Look who’s visiting this winter in Orion: Vesta, a bright asteroid with a dark side.
Winter begins at 10:27 p.m. EST on December 21st, the same day Vesta reaches opposition. But while the Sun smolders at its lowest point in the sky (for Northern Hemisphere observers), the brightest asteroid stands high at night in northern Orion. At magnitude 6.4 and 236.5 million kilometers distant it may be faintly visible with averted vision from moonless, rural skies. One thing's for sure — you'll have no problem spotting and tracking Vesta in binoculars as it tiptoes across the background stars.
Through early February 2024 Vesta moves west-northwest in retrograde motion as the speedier Earth laps the slower asteroid like a runner in a 1000-meter track race. In the next few weeks Vesta makes several fortuitous passes of naked-eye stars that both assist in finding it and make its nightly movement easily apparent.
* December 14 — 12′ northeast of 4.6-magnitude Chi 2(χ2) Orionis
* December 15 — 10′ northwest of Chi 2 Orionis
* December 23 — 22′ north of 4.4-magnitude Chi1 (χ1) Orionis
* January 7 — 16′ northeast of 3.0-magnitude Zeta (ζ) Tauri
* January 8 — 12′ north of Zeta Tau
* January 11–14 — passes about ½ ° south of the Crab Nebula (M1)
All asteroids look like pinpoints of light when observed visually through binoculars and telescopes because even the largest of them is too small and too far away to show a disk. That's exactly how they got their name, which derives from the Greek word asteroeidēs for "starlike." Like other astronomical specks we're eager to see — quasars, supernovae, novae, and planetary moons to name a few — it's what we know about them that helps us appreciate their one-dimensional facade. And when it comes to asteroids Vesta is deeply interesting.
First, it's bright. At perihelic opposition it can reach magnitude 5.3, but even during the current, more distant encounter it might still be glimpsed. Vesta's albedo — the amount of light it reflects from its surface — is 43 percent, making it considerably brighter than the Earth which reflects about 30 percent.
Over time most airless asteroids like Vesta develop a dark exterior from space weathering from the relentless radiation and particle bombardment by the Sun and from cosmic rays. Not Vesta. Early on, short-lived radioactive elements melted the orb causing it to differentiate into a metallic core surrounded by a rocky mantle and topped with a lighter, "flaky" crust. Currents stirring within its once-molten core may have generated at least a temporary magnetic field, shielding the surface from insidious solar ions and helping maintain its fresh and bright appearance.
Remnants of that ancient field may still exist embedded in the surface rocks. A team of scientists analyzed the Vesta-derived meteorite Allan Hills A81001 which still possesses remnant magnetism from its formation 3.7 billion years ago. Like Mars, the signature of its putative global magnetic field may be frozen into Vesta's crustal rocks and explain the asteroid's ease of visibility to this day.
Paradoxically, Vesta also has one of the highest albedo ranges of any known asteroid. It's not only highly reflective but exhibits streaks and patches of dark, carbon-rich material similar to carbonaceous chondrite meteorites found on Earth. Much of the coal-black stuff concentrates within the walls and rims of impact craters and their ejecta. Based on the global distribution of the material scientists think much of it originated when a water-rich, carbonaceous asteroid crashed into Vesta at a relatively low velocity several billion years ago. Upon impact it excavated the colossal Veneneia impact basin (around 400 kilometers across) and hurled chunks of debris across the landscape. Additional collisions may also have contributed to the asteroid's ebony bounty.
Carbonaceous asteroids like those that struck Vesta also likely transported water-rich materials to Earth, the very stuff that aided life's formation and has sustained it for more than 3.7 billion years. With no atmosphere the minor planet's carbon-water cache went to naught while here on the home planet I like to think we're still sipping both in our beer. As you track Vesta's night-to-night creep among the winter stars think of it as a distant cousin with a relatable tale to tell.