Brisset et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-018-0222-5(2018) 5:73

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Regolith behavior under asteroid-level
gravity conditions: low-velocity impact
experiments
Julie Brisset*, Joshua Colwell, Adrienne Dove, Sumayya Abukhalil, Christopher Cox and Nadia Mohammed

Abstract
The dusty regolith covering the surfaces of asteroids and planetary satellites differs in size, shape, and composition
from terrestrial soil particles and is subject to environmental conditions very different from those found on Earth. This
regolith evolves in a low ambient pressure and low-gravity environment. Its response to low-velocity impacts, such as
those that may accompany human and robotic exploration activities, may be completely different than what is
encountered on Earth. Experimental studies of the response of planetary regolith in the relevant environmental
conditions are thus necessary to facilitate future Solar System exploration activities.
We combined the results and provided new data analysis elements for a series of impact experiments into simulated
planetary regolith in low-gravity conditions using two experimental setups and a range of microgravity platforms. The
Physics of Regolith Impacts in Microgravity Experiment (PRIME) flew on several parabolic aircraft flights, enabling therecording of impacts into granular materials at speeds of ∼ 4–230 cm/s. The COLLisions Into Dust Experiment(COLLIDE) is conceptually close to the PRIME setup. It flew on the Space Shuttle in 1998 and 2001 and more recently
on the Blue Origin New Shepard rocket, recording impacts into simulated regolith at speeds between 1 and 120 cm/s.
Results of these experimental campaigns found that there is a significant change in the regolith behavior with thegravity environment. In a 10−2 g environment (with g being the gravity acceleration at the surface of the Earth), only
embedding of the impactor was observed and ejecta production was produced for most impacts at > 20 cm/s. Onceat microgravity levels (< 10−4 g), the lowest impact energies also produced impactor rebound. In these microgravity
conditions, ejecta started to be produced for impacts at > 10 cm/s. The measured ejecta speeds were somewhat
lower than the ones measured at reduced-gravity levels, but the ejected masses were higher. In general, the meanejecta velocity shows a power-law dependence on the impact energy with an index of ∼ 0.5. When projectile
rebound occurred, we observed that its coefficients of restitution on the bed of regolith simulant decrease by a factorof 10 with increasing impact speeds from ∼ 5 up to 100 cm/s. We could also observe an increased cohesion between
the JSC-1 grains compared to the quartz sand targets.
Keywords: Regolith, Low-velocity impacts, Microgravity experiments

Introduction
Small airless bodies of the Solar System are known
to be covered in a layer of regolith, composed ofgrains of varying sizes. Created by the bombardment of
micrometeoroids or thermal disintegration, micron-sized
and larger dust particles collect on the surfaces of asteroids, comets, and moons (Housen et al. 1979; Housen*Correspondence: julie.brisset@ucf.edu
Florida Space Institute, University of Central Florida, 12354 Research Parkway,
Partnership 1 Building, Suite 211, Orlando 32826, FL, USA

and Wilkening 1982). Any space mission probing or landing on the surfaces of these small bodies has to interact with this regolith, at the very low surface gravity
levels induced by the body’s small mass. The first softlandings on such small bodies were performed by the
NEAR-Shoemaker spacecraft on the asteroid Eros (Veverka
et al. 2001), Hayabusa on Itokawa (Kawaguchi et al. 2008),
and Philae on the comet 67P/Churyumov-Gerasimenko(Biele et al. 2015). In the latter instances, the landing
resulted in an unexpected response of the surface tothe low-velocity impact of the lander, in particular the© The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to theCreative Commons license, and indicate if changes were made.

Brisset et al. Progress in Earth and Planetary Science(2018) 5:73

remarkable recurrent bouncing of Philae on the comet’s
surface after a first touchdown at around 1 m/s.
Currently, two missions are en route to asteroids:
Hayabusa-2 carrying the MASCOT lander (Tsuda et al.
2013; Jaumann et al. 2016), due to arrive at Ryugu in
2018, and the OSIRIS-REx mission arriving at Bennu in
2018, planning to sample its surface using a Touch-AndGo mechanism (Lauretta et al. 2017). In addition, theincreased interest in In-Situ Resource Utilization (ISRU)
and mining prospects will soon lead to more reconnaissance and pathfinder missions to small bodies of the Solar
System. Understanding interactions with surface regolith
at very low gravity levels has therefore become an imperative. Impacts at a few meters per second down to afew centimeters per second into regolith layers are ofparticular interest to these current and future missions.
Motivated by these upcoming missions, theoretical andnumerical work has recently been performed in order
to investigate the behavior of granular material at thesurface of small asteroids, such as Ryugu and Bennu(Scheeres et al. 2010; Sanchez and Scheeres 2014;
Hirabayashi et al. 2015; Thuillet et al. 2017). In particular,
the response of a layer of regolith to a low-velocity impact
is investigated in, e.g., Maurel et al. (2017) for MASCOT
and Sánchez et al. (2013). Experimental works supportingthese efforts are scarce and often performed under conditions very different from what can be expected during alanding/contact at the surface of small bodies (1g, hypervelocity impacts). For example, the experiment campaign
performed by Housen and Holsapple (2003) to investigate crater formation in porous materials studied impacts
at > 2 km/s (see Holsapple et al. (2002) and Housen
and Holsapple (2011), for a review of other high-speed
impact experiments in porous materials). In order toinvestigate the relative influence of gravity on the impact
cratering and ejecta production processes in very large
impact events, Housen et al. (1999) performed high-speed
impacts into porous materials in a centrifuge at gravity levels up to 500g. While these experiments at highimpact velocities are relevant for meteorite and interasteroid impacts, the velocity ranges are not applicableto the landing or sample collection on a surface in low
gravity.
Theoretical and experimental studies that are more relevant to an asteroid landing situation are found in the
study of impactor deceleration in a granular medium: theKatsuragi and Durian (2007, 2013) and Katsuragi andBlum (2017) series studies the drag force exerted by agranular material on a penetrating impactor; Clark et al.
(2012) and Clark and Behringer (2013) study the impactor
stopping time and the energy transfer from the impactor
to the granular material at the grain level in quasi2D setups; Seguin et al. (2008) studies the influence ofcontainer walls on the impactor penetration depth; andPage 2 of 21

Machii et al. (2013) measures the influence of the target
strength on the impact outcome. All these experiments
were performed at impact speeds ranging from a few
10 cm/s to a few meters per second and provide therefore relevant data for the exploration of asteroid surfaces.
However, their experiments were performed in 1g, and theapplication of their findings to small body surfaces, which
can have gravity levels < 10−5 g, remains unexplored.
The study of low-velocity impacts into granular material in reduced gravity was started in 1998 with the
Shuttle experiment COLLisions Into Dust Experiment(COLLIDE) (Colwell and Taylor 1999; Colwell 2003).
Following experiments were performed during parabolic
flight campaigns in 2002 and 2003, using the Physics ofRegolith Impacts in Microgravity Experiment (PRIME)
hardware setup (Colwell et al. 2008). More recent datacollection was obtained with these two experiments during the PRIME-3 (Colwell et al. 2016) and COLLIDE-3
campaigns, on the NASA C-9 aircraft in 2014 and Blue
Origin’s New Shepard suborbital rocket in 2016, respectively. Both these experiment setups generate impacts ofapproximately centimeter-sized spherical projectiles onto
beds of granular material at speeds of 1–230 cm/s andgravity levels ranging from reduced gravity (∼ 10−2 g) tomicrogravity (< 10−4 g). The target materials used werequartz sand and JSC-1 lunar and JSC Mars-1 simulants,
sieved at particle size < 250μm. While the results ofeach of these experiment campaigns have been publishedseparately (except for the most recent COLLIDE-3 campaign), the collected data set has now reached a sizeof ∼ 130 data points and presents the opportunity fora new and combined analysis, with the perspectiveof applying these experimental results to interactions
with the regolith-covered surfaces of small Solar Systembodies.
This is the undertaking of the present paper. We combine the data sets of all the COLLIDE and PRIME campaigns and show the collision outcomes, coefficients ofrestitution, ejecta speeds, and estimated mass in dependance of the impact velocity and environmental g-level.
“Methods/Experimental” section describes the PRIME
and COLLIDE hardware setups and data collected. The“Results” section describes the data analysis and results,
which we then discuss in the “Discussion” section.

Methods/Experimental
The data results presented here were collected overalmost two decades during microgravity experimental
campaigns on a variety of flight platforms. The hardware used to collect data can be divided into two families: (1) the COLLIDE experiment series, which flew
on the Space Shuttle and a suborbital rocket, and (2)the PRIME experiment series, which flew on parabolic
aircrafts.

Brisset et al. Progress in Earth and Planetary Science(2018) 5:73

COLLIDE

Flight campaigns The first two COLLIDE flight campaigns were performed in 1998 and 2001, onboard theSpace Transportation System (Space Shuttle) STS-90
and STS-108 missions, respectively (COLLIDE-1 andCOLLIDE-2). The experiment hardware and performance
were described in Colwell and Taylor (1999) and Colwell(2003). During these flights, a total of nine impacts into
regolith simulant were recorded.
More recently, in April 2016, a modified setup was
flown on Blue Origin’s New Shepard suborbital rocket(COLLIDE-3). Parts of the Space Shuttle COLLIDE hardware were repurposed for this new campaign, and a totalof four impactor boxes were accommodated into the crew
capsule of New Shepard. Three of these boxes were similar to the Space Shuttle hardware, launching one projectile
into a bed of regolith simulant. The fourth box was outfitted with a multi-launcher instead of the single-launcher
configuration, now allowing for three impactors to besequentially launched onto the bed of regolith simulant
during the same experiment run.
Hardware For COLLIDE-3, four individual Impactor
Box Systems (IBSs) of COLLIDE-2 were modified forflight on suborbital launch vehicles. We manufactured
a vacuum chamber for each IBS in order to performthe impacts under vacuum conditions, because the New
Shepard crew capsule stays at atmospheric pressure during the flight. Each vacuum chamber was outfitted with aviewport allowing for a camera outside of the chamber torecord the impact. The IBS electronics were re-wired andconnected to custom electronics outside the chamber via

aPage 3 of 21

an electrical feedthrough. Both the chamber and IBS canbe seen in Fig. 1.
The camera system was entirely re-designed for
the COLLIDE-3 campaign. While COLLIDE-1 andCOLLIDE-2 used camcorders recording on digital video
tapes, the new design used a GoPro Hero 3+ black edition
recording data files on a microSD card. Both configurations required the retrieval of the tapes/memory card after
the experiment run.
Three of the IBSs were used as single-launchers (oneprojectile per experiment run, like in the COLLIDE-1 andCOLLIDE-2 campaigns), while the fourth one was modified to allow for the launching of three projectiles during
the same experiment run. This modified setup was named
multi-launcher. In a single-launcher IBS, the projectile is
kept on a compressed spring by an aluminum bar. Thisbar is held in its closed position by a pin attached to two
wires of memory shape alloy. When activated, these wires
retract the pin and the released aluminum bar swings in
its open position via a rotary spring. More details on the
single-launcher setup can be found in Colwell and Taylor(1999) and Colwell (2003).
The multi-launcher is composed of three individual
launchers (slightly smaller than a single-launcher), a central stepper motor, and a circular bottom plate. The stepper motor allows for the rotation of the bottom plate,which is outfitted with a drop hole. The projectiles inside
individual launchers are kept on a compressed linear
spring. They are held in place by an aluminum bar forced
in its closed position by the presence of the bottom plate.
This bar is held under tension by a rotary spring pressing
it towards an open position. When the drop hole alignsb

Launcher
LED panels
Memoryalloy wire

Mirror

Regolithtray door
Vacuum feed-throughElectronics

CameraVacuum chamber

Regolithtray

Fig. 1 COLLIDE-3 hardware. a The general setup showing the vacuum chamber with its viewport, the camera for data collection, and the experimentelectronics for autonomous experiment run. b The Impactor Box System (IBS) located inside the vacuum chamber with the launcher and regolith tray

Brisset et al. Progress in Earth and Planetary Science(2018) 5:73

with the launcher, the pin is freed thus allowing for the linear spring of the launcher to push the projectile outwards.
Figure 2 shows the multi-launcher setup.

Projectiles and simulants Table 1 details which projectiles and simulants were used for all three experiment campaigns. The projectiles used in the COLLIDE
campaigns were Teflon and quartz spheres of diameters
between 1 and 2 cm. These projectiles were chosen inorder to vary the impact energies over the largest range ofvalues possible while using the same hardware setup. Fora similar projectile size, 1 cm projectiles had masses about
one order of magnitude smaller than 2 cm projectiles.
The target materials used were quartz sand, the lunar
mare basalt simulant JSC-1, and the Mars soil simulant
JSC Mars-1. The choice of these materials was not motivated by their mineralogic similarity to asteroid regolith,
but rather by the shape of their grains. Indeed, from therelatively long history of inter-grain collision experiments(see Blum and Wurm (2008), for a review), it appears
that the actual mineral composition of the grains plays acomparatively negligible role in granular material cohesion compared to the size and shape of the grains. A
notable exception to this observation takes place if theregolith is composed of ices (in particular water ice) or
covered with organics, which are much stickier than rocky
regolith and therefore change the behavior of grain layers. Ices and organic materials are outside the scope of
the present work, and we therefore only considered grain
shape and sizes. The main difference between the chosen
simulants is the grain shapes and surface texture, which
are increasingly angular and irregular from quartz sandto JSC Mars-1 and to JSC-1 lunar simulants (Fig. 3). NoteaPage 4 of 21

that quartz sand and JSC Mars-1 are more similar in shapecompared with JSC-1.
The target materials were sieved to sizes between 75 and250 μm, which provides for a normal distribution between
these two sizes, and filled into the target tray of dimensions of 13 × 13 × 2 cm3 . Grain sizes on the surfaceof asteroids are known to have a size distribution ranging from micrometers to 10 m. The population of grains
smaller than 1 cm is difficult to infer from available imaging of surfaces, such as for Vesta (Hiroi et al. 1994) and
Eros (Dombard et al. 2010), as the image resolutions areusually >1 cm. However, from thermal inertia measurements, Gundlach and Blum (2013) has inferred averageregolith grain sizes covering the surfaces of 26 asteroids.
For 15 of these 26 bodies, they determined an averagegrain size of < 1 mm, which corresponds to the grain
size distributions we used in the COLLIDE and PRIME
experiments.
Seguin et al. (2008) studied the influence of the grain
container size on the penetration depth of projectiles
dropped into the container at speeds of < 3 m/s. They
showed that for a granular material target composed of
300–400 μm-sized grains and projectile sizes ranging
from 5 to 40 mm, the container bottom had an influenceon the projectile penetration depth only if located at a few
grain diameters from the surface. They also showed thatthe container side wall location had a vanishing influenceon the impactor penetration for a ratio between the tray
and projectile of > 5. Given their impact speeds and thesize of their projectiles and target grains, their results areapplicable to the COLLIDE and PRIME experiments. As
the target trays were 13 × 13 cm2 (13 > 5 × 2 cm fora projectile of 2 cm in diameter) in lateral size and 2 cm

b

1
2
3

Fig. 2 COLLIDE-3 multi-launcher system. a CAD side view showing the individual launchers (three, numbered) and the rotating plate at the bottom.
b The multi-launcher from the bottom showing the rotating plate and a projectile (marked by an arrow) in its individual launcher through theplate’s drop holeBrisset et al. Progress in Earth and Planetary Science(2018) 5:73

Page 5 of 21

Table 1 Projectiles and target materials of the three COLLIDE and the PRIME-3 campaigns
COLLIDE campaign

IBS

Projectile
Diameter [cm]

Mass [g]Material

Speed [cm/s]

Target material

OutcomeCoefficient of restitution

1

1

0.96

0.99

Teflon

14.80

JSC-1

R

0.028

1

4

0.96

0.99

Teflon

17.10

JSC-1

R

0.022

1

6

0.96

0.99

Teflon

90.00

JSC-1

R

0.03

2

1

2.00

10.71

Quartz110.00Quartz sand

E

0.01

2

2

2.00

9.64

Quartz3.60

Quartz sand

E

N/A

2

3

2.00

10.29

Quartz1.29

Quartz sand

E

N/A

2

4

2.00

8.66

Quartz

81.00

Quartz sand

E

0.015

2

5

2.00

8.98

Quartz

12.20

JSC-1

E

N/A

2

6

2.00

10.62

Quartz

28.00

Quartz sand

E

0.02

3

2

2.05

10.55

Quartz26.5

Quartz sandR

9.3×10−3

3

3

2.05

10.55

Quartz29.3

JSC Mars-1

E

N/A

PRIME-3 flight

Box

1

1

2.05

9.82

Quartz

48.45

Quartz sand

E

N/A

1

3

2.05

11.76

Quartz49.72

Quartz sand

E

N/A

1

4

2.05

30.75

Brass

43.19

Quartz and

A

N/A

1

5

2.05

9.9

Quartz43.75

Quartz and

E

N/A

1

6

2.05

28.2

Brass

19.87

Quartz sand

X

N/A

1

7

2.05

9.82

Quartz

33.32

JSC-1

R

0.078

1

8

2.05

28.2

Brass

19.56

JSC-1

X

N/A

2

2

2.05

28.2

Brass

16.21

Quartz sand

R

0.046

2

3

2.05

11.76

Quartz31.52

Quartz sand

A

N/A

2

4

2.05

30.75

Brass

38.4

Quartz sand

A

N/A

2

5

2.05

9.9

Quartz38.73

Quartz sand

E

0.025

2

6

2.05

28.2

Brass

13.6

Quartz sand

X

N/A

2

7

2.05

9.82

Quartz

5.55

JSC-1

R

0.276

2

8

2.05

28.2

Brass

14.45

JSC-1

R

0.091

3

1

2.05

9.82

Quartz24.75

Quartz sand

E

N/A

3

3

2.05

11.76

Quartz26.38

Quartz sand

E

N/A

3

4

2.05

30.75

Brass

31.84

Quartz sand

A

N/A

3

5

2.05

9.9

Quartz

48.18

Quartz sand

A

N/A

3

6

2.05

28.2

Brass

15.2

Quartz sand

X

N/A

3

8

2.05

28.2

Brass

4.02

JSC-1

R

0.229

4

1

2.05

9.82

Quartz

25.85

JSC-1

E

0.075

4

3

2.05

11.76

Quartz20.94

Quartz sand

E

0.020

4

4

2.05

30.75

Brass

27.77

JSC-1

A

N/A

4

5

2.05

9.9

Quartz52.93

Quartz sand

A

N/A

4

6

2.05

28.2

Brass

10.02

Quartz sand

X

N/A

4

8

2.05

28.2

Brass

13.38

JSC-1

R

0.055

The table of the 108 PRIME-1 campaign impacts can be found in Colwell et al. (2008). No coefficients of restitution could be measured for the PRIME-1 impacts. R reboundwith no ejecta, E ejecta production, A asteroid-level parabola (0.05g), X no data available (regolith pillar), N/A no rebound from the projectile on the target could be observed

Brisset et al. Progress in Earth and Planetary Science(2018) 5:73

aPage 6 of 21

c

500 m

b

d

500 m
Fig. 3 Optical and scanning electron microscope (SEM) pictures of the simulants used in COLLIDE and PRIME experiments. a Quartz sand and b JSC
Mars-1 and c JSC-1 lunar simulants. The samples were prepared in the same manner as for the experiments, by sieving the different grains to below
250 μm. d shows the details of the surface texture of JSC-1 grains

deep (>> 250 μm grain diameter), we expect that the tray
had no influence on the target response to the impacts
performed in COLLIDE and PRIME.
The trays were consistently filled using the same procedure, in order to reproduce the same target density.
This filling procedure consisted in the pouring of simulantmaterial into the tray using a funnel, followed by theleveling of the surface using a flat metal ruler. We measured the porosities of targets produced in this mannerin the laboratory (30 times) by filling the tray using
this procedure, then measuring the mass of the target
material using a scale. Knowing the container volume
and the bulk density of the grains, we determined target porosities consistently ranging from 0.4 to 0.5. Whilethe compaction level of fine grains at the surface ofasteroids is not well determined, the bulk porosity of“rubble-pile” asteroids was estimated to be of this order by
Britt et al. (2002), indicating that such porosity levels are
relevant for the study of low-velocity impacts on asteroid
surfaces.

In the COLLIDE experiment setup, the target was contained by a sliding door, with no particular measures
to prevent further compaction through post-preparation
handling and launch vibrations. However, the COLLIDE3 videos, which have a high enough resolution to show
details of the target tray filling after the door opened
during the rocket flight, show no volume reductionof the target sample. The target compaction during
COLLIDE-1 and COLLIDE-2 could not be quantified dueto the low resolution of the video data, but the projectile impact location indicates that the target surface was
not significantly shifted from its original position, so that
we do not estimate porosities much below 0.4 during theexperiment runs.
PRIME

Flight campaigns PRIME flew on three experiment campaigns on the NASA KC-135 and C-9 parabolic airplanes. The first flight campaign induced five flight weeks
between July 2002 and May 2003 and is described inBrisset et al. Progress in Earth and Planetary Science(2018) 5:73

Colwell et al. (2008). The PRIME-1 campaign produced
108 successful impact recordings.
From the results of this first campaign, it was clear thatthe regolith was very sensitive to the residual accelerations of the aircraft. Therefore, the PRIME-2 hardware
was modified to be partially free-floating. In this setup,
the individual impact boxes could be pulled out of theoverall payload structure, which was bolted to the airplane
floor. The boxes were still attached to the structure by anelectrical umbilical. Unexpectedly, the residual accelerations of the plane during parabolas were still high enoughto introduce torques onto the impact boxes via this umbilical, generating a worse acceleration environment thanduring PRIME-1, and no scientific data could be collected.
The PRIME-3 hardware was designed to be entirely
free-floating. Each impact box had its own electronics
attached to it, including a battery pack for power. The
following paragraph describes the hardware flown during the PRIME-3 1-week flight campaign in August 2014.
The PRIME-3 campaign produced 25 impact recordings.
During the flight campaign, the pilots flew a number ofparabolas at 0.05g (named “asteroid-level” parabolas by
the NASA flight support team) in addition to the usual
microgravity ones. Out of the 25 recorded impacts, 7 wereat asteroid level. Details on the PRIME-3 campaign can befound in Colwell et al. (2016).

Hardware Figure 4 shows the PRIME-3 hardware. It is
composed of eight individual impact boxes that are kept
in a structure bolted to the airplane floor. Each box canaPage 7 of 21

be slid out of the structure and used to perform anexperiment run while completely free-floating during aparabola. The impact boxes were inherited from PRIME1 and PRIME-2 and modified to allow for an autonomous
experiment performance: while the regolith tray andlaunch mechanisms are similar, the electronics werere-designed to actuate them externally using a wirelesssignal. One camera was used to record the impacts in theindividual chambers as it was attached to a bracket that
could be switched from box to box. The camera used was
also a GoPro Hero 3+ black edition. More details on theinside of the impact boxes can be found in Colwell et al.
(2008).
During asteroid-level parabolas, the impactor box wasplaced on the floor of the airplane and the projectile
launch initiated at the beginning of the parabola.

Projectiles and simulants The list of projectiles andsimulants for the 108 impacts recorded during the
PRIME-1 campaign can be found in Colwell et al. (2008).
Table 1 summarizes the projectiles and simulants usedduring the PRIME-3 campaign.
The target materials and preparation methods were thesame as for COLLIDE (see “Projectiles and simulants”
section for details). The only difference with the COLLIDE targets is an additional contraption on the tray door
that slightly compresses the target after preparation andthrough the pre-flight handling process. As the PRIME
tray doors open by rotation rather than sliding, a plateb
LaunchMechanism
Mirrors2
1
Tray Door
Regolith Tray

Fig. 4 PRIME-3 hardware. a The payload structure that is bolted to the aircraft floor and holds the eight impact boxes (only four are visible in thisfront view). b Interior of an impact box showing the regolith tray and the launch mechanism. During an experiment run, the tray door rotates toopen (1) before the projectile gets launched towards the regolith bed (2)Brisset et al. Progress in Earth and Planetary Science(2018) 5:73

was attached on the inner side facing the sample. Using
four weak springs, this plate applies pressure on the target sample when the tray door is closed. In this way, thesample is jammed and its porosity remains unchanged(0.4 to 0.5 as described in “Projectiles and simulants”
section) during loading of the experiment boxes onto
the aircraft.

Results
As the target size distribution and porosity were the samefor all the COLLIDE and PRIME campaigns, we combined
the collected data sets. This section presents how we analyzed the impacts and the results we obtained, integrating
all the data collected.
The platforms used to collect data in microgravity
induced residual accelerations on the experimenthardware attached to them. These accelerations werearound 10−3 g in the Space Shuttle (Colwell and Taylor
1999) and 10−2 g on the parabolic aircraft (Colwell

Page 8 of 212003) and on Blue Origin’s New Shepard rocket(Wagner and DeForest 2016). The free-floating experiment boxes of PRIME-3 offered the best microgravity
quality: with only air drag acting on the box moving atvery low speeds (< 1 mm/s), no residual acceleration
could be detected from our video data. This implies
that these experiment runs were performed at residual
accelerations under our detection threshold, 10−4 g.
In addition to these microgravity experiment runs,
the PRIME-3 flights also allowed us to perform sevenexperiment runs at 0.05g.
Across all the data collected, four types of collisionoutcome were observed:• Collisions producing ejecta with an embedded
projectile (squares in Fig. 5): in these collisions, nomotion of the projectile after the impact is detected.
Either the ejecta blanket or individual particles could
be tracked and their speed determined.

Fig. 5 Outcomes of all the low-velocity impacts observed in PRIME-1, PRIME-3, and the COLLIDE campaigns as a function of the impact velocity.
Circles: projectile embedded, no ejecta; diamonds: projectile rebound, no ejecta; squares: projectile embedded, ejecta production; triangles:
projectile rebound, ejecta production; open symbols: quartz sand target; filled symbols: JSC-1 target; green plus sign: JSC Mars-1 target; red: reduced
gravity (∼ 10−2 g); black: microgravity (< 10−4 g). All impacts producing no ejecta are shown at a mean ejecta velocity of 1 cm/s for the clarity of thegraph, even though no speed was measured as no ejecta was observed

Brisset et al. Progress in Earth and Planetary Science(2018) 5:73

• Collisions producing ejecta with projectile rebound(triangles in Fig. 5): the projectile can be tracked after
the collision. In addition to the ejecta speeds, a
coefficient of restitution could be measured for the
projectile.
• Collisions with no ejecta but projectile rebound(diamonds in Fig. 5). The coefficient of restitution forthe projectile could be measured.
• Collisions with no ejecta and embedded projectile(circles in Fig. 5).
Figure 5 shows these outcomes as a function of the impact
velocity. In this graph, all impacts not producing any
ejecta were placed at a mean ejecta velocity of 1 cm/s.
This is for the purpose of representing all the data on
the same graph only, as no speed could be measured on
absent ejecta (and a representation at 0 is not possible
on a logarithmic scale). The following paragraphs givesome additional details on these results for quartz sandand JSC-1 target materials.
JSC Mars-1 behaved similarly to quartz sand in reduced
gravity (the JSC Mars-1 data points seen in Fig. 5 aremostly from the PRIME-1 campaign; see Table 1 in
Colwell et al. (2008)). This can be explained by
the similar grain shape between these two simulants(Fig. 3a, b). In microgravity, only one impact was available
for data analysis, which was not enough for analyzing thisset of data.
Ejecta production

Impacts in COLLIDE-2, COLLIDE-3, PRIME-1, and
PRIME-3 resulted in the production of an ejecta blanket. In the PRIME-1 and most of the COLLIDE-2 data
(Colwell 2003), individual ejecta particles could not be
distinguished due to the low camera resolution. However, features of the ejecta blanket, in particular the fastest
particles forming the upper edge of the blanket (named“corner” in Colwell et al. (2008)), were identified andallowed for the measurement of the upper end of theejecta velocity distribution.
In PRIME-3, the camera resolution allowed for the individual tracking of ejecta particles. For these experiments,
we were able to measure a mean ejecta velocity by directly
tracking 30 particles for each impact, using the Spotlight software developed at NASA Glenn Research Center(Klimek and Wright 2006). However, due to the nature of
the video data collected, the tracking of individual particles is limited to optically thin portions of the ejecta
blanket. This includes but is not restricted to the ejecta“corner” mentioned above. Particle tracking was performed by three people independently in order to reducethe measurement errors. The ejecta grain velocity distribution obtained for each impact was normal and could be
fit by a Gaussian curve. Details on the tracking methods

Page 9 of 21

can be found in Colwell et al. (2016) (see their Figs. 5 and
6). For PRIME-3 and COLLIDE-3, the characteristic ejecta
velocity was chosen to be the Gaussian mean of velocity
distribution of the tracked particles.
Figure 6 shows the results of the ejecta speed measurements for PRIME-1 and PRIME-3. The 1-σ error
bars for the Gaussian mean determination are shown for
the PRIME-3 data points. COLLIDE-3 produced ejecta
for only one impact, and the target material was JSC
Mars-1. As stated above, JSC Mars-1 behaved similarly
to quartz sand, and we are not including JSC Mars-1
results here. The plateau formed by the PRIME-1 microgravity data points (black asterisks representing quartz
sand impacts) around 10 cm/s for impact energies below
104 ergs is due to the residual accelerations induced by
the parabolic aircraft on the experiment boxes (on theorder of 10−2 g). Only particles ejected with energies able
to overcome the effect of this ambient gravity field werelifted from the target. For this reason, the slowest moving ejecta has a minimum speed, set by this g-level, andthe measured velocities follow a plateau. This plateau
also formed in PRIME-3 microgravity data, but at ejecta
velocities around a few centimeters per second, indicating that residual accelerations were successfully reducedby the free-floating hardware configuration, compared to
PRIME-1, which was attached to the aircraft frame.
The scatter of the PRIME-1 data points towards higher
ejecta velocities was induced by the observational bias
produced by the tracking method used: tracking the ejecta“corner” described above limits the recorded speeds to thefastest moving particles and is therefore only representative of the upper end of the ejecta velocity distribution:
there is therefore a tendency to measure higher ejecta
velocities at the same impact energies. However, the data
points show a trend followed by the lower limit of the measured velocities: to guide the eye, we fit a power law to
the impacts into quartz sand in microgravity. The index
obtained is 1.35. For all impacts into JSC-1, a fitted powerlaw index is at 0.50. We collected only one data point for
JSC-1 ejecta at microgravity levels, which lies below thispower law fit. Further data collection on JSC-1 ejectaproducing impacts in microgravity will be required inorder to evaluate if this point is indicative of a different
ejecta behavior between the two gravity levels.
The ejecta speeds measured for impacts at 0.05g during the PRIME-3 campaign are of the same range as
the ones measured during the PRIME-1 campaign, where
microgravity parabolas were flown, but the experimenthardware was fixed to the aircraft frame. This demonstrates that the residual accelerations produced on theexperiment when attached to the aircraft are comparable
to 10−2 g and that our method for determining the average ejecta speeds provides similar values than the method
used in Colwell et al. (2008).

Brisset et al. Progress in Earth and Planetary Science(2018) 5:73

Page 10 of 21

Fig. 6 Combined PRIME-1 and PRIME-3 ejecta velocity measurements as a function of the impact velocity. PRIME-1 data is shown by red symbols.
For PRIME-3, black symbols show impacts in the free-floating box (< 10−4 g), while green symbols show impacts at 0.05g. Impacts into quartz sandare marked by asterisks, and impacts into JSC-1 by triangles. Error bars are not shown for PRIME-1: they were smaller than the symbol size (Colwell2003). PRIME-3 error bars represent the 1-σ error of the Gaussian fit to the velocity distribution of tracked particles. The dashed black line is a powerlaw fit to the microgravity impacts in sand (index 0.50), and the red dashed line is a power law fit to all the impacts into JSC-1 (index 1.35)

We defined the effective coefficient of restitution of the
impacts as the ratio between the average ejecta velocity and the projectile impact velocity and investigated
its relationship to the impact velocity. These effective
coefficients of restitution showed no correlation and a
uniform distribution over the available range of impact
velocities (Fig. 7). We calculated the overall average
effective coefficient of restitution to be 0.39 ± 0.15.
This value is about twice the one derived by Deboeuf
et al. (2009) for impacts of centimeter-sized spheres
into 100 μm glass bead beds. Deviations to our meanvalue were much higher than for the measurements performed in Deboeuf et al. (2009). When separating theimpacts into quartz sand from the ones into JSC-1, we
obtained average effective coefficients of restitution of
0.38 ± 0.15 and 0.45 ± 0.16, respectively. When separating between impacts in low gravity and microgravity,
we obtained values of 0.43 ± 0.14 and 0.15 ± 0.04,
respectively.

Projectile reboundA number of impacts during the COLLIDE-1, COLLIDE-3,
and PRIME-3 campaigns showed a rebound of the projectile after impact on the target and allowed for thevmeasurement of a coefficient of restitution  = vfi ,vi and vf being the projectile velocities before (initial)
and after (final) the impact, respectively (see Table 1).
Figure 8 shows the measured coefficients of restitution as a function of the impact energy. For speeds
above 30 cm/s, impacts systematically produced an ejecta
blanket and coefficients of restitution of the projectile could either not be measured or were of the orderof 10−2 .
At impact speeds between about 20 and 30 cm/s, bothejecta production and projectile rebound without ejecta
were observed. Below 20 cm/s, only rebounds withoutejecta were observed. In order to compare our results
to the experimental work of Katsuragi and Blum (2017),
we fit a power law to this set of data. In Katsuragi andBrisset et al. Progress in Earth and Planetary Science(2018) 5:73

Page 11 of 21

Fig. 7 Effective coefficient of restitution of the impacts depicted in Fig. 6 (the same colors and symbols were used). This coefficient of restitution isdefined as the ratio between the average ejecta velocity and the projectile impact velocity

Blum (2017), spherical projectiles of about 1 cm in diameter are dropped into aggregates composed of 750 μm
SiO2 monomer grains with a packing density of  = 0.35.
The index we obtain with our data is − 0.27, similar tothe value of − 1/4 found by Katsuragi and Blum (2017).
We note, however, that if we separate the data setsfrom COLLIDE and PRIME, we obtain indexes of − 0.10
and − 0.50, respectively, a difference mostly due to thetwo very low-energy impacts into JSC-1 of the COLLIDE1 flight. As mentioned in Katsuragi and Blum (2017),
this overall index value is twice as high as expected fromthe theory of an elastic sphere impacting a plane surface(Johnson 1985; Thornton and Ning 1998). This indicatesthat energy absorption in a bed of granular material is notentirely captured by the mechanics of elastic surfaces.
Projectile penetration depth

In the PRIME-3 video data, the resolution is high enoughto determine the maximum penetration depth of the projectile into the target for 21 impacts. This maximum
penetration depth is defined as the distance between thebottom of the projectile and the target surface. Figure 9shows these results as a function of the equivalent total
drop distance H. This distance was derived to be able tocompare our results with the ones obtained by Katsuragi
and Blum (2017): H = h+zmax , where h is the drop height(experiments in Katsuragi and Blum (2017) are performed
in 1g, and the drop height is used to control the impact
velocity), and zmax is the maximum penetration depth ofthe projectile into the target. As the impact velocity was
not determined by the drop height in PRIME experiments,
but by the stored potential energy in the spring of the
projectile launcher, we calculate an equivalent h from theimpact velocity vi by equating the 1g potential energy with
our kinetic impact energy as follows:
E1g = E0g
1
mgh = mv2i
2
v2
h= i
2g

(1)

Brisset et al. Progress in Earth and Planetary Science(2018) 5:73

Page 12 of 21

Fig. 8 Combined PRIME-3 (black) and COLLIDE (green) coefficient of restitution measurements. This coefficient of restitution is defined as the ratioof the projectile speed after and before the impact with the regolith bed. Impacts into quartz sand are marked by asterisks, and impacts into JSC-1
by triangles. Error bars are shown for the PRIME-3 and COLLIDE-3 measurements. For COLLIDE-1 and COLLIDE-2, they can be found in Colwell(2003). The straight line represents the power law fit to all data points shown, which has an index of − 0.27

with g = 9.81 m/s. As shown by the index 1 power law in
Fig. 9, H is dominated by zmax in half of the impacts. Thisis due to the very low impact velocities, inducing equivalent drop heights h  zmax , so that H ∼ zmax . We do
not observe any correlation between the maximum penetration depth and the computed total equivalent drop
distance. This is in contrast with the relation zmax ∝ H 1/3
observed in Katsuragi and Blum (2017), representative ofa scaling by impact energy. A scaling by momentum of theβform zmax = Amαp vi , with mp and vi being the projectile
mass and impact velocity, respectively, and A, α, and β fit
parameters, as performed in Güttler et al. (2009), was not
successful either.
Grain behavior with decreasing gravity level

From the experimental results presented here, we
observed differences in the response of the target to lowvelocity impacts in reduced gravity (∼ 10−2 g) and microgravity (< 10−4 g). Figure 5 shows that compared to thetotal number of impacts in reduced gravity (∼ 10−2 g),
only a few of them did not produce ejecta: the vast majority of impacts generated an ejecta curtain. In addition,none of the reduced-gravity impacts lead to a rebound of
the projectile (no red triangles or diamonds). In particular,when no ejecta was produced around the lowest impact
speeds observed (∼ 20 cm/s), only projectile embeddingin the target could be seen. For the same impact speeds, a
microgravity environment (< 10−4 g) lead to the reboundof the projectile. In fact, embedding into the target was
only seen in the two impacts under 4 cm/s. All otherimpacts than those without ejecta showed a rebound of
the projectile.
Projectile rebound was also observed in combination
with an ejecta curtain in half of the impacts observedin microgravity (black triangles). Even though Fig. 8
shows that the coefficient of restitution of these rebounds
is much smaller compared to the ones with no ejecta
production (diamonds), this behavior demonstrates theBrisset et al. Progress in Earth and Planetary Science(2018) 5:73

Page 13 of 21

Fig. 9 Maximum penetration depth of the projectile into the target for PRIME-3 impacts. H is the equivalent total drop distance as described in the
text. Impacts into sand are marked by asterisks, and impacts into JSC-1 by triangles. The line represents an index 1 power law

differences in target response in reduced- and microgravity environments.
It can also be noted that for the same impact speed,
ejecta is faster on average in reduced gravity compared
to microgravity (red symbols are above black symbols
in Fig. 5). In addition, less mass is ejected than in
microgravity. Figure 10 shows the average ejecta speeds
for four levels of ejected mass: as the ejecta mass cannot be measured after each experiment run (the returnto 1g mixes the target material from the tray and theejected particles), an estimation was performed from thevideo images. Four levels of ejecta mass were identified:no ejecta (0), individual particles detaching from the target (1), ejected mass of the order of the projectile mass (2),and ejected mass much higher than the projectile mass (3)(we followed the same numbering as in Colwell (2003)).
In Fig. 10, the red symbols show reduced-gravity impacts(∼ 10−2 g), and black symbols microgravity impacts(< 10−4 g). The impact velocity given for each target typeand ejecta mass level is the average of all impacts thatcreated the same amount of ejecta. From the sand targets,
we can see that the same average impact speed results inhigher ejecta masses in microgravity compared to reduced
gravity. Together with Fig. 5, this shows that fewer target
particles are ejected in reduced gravity, but their ejection
speed is higher.
Figure 10 also shows that no ejecta blanket was observedin microgravity with JSC-1 targets. While this is also dueto the fact that only very few impacts were performedin JSC-1 in microgravity at speeds > 50 cm/s, the natureof the target plays a role in the ejecta mass produced:
compared to quartz sand particles, which are rounded andconsidered cohesionless in vacuum (once the air humidity
is removed), JSC-1 particles are more angular and behave
like a cohesive powder. Figure 5 shows that only twoout of eight impacts into JSC-1 at > 10 cm/s resulted inejecta production, and these were only individual particles
detaching instead of an ejecta blanket. As we can see in
Fig. 10, the impact speed does not seem to influence theproduction if these individual particles in JSC-1 targets,Brisset et al. Progress in Earth and Planetary Science(2018) 5:73

Page 14 of 21

Fig. 10 Arbitrary quantified amount of ejecta for all collected data on sand and JSC-1 targets. The amount of ejecta mass was quantified as follows:
0 = no ejecta; 1 = ejection of single particles; 2 = ejecta blanket, mass of the order of the projectile mass; and 3 = ejecta blanket, mass much higher
than the projectile mass. The impact velocity given for each target type is the average of all impacts that created the same amount of ejecta. Asterisk= quartz sand target, triangle = JSC-1 lunar simulant target, black = microgravity impacts, red = reduced-gravity impacts. The dotted line marks thelimit between no significant ejecta produced (either none at all or only individual particles detaching) and an ejecta blanket

compared to quartz sand targets, which display a moreconsistent trend of increasing ejecta mass with increasing impact velocity. When an ejecta blanket is produced
though, we can see that higher impact velocities resultin higher ejected masses for both quartz sand and JSC-1
targets.

Discussion
Observing ejecta

Due to the hyper-g levels experienced after the low-gravity
phase, the state of the target and projectile after each
impact is destroyed and the PRIME and COLLIDE datacollection relies solely on video recordings. In particular,
it is not possible to measure the ejected mass, nor the trajectories of all ejected particles in the case of an ejecta
blanket forming. As described in “Ejecta production”
section, ejected particles could only be tracked in thinparts of the ejecta blanket or in the case of single
particles being lifted rather than a blanket. Figure 11ashows an example of particle tracks superposed with theejecta blanket: even though we can get particle speeds
from these partial tracks, there is a high uncertainty inplacing the origin of each track with respect to the surfaceof the target material and embedded projectile, which
location can only be estimated as it is obscured by theejecta blanket. For this reason, it is not possible to generate mass vs. speed or speed vs. launch position diagramsfor ejecta, in order to compare them to 1-g data and thescaling laws derived in Housen and Holsapple (2011).
Observing projectile reboundAs described in the previous section, a rebound of the
projectile from the target was never observed in reducedgravity conditions, compared to microgravity. This could

Brisset et al. Progress in Earth and Planetary Science(2018) 5:73

aPage 15 of 21

c t0

t0+183 ms

69 px

b

69 px

t0

t0+83 ms

91 px

95 px

Fig. 11 Data analysis examples. a COLLIDE-3 IBS 3 impact into JSC Mars-1 at 29.3 cm/s. Particle tracks are superposed in green, and the approximate
projectile position is shown with a green circle, as the ejecta curtain hides it entirely. The mirror above the target tray shows a top view of the
impact, in which the projectile is visible. b COLLIDE-3 IBS 2 impact into quartz sand at 26.3 cm/s. Note the mass transfer onto the projectile after
rebound. c PRIME-3 rebound observation: t0 indicates the moment of deepest penetration of the projectile into the target. (Top) Case of ejecta
production without projectile rebound (flight 1, box 1): no marble motion can be detected for 183 ms. In the following frames, the ejecta curtain
hides the projectile, which cannot be tracked anymore. (Bottom) Case of ejecta production with projectile rebound (flight 2, box 5): we can track the
motion of the projectile over 10 frames and 4 pixels

be due to two factors: either no rebound took place or therebound was too small to be observed at the space andtime resolution of the video recordings of the impacts.
In the case that the absence of observed rebound in
reduced gravity is only due to resolution limitations of
the experiment cameras, we can assume that the reboundtakes place with the same coefficient of restitution as in
microgravity and calculate the distance and time travelledby the projectile after the impact before it gets stopped bygravity. As the only force on the projectile after it leaves
the target again is gravity, simple Newtonian mechanics
give us the time t0 at the apogee of its trajectory and theheight h above the target it reaches at t0 :
t0 =

vr
gl

;

h=

1 v2r
2 gl

(2)

where vr is the rebound speed and gl the value of the local
gravity acceleration.
For impact speeds around 40 cm/s, we measure coefficients of restitution around 0.02 (see Fig. 8). This would
set vr to 8 mm/s. Reduced-gravity environments had gravity levels around 10−2 g; thus, gl ∼0.1 m/s2 . This leads toa travel time of t0 = 80 ms and a rebound travel heightBrisset et al. Progress in Earth and Planetary Science(2018) 5:73

of h = 320 μm. During the PRIME-3 campaign, the camera resolution was around 80 μm/pixel and the recording
was performed at 120 fps (about 8 ms between frames). In
addition, reduced-gravity environments during this campaign lead to short parabolic trajectories of the ejectedparticles consistent with a ∼ 0.05g acceleration environment, leaving a clear view of the projectile during theentire recording. This means that a rebound would havebeen observed and a rebound motion could have beenmeasured for 10 frames and over 4 pixels. As no reboundwas observed, it appears that they were due to a different behavior of the target and no rebound actually took
place in reduced gravity. To illustrate this, we show two
PRIME-3 impacts with different outcomes in Fig. 11c andtheir measured projectile positions, at deepest penetration (t0 ) and after 22 (no rebound) and 4 (rebound) frames.
Some impacts generating few ejecta allow for a long-term
tracking of the projectile after the impact, allowing forthe detection of rebound speeds as low as a few millimeters per second. However, large ejecta blankets obstruct
the view of the projectile (see Fig. 11a), thus limiting theobservation of very low rebound speeds in these cases.
Scaling the penetration depth

As shown in Fig. 9, scaling the maximum penetration
depth of the projectile into the target (zmax ) with either
the impact energy or momentum was not possible. Here,the difference in gravity levels (reduced or microgravity)
did not appear in the distribution of the data points. Following Katsuragi and Durian (2013), Fig. 12a shows zmax
2/3
as a function of the quantity μ−1 (ρp /ρg )1/2 Dp H 1/3 . InaPage 16 of 21

this quantity, μ is the coefficient of friction of the target material. From angle of repose measurements, we canestimate that μ = 0.84 for JSC-1 and 0.67 for quartz
sand (Brisset et al. 2016). ρp and ρg are the densities ofthe projectile and target material, respectively; Dp is the
projectile diameter; and H is the equivalent total drop
distance as defined in the paragraph on penetration depth
results. Figure 12a allowed us to recognize two groupsof data points, which we distinguished by color: impacts
with projectiles of masses around 10 g (see the PRIME-3
data in Table 1: 9.82 to 11.76 g) in black and impacts with∼ 30 g projectiles (28.2 to 30.75 g). Power law fits to each
population yielded similar indexes of about 2.5 and a factor of about 3 between both. We concluded that our data
2/3
set scales with μ−1 (ρp /ρg )0.1 Dp H 1/3 , the exponent forρp /ρg being 0.1 rather than 0.5 (all PRIME-3 projectiles
have the same diameter of 2 cm). The result of this scaling
is shown in Fig. 12b.
This scaling and the associated power law index are incontrast with the results by Katsuragi and Durian (2013).
In particular, the contribution of the projectile to target
densities ratio is reduced from an index 0.5 to 0.25, while
the projectile diameter and impact energy have increased
contributions from indexes of 0.67 to 1.67 and 0.33 to 0.83,
respectively. The coefficient of friction has an increased
contribution from an index − 1 to − 2.5 (μ < 1). Thismeans that in low gravity (no distinction between reduced
gravity and microgravity), the diameter and kinetic energyof the projectiles play a more significant role in the penetration depth than the density ratio with the target material than in 1g. This reflects the fact that the gravity pullb

Fig. 12 Scaling of the maximum penetration depth during PRIME-3 impacts. a With the quantity μ−1 (ρp /ρg )1/2 Dp H1/3 (see text for details) and2/3
b with μ−1 (ρp /ρg )0.1 Dp H1/3 . Asterisks and triangles represent quartz sand and JSC-1 targets, respectively. Impacts performed with projectiles ofmasses ∼ 10 g are marked by black symbols, and ∼ 30 g by red symbols. The black and red lines in a are power law fits to these two types ofimpacts (10 g and 30 g projectiles), both with indexes ∼ 2.5. The line in b shows the power law fit to the entire data set, with an index of 2.5
2/3

Brisset et al. Progress in Earth and Planetary Science(2018) 5:73

on the projectile during penetration is much reduced, andthe density ratio between projectile and target material
is a less significant factor during penetration. Large andhigh-velocity projectiles will penetrate the target easier in
reduced gravity. The role of the cohesion between the target grains (which is implicit in the coefficient for frictionμ) is increased by the reduced-gravity environment: cohesive targets are more efficiently stopping a projectile thanin 1g. This supports the notion that cohesive forces canbecome significantly more important in low gravity compared to the weight of the target particles (see following
paragraph).
Gravity-cohesion competition

In Fig. 13, we compare the ambient gravity forces to interparticle cohesion forces (van der Waals), following theexample of Scheeres et al. (2010), using measured interparticle force values rather than model estimations. The
straight black lines in this figure represent the cohesion

Page 17 of 21

force between two grains, which is linearly proportional tothe particle radius, as measured for two different types ofparticles. We are using the Johnson-Kendall-Roberts theory(Johnson et al. 1971), estimating the force between two
grains to be the pull-off force, Fpo = 3πγ r, γ being thesurface energy of a grain and r its radius. The surface energies for quartz sand was measured in Kendall et al. (1987)
and in Brisset et al. (2016) for polydisperse SiO2 particles
in similar experiment conditions (vacuum levels of around
10 mTorr at room temperature). These polydisperse SiO2
grains were similar to JSC-1 lunar simulant in shape, whilequartz sand has more rounded, less cohesive grains. The
red dashed lines mark the force of gravity on individual
grains for three g-levels, and the green vertical line marks
the largest particles present in the target materials used in
the present work.
In this figure, we can see that, for quartz sand, the forcesinduced on the target grains due to gravity are several
orders of magnitude larger than inter-particle adhesiveFig. 13 Comparison between gravitational and cohesive forces depending on the grain radius. The black lines correspond to the measured pull-off
force between SiO2 polydisperse (Brisset et al. 2016) and quartz sand grains (Kendall et al. 1987). The grain sizes at which the measurements were
performed are marked as diamond symbols. The dashed red lines show the gravitational forces on the regolith grains for different g-level
environments. The green line marks the largest particles of our target size distributions

Brisset et al. Progress in Earth and Planetary Science(2018) 5:73

Page 18 of 21

forces at 1g and 10−2 g. Only at 10−5 g, these two forces
are of the same order for the smaller target particles. Thiscan explain the reason for lower ejected masses and larger
average ejecta speeds (at the same impact speed) observedin reduced gravity compared to microgravity: in reduced
gravity, only the fastest, lighter grains can overcome thegravity field and be ejected from the target. In microgravity, the gravity force is low enough to allow for most grains
to get lifted, even the slower ones, and the average ejecta
speed is accordingly lower. In addition, as the gravity andcohesion forces are of the same order in microgravity, thetarget material displays a more elastic behavior: cohesion
forces between the particles seem to allow for a partiallyelastic response of the surface to the impact, thus leading
to a rebound of the projectile (see Fig. 5, open triangles
and diamonds), while in reduced-gravity environments,
the presence of a constant downward force locks the particles in place (e.g., jamming; (Cates et al. 1998)) andincreases the inter-particle forces due to friction inside thematerial (Murdoch et al. 2013), thus preventing projectile
rebound.
In Fig. 13, we can also see that JSC-1 targets (whichapproximately behave like SiO2 grains) have stronger
cohesion forces between grains than quartz sand: thecohesion and gravity forces are of the same order alreadyin reduced gravity, while cohesion dominates by several
orders of magnitude in microgravity. The strong dominance of the cohesive forces can explain why none of the
impacts in JSC-1 in microgravity created an ejecta curtain. While most impacts in JSC-1 were performed at
speeds below 40 cm/s and higher impact speeds may havepossibly triggered larger ejecta masses, impacts at similar
speeds and in the same microgravity environment generated an ejecta curtain with sand target (Fig. 10). The
predominance of cohesive forces between JSC-1 grains
kept the target material from escaping.
We can also see a difference in behavior of the JSC-1 targets in reduced- and microgravity environments: for thesame impact velocities, rebound is observed in microgravity, while the projectile embeds into the target in reduced
gravity (filled black diamonds and red circles in Fig. 5).
Even though JSC-1 displays a more cohesive behavior thansand in reduced gravity, the data indicates that these cohesion forces do not dominate gravity enough at 10−2 g tolead to the rebound of the projectile.

the surface of 67P/Churyumov-Gerasimenko for the firsttime at ∼ 1 m/s, OSIRIS-REx’s Touch-And-Go (TAG)
mechanism is planned to impact the surface of Bennu ataround 20 cm/s, and Hayabusa-2’s MASCOT will landon Ryugu also at several 10 cm/s. The present experiments are therefore in a relevant impact speed regime.
While the regolith size distribution on asteroid surfaces
seems to heterogeneously include large and fine particles,
images of the surfaces of asteroids visited by spacecraft
show the presence of regions of very smooth terrain covered in grains of sizes below a millimeter (e.g., the smooth
ponds on Eros; Cheng et al. (2002)). These smooth terrains also represent ideal surface interaction sites as they
reduce the risks related to landing on uneven surfaces.
Therefore, the target size distributions of our experiments
are relevant to understanding spacecraft interactions withasteroid surfaces.
At these relevant impact speeds and grain sizes, weobserved the new phenomenon of projectile bouncing
off of granular material surfaces in microgravity. While
numerical simulations are currently the only way of studying low-velocity impacts of spacecraft landers at thecorrect size scales, those simulations are mostly using
hyper-velocity or 1g experiment impact data for calibration. The only simulations predicting and reproducing
projectile bouncing from the surface (Maurel et al. 2017),
for example, are using Earth gravity impact experiments
at > 10 m/s into granular material (Yamamoto et al. 2006)
or impacts on hard surfaces as input parameters (Biele
et al. 2017). Maurel et al. (2017) specifically indicate theneed for parameter refinement based on microgravity
data from experiments.
It can be noted that the experimental conditions,in particular the level of residual accelerations, havea significant impact on the target behavior. Even for
irregular grains that behave like cohesive powders,the granular material displayed a different behavior in
reduced gravity and microgravity. This means that the
behavior of granular material at the g-levels present on
the surface of small asteroids (10−4 g and below) cannot be extrapolated from its behavior at lunar g-level(∼ 10−2 g). Therefore, experiments relevant to the interaction with surfaces of bodies like Ryugu and Bennu
will require the high-quality microgravity environment of
free-fall drop towers or free-floating experiment hardware.

Interacting with small body surfaces

Conclusions

The experiments presented here are limited by the sizeof the target tray and projectile. However, it is possible to learn from their results about possible outcomesof surface interactions by spacecraft landers or samplingmechanisms. In particular, impact speeds under 1 m/s
are typical for past and planned missions interacting withthe surface of a small body; Philae touched down on

We have combined and analyzed low-velocity impacts
into regolith from five experimental campaigns. The
PRIME and COLLIDE experiment setups allowed forrecording impacts of centimeter-sized spherical projectiles into a bed of regolith simulant (quartz sandand JSC-1 lunar and JSC Mars-1 simulants), sieved tosizes under 250 μm. Due to their flight platform, theseBrisset et al. Progress in Earth and Planetary Science(2018) 5:73

experiments were run either at reduced-gravity levels of∼ 10−2 g when fixed to the parabolic aircraft or during
0.05g parabolas, or at microgravity levels of < 10−4 g,when free-floating in the airplane or during Shuttle andsuborbital rocket campaigns. The impacts into regolith
simulant resulted into four types of outcomes, someproducing ejecta and some displaying projectile rebound.
These results revealed major differences in the target
behavior depending on if the experiment was run in
reduced- or microgravity conditions. The main results ofthe data analysis can be summarized as follows:• We observed projectile rebound off the target in
microgravity. None of the impacts in reduced gravity
displayed a similar behavior. The coefficient ofrestitution of the impacts varied with the impact
energy following a power law of index − 1/4 (Fig. 8);• The maximum penetration depth was observed todepend more on the projectile size and energy andless on its density difference with the target than in 1g ;• For the same impact speeds, more mass was ejectedin microgravity compared to reduced gravity, but the
average ejecta speed was lower (Figs. 6 and 10);• The difference in cohesive forces in quartz sand andJSC-1 could be observed from the experiment results,
with quartz sand behaving like a cohesionlessmaterial and JSC-1 displaying cohesive behavior.
None of the impacts into JSC-1 generated an ejecta
curtain in microgravity, and most of them displayed
projectile rebound.
The results of these flight campaigns show thatthe acceleration environment at which experiments are
performed has a significant influence on their outcome. The study of regolith behavior at the surfaceof small bodies (< 1 km-sized asteroids) will require
acceleration environments below 10−4 g, as data cannot be extrapolated from behavior in reduced-gravity
environments (> 10−3 g).
From our current data set, it clearly appears that more
low-velocity impact data is required in microgravity conditions. This is due to the fact that clean microgravity
environments are difficult to obtain, with most platforms
retaining residual acceleration levels around 10−2 g. In
addition, more data is currently available for sand targets than for JSC-1 lunar simulant. As JSC-1 is more
representative of the granular material expected on asteroids (in particular due to the irregular particle shape),
data on JSC-1 targets is more relevant for application tosmall body surfaces. We can also note that all experiments presented here were run using similar target size
distribution: particles sieved to sizes < 250μm. From
images returned of asteroid Itokawa (Fujiwara et al. 2006)
and spectral analysis of Ryugu, we can infer that somePage 19 of 21

asteroid surface regoliths have larger average grain sizes.
For a future contribution of our experimental results into
small body surface exploration missions, the investigation of grain behavior at sizes between 1 mm and 1 cm
will be necessary. Such future experiments will be evenmore sensitive to the quality of the microgravity environment they will be run in due to the larger grain sizes(see Fig. 13).
Abbreviations
CMR: Center for Microgravity Research; COLLIDE: COLLisions Into DustExperiment; IBS: Impactor Box System; ISRU: In situ resource utilization; JSC:
Johnson Space Center; MASCOT: Mobile Asteroid Surface Scout; NASA:
National Aeronautics and Space Administration; OSIRIS-REx: Origins, Spectral
Interpretation, Resource Identification, Security, Regolith Explorer; PRIME:
Physics of Regolith Impacts in Microgravity Experiment; TAG: Touch-And-Go
Acknowledgements
We would like to thank our two anonymous reviewers for their valuable input
and comments.
Funding
This research is based in part upon work supported by the National
Aeronautics and Space Administration under Grant No. NNX11AQ87G issued
by the Planetary Geology and Geophysics Program and under grant
NNX12AK43G issued by the Outer Planets Research Program, by the NASA
Flight Opportunities Program, and by funding from Space Florida for theCenter for Microgravity Research.
Availability of data and materials
Please contact author for data requests.
Authors’ contributions
JC secured funding for all performed experiments and provided the
COLLIDE-1 and COLLIDE-2, and PRIME-1 data. JC, AD, and JB performed theexperimental data collection during the PRIME-3 and COLLIDE-3 campaigns.
JB, SA, CC, and NM worked on the data analysis. All authors read and approved
the final manuscript.
Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Received: 31 October 2017 Accepted: 24 September 2018

References
Biele J, Ulamec S, Maibaum M, Roll R, Witte L, Jurado E, Muñoz P, Arnold W,
Auster H-U, Casas C, Faber C, Fantinati C, Finke F, Fischer H-H, Geurts K,
Güttler C, Heinisch P, Herique A, Hviid S, Kargl G, Knapmeyer M,
Knollenberg J, Kofman W, Kömle N, Kührt E, Lommatsch V, Mottola S,
Pardo de Santayana R, Remetean E, Scholten F, Seidensticker KJ, Sierks H,
Spohn T (2015) The landing(s) of Philae and inferences about cometsurface mechanical properties. Science 349(1). https://doi.org/10.1126/
science.aaa9816
Biele J, Kesseler L, Grimm CD, Schröder S, Mierheim O, Lange M, Ho T-M (2017)
Experimental determination of the structural coefficient of restitution of abouncing asteroid lander. ArXiv preprint arXiv:1705.00701
Blum J, Wurm G (2008) The growth mechanisms of macroscopic bodies inprotoplanetary disks. Annu Rev Astron Astrophys 46:21–56. https://doi.
org/10.1146/annurev.astro.46.060407.145152
Brisset J, Colwell JE, Dove A, Rascon A, Mohammed N, Cox C (2016)Low-velocity impacts into cryogenic icy regolith. In: AAS/Division forBrisset et al. Progress in Earth and Planetary Science(2018) 5:73

Planetary Sciences Meeting Abstracts, vol. 48. American Astronomical
Society. http://adsabs.harvard.edu/abs/2016DPS....4851805B
Brisset J, Heißelmann D, Kothe S, Weidling R, Blum J (2016) Submillimetre-sized
dust aggregate collision and growth properties. Experimental study of amulti-particle system on a suborbital rocket. Astron Astrophys 593:A3.
https://doi.org/10.1051/0004-6361/201527288
Britt DT, Yeomans D, Housen K, Consolmagno G (2002) Asteroid density,
porosity, and structure p. 485–500
Cates ME, Wittmer JP, Bouchaud J-P, Claudin PH (1998) Jamming, force chains,and fragile matter. Phys Rev Lett 81(9):1841
Cheng AF, Izenberg N, Chapman CR, Zuber MT (2002) Ponded deposits onasteroid 433 Eros. Meteorit Planet Sci 37:1095–1105. https://doi.org/10.
1111/j.1945-5100.2002.tb00880.x
Clark AH, Kondic L, Behringer RP (2012) Particle scale dynamics in granularimpact. Phys Rev Lett 109(23):238302. https://doi.org/10.1103/
PhysRevLett.109.238302
Clark AHM, Behringer RP (2013) Granular impact model as an energy-depth
relation. EPL (Europhys Lett) 101:64001. https://doi.org/10.1209/02955075/101/64001
Colwell J, Brisset J, Dove A, Whizin A, Nagler H, Brown N, Rascon A, Brightwell K,
Seward L (2016) Low-velocity impacts into regolith under microgravity
conditions. In: 15th Biennial ASCE Conference on Engineering, Science,
Construction, and Operations in Challenging Environments, volume 82 ofEarth and Space Conference. https://ascelibrary.org/doi/10.1061/
9780784479971.010
Colwell JE (2003) Low velocity impacts into dust: results from the COLLIDE-2
microgravity experiment. Icarus 164:188–196. https://doi.org/10.1016/
S0019-1035(03)00083-6
Colwell JE, Sture S, Cintala M, Durda D, Hendrix A, Goudie T, Curtis D,
Ashcom DJ, Kanter M, Keohane T, Lemos A, Lupton M, Route M (2008)
Ejecta from impacts at 0.2 2.3 m/s in low gravity. Icarus 195:908–917.
https://doi.org/10.1016/j.icarus.2007.12.019
Colwell JE, Taylor M (1999) Low-velocity microgravity impact experiments into
simulated regolith. Icarus 138:241–248. https://doi.org/10.1006/icar.1998.
6073
Deboeuf S, Gondret P, Rabaud M (2009) Dynamics of grain ejection by sphere
impact on a granular bed. Phys Rev E 79(4):041306. https://doi.org/10.
1103/PhysRevE.79.041306
Dombard AJ, Barnouin OS, Prockter LM, Thomas PC (2010) Boulders and pondson the Asteroid 433 Eros. Icarus 210:713–721. https://doi.org/10.1016/j.
icarus.2010.07.006
Fujiwara A, Kawaguchi J, Yeomans DK, Abe M, Mukai T, Okada T, Saito J,
Yano H, Yoshikawa M, Scheeres DJ, Barnouin-Jha O, Cheng AF, Demura H,
Gaskell RW, Hirata N, Ikeda H, Kominato T, Miyamoto H, Nakamura AM,
Nakamura R, Sasaki S, Uesugi K (2006) The rubble-pile asteroid Itokawa as
observed by Hayabusa. Science 312:1330–1334. https://doi.org/10.1126/
science.1125841
Gundlach B, Blum J (2013) A new method to determine the grain size ofplanetary regolith. Icarus 223:479–492. https://doi.org/10.1016/j.icarus.
2012.11.039
Güttler C, Krause M, Geretshauser RJ, Speith R, Blum J (2009) The physics ofprotoplanetesimal dust agglomerates. IV. Toward a dynamical collisionmodel. Astrophys J 701:130–141. https://doi.org/10.1088/0004-637X/701/
1/130
Hirabayashi M, Sánchez DP, Scheeres DJ (2015) Internal structure of asteroids
having surface shedding due to rotational instability. Astrophys J 808:63.
https://doi.org/10.1088/0004-637X/808/1/63
Hiroi T, Pieters CM, Takeda H (1994) Grain size of the surface regolith ofasteroid 4 Vesta estimated from its reflectance spectrum in comparison
with HED meteorites. Meteoritics 29:394–396
Holsapple K, Giblin I, Housen K, Nakamura A, Ryan E (2002) Asteroid impacts:
laboratory experiments and scaling laws
Housen KR, Holsapple KA (2003) Impact cratering on porous asteroids. Icarus
163:102–119. https://doi.org/10.1016/S0019-1035(03)00024-1
Housen, KR, Holsapple KA (2011) Ejecta from impact craters. Icarus
211:856–875. https://doi.org/10.1016/j.icarus.2010.09.017
Housen, KR, Wilkening LL (1982) Regoliths on small bodies in the solar system.
Ann Rev Earth Planet Sci 10:355–376. https://doi.org/10.1146/annurev.ea.
10.050182.002035
Housen KR, Wilkening LL, Chapman CR, Greenberg R (1979) Asteroidal regoliths.
Icarus 39:317–351. https://doi.org/10.1016/0019-1035(79)90145-3

Page 20 of 21

Housen KR, Holsapple KA, Voss ME (1999) Compaction as the origin of theunusual craters on the asteroid Mathilde. Nature 402:155–157. https://doi.
org/10.1038/45985
Jaumann R, Bibring JP, Glassmeier KH, Grott M, Ho TM, Ulamec S, Schmitz N,
Auster HU, Biele J, Kuninaka H, Okada T (2016) A Mobile Asteroid Surface
Scout (MASCOT) on board the Hayabusa 2 Mission to the near Earthasteroid (162173) Ryugu. in AGU Fall Meeting Abstracts
Johnson KL, Kendall K, Roberts AD (1971) Surface energy and the contact ofelastic solids. Proc R Soc Lond 324:301–313
Johnson KL (1985) Contact mechanics. Cambridge university press, Cambridge
Katsuragi H, Blum J (2017) The physics of protoplanetesimal dust agglomerates.
IX. Mechanical properties of dust aggregates probed by a solid-projectile
impact. Astrophys J 851:23. https://doi.org/10.3847/1538-4357/aa970d
Katsuragi H, Durian DJ (2007) Unified force law for granular impact cratering.
Nat Phys 3:420–423. https://doi.org/10.1038/nphys583
Katsuragi, H, Durian DJ (2013) Drag force scaling for penetration into granular
media. Phys Rev E 87(5):052208. https://doi.org/10.1103/PhysRevE.87.
052208Kawaguchi J, Fujiwara A, Uesugi T (2008) Hayabusa - its technology andscience accomplishment summary and Hayabusa-2. Acta Astronaut
62:639–647. https://doi.org/10.1016/j.actaastro.2008.01.028
Kendall K, McN. Alford N, Birchall JD (1987) A new method for measuring thesurface energy of solids. Nature 6107:794–796. https://doi.org/10.1038/
325794a0
Klimek R, Wright R (2006) Spotlight–8 Image Analysis Software. NASA TM
214084 2005. http://microgravity.grc.nasa.gov/spotlight/
Lauretta DS, Balram-Knutson SS, Beshore E, Boynton WV, Drouet d’Aubigny C,
DellaGiustina DN, Enos HL, Golish DR, Hergenrother CW, Howell ES,
Bennett CA, Morton ET, Nolan MC, Rizk B, Roper HL, Bartels AE, Bos BJ,
Dworkin JP, Highsmith DE, Lorenz DA, Lim LF, Mink R, Moreau MC, Nuth JA,
Reuter DC, Simon AA, Bierhaus EB, Bryan BH, Ballouz R, Barnouin OS,
Binzel RP, Bottke WF, Hamilton VE, Walsh KJ, Chesley SR, Christensen PR,
Clark BE, Connolly HC, Crombie MK, Daly MG, Emery JP, McCoy TJ,
McMahon JW, Scheeres DJ, Messenger S, Nakamura-Messenger K,
Righter K, Sandford SA (2017) OSIRIS-REx: sample return from Asteroid(101955) Bennu. Space Sci Rev 212(1-2):925–984. https://doi.org/10.1007/
s11214-017-0405-1
Machii N, Nakamura AM, Güttler C, Beger D, Blum J (2013) Collision of achondrule with matrix: relation between static strength of matrix andimpact pressure. Icarus 226:111–118. https://doi.org/10.1016/j.icarus.2013.
05.006Maurel C, Michel P, Biele J, Ballouz R-L, Thuillet F (2017) Numerical simulationsof the contact between the lander mascot and a regolith-covered surface.
Adv Space Res 62(8):2099–2124
Murdoch N, Rozitis B, Nordstrom K, Green SF, Michel P, de Lophem T-L,
Losert W (2013) Granular convection in microgravity. Phys Rev Lett
110(1):018307. https://doi.org/10.1103/PhysRevLett.110.018307
Sanchez P, Scheeres DJ (2014) The strength of regolith and rubble pile
asteroids. Meteorit Planet Sci 49:788–811. https://doi.org/10.1111/maps.
12293
Sánchez P, Scheeres DJ, Bierhaus EB, Clark B (2013) Regolith penetrometry inmicrogravity. Eur Planet Sci Congr 8:EPSC2013-882
Scheeres DJ, Hartzell CM, Sánchez P, Swift M (2010) Scaling forces to asteroid
surfaces: the role of cohesion. Icarus 210:968–984. https://doi.org/10.1016/
j.icarus.2010.07.009
Seguin A, Bertho Y, Gondret P (2008) Influence of confinement on granularpenetration by impact. Phys Rev E 78(1):010301. https://doi.org/10.1103/
PhysRevE.78.010301
Thornton C, Ning Z (1998) A theoretical model for the stick/bounce behaviourof adhesive, elastic-plastic spheres. Powder Technol 99(2):154–162
Thuillet F, Maurel C, Michel P, Biele J, Ballouz R-L, Richardson DC (2017)
Numerical simulations of surface package landing on a low-gravity
granular surface: application to the landing of MASCOT onboard
Hayabusa2. In: Lunar and Planetary Science Conference, Vol. 48. 48th Lunarand Planetary Science Conference. https://www.hou.usra.edu/meetings/
lpsc2017/pdf/1810.pdf
Tsuda Y, Yoshikawa M, Abe M, Minamino H, Nakazawa S (2013) System designof the Hayabusa 2 - asteroid sample return mission to 1999 JU3. Acta
Astronaut 91:356–362. https://doi.org/10.1016/j.actaastro.2013.06.028
Veverka J, Farquhar B, Robinson M, Thomas P, Murchie S, Harch A,
Antreasian PG, Chesley SR, Miller JK, Owen WM, Williams BG, Yeomans D,

Brisset et al. Progress in Earth and Planetary Science(2018) 5:73

Dunham D, Heyler G, Holdridge M, Nelson RL, Whittenburg KE, Ray JC,
Carcich B, Cheng A, Chapman C, Bell JF, Bell M, Bussey B, Clark B,
Domingue D, Gaffey MJ, Hawkins E, Izenberg N, Joseph J, Kirk R, Lucey P,
Malin M, McFadden L, Merline WJ, Peterson C, Prockter L, Warren J,
Wellnitz D (2001) The landing of the NEAR-Shoemaker spacecraft onasteroid 433 Eros. Nature 413:390–393. https://doi.org/10.1038/35096507
Wagner E, DeForest CE (2016) Opportunities for suborbital space andatmospheric research facilities on Blue Origin’s New Shepard crew capsule.
AGU Fall Meeting Abstracts P53C-2216. http://adsabs.harvard.edu/abs/
2016AGUFM.P53C2216W
Yamamoto S, Wada K, Okabe N, Matsui T (2006) Transient crater growth in
granular targets: an experimental study of low velocity impacts into glass
sphere targets. Icarus 183:215–224. https://doi.org/10.1016/j.icarus.2006.
02.002

Page 21 of 21