The decade-long search for Arrokoth, a strange new world in the outer solar system – Sky & Telescope

Just over nine years ago, a plucky robot with a massive satellite dish for a head and a heart made of radioactive plutonium completed one of the most audacious space missions yet conceived. After hibernating for nearly a decade of interplanetary travel, the probe, which its dedicated creators dubbed New Horizons, awoke as it approached the dwarf planet Pluto. By design, it was headed toward its destination at more than 50,000 miles per hour — too fast to slow down using its limited remaining fuel.

And so it was. In the blink of an eye, New Horizons snapped as many pictures as it could, frantically recorded data from its other instruments, and served as humanity’s first ambassador to the king of the Kuiper Belt before continuing its march toward our outer solar system.

And then it was over. Although the New Horizons probe was passing through a region that had never been visited by another mission before, there was no second destination planned in mind. Pluto had been the target, and after the successful flyby, the mission could have been declared a triumph and the team could have put New Horizons to rest again.

But the probe still had some fuel in its tanks—in a very literal sense. Although New Horizons was traveling on a safe route that wouldn’t take it near any known objects, its controllers could potentially use the onboard thrusters to steer the probe closer to another Kuiper Belt object for a first-of-its-kind investigation. The only glitch in this plan was that when New Horizons left Earth in 2006, astronomers didn’t know of any objects close enough to Pluto to be reachable once the main flyby was complete. The first Kuiper Belt objects (other than Pluto) had been discovered just a few years earlier, in 1992, and although scientists were beginning to suspect that millions of tiny planetesimals might dwell in the forests of our solar system beyond our planets, they hadn’t found many of them yet.

Unable to pass up the opportunity to study one of these likely ancient and preserved relics of the formation of our solar system, an international team of astronomers set out to find a second target for New Horizons.. Incredibly, most of this work was done While the spacecraft was actively flying towards its final destinationThe New Horizons team had thrown their hats over the wall, and with limited time before their robot drifted into interstellar space or succumbed to one of the many dangers of the vacuum, they needed to discover a new object quickly.

To spoil the ending: this industrious team was ultimately successful. They discovered a strange 32km-long snowman-shaped object called Arrokoth, and New Horizons performed its second flawless flyby of the Kuiper Belt on New Year's Day 2019. However, while this end result is well known, the effort that was put into this search was less well documented previously. This has recently been remedied with a narrative-type paper published in The journal of planetary science, detailing the decade of frantic work building the plane while it was flying that led to the discovery of Arrokoth, along with the creation of a comprehensive set of tools and best practices that guide many of today's solar system mapping efforts.

First days

The astronomers' work began in 2004, in the early days of Kuiper Belt research, when all the models were constantly being revised as new discoveries were made. How many Kuiper Belt objects were there? How big were they? What orbits did they travel? These were all fundamental and uncertain questions, but armed with the Subaru Telescope on Mauna Kea and optimism that discoveries would be made quickly, the team set out to answer them.

Between 2004 and 2005, researchers took deep images of wide swaths of the sky as they searched for the exact object New Horizons would track. With each new telescope they pointed, they captured images of stars that had not yet been recorded in any catalog and that had likely never been observed by any human before them. While exciting, this abundance of secondary discoveries actually complicated their search. There were so many stars in their images that it was extremely difficult to detect the faint, moving Kuiper Belt objects moving among them.

A thorough manual inspection of all the images ultimately turned up 24 new objects, but none were close enough to New Horizons’ trajectory to be a feasible target. After experimenting with some citizen science initiatives and a handful of new software routines to automate a portion of their workflow, the team regrouped and devised a new strategy for their search.

Warming up

Between 2011 and 2015, astronomers began a more intensive observing campaign. In the intervening years, several factors had emerged that tilted the odds of a successful detection in their favor. For one thing, new instruments had been designed and installed on several large telescopes, speeding up the search. For another, the area of ​​sky they had to scan was shrinking thanks to the changing arrangement of Earth, Pluto, and New Horizons.

In addition to the advantages, this time the researchers incorporated complex software tools into their analysis from the start. The huge amount of data expected to be obtained from this new search would put enormous pressure on the old method of carefully comparing two images with the naked eye to look for moving objects; now it was time for computers to take the lead.

These new instruments and analysis techniques, along with more manual checks, yielded more than 50 additional discoveries. However, further analysis of all of these objects confirmed that they were out of range for New Horizons. The probe could wave as it cruised, but it wouldn’t get close enough to take useful images of the surfaces. By now, as the mission team neared the actual flyby of Pluto and the looming deadline for making final course corrections, time was running out.

Big guns, big future

Since “…we had to find an object in 2014” (Buie et al. 2024), it was time to leave the ground-based telescopes behind and bring in a closer one: the most famous telescope in the world (and elsewhere), the Hubble Space Telescope. The team was given observing time equivalent to over 200 orbits in 2014, and their efforts were eventually rewarded with a handful of viable potential targets. One of them would end up being known as Arrokoth, and the rest is history.

This decade of exploration, searching, waiting, and reorganizing ultimately left the planetary science community with another flyby and 80 new Kuiper Belt objects, yes, but also with techniques less tangible than these discoveries. As astronomers reach ever deeper into the night sky and search for increasingly star-filled regions, they often use the observing plans, algorithms, and sometimes actual code developed as part of this search. It was a tremendous effort that paid off handsomely, and as we approach the 10th anniversary of our robotic arrival at the Kuiper Belt, it’s worth reflecting on and celebrating how we’ve learned to study this strange place.

Citation

“The New Horizons Extended Mission Goal: Search for and Discovery of Arrokoth,” Marc W. Buie et al. 2024 Planet. Sci. J. 5 196. doi:10.3847/PSJ/ad676d

This post originally appeared on AAS Newfeaturing highlights of research from the journals of the American Astronomical Society.