Sunday, November 22, 2015

Justin Atchison has been kind enough to provide background on the proposed NASA Double Asteroid Redirection Test (DART) mission.  As Justin states, the NASA mission may be paired with a European orbiter that would also deliver a lander and several CubeSats to explore both members of this binary asteroid pair.  ESA has a nice webpage describing their proposed mission: Asteroid Impact Mission if you would like additional information.

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.


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.

Conjunction – In October of 2022, Didymos will make a rare (every ~25 years) conjunction with Earth. If we time the encounter to occur during that conjunction, Earth-based instruments, including radar, can participate in the observations and add a great deal of science. 

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 explosionsSpecifically, 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.


[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.