Dip into this easy guide to seeing the brightest asteroid, Vesta, and understanding what makes it unique.
A new month is underway, with a bright minor planet strutting its stuff in Leo, the Lion. Grab your binoculars because that's all you'll need to see 4 Vesta, the second largest and brightest main belt asteroid. In early March it's a 6.1-magnitude point of light traveling westward near the 3rd-magnitude star Theta (θ) Leonis, also known as Chertan. Opposition occurs on March 4th, and it's nearest the Earth on the 9th at a distance of 203.6 million km (126.5 million miles).
Even at month's end, Vesta will still be as bright as magnitude 6.5. With the Moon now vacating the evening sky, timing couldn't be better for watching the asteroid's night-to-night perambulations.
Vesta orbits the Sun every 3.6 years with an orbital eccentricity of 0.09, very similar to that of Mars, so its distance and brightness vary significantly from opposition to opposition. This one falls in the middle between close and far with Vesta faintly visible without optical aid from the countryside. Although a fulsome 525 kilometers (326 miles) across, it subtends just 0.5″ this month, appearing completely stellar except in large instruments at high magnification under optimal seeing conditions.
While quite tiny, Vesta's apparent size is nearly nine times larger than that of R Doradus, the star with the largest apparent diameter (0.057″). This makes me wonder whether Vesta might be large enough to mimic a planet and twinkle less than a star of similar magnitude. Has anyone noticed this through the telescope?
To find Vesta the next clear night, start at Regulus then drop down about 16° to Chertan (θ Leonis), the bottom-right star of the scalene triangle that defines Leo's hindquarters. Vesta remains within 1.5° of the star through March 10th, making it a great place to launch your star-hop. Moving westward in retrograde motion about ¼° a day, a careful observer should be able to easily detect Vesta's night-to-night movement. Many observers, including myself, report that the asteroid appears pale yellow through a telescope.
For your best view of the asteroid zoom in with your mind's eye with the help of images returned by NASA's Dawn Mission. Dawn entered orbit around Vesta in 2011 and for 14 months took photos and gathered data on its surface and interior. I encourage you to browse the Photojournal entries for Vesta to prime the imagination before you take that first look in your telescope or binoculars.
Unlike most asteroids, which are composed of a mishmash of metallic, icy, and rocky materials, Vesta is differentiated or layered, with a metallic core wrapped in a dense, rocky mantle and topped by a crust of cooled lava. Earth has a similar build but grew large enough to achieve planetary status. Vesta got halfway there, stopping at the protoplanet stage.
Early protoplanets like Vesta served as the nuclei of the planets we know today. Astronomers speculate that if it wasn't for the constant agitation of Jupiter's gravity Vesta might have grown into a full-fledged planet itself. Instead, it suffered a case of arrested development and remains the only known embryonic planet to date.
Dawn also confirmed that a class of meteorites found on Earth called Howardites, eucrites, and diogenites — together known as the HED clan — derive from the protoplanet. A billion or so years ago another asteroid smashed into Vesta and excavated Rheasilvia Crater, a basin some 500 kilometers (300 miles) across. Nearly 1% of the asteroid was pulverized and blasted into space with enough fragments arriving on Earth over time to account for about 5% of all meteorites known.
At the center of Rheasilvia stands a 22-kilometer-tall peak slightly taller than Olympus Mons on Mars. Although not nearly as expansive, this jagged reminder of Vesta's near-catastrophic encounter is the tallest known peak in the solar system.
As you accompany Vesta this month, expect to see new sights along the way. On the evening of March 21st, the asteroid passes just 56″ north of the core of the 12.9-magnitude, edge-on spiral NGC 3501 around 10 p.m. EDT. Be sure to look for its neighbor, the face-on, barred spiral NGC 3507 (magnitude 10.9) just 12′ to the northeast at the same time.