Fermi Epicycles

The Fermi Gamma-ray Space Telescope (formerly the Gamma-ray Large Area Space Telescope, or GLAST) orbits planet Earth every 95 minutes. It rocks north and south on alternate orbits in order to survey the sky with its Large Area Telescope (LAT). The spacecraft rolls so that solar panels stay pointed at the Sun for power, and the axis of its orbit precesses like a spinning top, making a complete rotation once every 54 days. As a result of these multiple cycles, the paths of gamma-ray sources trace out complex patterns from the spacecraft’s perspective. The plot above shows the path of the brightest persistent gamma ray source in the sky, the Vela Pulsar. The plot is centered on the LAT instrument’s field of view and spans 180 degrees. It follows the pulsar over the two years from August, 2008, through August, 2010. The Vela Pulsar is a neutron star spinning 11 times a second. It is the remnant of the explosion of a massive star within our Milky Way galaxy.

Image Credits: NASA