Originally published 2018-07-05.
Can you see an asteroid without a telescope? With Vesta, yes, at least on its best nights. In June and July 2018 this giant of the asteroid belt became bright enough to spot with the naked eye from a dark site, sitting in Sagittarius not far from Saturn. It reached opposition on 19 June 2018 at a distance of about 170.6 million kilometres (106 million miles), its closest approach in at least two decades, and shone at around magnitude 5.3.
That event has passed, but Vesta returns to good viewing positions roughly every 17 months, and the same principles apply each time. Here is what made 2018 special, how observers found it, and what we know about this unusual world.
What Vesta is
Vesta orbits the Sun in the main asteroid belt between Mars and Jupiter. It was discovered on 29 March 1807 by the German astronomer Heinrich Olbers and named, at Carl Friedrich Gauss's suggestion, after the Roman goddess of the hearth.
It is the second most massive body in the belt after the dwarf planet Ceres, and it is often described as a protoplanet: a leftover building block that started to grow into a planet, melted inside and separated into a metal core, a rocky mantle and a basalt crust before its growth stalled. Its shape is a lumpy, flattened ellipsoid measuring roughly 573 by 557 by 446 kilometres, with a mean diameter of about 525 kilometres. Some older sources round the longest axis to 570 kilometres, which is why that figure often appears.
Why it was visible in 2018
Two things decide how bright an asteroid looks from Earth: how close it is and how much sunlight its surface reflects. In 2018 Vesta had both on its side.
Opposition is the moment when Earth sits between a body and the Sun, so the object is fully lit from our point of view and stays up all night. Because Vesta's orbit is slightly eccentric, some oppositions are closer than others. According to Sky & Telescope, the 2018 opposition brought it within 170.6 million kilometres, a touch nearer than in 2007, when it came to about 171.2 million kilometres and reached magnitude 5.4.
The naked-eye window ran roughly from early June until about 16 July, when Vesta faded to magnitude 6.0, the usual limit for good eyesight under dark skies. The best nights were 8 to 22 June, when there was no bright Moon.
How observers found it
From mid-northern latitudes, Vesta sat low in the south-east in the evening, in the rich Milky Way fields of Sagittarius, near the M24 star cloud. From the Southern Hemisphere it rode much higher, which made it easier to see.
Saturn was the easiest signpost. At opposition Vesta lay about 7.5 degrees north-west of the ringed planet, roughly the width of three fingers held at arm's length. Some popular articles said it was to the east of Saturn; Sky & Telescope's charts placed it to the north-west. Observers then hopped to the star Mu Sagittarii, with Vesta a few degrees beyond it, and confirmed the identification by noting its position on successive nights. Unlike a star, an asteroid shifts against the background from one evening to the next.
Binoculars made the job much easier, showing Vesta as a steady, star-like point. Even large telescopes reveal little more than that, although experienced observers with high magnification have reported a slightly oval shape and a yellowish tint.
Why Vesta's surface is so bright
Vesta reflects about 42% of the sunlight that hits it, measured as geometric albedo, compared with around 12% for our Moon. That brightness is the main reason it is the only main-belt asteroid regularly visible to the naked eye, despite being smaller than Ceres.
Older articles sometimes suggested that a magnetic field might shield Vesta's surface and keep it fresh. That idea is speculative. Results from NASA's Dawn mission point to a different explanation: Vesta's soil lacks the tiny particles of iron that darken the Moon's surface over time, and impacts in the asteroid belt are generally too slow to melt or vaporise rock. Instead, collisions mainly break up and stir the regolith, mixing bright original basalt with darker material delivered by carbon-rich impactors.
What NASA's Dawn mission revealed
Dawn launched in September 2007, entered orbit around Vesta on 16 July 2011 and left in early September 2012 for Ceres, which it reached in March 2015. It was the first spacecraft to orbit two different bodies beyond the Earth–Moon system. After running out of hydrazine, the fuel used to keep it pointed, the mission ended on 1 November 2018, and Dawn remains in a stable orbit around Ceres.
At Vesta, Dawn mapped the enormous Rheasilvia impact basin near the south pole: about 500 kilometres wide, roughly 19 kilometres deep and crowned by a central peak more than 20 kilometres high, among the tallest mountains known in the solar system. That impact blasted out around 1% of Vesta's volume.
Some of that debris has reached Earth. More than a thousand meteorites known as HEDs, short for howardites, eucrites and diogenites, match Vesta's surface chemistry, so scientists effectively hold pieces of the asteroid in their collections.
Tips for seeing Vesta at a future opposition
- Check a current chart. Vesta's position and brightness change with every opposition; the 2018 directions will not work for later years. Astronomy magazines and planetarium apps publish finder charts.
- Get away from lights. At magnitude 5 to 6, Vesta is near the limit of naked-eye vision, so dark skies matter more than anything else.
- Use binoculars first. Find it with optical help, then try to spot it without.
- Watch it move. Sketch or photograph the field on several nights; the "star" that changes place is your asteroid.
Seeing a 525-kilometre rock hundreds of millions of kilometres away with nothing but your eyes is a quietly remarkable experience, and Vesta remains the best chance most people will ever have.
Sources: Sky & Telescope: Vesta's 2018 opposition (Bob King); Wikipedia: 4 Vesta; Wikipedia: Dawn (spacecraft); Astronomy Now finder guide, June 2018; IFLScience report, 2018.
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