20 meters and both
hydrazine and xenon propulsion
systems. (The latter
is a technology demonstration
of an upgrade to the
system flown on NASA's
Dawn spacecraft.) The
DRACO imager sits on the
spacecraft's front. The small
box sitting on DART carries
LICIACube, which will pop
out en route to Dimorphos.
DIDYMOS SYSTEM Captured by the Arecibo Observatory
in 2003, these radar images are the best views we
have of Didymos and its moon, Dimorphos.
light curve. Dips in the light curve indicate when
the smaller moonlet passes in front of (or is hidden
behind) Didymos from Earth's point of view,
giving a precise gauge of the orbital period.
" When we see the binary orbit start going off
schedule in the data, that's when we know we
have, in fact, changed the motion of a celestial
body in space for the first time, " says Statler.
" That's the goosebumps moment for me. " With
initial estimates of how much DART changed
Dimorphos' orbit expected around two weeks
after impact, the NASA mission will be complete.
Uncertain Beta
The collision should slow the moonlet down and kick it into a
slightly lower orbit with a shorter period. Although the deflection
will be declared a success if DART shoves Dimorphos'
orbit off schedule by the minimum amount deemed detectable
from Earth - a measly 73 seconds - Statler is more optimistic:
" We're expecting a change in period in the vicinity of
10-ish minutes, " equivalent to bringing the asteroids' nearest
approach 22 meters closer. " But this is why we have to do the
test, because we don't know. "
Uncertainty can be boiled down to a single figure, beta (β).
Beta is a number that compares the momentum of Dimorphos
before and after the collision. More specifically, it is a
measure of how much additional momentum beyond what's
carried by the spacecraft itself is transferred to the asteroid.
If beta equals 1, DART will have a perfect inelastic collision
with Dimorphos, adding all of DART's momentum to the
asteroid. But what mission scientists are hoping for is that
DART's momentum gets a boost from a huge plume of ejecta
blasted off the surface. " It works like an extra, instantaneous
little rocket engine that pushes the asteroid in the other
direction, " explains Statler. " By having this ejecta help you,
you get beta greater than 1. "
But calculating beta is no simple task. It consists of many
factors linked to the properties of both the impactor and
asteroid in question. And right now, most of Dimorphos'
properties are completely unknown. Is it shaped like a potato,
a donut, a die, or some other wacky configuration? Is it solid
rock? Basalt? Is it porous or dense, cracked or solid, strong
or weak, tough or brittle? What happens if the probe hits the
moonlet at a different location or angle than simulated? All
these factors play a part in determining the beta value.
Depending on the best guesses physicists choose, simulaLICIACube
tions
suggest the beta of the DART collision could range from
a little extra push (β = 1.5) to a huge shove (β = 5), Statler
says. And this is a problem. If ever we need to deflect an asteroid
from Earth's path, we will require more precise simulations,
to tell us if our efforts are likely to succeed or fail.
sk yand tele scope .o r g * SEPTEMBER 2022 17
RADAR IMAGE OF SYSTEM: NAIDU ET AL. / ICARUS 2020 AND NASA;
DART: NASA / JHU APL
http://skyandtelescope.org
Sky & Telescope - September 2022

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