Science - 06.12.2019

possible explanation for the activity of the
smaller active asteroids ( 22 ). In this scenario,
rapidly rotating asteroids experience centrif-
ugal forces greater than the centripetal forces
from self-gravity, leading to particle ejection

preferentially from low latitudes. Particles

launched from Bennu’s surface would have a

maximum velocity equal to the equatorial

surface velocity of 10 cm s–^1 (on the basis of

Bennu’s ~250-m equatorial radius and ~4.3-hour

rotation period). This mechanism would pref-

erentially produce particles in equatorial orbits

in the rotational direction. It is not capable of

launching particles on retrograde or hyperbolic

trajectories, as we observed.

Laurettaet al.,Science 366 , eaay3544 (2019) 6 December 2019 5of10

X Z

Y

X

Z

Y

Particle 5

Particle 6

Particle 1

Particle 2

Particle 3

Particle 4

19 20 21 22 23 24 25

2019 Jan date

0

0.5

1

1.5

2

2.5

Alt. - km

Particle 1

20.5 21 21.5 22 22.5 23 23.5 24 24.5 25

2019 Jan date

0

0.05

0.1

0.15

0.2

0.25

0.3

Alt. - km

Particle 2

18 18.5 19 19.5 20 20.5 21 21.5

2019 Jan date

0

0.1

0.2

0.3

0.4

0.5

Alt. - km

Particle 3

25.5 26 26.5 27 27.5 28

2019 Jan date

0

0.05

0.1

0.15

0.2

0.25

0.3

Alt. - km

Particle 4

18.2 18.25 18.3 18.35 18.4

2019 Jan date

0

0.05

0.1

0.15

0.2

0.25

Alt. - km

Particle 5

10 10.2 10.4 10.6 10.8 11

2018 Dec date

0

0.5

1

1.5

Alt. - km

Particle 6

A B

C

D

E

G

H

F

Fig. 4. Gravitationally bound particle altitudes and trajectories.(A)Trajec-
tories and (BtoE) altitude above Bennu’s surface over time for four orbiting
particles (particles 1 to 4) (fig. S7 and tables S4 and S5). On the altitude
plots, red circles mark the times of observations used in the trajectory estimate.
Axis scales of the altitude plots differ. For particles 2 and 3, it is not clear whether
the last revolution depicted occurred or whether the particle impacted at the
previous periapsis passage. The ragged appearance of the curves is a result of

the rough topography of the surface of Bennu. (F) Trajectories and

(GandH) altitude above Bennu’s surface over time, as in (A) to (E) but for

two suborbital particles (particles 5 and 6) (fig. S7 and tables S4 and S5).

The trajectories are seen from above Bennu’s north pole [xaxis toward the

Sun,zaxis close to Bennu’s north (positive) pole,yaxis roughly in the

direction of Bennu’s heliocentric velocity]. Particle 6 is the earliest evidence

of a particle in imaging data (10 December).

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