Looking Back, Looking Forward

The largest gathering of astronomers ever is taking place right now! The 245th meeting of the American Astronomical Society (AAS) kicked off yesterday in National Harbor, Maryland. This is a chance for astronomers to share their discoveries, to talk shop with other specialists—and also to catch up on science that they might not be thinking about on a daily basis.

Plenary talks at a large conference often provide an opportunity to give colleagues a broad overview of the current state of a topic with which they may only have passing familiarity.

But sometimes, a plenary is about celebrating past achievements. In the first two days of the AAS conference, talks have focused on two very different NASA achievements—the asteroid-smashing Double Asteroid Redirection Test (DART) mission and the long-running Chandra X-ray Observatory.

(And of course, any good speaker looks forward as well as back, so both talks addressed what the future can look like in their disparate fields of study.)

So, first off, DART. (BTW, I gave the URL to the NASA website above, but it’s pretty cute if you justdo a Google search for “nasa dart” and see what happens…)

Yesterday, Jason Kalirai from the Johns Hopkins Applied Physics Laboratory gave a talk about DART. He began his talk describing the “global concern” of mitigating asteroid hazards. You may have heard what happened to the dinosaurs, but hey, they didn’t have a space program!

The basic objective of the DART mission was to change the orbit of an asteroid. Kind of a practice run for a future civilization-threatening event. DART’s specific target? Dimorphos, a smallish asteroid about 180 meters (a bit less than 600 feet) in diameter, which orbits a larger companion, Didymos, which is about 780 meters (a little over 1,900 feet) across.

Actually, Kalirai spelled out DART’s specific objectives in greater detail. First, impact Dimorphos! Second, change the binary orbital period, with the goal of creating a 273-second change in its orbital period. Third, measure the period change. And finally, measure the momentum enhancement factor.

What’s the momentum enhancement factor, you say? Glad you asked…

If you want to deflect an asteroid, you probably want to do it as efficiently as possible. In the simplest case, you might imagine transferring all of the momentum of a spacecraft to the asteroid. But you can actually do better than that! If you create a substantial amount of ejecta, the momentum carried away by the ejecta adds to that transferred from your spacecraft, making it possible to multiply the effect of your spacecraft impact.

Transferring just the momentum of the spacecraft wouldn’t be very, well, enhanced, so that’s a factor of one. Double the momentum? That’s two. Triple? Three. You get the idea…

DART succeeded in all those goals.

First off, it impacted Dimorphos at 7:14 p.m. Eastern on Monday, September 26, 2022. (You can watch a recording of the NASA livestream here.)

DART reduced the average distance between the two asteroids, which caused the orbital period to shorten by 32 minutes and 42 seconds (1,962 seconds, about eight times more than the proposed minimum change of 273 seconds), and they estimated the momentum enhancement to be 3.6. Not bad!

If we want to deflect a potentially hazardous asteroid, slamming a spacecraft into it might not be a bad idea…

Kalirai ended his talk advocating for a complete census of asteroids large enough to cause regional devastation. He highlighted the Near Earth Object Surveyor (NEO Surveyor) mission, which he said would find 65% of Potentially Hazardous Asteroids (PHAs), those greater than 140 meters in diameter, within five years—and more than 90% of PHAs in a decade.

Most talks at AAS don’t have such dire consequences.

At this morning’s first plenary, David Pooley from Trinity University talked about Chandra, but before we dig in, let’s cover some basics…

Most of what we know about the universe comes from studying light.

Light comes in many flavors and, well, colors—or, as astronomers would describe it, many wavelengths. Light with longer wavelengths carries less energy, while light with shorter waves carries more energy. In terms of light we can see, red light carries less energy with a longer wavelength than blue or violet. This is the basic idea behind the spectrum that is created when you use a prism to split up “white” light.

But there’s plenty of light in the Universe that we humans cannot see. Infrared light and radio waves, for example, carry less energy than red light, with even longer wavelengths. Ultraviolet light and x-rays are far more energetic than blue or violet light, with much shorter wavelengths. In total, we collectively refer to all wavelengths of light as the electromagnetic spectrum.

Toward the end of the 20th century and at the very beginning of the 21st, NASA launched its Great Observatories program. Each mission looked at a different part of the electromagnetic spectrum. Hubble quite famously observes at visible wavelengths, with a few instruments that observe in infrared and ultraviolet light. The Spitzer Space Telescope focused on infrared light, even less energetic than what Hubble can detect. And Chandra deals with x-rays. By studying the sky in different wavelengths of light, these great observatories (along with others I haven’t mentioned) give us a fuller picture of what is going on in the Universe around us.

Chandra launched in 1999, so last year, it celebrated 25 years of observing. With its unprecedented sensitivity and resolution, it revealed aspects of the Universe that had remained hidden from astronomers—and it’s still in operation! A quarter century is plenty of time for a spacecraft to produce amazing results.

As Pooley noted, Chandra can record the position on the sky, the energy, and the arrival time of every photon it detects. As an example, he showed a spectacular image of supernova remnant E0102-72 with its x-ray emission tracing the presence of oxygen, neon, and other elements at different levels of excitation—indicative of the high-energy environment around the dead star.

That’s a goldmine for astronomers!

Of course, Pooley went on to highlight some of Chandra’s significant discoveries.

He noted that “Chandra determined the increase in mass of galaxy clusters over the last seven billion years confirming that general relativity works as expected on large scales. The cluster work, in combination with other studies, provides the strongest evidence to date that dark energy is the cosmological constant.”

He also described how Chandra revolutionized the study of supernovae remnants, giving us a better understanding of how massive stars end their lives—and delivering some beautiful images along the way!

And for those of you who saw my article yesterday about supermassive black holes, it’s worth knowing that Chandra has given us a deeper understanding of the black hole at the center of the Milky Way.

And Chandra famously helped us get a “look” at the influence of dark matter in the Bullet Cluster.

Finally, looking to the future, Pooley advocated for the Lynx X-ray Observatory as a successor to Chandra because “Lynx will continue Chandra's legacy of transformative discovery.”

But he also noted that Lynx won’t come along for another couple of decades! So it’s important to keep Chandra running because “its greatest discoveries lie ahead.” NASA had considered shutting down the Chandra program last year, but a massive Save Chandra effort convinced lawmakers to preserve it.

Here’s to many more years of Chandra discoveries.

Stay tuned for more news from the largest gathering of astronomers tomorrow!