17 Jun 2002
A hollow aluminium ball covered in mirrors is helping NASA scientists study the upper atmosphere.
By studying the way in which an aluminium sphere covered in mirrors falls back to Earth, NASA scientists are gaining valuable information into how the density of the upper atmosphere changes in response to solar activity. Earth-based observers have been monitoring reflected sunlight from the spinning mirror-ball as part of a project called Starshine.
Project director Gil Moore said: "The thickness and density of Earth's atmosphere changes with solar activity. Storms on the Sun can produce outbursts of ultraviolet radiation that causes the upper layers of the Earth's atmosphere to expand. We don't have a lot of information about these changes and that's why Starshine is important."
The latest Starshine satellite is 49 cm in diameter, weighs 40 kg and is covered with 845 polished aluminium mirrors. After polishing, each mirror is coated with a layer of silicon dioxide to protect it from corrosion by atomic oxygen present in the atmosphere.
The Starshine satellite will explore a layer of the atmosphere called the thermosphere, roughly between 150 and 500 km from the Earth's surface. Following its deployment into an orbit of 387 km, a nitrogen gas system sets the satellite spinning at a rate of 5 degrees per second.
Moore explained: "We are very interested in spin. As the satellites skim along the upper atmosphere they lose kinetic energy, start to de-spin and begin to fall. As a result, by monitoring Starshine's orbit and studying how it decays we can calculate the density of the gas that is dragging it down."
Flashes of sunlight are reflected from the satellite every three or four seconds and are easily observed on Earth. Monitoring the exact time difference between the flashes gives the researchers information about the density of the gases in the thermosphere.
The Sun's solar cycle is 11 years long. The researchers' goal is to monitor the upper atmosphere during all phases of the solar cycle using Starshine satellites launched every year. Two further satellites are planned.
Jacqueline Hewett is news reporter on optics.org and Opto & Laser Europe