Some 50,000 years ago, a herd of mammoths trotted across the grassy plains in Northern Arizona, unaware of the impending doom hurtling towards them at 30,000 mph. As a jumbo jet-sized space rock slammed into Earth, it left behind an iconic scar that has become one of Arizona's most recognizable natural features: Meteor Crater, about 37 miles east of Flagstaff.

On Saturday at 12:51 p.m. Arizona time, an asteroid estimated to be about the same size will whiz past the Earth at less than half the distance between Earth and the moon. In astronomical terms, that's a near miss. A team of University of Arizona students is ready to observe the action and gather as much data as possible as part of the Rapid Response Characterization Campaign, organized by the International Asteroid Warning Network.

Vishnu Reddy, a professor in the UArizona Lunar and Planetary Laboratory, who is leading the global campaign, said the idea is to practice and test procedures that could be useful in mitigating an impending asteroid impact in the future. Reddy was quick to point out that the asteroid zipping past Earth this week has no chance of colliding with the planet.

The discovery of near-Earth asteroid 2023 DZ2 was announced March 16 by the Minor Planet Center, the single worldwide clearinghouse for asteroids and comets, which is managed by the International Astronomical Union. One day later, the campaign was launched, with the goal of using the close encounter as a "fire drill" of sorts, to spin up a worldwide network of observers that could prove vital in the future, should an object be discovered that is headed for impact instead of just zipping by. 

"The idea is to use this opportunity as a rehearsal of sorts," Reddy said. "We are using this asteroid as a stand-in for a scenario in which a space rock is detected that actually is headed toward the Earth. If that were to happen, we can apply lessons learned from exercises like this one – for example, what are the most important steps the international community would have to take to avert or minimize the risk of an impact and its possible aftermath?"

DZ2 was first observed on Feb. 27. Initial orbital calculations suggested that the asteroid had a high probability of hitting Earth in 2026. As subsequent observations allowed observers to calculate its orbit around the sun with more precision, it became clear that an impact is unlikely in the foreseeable future. And while a close shave is much preferred to an impact, the UArizona team is excited to have access to front-row seats to the cosmic encounter during the observing campaign.

As recently as last week, impact probability for 2026 was estimated at 1 in 430. A few days later, it stood at 1 in 71,000, according to Adam Battle and David Cantillo, two doctoral students at the Lunar and Planetary Laboratory who are leading the characterization of DZ2. Reddy's team also includes Benjamin Sharkey, a postdoctoral research associate in the College of Engineering, and Juan Sanchez, a staff scientist at the Planetary Science Institute.

Cantillo said he was pulling into a Safeway parking lot when he received a call from Reddy.

"He said, 'David, there is this new object, and it might pose a risk of impact in a few years. We really want to get some data on it, and closest approach is next week,'" Cantillo said. "And I just thought, wow!"

Although DZ2 does not pose a threat to Earth, it serves as a reminder that there is a sizeable population of asteroids out there that could. This is true particularly for the size range DZ2 falls into, which makes it not quite big enough to cause mass extinctions and small enough to slip through the cracks of routine asteroid detection programs such as NASA's Spacewatch program and the UArizona-led Catalina Sky Survey. DZ2 is believed to most likely be an asteroid that was part of the asteroid belt between Mars and Jupiter before orbital mechanics and gravitational tugs nudged it off course and toward an orbit around the sun that takes it past Earth on a regular basis.

Because of projects like Spacewatch, Catalina Sky Survey and other dedicated survey projects, most asteroids that could pose a significant threat to Earth larger than 1 kilometer (0.62 miles) have been discovered and cataloged and their orbits are being monitored.

"None of these asteroids typically pose a threat," Cantillo said. "Our goal is to learn more about their composition and specifically look at the smaller size range of near-Earth asteroids, because they're much harder to detect. DZ2 happened to check all of those boxes with our ongoing work, while also having this extra timely component of a close approach this week."

During the asteroid's closest approach on Saturday, Cantillo and his colleagues expect it to increase in brightness, although it likely won't be visible to the unaided eye.

"We hope to collect a lot of valuable data on the object," said Cantillo, who will observe remotely from Tucson with NASA's Infrared Telescope Facility on Mauna Kea in Hawaii. "Specifically, we're studying how the asteroid reflects light across different wavelengths, and that in turn will reveal certain features that are diagnostic of surface minerals."

"Once we have a better idea of the composition of the asteroid, it will tell us something about its density, which is helpful for understanding how much energy it would impart during an impact," said Battle, explaining that there are a wide variety of asteroids out there, each with their own implications for planetary safety.

Battle is using the RAPTORS telescope, designed and built by UArizona undergraduate students under Reddy's mentorship, to do spectroscopy – analyzing DZ2's reflection of sunlight in the visible light spectrum. Those observations include studying its light curve – the way its brightness changes over time.

"If you're looking at a spherical object, you won't see any or very little changes in brightness because the area reflecting the light is more or less the same while the asteroid rotates," Battle said. "But let's say you have something that's shaped like a potato and tumbling end-over-end – it'll appear brighter when the long edge is facing us than when its narrow end is facing us. So, as that asteroid tumbles through space, you would see a periodic brightening and dimming."

The speed at which an asteroid spins can be a giveaway about what it's made of, Battle said.

"Rubble pile asteroids can only spin so fast, or they would fly apart," he said. "If it rotates very fast, on the other hand, that tells us it's likely monolithic, just one big boulder. Having just one huge rock hurtling toward us carries a lot more significance than if we're facing a ball of debris, even if the two are of similar size."

For now, Battle and Cantillo are hoping for good weather and clear skies.

"It's been a really fun process to be a part of," Cantillo said. "Having the chance to work with NASA and other collaborators and being part of that team that gets a close-up and personal look at it has been very rewarding."