How solid is the surface of asteroid Bennu?
When NASA's OSIRIS-REx touched asteroid Bennu, its sampler sank easily into rubble that was loosely packed and barely stuck together, more like a ball pit than solid ground.
Source
Near-zero cohesion and loose packing of Bennu's near subsurface revealed by spacecraft contact
Study at a glance
- Design
- Other — Analysis of OSIRIS-REx images and inertial measurement data from a single 10 cm/s sampling touchdown, interpreted with soft-sphere discrete element simulations and an empirical granular drag law
- N
- A single contact event on one asteroid; no sample size
- Population
- Regolith in the top 0 to 10 cm at the Hokioi crater sample site on asteroid Bennu
- Outcome
- Contact forces, penetration depth, inferred bulk density, packing fraction, compressive strength and cohesion
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
The team reconstructed the short window after the sampler head touched down, before its gas burst fired, using camera images taken every 1.2 seconds and the spacecraft's accelerometer. Shadows and particle sizes in the images fixed the penetration depth at a known time, and integrating accelerations gave how deep and how hard the sampler pushed. They compared the forces with discrete-element simulations of a sampler hitting granular beds of different packing, friction and cohesion, and with an empirical low-speed granular drag law.
What they found
The sampler reached about 6 to 7 cm depth in under a second while experiencing forces of only 10 to 15 N, and it disturbed an area about six times its own footprint. The simulations place this in the loose regime with packing fraction at or below about 0.5, and the drag law implies a near-surface bulk density of 440 to 600 kg per cubic metre, well below Bennu's overall density of 1190. The implied packing fraction is 0.2 to 0.45, compressive strength 2 to 200 Pa and cohesion roughly 0.2 to 20 Pa, far weaker than the Moon's surface.
The limits
What it doesn't show
This is one touchdown at one site, so the properties may not represent Bennu's whole surface. The results depend on assumptions such as a friction angle of about 40 degrees taken from local slopes, porosity of individual boulders inferred from thermal data, and an empirical force law developed from laboratory impacts. Contact with a 5 cm rock tilted the sampler and complicated the force record, and the cause of the very low cohesion (particle roughness, fine dust) remains unresolved pending analysis of returned samples.
Key terms
- Regolith
- The layer of loose rock fragments and dust covering a planetary or asteroid surface.
- Rubble-pile asteroid
- An asteroid made of boulders and gravel loosely held together by gravity after reassembling from a disrupted parent body.
- Packing fraction
- The fraction of a volume actually filled by solid particles rather than empty space.
- Cohesion
- The strength with which particles stick together independent of pressure, measured in pascals.
- Soft-sphere discrete element method
- A simulation that models each grain as a sphere allowed to overlap slightly, computing contact, friction and cohesive forces between them.
Flashcards
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Quiz yourself
What data were used to measure forces on the sampler?
Common questions
How can a spacecraft's accelerometer measure soil strength?
The forces resisting the sampler as it pushes into the surface slow the spacecraft; comparing those forces with granular physics laws and simulations reveals the density, packing and strength of the material.
Why is the surface density so much lower than Bennu's average?
The top layer is packed only about half as densely as normal random packing, so it contains large voids between particles.
Why does low cohesion matter for spacecraft?
A nearly cohesionless surface offers little resistance, so landers could sink or trigger large disturbances, as happened here.
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