The strength of gravity is different on every body in the solar system.
Whether it’s the crushing weight of Jupiter or the miniscule pull of a small asteroid, this fundamental force of physics still has a major impact on the material those bodies are made up of.
A new paper from researchers at the University of Duisburg-Essen and the German Aerospace Center (DLR) showcases just how different it can be by letting planetary simulants freefall inside a giant drop tower and measuring how “fluffy” the space dirt got.
To start, the researchers chose three separate kinds of space dirt (also known as regolith) – fine basalt, coarse basalt, and spherical glass beads.
Basalt is the gold standard planetary simulant, at least for the Moon and Mars, as it has a similar density and angular crushing profile as samples returned from the Apollo missions.
The glass beads were added as a perfectly smooth and uniform control group.
The drop tower itself was the GraviTower Pro Bremen (GTB) in Germany, which provides up to 2.5 seconds of “free fall” – or as space researchers would call it, microgravity.
For this particular experiment, another feature of the tower was crucial – a linear motor was placed inside the capsule that changed the acceleration experienced by the samples from full free-fall to a partial gravity of either 150, 250, 500, 750, or 1000 mm/s2.
Typically these types of experiments are done via planes flying in parabolic arcs that allow for even longer periods of freefall.
However, planes themselves have mechanical vibrations that could act as something equivalent to a finger-tap, causing the grains of regolith settle more so than they would otherwise. In this case, a drop tower, even with the necessary acceleration modifications, seemed the best bet.
Also inside the capsule was another vibratory motor that vigorously shook the samples while in midair – and a camera that watched the entire process during the microgravity trip.
One expectation is that the van der Waals forces – the electrostatic forces that bond particles to one another but are typically completely overshadowed by gravity – would play an important role in the distribution of particles.
Simply put – it did. The authors noted that every sample – even the glass beads – occupied significantly more volume in lower gravity. However, each sample reacted differently to that environment.
The fine basalt increased in volume the most (by 19.6%) when settling at an acceleration of 250 mm/s2, whereas the coarse basalt had its largest volume increase (12.2%) at the 150 mm/s2 setting. The glass beads, on the other hand, exhibited a paltry 4.25% increase in volume.
So why the discrepancy? Geometry – jagged basalt grains can easily hook onto one another, and in low gravity this friction and the van der Waals forces are enough to push back against structures’ own weight.
The smooth spheres, on the other hand, lacked those jagged edges, so they lacked many of the hooks that allowed the other particles to maintain a larger volume.
There’s still some more work to be done, though. 2.5 seconds is not really a long enough time to let the samples fully settle. And the fact that the sample containers themselves could have introduced “wall effects” that would have skewed the data. But overall, this was a step towards understanding how regolith works on small bodies throughout the solar system.
Understanding that in detail will be critical for any asteroid mining efforts in the future, as designing the right type of excavation rig is one of the most important features of that effort.
Systems can easily fail if not equipped to deal with incredibly loose, cohesive dirt from outer space. So it’s best we continue to run more experiments to build on this one to make sure we are prepared for whatever’s out there.
Written by Andy Tomaswick/Universe Today.
