NASA engineers have built a device that can suspend mice in the air for hours. The purpose is to understand how zero gravity affects the bone density and muscle mass of astronauts.

The levitation device, built by Yuanming Liu and colleagues at NASA’s Jet Propulsion Laboratory, uses a magnetic field that distorts the movement of electrons in water molecules to let the mice float.

According to New Scientist:

[The researchers] used a purpose-built levitation device containing a coil of wire, or solenoid, cooled to a few degrees above absolute zero so that it became superconducting. Running a current through the solenoid creates a magnetic field of 17 teslas, ten thousand times as strong as a typical fridge magnet and 10 million times that of the Earth.

The researchers have shown previously that the device can levitate water-based items for hours, but were skeptical that it would be able to make a mouse, weighing10-grams, float for long periods of time. Yet, they were able to “fly” the mouse for hours, allowing it to roam freely, and giving it food and water.

The experiment is a significant step to study bone and muscle loss, and even changes in blood flow in zero gravity, which is a common problem for astronauts when they return for space missions or extended stays on the space station. Engineers have built exercise equipment to combat the losses, which can result in long-term health issues, but there has been limited ways to actually study zero-gravity effects on humans on Earth.