When you think of objects close to the sun, Mercury, the solar system's innermost planet, probably comes to mind. But has any other object — natural or artificial — gotten closer to our star? And what's the closest an object has gotten to the sun while still escaping?
Many objects have reached much closer than Mercury, including asteroids, comets and spacecraft, experts told Live Science.
At its closest point to the sun, called perihelion, Mercury comes within 29 million miles (47 million kilometers) of our star, according to NASA.
But several known asteroids come even closer. Scott Sheppard, an astronomer at Carnegie Science, an independent research institute in Washington, D.C., said such asteroids, with orbits that cross Mercury's, are called Mercury-crossers. They have highly eccentric orbits, meaning "they did not form in this location but were likely scattered from the main asteroid belt towards the Sun," he told Live Science in an email. A search through NASA's Small-Body Database for such asteroids shows 2024 YL3 is the current record holder. This 463-foot-long (141 meters), skyscraper-size rock has a perihelion of 6.4 million miles (10.3 million km).
Another group of celestial objects, though, may beat 2024 YL3. These are proposed asteroids that are thought to have circular orbits within Mercury's path. Dubbed Vulcanoids, their name comes from Vulcan, a theoretical planet once hypothesized to exist in this region.
A rendering of the Mercury-crossing asteroid Icarus. Over longer timescales, such asteroids also turn out as "winners." Sheppard said that because they are dynamically unstable, they will likely collide with the sun within a few million years, although they may instead crash into a planet or be ejected.
Martin Beech, a professor emeritus of physics at the University of Regina in Canada, told Live Science in an email that numerical models show such asteroids would have stable orbits between 5.58 million and 23.23 million miles (8.98 million to 37.39 million km) from the sun's center. This means some Vulcanoids may whiz even nearer the sun than Mercury-crossers do. There's just one glitch: Like their namesake planet, Vulcanoids haven't been shown to exist, and a 2013 study found no trace of them.
But some spacecraft have come much, much closer. For instance, NASA's Parker Solar Probe, which launched in 2018, studied the sun's outer atmosphere, or corona. For the primary mission's final three orbits, the probe took an eccentric path with a perihelion of merely 4.5 million miles (7.3 million km) from the sun's center. At this distance, illustrated graphically on the mission's official webpage, the spacecraft flew through the corona, where a carbon foam shield protected the probe from the extreme heat.
"Sungrazers" and "sun-diving" comets
Sungrazing comets often become spectacularly bright as they approach perihelion.
Another class of natural objects — dubbed sungrazing comets — reach closer still. A 2018 study described such comets as coming closer than 1.49 million miles (2.39 million km) of the sun's center, although any of these comets' precise solar proximity would depend on their orbital parameters.
Man-To Hui, a researcher at the Shanghai Astronomical Observatory, told Live Science that small sungrazers — some of which are just 30 feet (10 m) wide —break up as they rotate because they get nonuniformly heated, while the sun's powerful gravity pulls apart larger ones. Nonetheless, several escape unscathed; one of the closest observed so far was the Great Southern Comet of 1887, which came within an incredible 446,187 miles (718,070 km) from the sun.
But the real "winners" are "sundiving," or "sunstriking," comets. Such comets likely abounded in the early solar system, although they're thought to be rare nowadays. These comets would cross the sun's visible surface, or photosphere, plunging into the sun. Although no one has detected such a sundiver yet, the 2018 study noted that a comet would need a perihelion less than the sun's radius, or 432,244 miles (695,630 km) from the sun's center. There are no second passes, though — all sundiving comets are destroyed within the sun.
So, the detected object that got closest to the sun was the Great Southern Comet of 1887. If a sundiver is ever spotted, however, it would become the new record-holder, edging out the Great Southern Comet by 13,943 miles (22,439 km).
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Abha Jain is a freelance science writer. She did a masters degree in biology, specializing in neuroscience, from the Indian Institute of Science, Bengaluru, India, and is almost through with a bachelor's degree in archaeology from the University of Leicester, UK. She's also a self-taught space enthusiast, and so loves writing about topics in astronomy, archaeology and neuroscience.
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