After 8 and a half minutes, Booster 1061 landed on the droneship Just Read The Instructions, successfully completing its 5th flight into space.
Back in space, the fairings separated, exposing the 300-kilogram satellite to space. Then the second stage shut down and began a twenty minute coast phase. At second stage restart over the middle of Africa, the Falcon 9 did something remarkable: a 27-degree plane change to almost completely zero out its inclination and circularize the orbit at 600 kilometers. Although this required about 3.5 kilometers of delta velocity, because the second stage was nearly perpendicular to the direction of travel during the burn, the vehicle’s orbital speed hardly changed.
Why go to all of this effort to launch a small satellite? Part of the answer is because of money, as many things are. The mission was originally designed to be launched on a much smaller rocket, a Northrop Grumman Pegasus rocket, from under a plane over the ocean at the equator. Ultimately Pegasus was not selected due to cost, and SpaceX was awarded the mission launching on a much bigger Falcon 9 rocket from Florida.
The other part of the answer is because of the scientific data it is intended to collect: X-rays. The telescope needed to go into a low inclination orbit to avoid the South Atlantic Anomaly (SAA), an area where the lower Van Allen radiation belts get closest to the Earth. Flying through the SAA would interfere with the sensitive detectors. In order to avoid contaminating the instruments, IXPE does not have any propulsion either for pointing or orbital maneuvering. IXPE will decay from orbit in less than 25 years, in compliance with orbital debris regulations.
X-ray telescopes are pretty cool. Because of the high energy of an X-ray photon, it is much more difficult to collect and focus the energy to create an image as they go right through a typical mirror. The mirrors in an X-ray telescope, however, are nearly parallel to the oncoming light, using what is known as grazing incidence to direct the X-ray photons to a focal point behind the mirrors, like a Cassegrain-type telescope.
The parallel mirrors don’t have a lot of surface area along the scope axis so instead of having one big flat(ish) mirror, an X-ray telescope has many nested rings of mirrors like an onion to collect as much light as possible. IXPE’s telescopes have 24 rectangular mirror segments each, arranged around a cylindrical support.
IXPE brings a new way of observing X-rays to orbit. This new mission improves on Chandra by making it possible to measure the orientation – technically called polarization – of the X-ray photons. It also has the ability to simultaneously make spectral, spatial, and time-based measurements. This will allow it to accomplish tasks like determining how pulsars accelerate particles and to measure the magnetic field configuration in magnetars. With IXPE, we’ll have a whole new set of ways to see the higher energy parts of our Universe.