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The impressive December geminid meteor shower is born of a humble asteroid


On Sunday night, December 13, countless meteors will shoot through the sky as space particles burn in our atmosphere and find an end to fire. Most meteor showers occur when the Earth strikes against debris left by a comet.

But not this meteor shower, which will probably be the most spectacular of the year. Known as Gemini rain, it collides every December and does not arise from an extravagant comet but from a common asteroid: the first, but not the last, linked to a meteor shower.

Although both comets and asteroids are small objects that orbit the sun, icy comets sprout beautiful tails when their ice vaporizes in the heat of the sun. Instead, asteroids earned the name “soul of the heavens” for traversing and ruining photographs of celestial views reflecting sunlight.

So how can a mere asteroid beat all the glamorous comets and generate a meteor shower that beats its rivals? "It's still a mystery," says David Jewitt, a UCLA astronomer. It’s like raising an ugly duckling that usurps the beautiful swan to win first place in a beauty pageant.

Astronomers still don’t know the secret to the asteroid’s success in creating a rain that at its highest point typically produces more meteors per hour than any other rain of the year. However, three years ago, the asteroid got closer to Earth and gave scientists the best opportunity to study the humble space rock. Now they are waiting for the launch of a spaceship that will imagine the surface of the asteroid.

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Cosmic connections

Astronomers first related a meteor shower to a comet in 1866. They connected the well-known Perseid meteors, visible to most of the world each August, with a comet called Swift-Tuttle that had passed to Earth four years earlier. Astronomers later combined most meteor showers with one comet or another.

When a comet’s ice vaporizes in sunlight, dust grains also fly from the comet. These dust particles, called meteoroids, scatter along the comet's orbit like a dandelion that has been sowing. If the Earth enters this long stream of dust, we see a rain of fire as the particles collide against our atmosphere. The typical meteoroid is no larger than a grain of sand, but travels so fast that it energizes electrons both in their own atoms and disintegrates as in atmospheric atoms and molecules. When these electrons lose energy, they emit the ray of light, the meteor, which looks like a star has fallen from the sky.

Still, as comet after comet was linked to different meteor showers, the Geminids remained separate; no one knew its source.

Geminid meteors also stood out in other ways. Unlike Perseid meteors, which people have been observing for almost 2,000 years, geminids are relatively new. The first reports of its existence came from England and the United States in 1862. The rain at that time was light, producing at most only one or two dozen meteors per hour. However, during the 20th century, the shower was strengthened. Today, at the peak of the shower, a single observer under a dark sky can see more than 100 meteors per hour. That’s better than most Perseid performances.

In addition, the geminid meteoroid current, the dust band that traces the asteroid's orbit around the sun, is younger than many other currents. Over time, currents have spread, but this one is so narrow that it must have formed less than 2,000 years ago and perhaps only a few hundred years ago. And based on how little meteoroids slow down when they hit the air, astronomers have deduced that geminid meteoroids are quite dense, about twice as much as water and twice as much as chased meteoroids.

In 1983, astronomers finally found the father of the Geminids. Jewitt, then a graduate student at Caltech, recalls walking home one January night when he saw a rocket coming out of a military base. “I assumed it was an ICBM or something the Air Force was launching to test,” he says. Instead, it was a heat-seeking spacecraft called the Infrared Astronomical Satellite.

In October of that year, the satellite discovered a small asteroid. For Harvard astronomer Fred Whipple, best known for his “dirty snowball” comet model (SN: 14/03/92, p. 170), that small object stood out. It followed the same path around the sun as the particles in the flow of geminid meteoroids. Half a century earlier, Whipple himself had determined the orbit of meteoroids by photographing the paths of meteors against the sky. The new asteroid, Whipple said, must be its much-sought-after source. The discovery also explained why meteoroids were so dense: they come from a space rock instead of an icy comet.

The asteroid revolves around the sun every 1.43 years and gets very close to the sun, cutting well into the orbit of Mercury, the innermost planet. Therefore, astronomers named the asteroid Phaethon, a son of Helios, the sun god in Greek mythology. In its farthest reaches, Phaeton ventures beyond the orbit of Mars and reaches the asteroid belt, home to the largest space rocks, between the paths of Mars and Jupiter.

However, for a quarter of a century after the discovery of Phaethon, no one saw him shedding dust particles or pebbles that could account for the many meteors that make up the December spectacle. Due to the brightness of the sun, astronomers could not observe Phaethon when he was closer to the sun. Observing for a close step may be especially interesting because calculations have indicated that intense sunlight caused Phaeton's surface temperature to rise to about 1,000 kelvins (1,340 ° Fahrenheit), warmer than any planet in the solar system. The torrid temperature can cause the asteroid to throw particles into space.

Luck happened because Jewitt married an astrophysicist who studies the sun. “Really, the key was to talk to my wife about this,” he says. Jing Li, also at UCLA, and Jewitt realized that a solar spacecraft could be able to collect details about the asteroid when it is closer to the sun and thus provide clues as to why the space rock is such a fertile meteor maker.

Indeed, in 2009 and again in 2012, images taken by a NASA solar spacecraft called STEREO A captured Phaeton illuminating when it was close to the sun, suggesting that the asteroid was emitting dust particles. So in 2013, Jewitt and Li noticed a short tail of dust in that data. The queue lasted only two days. “It’s really very weak in the silliest data in the world,” Jewitt says. The bright background sky makes it difficult to see the tail.

Researchers attribute Phaethon's dust production to extreme heat, which breaks rocks on the asteroid's surface and sends particles upward. Phaeton has so little gravity that those particles can escape into space. Jewitt says, “In the presence of such heat, the asteroid’s hydrated minerals can dry out and crack, as do the empty beds of lakes on Earth, releasing more particles.

Gemini meteor seen from the Mojave DesertAs seen from the Mojave Desert in California in 2009, a geminid meteor passes through the Orion Belt. Walter Pacholka, Astropics / Science Source

Phaethon’s quick turn causes more stress. The asteroid makes a complete spin every three hours and 36 minutes. This rapid rotation is typical of small asteroids and means that the surface freezes and cools in a short period of time. The rotation also creates a centrifugal force that can help lift the particles into space.

However, these findings do not solve the mystery of how a modest asteroid produces such a stunning meteor shower, says Jewitt. On the one hand, as he and his colleagues pointed out in 2013 in the Astrophysical Journal Letters, the particles in Phaethon’s temporary tail are too small.

Most of the geminid meteors we see come from particles about a millimeter in diameter. But the tail particles are even smaller and span only about a thousandth of a millimeter. Jewitt and Li deduced the small size because sunlight exerts a weak, radiant pressure that pushes the tail back toward the sun; if the particles were larger, they would withstand weak pressure and the tail would be curved.

In addition, steps close to the Phaethon sun do not expel nearly enough particles to populate the geminid current. This suggests that some catastrophe has hit the asteroid in the recent past and produced so many meteoroids that they continue to delight meteor observers today.

In 2014, astronomer Richard Arendt of the University of Maryland, Baltimore County, reported the first direct sighting of the geminid meteor stream itself. He reanalyzed ancient data from a spacecraft whose main mission had nothing to do with the solar system: the Cosmic Background Explorer, which NASA had launched a quarter of a century earlier to study the glow of the Big Bang and probe the birth of the universe.

“They didn’t really have the tools to look at the data the right way back then,” Arendt says. With modern computers, he made movies with the data and glimpsed bright wires of dust that threaded the solar system that emit infrared light as the sun warms them. He used this approach to see the never-before-seen trace of dust along Halley's comet's orbit, as well as the Phaethon's dust trail: the stream of geminid meteoroids, which looked like a narrow filament along Phaethon's orbit. Arendt published his work in the Astronomical Journal.

More recently, NASA’s Parker solar probe also detected current (SN: 18/1/20, p. 6). “This is the first time it’s seen in visible light,” says Karl Battams, an astrophysicist at the U.S. Naval Research Laboratory in Washington, D.C. Sunlight hits the dust, reflecting light into the probe. Observations placed the mass of the flow at about 1 percent of Phaeton itself. This is much more material than the asteroid produces when it is closer to the sun, which Battams says again favors the idea that most of the flow of geminid meteoroids owes its existence to some past catastrophe.

Phaethon visits Earth

In December 2017, the asteroid helped astronomers fly just 10 million kilometers from Earth, the nearest rock will reach 2093. "This was a great opportunity to look at Phaeton," says Patrick Taylor, then an astronomer at the Arecibo Observatory in Puerto Rico.

Hurricane Maria devastated the island and damaged the radio telescope just three months earlier, but observations were successful. “That was the result of a huge effort on the part of the observatory staff, the community and local government,” Taylor says. The telescope was repaired and commercial power was restored at the observatory by clearing roads and replacing downed poles and cables. “Everyone was aware of how important this observation was going to be,” he says.

Over a five-day period, his team skipped the asteroid’s radar signals, seeing how different features appeared as the rock spun. As published in 2019 in Planetary and Space Science, observations indicate that Phaethon's equatorial diameter is about 6.25 kilometers, meaning the asteroid is just over half the size it reached Earth and reached dinosaurs. (SN: 2 / 15/20, p. 7). The images show what may be craters, more than a mile in diameter, on the surface of Phaethon. There is also a 300 meter wide boulder field.

Radar images suggest that Phaethon is not perfectly round. On the contrary, it may look like a horn, like Bennu and Ryugu, two even smaller asteroids that the spacecraft recently visited. The two asteroids have equatorial diameters larger than their polar diameters. More than a thousand Bennus would fit inside Phaethon, but the two asteroids have similar shapes, Taylor points out. He thinks Phaethon owes his form to his quick turn.

Jewitt also tried to take advantage of Phaethon's close visit. “It was a little disappointing,” he says laughing. "We saw absolutely nothing." Neither the Hubble Space Telescope nor Chile’s very large telescope discerned dust or rocks coming out of the asteroid.

But the future should have a much better outlook. In 2024, Japan will launch the DESTINY + spacecraft, which will fly over Phaethon several years later. Japan has already sent spacecraft to two other small asteroids and the new mission promises sharp images that should reveal the shape, structure, geological features and dust trail of Phaethon. The spacecraft can even see the asteroid emit particles in real time, as did NASA’s OSIRIS-REx mission to Bennu (SN: 13/04/19, p. 10).

The DESTINY + spacecraft will look for signs of a recent catastrophe that may have excavated enough material to create the flow of geminid meteoroids. The most obvious possibility – an impact with another asteroid – is also the least likely, Jewitt says, because Phaethon is a small target and the impact would have to occur less than 2,000 years ago. However, if such an impact had occurred, he would surely have carved out a new scar that a spaceship could catch.

Maybe some other catastrophe caused the meteoroids. Perhaps the asteroid was once a larger object that broke because sunlight was stressing it or spinning too fast. In fact, another or two asteroids, smaller than Phaeton, follow similar paths around the sun and could be remnants of a superfaith. After Phaethon's DESTINY + zippers, you can visit one of these other asteroids to investigate.

There’s another question the spacecraft might address: Geminids come from Phaethon, okay, but where did Phaethon come from? He was not born where he is, because he crosses the paths of four planets. In a few tens of millions of years, it will collide with one of them or its gravity will throw the rock into the sun or far away from it.

Some astronomers have proposed that Phaeton is actually a piece thrown at the large asteroid Pallas, a resident of the asteroid belt. “Could it be Phaeton a piece of Pallas? Yes, "says Jewitt." Is it probably a piece of straw? I'm not really sure about that. " The two asteroids are similar in composition, but there are also differences. Such distinctions may simply mean that strong sunlight has altered the surface of Phaethon. Or they may indicate that the two asteroids have nothing to do with each other.

Either way, this month’s program should be especially good because the moonlight won’t interfere. Any astronomer you see may wish the fallen stars a better understanding of what those meteors were like and their unlikely father.


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