On Sunday evening, December 13th, countless meteors shoot across the sky as space particles burn up in our atmosphere and come to a fiery end. Most meteor showers occur when the earth smashes into rubble left behind by a comet.
But not this meteor shower, which is probably the most spectacular of the year. Known as the Geminid Rain, it hits every December and is not from an extravagant comet, but from an ordinary asteroid – the first, but not the last, associated with a meteor shower.
Although both comets and asteroids are small objects orbiting the sun, icy comets sprout beautiful tails when their ice evaporates in the heat of the sun. In contrast, asteroids have earned the name “vermin of the sky” because they roam and ruin photos of celestial views by reflection of sunlight.
How can a mere asteroid outperform all glamorous comets and create a meteor shower that surpasses its rivals? “It remains a mystery,” says David Jewitt, an astronomer at UCLA. It can be compared to the offspring of an ugly duckling who usurp the beautiful swans to win first place in a beauty pageant.
Astronomers still don’t know the secret of the asteroid’s success in creating a shower that, at its peak, typically produces more meteors per hour than any other shower of the year. However, three years ago the asteroid swung particularly close to Earth, giving scientists the best opportunity to study the humble space rock. They are now looking forward to the launch of a spacecraft that will map the surface of the asteroid.
Cosmic connections
In 1866, astronomers first associated a meteor shower with a comet. They linked the well-known Perseid meteors, which were visible to most of the world each August, with a comet called Swift-Tuttle, which had passed Earth four years earlier. Astronomers later compared most major meteor showers to one comet or another.
When a comet’s ice evaporates in sunlight, grains of dust also fly off the comet. These dust particles, called meteoroids, scatter over the comet’s orbit like a dandelion. As the earth plows into this long stream of dust, we see a fiery shower as the particles hit our atmosphere. The typical meteorite is no bigger than a grain of sand, but it moves so fast that it energizes electrons both in its own atoms as they decay and in atmospheric atoms and molecules. When these electrons lose energy, they send out the streak of light – the meteor – that looks like a star has fallen from the sky.
Since comet by comet was associated with different meteor showers, the Geminids remained separate. nobody knew its source.
The geminid meteors also stood out in other ways. In contrast to the Perseid meteors, which have been observed for almost 2,000 years, the Geminids are relatively new. First reports of their existence came from England and the United States in 1862. The shower at the time was weak, producing at most a dozen or two dozen meteors an hour. However, during the 20th century the shower intensified. Nowadays a single observer can see more than 100 meteors per hour under a dark sky at the height of the shower. That’s better than most of the Perseid gigs.
In addition, the Geminid Meteorite Stream, the ribbon of dust that tracks the asteroid’s orbit around the Sun, is newer than many other streams. Over time, the streams expanded, 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 the meteoroids slow down when they fly in the air, the astronomers concluded that Geminid meteoroids are quite dense, about three times as dense as water and twice as dense as Perseid meteoroids.
In 1983, astronomers finally found the Geminids’ parents. Jewitt, then a PhD student at Caltech, recalls walking home one January evening when he happened to see a missile taking off from a military base. “I assumed it was an ICBM or something the Air Force was launching for testing,” he says. Instead, it was a heat-seeking spaceship called the Infrared Astronomical Satellite.
In October of this year, the satellite discovered a small asteroid. To the Harvard astronomer Fred Whipple, who is best known for his comet model “Dirty Snowball” (SN: 14.03.92, p. 170), the small object stood out. It followed the same path around the sun as the particles in the Geminid meteor stream. Half a century earlier Whipple had determined the orbit of the meteoroids himself by photographing the paths of the meteors against the sky. The newly discovered asteroid, Whipple explained, must be their long-awaited source. The find also explained why the meteoroids were so dense: They came from a space rock rather than an icy comet.
The asteroid revolves around the Sun every 1.43 years and comes very close to the Sun. It cuts well into the orbit of Mercury, the innermost planet. Astronomers therefore baptized the asteroid Phaethon, a son of Helios, the sun god in Greek mythology. Phaethon ventures furthest beyond the orbit of Mars and reaches the asteroid belt, home of the largest space rocks, between the paths of Mars and Jupiter.
However, a quarter of a century after Phaethon’s discovery, no one saw it shedding dust particles or pebbles, which could be responsible for the many meteors that make up the December show. Because of the sun’s glare, astronomers could not see Phaethon when it was closest to the sun. Observing during a close pass could be particularly interesting, as calculations showed that the intense sunlight caused the surface temperature of Phaethon to rise to approximately 1,000 Kelvin (1,340 degrees Fahrenheit), hotter than any other planet in the solar system. The hot temperature could cause the asteroid to shoot particles into space.
A happy break came because Jewitt married an astrophysicist who studies the sun. “Really, the key was to talk to my wife about it,” he says. Jing Li, also at UCLA, and Jewitt realized that a solar spaceship could potentially capture details about the asteroid when it is closest to the Sun, thus providing clues as to why the space rock is such a fertile meteor maker.
In 2009 and 2012, images of a NASA solar spacecraft called the STEREO A Phaethon near the Sun brightened up, suggesting the asteroid was throwing dust particles. Then, in 2013, Jewitt and Li noticed a short tail of dust in these dates. The tail only lasted two days. “It’s really, really weak on the worst data in the world,” Jewitt says. The bright background sky makes the tail difficult to see.
The researchers attribute Phaethon’s dust production to the extreme heat that breaks stones on the surface of the asteroid and sends particles upwards. Phaethon has so little gravity that these particles can escape into space. Additional dust can be created by dehydration, says Jewitt: In such heat, hydrated minerals on the asteroid can dry out and crack like empty seabeds on Earth, releasing more particles.
Phaethon’s fast spin causes further stress. The asteroid makes a full revolution every three hours and 36 minutes. Such rapid rotation is typical of small asteroids and means that the surface will freeze and then fry over a short period of time. The spin also creates a centrifugal force that can help lift particles into space.
However, these findings don’t solve the mystery of how a humble asteroid creates such a breathtaking meteor shower, says Jewitt. On the one hand, as he and his colleagues did in 2013 in the Astrophysical diary lettersthe particles in Phaethon’s temporary tail are way too small.
Most geminid meteors come from particles about one millimeter in diameter. But the particles in the tail are even smaller and only span about a thousandth of a millimeter. Jewitt and Li inferred the small size because the sunlight exerts weak radiation pressure that pushes the tail directly away from the sun. If the particles were larger, they would withstand the weak pressure and the tail would be curved.
In addition, Phaethon’s narrow passages to the Sun don’t emit nearly enough particles to populate the Geminid Current. This suggests that in the recent past a catastrophe struck the asteroid and created so many meteoroids that they are still delighting meteorite watchers today.
In 2014, astronomer Richard Arendt of the University of Maryland, Baltimore County reported the first direct sighting of the Geminid meteor stream. He had re-analyzed old 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 afterglow of the Big Bang and investigate the birth of the universe.
“Back then they didn’t really have the tools to look at the data properly,” says Arendt. Using modern computers, he made films with the data and saw glowing strands of dust that threaded the solar system and emitted infrared light when the sun warmed them. He used this approach to look at the never-before-seen dust trail along the orbit of Halley’s comet as well as Phaethon’s dust trail: the Geminid meteorite stream, which looked like a narrow filament along the orbit of Phaethon. Arendt published his work in the Astronomical Journal.
More recently, NASA’s Parker Solar Probe also recognized the stream (SN: 01/18/20, p. 6th). “This is the first time it has been seen in visible light,” says Karl Battams, an astrophysicist at the US Naval Research Laboratory in Washington, DC. Sunlight hits the dust and reflects the light back to the probe. The observations assume that the mass of the stream is approximately 1 percent of the mass of Phaethon itself. This is much more material than the asteroid produces when it is closest to the Sun, which Battams says again favors the idea that most of the Geminid meteor shower owed its existence to a past disaster.
Phaethon visits the earth
In December 2017, the asteroid helped astronomers by flying just 10 million kilometers from Earth, with the rock being the closest by 2093. “This was a great opportunity to look at Phaethon,” says Patrick Taylor, an astronomer who was then at the Arecibo Observatory in Puerto Rico.
Hurricane Maria had devastated the island and damaged the radio telescope only three months earlier, but the observations were successful. “It was the result of a tremendous effort by the observatory, community, and local government,” says Taylor. The telescope was repaired and commercial power was restored to the observatory by clearing roads and replacing derelict masts and cables to the site. “Everyone knew how important this observation would be,” he says.
Over a five-day period, his team bounced radar signals from the asteroid, watching various features become visible as the rock spun. As published in 2019 in Planetary and space scienceThe observations suggest that Phaethon’s equatorial diameter is about 6.25 kilometers, which means the asteroid is just over half the size of the one that hit Earth and killed the dinosaurs (SN: 02.15.20, p. 7th). The images show craters more than a kilometer in diameter on Phaethon’s surface. There is also a possible 300 meter wide boulder.
The radar images suggest that Phaethon is not perfectly round. Instead, it may resemble a top like Bennu and Ryugu, two even smaller asteroids that spacecraft recently visited. Both asteroids have equatorial diameters that are larger than their polar diameters. More than a thousand Bennus could fit in Phaethon, but the two asteroids have similar shapes, Taylor notes. He believes Phaethon might owe its shape to its quick spin.
Jewitt also tried to take advantage of Phaethon’s close visit. “It was a bit of a disappointment,” he says with a laugh. “We didn’t see anything at all.” Neither the Hubble Space Telescope nor the Very Large Telescope in Chile could detect dust or stones from the asteroid.
But the future should have much better prospects. In 2024, Japan will launch the DESTINY + spacecraft, which will fly past Phaethon a few years later. Japan has already sent spaceships to two other small asteroids, and the new mission promises sharp images that should reveal Phaethon’s shape, structure, geological features, and dust trail. The spaceship can even see the asteroid emitting particles in real time, as NASA’s OSIRIS-REx mission did for Bennu (SN: 4/13/19, p. 10).
The DESTINY + starship will be looking for signs of a recent disaster that may have excavated enough material to create the Geminid Meteorite Stream. The most obvious possibility – impacting another asteroid – is also the most unlikely, according to Jewitt, since Phaethon is a small target and the impact should have occurred less than 2,000 years ago. If such an impact did occur, it would certainly carve a fresh scar that a spaceship could pick up.
Perhaps another disaster hit the meteoroids. Perhaps the asteroid was once a larger object that broke apart because sunlight emphasized it or it spun too quickly. In fact, one or two other asteroids smaller than Phaethon follow similar paths around the Sun and could be remnants of a super-Phaethon. After DESTINY + zips off Phaethon, it may visit one of these other asteroids to investigate.
There is another question that the spaceship could answer: The Geminids may be from Phaethon, but where does Phaethon come from? It was not born where it is because it crosses the paths of four planets. Within ten million years it will either hit one of them, or their gravity will throw the stone into the sun or far away from it.
Some astronomers have suggested that Phaethon is really a piece of the large asteroid Pallas that lives in the asteroid belt. “Could Phaethon be a piece of Pallas? Yes, ”says Jewitt. “Is it probably a piece of Pallas? IM not sure. “The two asteroids are similar in composition, but there are also differences. These differences can only mean that strong sunlight has altered Phaethon’s surface. Or they indicate that the two asteroids have nothing to do with each other.
In any case, this month’s show should be especially good as the moonlight doesn’t interfere. Any astronomer watching the shooting stars could wish for a better insight into how these meteors and their unlikely parents came about.
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