The instrument that spots killer asteroids and star-eating black holes

Editor’s Note: Don Lincoln is a senior scientist at Fermi National Accelerator Laboratory. He is the author of “The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind” and produces a series of science education videos. Follow him on Facebook. The opinions expressed in this commentary are his. View more opinion articles on CNN.

Astronomers have it rough. Their job is to scan the heavens, looking for something interesting. But often they see the same thing, night after night. After all, stars can burn placidly and essentially unchanged for billions of years. An unlucky astronomer could spend a lifetime turning his or her telescope here and there and, except for the march of the planets and an occasional comet crossing the sky, see nothing new.

The situation is a little better in our connected world. If an astronomer does happen to see something interesting, say a supernova, they can notify other night owls and they can all turn their cameras to the same spot in the sky to study the phenomenon. However, even that requires luck. Somebody has to first observe a notable event.

Don Lincoln

If only there were a way to watch the whole sky and automate the process. If only…

And, of course, there is. A massive sky-survey instrument, called the Zwicky Transient Facility (ZTF) is designed to scan the heavens and look for “transients” or things that brighten unexpectedly. When the instrument sees a change, alerts go out to other astronomers subscribed to the service, who can then use even more powerful telescopes to study the transient event in detail. Even the public can get a daily summary of the previous night’s happenings.

The facility began operations in March of 2018 and the astronomical team has recently publicly released the results of nearly a year of use. It’s an incredible contribution to the worldwide astronomical community. Using the instrument, astronomers have already made many important observations, including the detection of over a thousand supernovae.

ZTF uses the 48-inch Samuel Oschin Telescope, located at the Palomar Observatory. It is supported by the National Science Foundation and other international partners, and managed by Caltech.

Using a 576-megapixel camera, ZTF can take photos of a huge amount of the sky, each covering an area about 240 times more than the moon; the entire northern sky can be surveyed every three days and the plane of the Milky Way galaxy is imaged twice a night.

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The team of astronomers operating the ZTF spent some of 2018 taking baseline images of the sky, paying particular attention to the natural variability of the stars and galaxies they imaged.

Because alerts go out when ZTF observes something unexpectedly bright, knowing the baseline variability of objects in the sky is very important. After all, stars twinkle due to the effects of the atmosphere and some stars have varying brightness by nature. Alerts should be reserved for less mundane phenomena. So, once the natural variability is known, astronomers set the threshold for alerts high enough that only noteworthy events trigger a notification.

Each night, over 100,000 alerts go out. When they perform a focused survey in the plane of the Milky Way galaxy, that number can rise to a million. Then individual astronomers scan the alerts, find the ones that are most interesting, and take a look.

To date, ZTF has observed new supernovae, binary stars and even two black holes caught in the act of “eating” nearby stars. As stars wander near black holes, the strong gravitational fields can rip apart the star and the material will then spiral down into the black hole.

An artist's rendering of a black hole.

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This incredible process is one of many fascinating events that ZTF can detect. But finding big and salient events in the sky isn’t ZTF’s only capability. It can also search for small and thus far undiscovered objects in nearby space.

Our solar system consists predominantly of eight planets and a swarm of much smaller rocks clustered in an orbit between Mars and Jupiter called the asteroid belt. However, there are other asteroids that aren’t located in the belt. Some of them orbit in relatively close proximity to the Earth and are called Near Earth Objects, or NEOs. And sometimes they can even hit the Earth.

NEOs can be very small and burn up in our atmosphere as meteors. But some are bigger. For instance, in February 2013, a NEO about 50 feet in diameter entered the atmosphere above Chelyabinsk, Russia, before exploding with the equivalent force of 300 kilotons of TNT, shattering windows and knocking down some walls. And, about 50,000 years ago, a meteor about 150 feet across hit the Colorado Plateau in Arizona, releasing about 10 million tons of TNT worth of energy and making Meteor Crater.

ZTF has identified 45 NEOs – asteroids – with a size of about 300 feet in diameter and two NEOs that passed closer to the Earth than the moon. These were near misses. The asteroids came awfully close to the planet and each of them was double the size of the one that made the Meteor Crater. If they had hit the Earth, the energy release would have been larger than the largest hydrogen bomb mankind has ever made.

More are certain to be discovered. Luckily, space is big and the Earth is small, but we should still be aware of these potentially dangerous NEOs.

ZTF also found a very odd new denizen of the solar system zoo. Most solar system objects orbit more or less in the same plane as the Earth does. However, the instrument discovered an asteroid called 2019 AQ3 orbiting between Mercury and Venus, but with an orbit that is tipped not quite 90 degrees – almost perpendicular to the plane in which the planets orbit. While astronomers have some ideas as to how 2019 AQ3 could have come into being, the object wasn’t discovered until early January 2019, which means that they’re still in the head-scratching phase of understanding this curious object.

The data taken each night by ZTF is enormous – about 4 terabytes. And the alerts have to be processed in near-real time. They generally go out about 15 to 20 minutes after the data is taken. In many respects, the facility acts like a newsroom of a major network. The fast reaction capabilities are a big contribution to observational astronomy.

As if astronomy needed help to be fascinating.