Our recent discussions of X-ray beaming to propel interstellar lightsails seem a good segue into Don Wilkins’ thoughts on the Chandra mission. Chandra, of course, is not a deep space probe but an observatory, and a revolutionary one at that, with the capability of working at the X-ray wavelengths that allow us to explore supernovae remnants, pulsars and black holes, as well as making observations that advance our investigation of dark matter and dark energy. This great instrument swims into focus today because it faces a funding challenge that may result in its shutdown. It’s a good time, then, to take a look at what Chandra has given us since launch, and to consider its significance as efforts to save the mission continue. We should get behind this effort. Let’s save Chandra.

by Don Wilkins

On July 23, 1999, the Chandra X-ray Observatory deployed from Space Shuttle Columbia. Chandra along with the Hubble Space Telescope, Spitzer Space Telescope (decommissioned when its liquid helium ran out) and Compton Gamma Ray Observatory (de-orbited after gyroscope failure), composed NASA’s fleet of ‘Great Observatories,’ so labeled because of their coverage from gamma rays through X-rays, visible light and into the infrared, spanning the electromagnetic spectrum. This scientific fleet expanded – and continues to expand – our understanding of the cosmos.

A two-stage Inertial Upper Stage booster put the observatory into an elliptical orbit. Apogee is approximately 139,000 kilometers (86,500 miles) — more than a third of the distance to the Moon – with perigee at 16,000 kilometers (9,942 miles) and an orbital period of 64 hours and 18 minutes. The numbers are significant: The consequence of this orbit is that about 85 percent of the time, Chandra orbits above the charged particle belts surrounding the Earth, allowing periods of uninterrupted observing time up to 55 hours.

Figure 1. The Chandra Observatory monitors X-rays, a critical and unique window into the hottest and most energetic places in the Universe. Credit: NASA.

A Huge Return on Investment

Chandra’s achievements include:

• The first image of the compact object, possibly a neutron star or black hole, at the center of the supernova remnant Cassiopeia A.
• The detection of jets from and a ring around the Crab Nebula.
• The detection of X-ray emitting loops, rings and filaments encircling Messier 87’s supermassive black hole.
• The discovery, by high school students, of a neutron star in the supernova remnant IC 443.
• The detection of X-ray emissions from Sagittarius A*, the black hole at the center of the Milky Way. Chandra observed a Sagittarius A* X-ray flare 400 times brighter than normal, and found a large halo of hot gas surrounding the Milky Way.
• The X-ray observation of a supernova, SN 1987A, shock wave.
• The observation of streams of high-energy particles light years in length flowing from neutron stars.
• Observations of planets within the Solar System found X-rays associated with Uranus that may not be the reflection of solar X-rays. Chandra also found that Jupiter’s X-ray emissions occur at its poles, and it detected X-ray emissions from Pluto.
• Found a dip in X-ray intensity as the planet HD 189733b transited its parent star. This is the first time a transit of an exoplanet was observed using X-rays.
• Detected the shadow of a small galaxy as it is cannibalized by a larger one.
• Found a mid-mass black hole, between stellar-sized black holes and galactic core black holes, in M82.
• Associated X-rays with the gamma ray burst GRB 91216 (Beethoven Burst). Chandra observations also associated X-rays with a gravitational source (Figure 2).
• Produced controversial data suggesting RX J1856.5-3754 and 3C58 are quark stars.
• Measured the Hubble constant as 76.9 km/s per megaparsec (a megaparsec is equal to 3.26 million light years).
• Found strong evidence dark matter exists by observing super cluster collisions. Observations placed limits on dark matter self-interaction cross-sections.
• Combined with the outputs of other telescopes yielded insights into astronomical phenomena, Figure 3.

Building Chandra was a significant technical challenge. Mirror alignment accuracy, from one end of the mirror assembly to the other, a distance of 2.7 meters or 9 feet, is accurate to 1.3 micrometers (50 millionths of an inch). X-rays are focused using complex geometrical shapes which must be precisely formed and kept ultraclean. Even today, no X-ray telescope provides the sharpness of image that Chandra does.

Figure 2 Chandra made the first X-ray detection of a gravitational wave source. The inset shows both the Chandra non-detection, or upper limit, of X-rays from GW170817 and its subsequent detection. The main panel is the Hubble image of NGC 4993. Credit: NASA.

Figure 3. The image is a combination of the outputs of three telescopes. Webb highlights infrared emission from dust; Chandra the X-ray emissions and Hubble shows stars in the field. Credit: NASA/CXC/SAO

Chandra’s Demise?

This list of discoveries may be ending. Although it is estimated Chandra has another decade of operation, proposed budget cuts eliminate the $74 million necessary for Chandra operation, maintenance and analysis of data. Unless the funding is restored by way of a budget line item directing NASA to spend the funds on Chandra, the agency’s X-ray observation is stopped for years until a new telescope can be brought on line. The skills of highly talented scientists and engineers working on Chandra will be dissipated.

The problem is significant. NASA’s own NuStar instrument (Nuclear Spectroscopic Telescope Array) and the joint NASA/JAXA XRISM effort (X-Ray Imaging and Spectroscopy Mission) have their virtues, as does the European Space Agency’s XMM-Newton (X-ray Multi-Mirror Mission). But Chandra’s eyes are sharper, with imagery 10 times crisper than XMM-Newton’s, 30 times sharper than NuStar, and fully 150 times sharper than XRISM.

A possible successor is in the planning stages. With 100 times Chandra’s resolution, the Lynx X-ray Observatory could study individual stars more than 16,000 light-years away, or 12.5 times the range of Chandra, using an X-ray Mirror Assembly that is the most powerful X-ray optic ever conceived. Even so, it is probable three decades or more would elapse before the new observatory would orbit and it might well involve significant cost overruns. ESA plans the Athena mission (Advanced Telescope for High ENergy Astrophysics), a large-aperture grazing-incidence X-ray telescope, but Athena would not come online until the late 2030s. In the meantime, we have a healthy Chandra.

Although we are dealing with a 25 year old spacecraft, its efficiency is both high and stable, and enough fuel remains for another decade of operations, while the cost of operations has remained stable for decades. Considered as a bridge to Lynx, Chandra is at the mercy of Congress largely through the FY25 appropriations process. The particulars of how to support this great observatory are available on the Save Chandra website, which is the voice of a grassroots effort within the astronomical community.

Saving Chandra would avert what some are describing as “an extinction-level-event for X-ray astronomy in the US.” An open community letter with 87 pages of signatures recently submitted to NASA Science Leadership strongly supports this statement.