WASHINGTON — A SpaceX Falcon 9 launched a small NASA X-ray astronomy satellite Dec. 9 to study black holes and neutron stars, an “appetizer” for the launch later this month of the much larger James Webb Space Telescope.
The Falcon 9 lifted off from Launch Complex 39A at the Kennedy Space Center at 1 a.m. Eastern. The rocket’s upper stage released the payload, NASA’s Imaging X-ray Polarimetry Explorer (IXPE) spacecraft, 33 minutes later.
IXPE, built by Ball Aerospace with telescopes from NASA’s Marshall Space Flight Center and detectors from the Italian space agency ASI, will study several dozen objects, such as black holes and neutron stars, looking to see how the X-rays those objects emit are polarized.
“By doing this mission, we are adding two variables to the astrophysics tool kit to understanding these sources, that’s the degree of polarization and direction associated with the polarization,” said Martin Weisskopf, IXPE principal investigator at NASA Marshall, during a prelaunch news conference Dec. 7.
An example he gave were pulsars. “We have three theories about how the X-rays are produced” by those rapidly spinning neutron stars, he said. “They all predict different polarization dependence.”
The science and operational requirements of IXPE drove it to an unusual orbit, at an altitude of 600 kilometers and an inclination of about zero degrees. “The inclination is very important to us because, in an equatorial orbit, the cosmic ray background is minimum,” Luca Baldini, a co-principal investigator for the mission at Italy’s National Institute for Nuclear Physics, said at another briefing Dec. 7.
The altitude, added Brian Ramsey, deputy principal investigator at NASA Marshall, was driven by a NASA orbital debris mitigation requirement to deorbit in 25 years. “This is the highest orbit we can put it in and still meet that requirement,” he said, with current predictions estimating it will reenter in 18 years.
That orbit drove the development of IXPE, with the spacecraft designed to fit in a Pegasus XL rocket. That air-launched vehicle would have flown out of Kwajalein Atoll in the Pacific Ocean to place the spacecraft into that equatorial orbit.
However, in a surprise, NASA selected SpaceX in 2019 to launch the mission on a Falcon 9 from Florida. SpaceX bid $50.3 million for launching IXPE, significantly less than previous NASA awards for Pegasus XL launches.
At an estimated mass of 325 kilograms, IXPE is the smallest dedicated payload launched on a Falcon 9, said Julianna Scheiman, director of civil satellite missions at SpaceX, during the prelaunch briefing, but the inclination change took up much of the available performance on the vehicle.
That required the first stage to land on a droneship in the Atlantic Ocean rather than return to Cape Canaveral, an option for Falcon 9 launches of some smaller payloads. The booster, which previously launched the Crew-1 and Crew-2 commercial crew missions, the SXM-8 communications satellite and the CRS-23 cargo mission, did land successfully on the droneship.
Another IXPE requirement led to the use of LC-39A, rather than nearby Space Launch Complex 40, for the launch. “The IXPE spacecraft is very sensitive to acoustic effects” from launch, said Tim Dunn, launch director in NASA’s Launch Services Program, at the prelaunch briefing. LC-39A has a better sound suppression system than SLC-40.
IXPE has a two-year primary mission, but Baldini said there were no consumables on board to exhaust and the X-ray detectors are not expected to degrade. “If everything goes well, I think it’s very possible we can aim for an extension.”
IXPE is the first of two NASA astrophysics missions scheduled to launch in December. In French Guiana, preparations continue for the launch the James Webb Space Telescope on an Ariane 5 Dec. 22. NASA and ESA announced Dec. 6 that they had completed fueling JWST with hydrazine and dinitrogen tetroxide for its thrusters. That fueling is one of the final steps before JWST is installed on the Ariane 5.
While both IXPE and JWST are astrophysics missions, they are very different programmatically. IXPE is a Small Explorer, or SMEX, mission, with a total cost including launch of $214 million. The flagship-class JWST is projected to cost $9.7 billion through launch and its first five years of operations.
“We have a two-course dinner here,” said Thomas Zurbuchen, NASA associate administrator for science, during coverage of the launch on NASA TV. “This is the appetizer and the main dish is coming in two weeks, when we’re going to launch the Webb Space Telescope.”