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| LARES |
One of the most subtle effects predicted by general relativity is a phenomenon known as rotational frame-dragging. This is caused by a massive spinning body, such as a planet, dragging space-time with it as it turns. That causes any small rotating particles in the vicinity to precess.
This disco ball is an extraordinary object. It is entirely passive, with no thrusters or electronic components. Instead, it is a tungsten sphere about the size of a football, weighing 400 kg and covered with 92 reflectors that allow it to be tracked using lasers on Earth. These reflectors also make it look like a disco ball.
The ball’s small size large mass make it the most perfect test particle ever placed in orbit, the first aerospace structure ever made from tungsten and the densest object orbiting anything anywhere in the Solar System.
The ball is known as the LAser RElativity Satellite or LARES. The Italians launched it in February last year and have been carefully measuring its orbital characteristics ever since.
Physics arXiv:
LARES Successfully Launched Into Orbit: Satellite and Mission Description
Read more: The Extraordinary "Disco Ball" Now Orbiting Earth
From MIT Technology Review
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