New Horizons and Ultima Thule

[Recusant]

I watched the New Horizons flyby of the Kuiper Belt object now called Ultima Thule on NASA TV live when it took place (about 5:30 AM GMT on January 1). I thought about posting something beforehand, but I knew it would just be some NASA boffins sitting in a room in front of monitors talking to each other, then either expressing dismay or celebrating. Which is exactly what happened, with the celebrating part rather than the other. It would take at least a day or so before anything was downloaded and converted into usable images.

Ultima Thule was intriguing because previously it had shown a variation in its albedo (reflectivity), but as New Horizons approached the information sent back showed no evidence of such variation. The route of the spacecraft meant that its approach was from a different angle than what had been available previously.

The first images published were described as showing an object shaped like a bowling pin.

The Kuiper Belt object Ultima Thule takes on a bowling pin shape (left) in this view from the New Horizons spacecraft taken on Dec. 31, 2018 just before its flyby closest approach on Jan. 1, 2019. At right is an artist's sketch of the object. . . . It is approximately 20 miles long by 10 miles wide (32 kilometers by 16 kilometers).
Credit: NASA/JHUAPL/SwRI; sketch courtesy of James Tuttle Keane

This explained the discrepancy between earlier observations and New Horizons data. New Horizons was approaching from the "side" of the bowling pin as Ultima Thule rotated, while previously the view had been from either the top or bottom as it rotated.

When the data from the flyby had been processed, it became clear that Ultima Thule was not a bowling pin, but more like a snowman.

This first color photo of the Kuiper Belt object Ultima Thule reveals the object's red color as seen by NASA's New Horizons spacecraft from a distance of 85,000 miles (137,000 kilometers) during a Jan. 1, 2019 flyby. From left to right: an enhanced color image, a higher-resolution black and white image, and an overlay that combines both into a more detailed view.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

From Space.com:

We now know what Ultima Thule looks like, and it's not a bowling pin.

The first resolved photos of Ultima Thule have come in from NASA's New Horizons spacecraft, which zoomed past the frigid faraway object just after midnight yesterday (Jan. 1). The historic imagery reveals that the 21-mile-long (33 kilometers) Ultima is a "contact binary" composed of two roughly spherical lobes.

Photos taken by New Horizons over the previous week or so had suggested that these two lobes are connected by a relatively narrow neck. But the new imagery shows they're glommed tightly together, dashing earlier analogies.

"That bowling pin is gone," New Horizons principal investigator Alan Stern, of the Southwest Research Institute (SwRI) in Boulder, Colorado, said during a news conference today (Jan. 2). "It's a snowman, if it's anything at all."

[Continues . . .]

[Icarus]

New Horizons is a miraculous vehicle.  It is a tribute to human ingenuity and the determined quest for  knowledge.

[xSilverPhinx]

Out of this world. 

[Tank]

It's things like this that make it worthwhile getting up in the morning.

[Recusant]

A collaboration between New Horizons and the Subaru telescope on Hawai'i has found what may be an outer ring of the Kuiper Belt. If confirmed this would show that the overall size of our Solar System isn't unusually small (per this article):

A lot of our observations of the Milky Way galaxy suggest that our Solar System is unusual in many ways. Since the Solar System is the only known planetary system to host life, these oddities could be contributing factors to the Solar System's habitability.

But our technology for observing space has limitations that could result in significant observation biases, suggesting peculiarities that don't actually exist. If the new observations of the Kuiper Belt are confirmed, we have just ruled out one of those peculiarities – an unusually small solar nebula.

"Structure of the Outer Solar System Revealed - The Subaru Telescope and New Horizons' 20-Year Challenge" | Subaru Telescope

Survey observations using the Subaru Telescope's ultra-widefield prime focus camera have revealed that there may be a population of small bodies further out in the Kuiper Belt waiting to be discovered. The results, which are important for understanding the formation of the Solar System, were obtained through an international collaboration between the Subaru Telescope and the New Horizons spacecraft traveling through the outer Solar System.

NASA's New Horizons spacecraft was launched in 2006 with the critical mission of observing the surfaces of outer Solar System bodies up close for the first time in human history; it successfully completed a flyby of the Pluto system in 2015, and in 2019 it made a flyby of one of the Kuiper Belt objects, (486958) Arrokoth. There have been five spacecrafts that have flown to the outer Solar System (including New Horizons), but New Horizons is the only spacecraft that has flown through the Kuiper Belt while observing Kuiper Belt objects.

When observing Kuiper Belt objects from the ground, we can only observe them at small solar phase angles (the angle between the sun, the object, and the observer). On the other hand, when observing a Kuiper Belt object from a spacecraft in the Kuiper Belt, the same object can be observed at various phase angles and its reflection characteristics can be used to estimate the surface properties of the object. This is something only New Horizons can do.

However, the camera on the spacecraft has a narrow field-of-view and cannot discover Kuiper Belt objects on its own. This is where the Subaru Telescope comes in. The Subaru Telescope uses its wide-field camera to find many Kuiper Belt objects and then narrow down the list of objects that the spacecraft can fly by and observe. This collaboration between New Horizons and the Subaru Telescope began in 2004.

Unfortunately, the Kuiper belt objects so far found during this observation require too much fuel for the spacecraft to flyby, but new ones at great distance may fall within the available fuel reach of New Horizons. In 2020, deeper observations began with Hyper Suprime-Cam (HSC) on the Subaru Telescope, and by 2023, there had been 239 Kuiper Belt objects discovered.

"The most exciting part of the HSC observations was the discovery of 11 objects at distances beyond the known Kuiper Belt," says team member Dr. Fumi Yoshida (University of Occupational and Environmental Health Sciences; Planetary Exploration Research Center, Chiba Institute of Technology).

Many of the objects discovered with HSC are located at distances of 30-55 astronomical units (au) from the Sun (1 au corresponds to the distance between the Sun and Earth) and are thought to be within the known Kuiper Belt. On the other hand, the team was not expecting what appears to be a cluster of objects in the 70-90 au region and a valley between 55 au and 70 au (where only a small number of objects are distributed). Such a valley had not been reported in other observations.

There may be a new population of Kuiper Belt objects at 70-90 au. "If this is confirmed, it would be a major discovery. The primordial solar nebula was much larger than previously thought, and this may have implications for studying the planet formation process in our Solar System," says Dr. Yoshida.

[Continues . . .]

A pre-print version of the paper is open access:

"Candidate Distant Trans-Neptunian Objects Detected by the New Horizons Subaru TNO Survey" | ArXiv

Abstract:

We report the detection of 239 trans-Neptunian Objects discovered through the on-going New Horizons survey for distant minor bodies being performed with the Hyper Suprime-Cam mosaic imager on the Subaru Telescope. These objects were discovered in images acquired with either the r2 or the recently commissioned EB-gri filter using shift and stack routines.

Due to the extremely high stellar density of the search region down stream of the spacecraft, new machine learning techniques had to be developed to manage the extremely high false positive rate of bogus candidates produced from the shift and stack routines. We report discoveries as faint as r2∼ 26.5. We highlight an overabundance of objects found at heliocentric distances R ≳ 70 au compared to expectations from modelling of the known outer Solar System.

If confirmed, these objects betray the presence of a heretofore unrecognized abundance of distant objects that can help explain a number of other observations that otherwise remain at odds with the known Kuiper Belt, including detections of serendipitous stellar occultations, and recent results from the Student Dust Counter on-board the New Horizons spacecraft.

[Tank]

Wonderful.