NASA’s award to SpaceX for the launch contract for their future X-ray telescope, the Imaging X-ray Polarimetry Explorer (IXPE), points to SpaceX’s growing launch profile options for customers. This spacecraft would normally be a Pegasus mission due to its orbital requirements. However, SpaceX offered the capability via Falcon 9 and at a lower cost. IXPE – to be built by Ball Aerospace, who recently completed the spacecraft’s design review – is slated to launch on a Falcon 9 from LC-39A in April 2021.
Falcon 9 Capability:
The growing range of rockets available for NASA spacecraft has allowed the agency to shop around.
This particular spacecraft requires a launch to a Low Altitude Equatorial Orbit with zero degrees inclination, which – as experts such as Scott Manley point out – is normally the role provided by an air-launch system, such as NASA’s usual go-to launch vehicle, the NGIS Pegasus.
As noted, SpaceX has shown Falcon 9’s can provide the post-flight orbit capability, which Manley predicts will be about 3.7 kms per second Delta-v.
IXPE History
IXPE’s selection was announced by NASA on January 3, 2017, as part of the agency’s Explorers program – a low-cost set of scientific missions. It will have a 2-year primary mission – which may be extended, pending IXPE’s performance.
IXPE will contain 3 X-ray telescopes to measure the polarization of X-rays. This will allow scientists to conduct further research into the universe’s strangest objects, including black holes, pulsars, and neutron stars. The telescopes will have a 2-8 keV range, an 11-arcminute field of view, and ≤30-arcsecond angular resolution. IXPE’s detectors will be two orders of magnitude more sensitive than those on the Orbiting Solar Observatory.
A labeled diagram of IXPE, showing the major pieces of the spacecraft. Image credit: IXPE team
IXPE will be NASA’s newest X-ray telescope – its first since NICER, which launched to the International Space Station on the CRS-11 Dragon in June 2017.
X-ray astronomy offers a unique perspective on the universe that is unattainable through other methods. This type of astronomy is useful for imaging extremely hot objects in space, which begin to emit X-rays around temperatures of 1 million Kelvin. These objects include the centers of galaxies, supernova remnants, neutron stars, and black holes.
Since Earth’s atmosphere blocks almost all extraterrestrial X-rays from reaching the surface, X-ray astronomy is only possible with space telescopes.
IXPE will study a specific area of X-ray astronomy, called X-ray Polarization. Studying polarization will allow scientists to observe matter distribution, the spin of black holes, and more.
An example of an X-ray image. This picture of Centaurus A is a combination of 3 different energies of X-rays and was taken by the Chandra X-ray Observatory. Credit: NASA & Chandra team
IXPE will be built by Ball Aerospace, who is no stranger to building spacecraft. Their past experience includes building the Kepler space telescope, portions of the Hubble, Spitzer, and James Webb space telescopes, and the HiRISE camera on the Mars Reconnaissance Orbiter (MRO).
On July 10, 2019, Ball announced that they had completed the Critical Design Review (CDR) for IXPE.
A CDR is a detailed analysis of all parts of a mission. It ensures that the spacecraft design is acceptable and within the budget constraints for the mission. Once a spacecraft passes its CDR, major production, assembly, and testing can begin for the spacecraft.
Two days earlier, on July 8, NASA announced that SpaceX had been awarded the launch contract for IXPE. It will be launched on a Falcon 9 to a 540km circular equatorial orbit. The launch will take place from LC-39A at Kennedy Space Center in April 2021.
In the original proposal, IXPE was to launch on a Pegasus rocket from Kwajalein sometime around November 2020. However, this was likely switched to a Falcon 9 due to the substantial delays of the Pegasus that will launch the ICON spacecraft. ICON was planned to launch in late 2017, but that date has been pushed back multiple times due to issues with its Pegasus rocket.
Since IXPE was designed to launch on Pegasus, it had to fit within the rocket’s small fairing. Because of this, it will launch in a compact configuration. Once it separates from the Falcon 9 second stage, it will deploy its solar panels, payload boom, and X-ray shield.
IXPE shown in its launch configuration. In this diagram, it is shown inside of a Pegasus fairing. Image credit: IXPE team
In addition to having fewer delays, the Falcon 9 also offers a very low launch price for its capabilities. The total launch costs for the mission are only $50.3 million – lower than the SpaceX’s advertised price of $62 million. This lower number indicates that the vehicle will likely include a flight-proven first stage.
Compared to Falcon 9’s regular payloads, IXPE is very small. Weighing in at approximately 320kg, it will make for an easy mission for the workhorse rocket. Due to its low mass and orbit, this mission could likely feature a Return-to-Launch-Site (RTLS) landing of the first stage at SpaceX’s Landing Zone 1. However, this depends on both the flight profile and how much extra performance margin NASA requests.
Currently, SpaceX has 13 missions planned for 2021 – including IXPE. This number excludes potential ISS commercial resupply, crew transportation, and Starlink missions. Should the current schedules hold, IXPE will be SpaceX’s seventh NASA spacecraft launch.