Small Body Rings, Visits, and Close Approaches

Happy 2023! I started out my new year by breaking a toe, so let's all hope that's the worst thing that happens to me this year. Three pieces of good news news about small solar system bodies landed in my inbox this week, so that's my theme for this newsletter.

Our Asteroid Watch System Works!

On Saturday, January 22, just past midnight UTC, Crimean amateur astronomer Gennadiy Borisov discovered a previously unobserved near-Earth object encroaching on Earth's space. He reported it out and almost immediately, several other European amateurs spotted the object; soon after, telescopes in California and Australia also observed it. That was enough observations to establish a secure prediction of the asteroid's path, and the Minor Planet Center issued a Minor Planet Electronic Circular announcing the (then-) newest-known member of the solar system, 2023 BU.

There's a lot of numbers on a MPEC. One key line is this one, right below where it says "Orbital elements":

What those numbers mean: 2023 BU orbits the Sun at about 1.11 AU, but its orbit isn't quite circular. Eccentricity (e) varies from 0 to 1, with 0 meaning a circular orbit and 1 being hyperbolic; at e=0.12, 2023 BU's orbit is as close to circular as a planet's, more elliptical that Mars' but less so than Mercury's. Its inclination, only 4 degrees, is quite low, meaning it orbits pretty much in plane with the planets. And its perihelion, or closest approach distance to the Sun, is 0.9842 AU, which is to say, just barely inside Earth's.

So we have an object on a near-circular orbit in the same plane as Earth that has a path that lies mostly outside Earth's orbit but spends time on its orbit every perihelion crossing inside Earth's path. Now, Earth and 2023 BU will usually not be close to each other in their orbits when this happens, but this is definitely what you'd call a potentially hazardous object. Indeed, another number you can read off the MPEC is the predicted Minimum Orbit Intersection Distance, or MOID, of only 0.0004 AU (or 60,000 kilometers). That's much closer to us than the Moon, and warranted urgent further study.

The MPEC spurred dozens more astronomers around the world to make observations, which quickly allowed the Center for Near Earth Object Studies to improve the quality of the predicted orbit and determine that 2023 BU will not hit Earth, but it will come awfully close, passing within 3600 kilometers of Earth just past midnight on Friday, January 27 (UTC). Ironically, its path across the southern tip of South America means that even though it'll be on Earth's night side when it passes by, the European telescopes that discovered it won't be able to spot it; it's too far south.

Read more:

• Here's the NASA JPL Small-Body Database page on 2023 BU

• OliveAstro posted some lovely visualizations on Twitter

• So did Tony Dunn

• More from Gianluca Masi on the impending 2023 BU flyby, which Virtual Telescope will image live!

Jelly Welly Space Telly Spotted Chariklo's Rings!

Chariklo is a Centaur, an object that orbits in the outer solar system among the planets. Centaurs likely began their existence beyond the orbit of Neptune and were flung into their current paths during the migration of the outer planets. Chariklo's likely rings were first reported in the journal Nature in 2014; at the time, I asked planetary astronomer Alex Parker to write an explanation of the discovery for my blog. It was the first non-planetary body known to have rings. (Rings were reported in the past for Saturn's moon Rhea, but that idea didn't stand the test of time.) Since then, rings have been discovered around the largest Centaur, Chiron, as well as the trans-Neptunian object Haumea.

Well, JWST just captured more evidence for Chariklo's rings. (Well actually the evidence was captured in October and published today.) Although I think the fact of Chariklo having rings was considered pretty firmly settled already, it's such a dim, distant world, and those rings are so narrow and tenuous, that I think I always had an asterisk by the existence of those rings in my head. But this observation cinches it for me. Chariklo has rings! (Want to know where Chariklo orbits? Here's the NASA JPL Small-Body Database page on Chariklo.)

Lucy Mission Gets Another Asteroid Encounter!

When the Lucy mission to Jupiter's Trojan asteroids launched in October 2021, they did not expect to have their first, practice encounter with an asteroid until 2025, when it will pass asteroid (52246) Donaldjohanson. Following launch, colaborator Raphael Marschall searched the database of known asteroids — more than half a million of them! — to see if Lucy would have an earlier opportunity. It turned out that without any adjustments, Lucy's predicted trajectory would take it within 60,000 kilometers of asteroid (152830) 1999 VD57 late in 2023. But in May 2023, very tiny adjustment of the spacecraft's planned path will make that encounter much closer, within 450 kilometers. That's great because it will allow the mission to perform a real-world test of the "terminal tracking system" that will use computer vision to automatically track the positions of all of Lucy's targets during its future Trojan encounters.

So there are good engineering reasons to do the work (and it's a lot of work!) necessary to prepare for and perform this flyby. The science will be cool, too, because no main-belt asteroid as small as (152830) 1999 VD57 has been visited by a spacecraft. It's estimated to be only 700 meters in diameter. We've seen several near-Earth asteroids of similar sizes — Itokawa, Ryugu, and Bennu — but they have all had recently violent pasts that sent them on their Earth-neighboring paths. Will (152830) 1999 VD57 look the same? We'll find out in November! This article from the Lucy team gives a lot more information about the importance of the terminal tracking system, and what level of detail Lucy will be able to see at that encounter distance. (TL;DR: details as small as half an elephant.)

Bonus Sky and Telescope Article!

Got questions about the Lucy mission? I wrote about Lucy and Psyche for Sky and Telescope about a year ago. (This was before the Psyche mission was delayed, but most of the article is still relevant.) Download a PDF copy of my feature article about Lucy and Psyche here!