Flyby Reveals Phaethon, Our Solar System’s Weirdest Asteroid-Comet Hybrid

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The Solar System is full of some really weird stuff. Venus, despite being farther from the Sun than Mercury, has a considerably higher surface temperature. Uranus is tilted on its side and rotates in one direction, while its moons orbit it normally. Jupiter’s moon Io is the most volcanically active body in the solar system, Mars has a volcanic bulge the same mass as Ceres that may be responsible for changing the orientation of the planet’s crust with respect to its own axis of rotation. The existence and composition of our own Moon is an ongoing puzzle we’re trying to solve, and we haven’t yet found a solar system that looks anything like our own. Other solar systems often feature so-called “hot Jupiters” orbiting tightly around their host star, our Jupiter sits in the middle of the solar system — which is probably why life exists on this planet in the first place. It may even be why this planet exists in the first place.

Given these facts, it shouldn’t be surprising that there’s a really weird asteroid-comet hybrid whizzing around complicating things. And thanks to a close flyby last December, we’re a little bit closer to understanding this intriguing rock. 3200 Phaethon is unambiguously a rocky asteroid with a closer approach to the Sun than any other named asteroid. Its perihelion distance is just 0.14 AU — less than half of Mercury’s own approach distance. Its orbit closely resembles that of a comet rather than an asteroid, however, and its orbit and position correspond well with it being the source of the Geminid meteor shower. But that’s also where the confusion comes in because if 3200 Phaethon is the source of the Geminids, it’s the only asteroid known to cause a meteor shower — that’s a characteristic of comets.

3200 Phaethon’s orbit. Image by NASA JPL

The difference between a comet and an asteroid is typically their composition: Asteroids are principally composed of rock and metals, comets are a collection of water ice, some rock, and dust. Asteroids formed closer to the Sun, where the star’s heat meant there was little to no water ice present. The coma of a comet — the bright fuzzy gaseous blur we see around its head — forms when water ice and dust sublimate from inside the core or the surface. Asteroids don’t contain these ice reserves and therefore don’t sublimate in the same fashion. Or so we thought. But 3200 Phaethon blurs these distinctions in some interesting ways.

The team’s new findings show that 3200 Phaethon may not be related to the Pallas meteor family, as was previously thought. The meteor’s albedo is only slightly higher than charcoal, even if it shares a blue hue with the much brighter 2 Pallas. An investigation revealed that the asteroid is the same color everywhere, implying it’s been fried by repeated exposure to the Sun at such close range (Space.com reports that at close range, Phaethon’s surface temperature would exceed 1500F / 800C). It’s thought that 3200 Phaethon could be a comet remnant that’s been cooked to a crisp by the Sun over time — while the asteroid does give off some dust as it approaches our star, it’s not nearly enough to sustain the Geminids over time.

The aggregate findings mostly confirm that 3200 Phaethon isn’t exactly a comet or an asteroid, but an object with some of the characteristics of each that puts a foot in both camps. The more we discover about our own backyard, the more fascinating it looks.

Feature image by Heather Roper/University of Arizona

Now read: NASA Sets New Roadmap for Moon Base, Crewed Missions to MarsInterstellar Visitor ‘Oumuamua Came From 1 of 4 Nearby Stars, and Elon Musk Says a Mars Base Could Happen by 2028

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