Try this stumper on an astronomy-savvy friend: What is the smallest celestial body visible to the naked eye? Hint: It lies farther away than the moon, beyond Mars.
The answer is Vesta, an asteroid between Mars and Jupiter. Although it is only 325 miles across, hardly wider than the state of Arizona, Vesta is a world with a complicated geologic history and an intimate relationship with Earth. This month it makes its closest approach in seven years, briefly coming into view as it glides dimly through the constellation Virgo.
The saga of Vesta stretches back to the late 18th century, when scientists speculated that an unseen planet might reside within the wide gap between Mars and Jupiter. On the first day of the 19th century, Italian astronomer Giuseppi Piazzi found a body in the right location and named it Ceres. The situation quickly grew confusing, however, as observers spotted additional small objects in the same region. In 1807 German astronomer Heinrich Olbers discovered Vesta, the fourth of these asteroids (so named because they each resemble a star—aster in Latin—through a telescope). It became obvious the missing planet was actually a collection of sub-planetary debris. Researchers have since cataloged more than 40,000 asteroids, some no larger than a house.
Vesta has always stood out from the crowd. First of all, it is peculiarly bright. No other asteroid is visible to the unaided eye, not even Ceres, which is nearly twice as wide. Composition makes the difference. Most asteroids are dark gray, but Vesta is as white as chalk. Once astronomers learned to recognize the milky coloration, they started finding bits of Vesta in surprising places. The mile-wide asteroid Braille, visited by the Deep Space 1 spacecraft in 1999, appears to be a big chip that broke off Vesta. Smaller pieces of Vesta have even made visits to Earth— and witnesses have watched it happen.
In October 1960, two fence workers in Western Australia witnessed a bright shooting star. Ten years later, when bits of that meteorite were finally found, they proved to be chunks of solidified lava whose colors strongly linked them to Vesta. Only two other bodies, Mars and the moon, are known to have produced fragments that landed on Earth. With pieces of Vesta in hand, scientists could begin to interpret its geology. Apparently it was once a molten world, similar to early Earth, whose interior separated into a mantle and a crust covered with volcanic rock. Planetary researcher Benjamin Zellner of Georgia Southern University calls Vesta "the sixth terrestrial planet."
In September 1997, Zellner published images taken by the Hubble Space Telescope, which provided the first clear look at Vesta. The images showed dark markings—probably plains of ancient lava—and an enormous cavity, 285 miles across and eight miles deep, covering virtually its entire southern hemisphere. Whatever smashed into the asteroid must have hurled tremendous amounts of debris into space. Computer simulations reveal that Jupiter's gravity would have caught some of that material and redirected it into Earth-crossing trajectories.
The huge Keck II telescope joined the Vesta patrol in 1999, producing spectra that showed Vesta's surface is covered with pyroxene and olivine, minerals common in terrestrial volcanoes. These spectra also indicated the ratio of oxygen isotopes in Vesta's rocks. That ratio precisely matches the composition of the Australian meteorites, clinching their identification.
To spot the sixth terrestrial planet for yourself, go out on a clear, dark night and look for the bright orange star Arcturus. Trace a line from there to Virgo's medium-bright star Vindemiatrix, and then extend the line another one-quarter of that span. Sketch the scene and note the very faint star that changes position nightly. That is Vesta.
The tracking process is not easy. At its closest, on March 26, Vesta will still be 119 million miles from Earth, equivalent to a golf ball floating 10 miles away. But when else are you going to see an asteroid?