In 2024, ESA flew bouncing hexapods to 256 km on a Swedish sounding rocket; six minutes of microgravity showed how complex grains collide and transfer energy

On November 26, 2024, a sounding rocket launched from a spaceport in northern Sweden, carrying an experiment that sounds almost like a toy: hundreds of tiny brass crosses, shaped like children's jacks, bouncing around inside a sealed box. The rocket climbed to 256 kilometers, providing scientists with some six minutes of weightlessness to see these particles collide, according to the European Space Agency. It’s called the JACKS experiment, and it’s trying to answer a big question with very tiny objects: how do grains of matter behave when there’s no gravity around to settle them down?

Why scientists needed weightlessness for this

On Earth, free particles like sand and gravel fall because gravity pulls them down, making it difficult to study how particles interact through collisions. However, it is possible in space. The rocket was named SubOrbital Express-4, and it was launched from Esrange Space Center, which is located near Kiruna and operates under the Swedish Space Corporation. The European Space Agency says it carried six scientific experiments in all, but JACKS concentrated on the so-called granular gas, which is a cloud of particles colliding with each other instead of forming a heap.


The particle structure of JACKS resembled that of hexapods, three rods arranged at right angles to one another, a structure researchers sometimes just call jacks. This form matters because many particles in space, such as dust in comet tails and material in planetary rings, are not perfectly round; they have irregular shapes. This experiment, conducted by a group of researchers from Brandenburg University of Applied Sciences and Otto von Guericke University Magdeburg in Germany, was aimed at exploring how these more realistic, complex shapes collide and dissipate energy compared to smooth, round particles.

What earlier tests with simpler shapes already showed

JACKS did not start from scratch. The same German research group had already tested simpler rod-shaped particles in earlier microgravity drop-tower experiments. The team describes what happened when they mixed rods of different sizes and weights inside a container, shook it, and watched the rods cool down in a 2024 study titled ‘Cooling of a granular gas mixture in microgravity’ published in npj Microgravity. According to the paper, the cooling of the mixture followed a pattern similar to a previously known mechanism of dissipation of energy from granular material, described back in 1983 as Haff's law. The other finding was that thicker, heavier rods had slightly higher kinetic energy than thinner, lighter rods across all mixtures as they settled down.

Bouncing crosses and a plot twist in the data

JACKS built on this research by replacing simple sticks with the more complex hexapod shape. A 2025 paper presented at the Powders and Grains conference, published in the EPJ Web of Conferences by members of the same JACKS team, offers an early look at how these cross-shaped particles behave. This paper also notes that previous work in this line of research has already shown that energy is not equally divided between a particle's rotational motion and its translational motion. Generally, the rotational share of a particle's energy runs lower than its translational share. In the hexapods in particular, it was observed that these crossings form clusters much tighter than rods, and their tangled nature makes them harder to detect on film. What is interesting, though, is that according to early computer modeling, the average kinetic energy of the cluster appears to be decreasing over time in a pattern similar to Haff's law, even in the presence of many clustered particles, a condition where that pattern is not usually expected to hold. The team was cautious to emphasize that this was a preliminary finding that needed further investigation, not a definitive one.

Why any of this matters outside a lab in Germany

Granular gas research may sound niche, but it has its fair share of big questions. Asteroids, comets, and the icy rings around planets like Saturn are essentially loose collections of grains held together mainly by gravity and gentle collisions, not by anything solid. Scientists need to understand how the irregular particles collide, spin and transfer energy so they can develop models for how such objects form and how they might behave if a spacecraft ever has to land on or near one. The same physics is also observed on Earth in factories with powders and granular materials, where collisions between oddly shaped particles often cause processing headaches.

What comes next

According to ESA, the JACKS experiment payload was retrieved from the launch site soon after the flight using a helicopter, and the team has said a full, detailed account of the results will be published separately. So the full story of what those six minutes showed is still being written. For now, the flight is a reminder that you don’t always have to send astronauts, or go to distant planets, to do useful space science. Sometimes all it takes is a box of little brass crosses, a Swedish rocket, and a few careful minutes of watching what happens when things bump into each other, when nothing is holding them down.