March — A Good Time to Visit Vesta – Sky & Telescope


Vesta Moon comparison
The asteroid 4 Vesta and the Moon are shown to scale. Although more than 6½ times smaller than Earth’s satellite, Vesta has a much higher surface brightness. German astronomer Heinrich Olbers discovered Vesta in northern Virgo on March 29, 1807.
Moon image: Gregory H. Rivera; Vesta: NASA / JPL-Caltech / UCAL / MPS / DLR / IDA with additions by the author

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).

Vesta animation February 2021
Amateur astronomer Piqui Diaz of Ezeiza, Argentina used a cellphone camera held up to one eyepiece of her 7×50 binoculars to capture this image sequence of Vesta from February 18–21, 2021. South is up.
Piqui Diaz

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 orbit
Vesta’s position is shown for March 4, 2021, the date of opposition. Its orbit is inclined 7.14° to the ecliptic, with the asteroid currently north of the ecliptic plane.
JPL Horizons

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.

Vesta resolved
On June 15, 2018, during Vesta’s perihelic opposition, Michel Deconinck used a 250-mm aperture telescope and magnification of 830× to discern a small disk (upper right) in moments of good seeing. Vesta’s was 0.6″ across at the time. He also noted the asteroid’s gold color, describing it as “conspicuous.”
Michel Deconinck

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?

Vesta wide locator map
To find Leo so that you can navigate to Vesta, use the Big Dipper. Mark off 3½ fists (held at arm’s length) from the Dipper Bowl to Regulus, then work your way down to Theta in the Lion’s tail.
Stellarium with additions by the author

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.

Vesta Finder Feb - June 2021
Third-magnitude Theta (θ) Leonis (Chertan) makes it easy to find and track Vesta in the coming weeks. Dates are shown for 0h UT. To convert to EST, subtract 5 hours (4 for Eastern Daylight Time starting March 14th) and back the date up one day. For example, March 4 at 0h UT = March 3 at 7 p.m. EST. Click here for a high resolution map.

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.

Vesta is a heavily cratered asteroid shaped like a smooshed sphere or oblate spheroid. Its largest peak (described below) juts out from the south polar region at bottom.
NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

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.

Eucrite meteorite from Vesta
This slice of the eucrite meteorite NWA 2724, found in Northwest Africa in 2004, was once part of a lava flow on the surface of Vesta. After the lava cooled, an enormous impact sprung it free and ultimately sent it to Earth.
Bob King

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.

Rheasilvia Crater and peak
The solar system’s tallest peak looks like a rocky blister at the center of the 500-kilometer-wide Rheasilvia Crater on Vesta.
NASA / JPL-Caltech

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.

Vesta visits galactic duo
Watch for Vesta to pass between these two galaxies (but much closer to NGC 3501) on the evening of March 21st.
DSS2 / Aladin Sky Atlas

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.

Happy hunting!

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