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SYFY WIRE Bad Astronomy

The case of the Sun-diving asteroid that thinks it's a comet

By Phil Plait
The comet-like asteroid 322P/SOHO was seen once again by SOHO in August 2019. Credit: ESA/NASA SOHO and Karl Battams

In the year 1999 — on September 4, so 20 years ago today — citizen scientists with the NASA funded Sungrazer Project discovered a new comet that, at the time, was dive-bombing the Sun. Screaming past the Sun's surface by less than 8 million kilometers, it appeared in the field of view of the NASA/ESA Solar and Heliospheric Observatory (SOHO), as many sun-diving comets do. Some get so close to the Sun they "burn up," turning to vapor under the relentlessly pounding heat of the Sun from that close.

But this new object — now called 322P/SOHO — was unusual in that it was the first object discovered in SOHO images that was periodic, meaning it was on an elliptical orbit that would regularly bring it back to such a close distance to the Sun. It's a tough bugger: Instead of just fizzling away, it actually survives its encounters.

So right away this was an unusual object. With a period of almost exactly four years, it was spotted four more times in SOHO data (in 2003, 2007, 2011, and 2015), and now, just a few days ago, it was picked up once again.

Mind you, at perihelion (when it’s closest to the Sun) temperatures on its surface should reach well over 700°C (1340°F) — hot enough to melt aluminum! Yet every four years when it nears the Sun again it's almost exactly the same brightness, meaning it hasn't lost much material. A normal comet wouldn't do that; it would either have so much ice vaporized that it would get smaller and therefore dimmer every return, or it would flare suddenly as some buried pocket of ice vaporized and created a big cloud around the solid nucleus.

The orbit of 322P is so elongated that even though it gets so close to the Sun, the other end of its orbit stretches out nearly to the distance of Jupiter. Some astronomers realized that meant they could try to see it using other telescopes, so they gathered their forces and observed it in 2015 using the Very Large Telescope (a 10-meter behemoth), the 4.3-meter Discovery Channel Telescope, and the Spitzer Space Telescope.

What they found is really weird — or rather, what they didn't find. 322P was quiet. No of cometary activity, no fuzzy coma surrounding the nucleus, no tail, nothing, even though it was close enough to the Sun to show such features.

Left: An image of 322P/SOHO from the Very Large Telescope taken in May 2015. Right: Spitzer Space Telescope image from July 2015. In neither is there any sign of cometary activity. Credit: Knight et al.

And that's not the only odd thing. They measured how it changed in brightness over time, and found that it got slightly brighter and dimmer every 2.8 hours. This is called a light curve, and the change indicated that 322P is slightly elongated and rotating. Since it reflects sunlight, when we see it end-on it’s dimmer, and side-on it’s brighter. So the 2.8 hours is its rotation period, which is faster than comets usually rotate.

The Spitzer observations in infrared can be used to determine the object’s size — unlike visible light, where it reflects sunlight, in the infrared it emits light due to its own heat. In visible light the brightness depends on how shiny it is and how big it is, but in the IR the size is what affects an object’s brightness.

What they found is that it’s 150–320 meters across… and that presents another weird issue. An object that big and rotating that quickly would fly apart unless its density is high enough to hold itself together. The density they found is at least 1 gram per cubic centimeter (and probably more). But comets aren't that dense! They're porous with a lot of ice, so a typical comet might have a density of 0.6 g/cc.

The evidence was pointing in one direction: 322P/SOHO isn't a comet at all. It's an asteroid.

That would explain a lot. No comet activity, spins too fast, too dense, on a periodic orbit instead of a singular death-dive… also, the colors of 322P measured via telescope were more like an asteroid, as was its reflectivity (what's called the albedo, how much light it reflects). Its size would be small for a comet, but typical for asteroids. And this would make it a record holder as the asteroid with the closest approach to the Sun known.

So why was it ever classified as a comet? Well, when looking at the 3,000 objects discovered dive-bombing the Sun in SOHO observations, they're always comets. Except now we know it’s almost always.

The orbit of the weird object 322P/SOHO takes it as close as 7.5 million kilometers of the Sun’s surface, and nearly as far out as Jupiter. Credit: NASA/JPL-Caltech

Now let me confuse you: It might yet be a comet! A dead one. Sometimes, after repeated passages of the Sun, all the ices are sublimated away, turned into vapor and blown away by the solar wind. What’s left is a husk made of rock, its surface toughened over time by the heat, preventing anything from deep inside it from getting out. These can look a lot like asteroids. 3552 Don Quixote, for example, was first classified as an asteroid but it's more likely a dead comet.

Let me confuse you more: It's possible that 322P does emit material! Some asteroids do this when they get close to the Sun; it gets hot enough that even silicates can vaporize and form a dust cloud. The observations of 322P didn't see anything like that, but that only means there's an upper limit to how much stuff it might be blowing off, or else it would be detected. The astronomers calculate that it could lose from 2 million to 200 million kilograms of dust every orbit. If true, that means it could evaporate away completely in 1,000 to 100,000 years.

Or it may just be a solid, charred chunk of rock that is all done losing material, in which case it could be around a long time. It gets close enough to Jupiter that its orbit could be significantly altered enough to either drop it into the Sun or eject it from the solar system. On average that takes about a million years in orbits like this. A long time for us, but the blink of an eye for an asteroid. Or comet. Whatever.

More observations of this interesting critter would be useful, so maybe in a year or so when it's far enough from the Sun we can take another look. Hopefully, too, we'll find more objects like this one with time, rare as they are. We still have lots of questions. How did it get into such an elongated orbit (probably Jupiter, but knowing the details would be nice)? How long has it been on this orbit? How long will it last? What’s it made of? Is this a common way for asteroids like this to "die"? How many more 322Ps are out there?

It seems there's always more questions than answers, but that's going to happen when you find a new kind of object, especially one of a singular nature. Hopefully that latter part won't be true for too long.

Tip o' the Whipple Shield to Karl Battams for pointing this interesting object out to me and for making the cool animation of its motion around the Sun for me.