Preface: Greetings,
I’m Justin Atchison, an aerospace engineer at the Johns Hopkins University
Applied Physics Laboratory. I’m excited to be guest-writing an article about the
Double Asteroid Redirection Test (DART). I recently presented some research
about DART at the International Astronautical
Congress, which I was able to attend thanks to a travel fellowship through
the Future Space Leaders
Foundation (FSLF). I’d strongly encourage any student or young-professional
(under 35) to apply for this grant next year. It’s a great opportunity to attend
this premier conference and interact with a variety of leaders in the aerospace
field. FSLF also hosts the Future Space Event on Capitol Hill each summer, which offers engagement
with US Congress and aerospace executives on the latest and most relevant
space-related topics.
AIDA
The Asteroid Impact Deflection Assessment (AIDA) mission is
a proposed joint program between NASA and ESA to study and demonstrate kinetic
asteroid deflection as a means of Earth impact mitigation [[1]]. Basically, we want to understand
what’s involved with using a high speed impacting spacecraft to change the
orbit of an asteroid that is (hypothetically) threatening Earth. DART is the proposed NASA component of AIDA.
Its role is to hit a small asteroid at high enough velocity to create a
measurable deflection. The proposed ESA component of AIDA is called the Asteroid Impact Mission (AIM), and its role
is to rendezvous in advance of DART and study the pre- and post-impact asteroid
conditions in-situ. The final
component of AIDA is Earth-based observations.
 |
| AIDA consists of DART, AIM, and Earth Based Observations. AIM will rendezvous with the binary asteroid Didymos and observe as DART impacts the smaller secondary asteroid. |
What makes this concept unique is that the target is a
binary asteroid system, specifically Didymos (1996 GT). Didymos was one of the
first Near Earth binary asteroids to be discovered; we now estimate that
roughly 15% of all Near Earth Asteroids (NEAs) are binary systems [[2]].
The primary body of the Didymos system is about 800 m in diameter, and the
secondary body (playfully called “Didymoon” on the mission) is only 170 m in
diameter [[3]]. We want to impact Didymoon using
the DART spacecraft and measure the change in orbit about the primary.
Here’s the point—the binary system represents a sensitive
experimental setup. The DART impactor is about 300 kg and will impact the
asteroid at nearly 7 km/s. That is a lot of
linear momentum, but we estimate that it will only impart ~1 mm/s to Didymoon.
For a single asteroid, the heliocentric velocity is ~30 km/s. Measuring 1 mm/s
out of 30 km/s (1 part in 10,000,000) would be exceedingly difficult. You could
pick an even smaller target, so that you impart a higher amount of momentum,
but then you have the challenge of trying to observe a very small body at long
ranges. However, in the case of a binary, the velocity of Didymoon with respect
to the primary is only about 10 cm/s. The effective signal-to-background is now
just 1 in 1000. (Ok…the order-of-magnitude argument isn’t entirely accurate
here, but it communicates the point.) Further, Didymoon is observable at long
ranges as a signature in the system’s light-curve. In fact, that’s how it was
first confirmed to be a binary system.
Why Didymos?
Of the known binary asteroids, Didymos makes an excellent
candidate target for the experiment for a few reasons:
Accessibility – Didymos falls in a class of
asteroids that requires less total ∆V with which to rendezvous than the Moon.
That means that AIM can be kept relatively small and affordable. Likewise,
there are very feasible trajectories for DART to impact with high relative
velocities.
 |
Orbits
of Earth and Didymos from above the ecliptic (top), and from the “side” along
the ecliptic y-axis [[4]].
The dashed line shows the line where the two orbit planes intersect. Didymos is
inclined about 3.4 degrees relative to Earth’s orbit plane.
|
 |
| Earth-Didymos range between 2000 and 2050. The 2022 conjunction is especially close. |
The Experiment
Although I described the kinetic impact measurement in terms
of velocity, we’re really going to be measuring the change in orbit period of Didymoon.
This approach has the added advantage that the measurement sensitivity
“integrates” with time. After enough time, the phase of Didymoon will be
clearly different from what one would predict without the impact. Within
practical limits, the longer we watch, the more clear the effect becomes. (The
practical limits are associated with the native acceleration environment and
the quality of the pre-impact characterization.)
At this point, if you’re like me, you’re probably wondering
why there’s any uncertainty in the imparted momentum. (In fact, there’s a Reddit
thread dedicated to this exact question). After all, didn’t high school
physics pretty clearly describe this type of event using inelastic collisions
and conservation of linear momentum? The one word answer is: hypervelocity.
That’s meant to say that the velocity of the impact, 7 km/s, is significantly
higher than the speed-of-sound in an asteroid. Bear in mind that a bullet travels
at something like 1 km/s. When DART
impacts the surface, its energy will propagate into the asteroid where it will
generate a new crater. Some fraction of the material from the crater will be
ejected off the asteroid at high speed. This ejected material acts like a thruster,
imparting additional momentum to Didymoon. Total momentum is conserved…but
Didymoon gets more than DART’s momentum added to it because it’s ejecting some
of its material.
The amount of momentum amplification is called β and it
depends principally on the speed of impact, the asteroid composition, and the
asteroid structure (porosity and cohesion) [1]:
(mS and vS are the mass and velocity of the
spacecraft. mA is the mass of the bulk asteroid. ∆v is the change in
the asteroid’s velocity.)
Estimates suggest that β can be anywhere between 1 and 5
for the types of conditions you’d expect for kinetic deflection. (Yes, it’s
hypothetically possible to get cases where β < 1, in the event that
material is ejected from the asteroid’s side opposite the impact.)
 |
There are roughly
three different cases of expected impact momentum exchange. The low-speed case
(at top) matches simple expectations for an inelastic collision (β = 1). The middle hypervelocity case
is more likely; a crater is formed at the impact site and debris is ejected
away. This ejected material imparts additional momentum to the remaining
asteroid, giving an effective “amplification” to the momentum exchange (β > 1). The bottom hypervelocity
case is unlikely, but shows a scenario where the impact’s shock wave crosses
the asteroid and ejects material on the far side of the asteroid. In this case,
some of the spacecraft’s momentum is transferred to the ejected material and
the bulk asteroid receives a reduced change in momentum (β < 1).
|
Right now, the DART team is parametrically modeling the
encounter using really impressive numerical simulations. The codes were developed
from explosives and weapons modeling, which are some of the most complex
computer models to create. These simulations require massive supercomputing
resources and help us understand what happens to different kinds of materials
as they undergo complex physical events such as high-velocity impacts and
explosions. Specifically,
the region around the impact is divided up into a grid consisting of millions
of little asteroid “cells”. The simulation evaluates the propagation of energy
and momentum through each cell, determining the interaction of each cell with
its neighbors. The fraction of cells containing material that are ejected from
the surface represents the amount of material that affects the value of β.
The simulations are very useful, but it’s quite difficult to
validate them in 1-g. Scientists conduct scaled experiments with high
energy mechanical contraptions like air-guns and catapults, but it’s hard to
accurately reproduce the expected structural properties of the target--a
gravitationally bound “rubble pile”. With that in mind, DART will represent a
chance to validate our hypervelocity impact modeling and enable us to then more-accurately extrapolate to other asteroid
types.
 |
| Various computational hydrocode results for DART impacts at differing geometries [[5]]. |
Program Status
Together, the two proposed missions form a coherent
experiment. However, they’re being developed such that they aren’t codependent:
That is, AIM will be the first spacecraft mission to a binary asteroid, which
would be pretty exciting with or without DART. AIM carries a suite of instruments including a
visible imager, thermal infrared imager, two radar systems, and a landed
package. It also will be carrying a set of 2-6 cubesats [[6]].
Should only DART launch, the change in orbit period can be
observed without AIM by using Earth-based observatories. Amateur and
professional astronomers worldwide will want to study the event to try to
characterize the post-impact environment. (Think Shoemaker-Levy 9’s impact at
Jupiter in 1994.) Between light curves and radar, we intend to measure the orbit
period change to better than 10%. If AIM is there, the results are obviously
much clearer and more accurate, but nonetheless the experiment can be conducted
using DART only.
DART itself is a relatively simple spacecraft with only a
single instrument, DRACO, which is an imager derived from the
narrow-field-of-view telescope on New Horizons. DART has the challenge of
reliably targeting and impacting a slight 170 m diameter target. To
successfully achieve this, we’ve been working hard to develop and prove out algorithms
for autonomous optical guidance, navigation, and control (GNC). The GNC
software must distinguish between the two bodies at Didymos and then drive the
spacecraft towards the image centroid corresponding to Didymoon, all within a
matter of hours. For this complex problem, we’re leveraging decades of missile
guidance algorithms, namely something developed in the 1970’s called Proportional Navigation. I can’t help
but call to mind a quote from the opening of the Arthur C. Clarke book Rendezvous
with Rama (1973):
“A
hundred years earlier, a much poorer world, with far feebler resources had
squandered its wealth attempting to destroy weapons launched suicidally by
mankind against itself. The effort had never been successful, but the skills
acquired then had not been forgotten. Now, they could be used for a far
nobler purpose, and on an infinitely vaster stage. No meteorite large enough to
cause catastrophe would ever again be allowed to breach the defenses of Earth.”
Lofty quotes aside… AIM is currently a Phase A/B1 study
within ESA, with two companies developing conceptual designs. DART is also a
Phase A study, managed by the NASA Planetary Defense Coordination Office, within
the Science Mission Directorate at NASA Headquarters. Both projects will proceed
over the next year towards their respective key decision points.
I hope
you’ll agree that this is a compelling concept. To me, it seems to answer that
old criticism,
“Everyone complains about the weather
asteroid-impact-threat, but no one
does anything about it.”
Let’s do something about it.
References:
[1]
A. F. Cheng, J. A. Atchison, B. Kantsiper, A. S. Rivkin, A. M.
Stickle, C. Reed, A. Galvez, I. Carnelli, and P. Michel, “Asteroid Impact and
Deflection Assessment Mission," Acta Astronautica, vol. 115, pp. 262-269,
2015.
[2]
Bottke W. and H. J. Melosh, Binary Asteroids and the Formation of Doublet
Craters, Icarus 124: 372–391 (1996)
[3]
Scheirich, P., and P. Pravec, 2009, Modeling of lightcurves of binary
asteroids, Icarus, 200:531-547
[4] J.
A. Atchison, M. T. Ozimek, B. Kantsiper, and A. F. Cheng, “Trajectory Options
for the DART Mission,” International Astronautical Congress, Jerusalem, Israel,
IAC-15-C1.1.31080.
[5]
A. M. Stickle, J. A. Atchison, O. S. Barnouin, A. F. Cheng, D. A.
Crawford, C. M. Ernst, Z. Fletcher, and A. S. Rivkin, “Modeling momentum
transfer from kinetic impacts: Implications for redirecting asteroids," 13th
Hypervelocity Impact Symposium, 2014.
[6] I.
Carnelli, A. Galvez, K. Mellab, M. Kueppers, “Industrial Design of ESA Asteroid
Impact Mission,” International Astronautical Congress, IAC-15-A3.4.9.x30901,
2015.
