Space & Astronomy

The best way to know a world is to touch it. Scientists have observed the planets and moons in our solar system for centuries, and have flown spacecraft past the orbs for decades. But to really understand these worlds, researchers need to get their hands dirty — or at least a spacecraft’s landing pads.
Since the dawn of the space age, Mars and the moon have gotten almost all the lander love. Only a handful of spacecraft have landed on Venus, our other nearest neighbor world, and none have touched down on Europa, an icy moon of Jupiter thought to be one of the best places in the solar system to look for present-day life (SN: 5/2/14).
Researchers are working to change that. In several talks at the virtual American Geophysical Union meeting that ran from December 1 to December 17, planetary scientists and engineers discussed new tricks that hypothetical future spacecraft may need to land on unfamiliar terrain on Venus and Europa. The missions are still in a design phase and are not on NASA’s launch schedule, but scientists want to be prepared.

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Navigating a Venusian gauntlet
Venus is a notoriously difficult world to visit (SN: 2/13/18). Its searing temperatures and crushing atmospheric pressure have destroyed every spacecraft lucky enough to reach the surface within about two hours of arrival. The last landing was over 30 years ago, despite increasing confidence among planetary scientists that Venus’ surface was once habitable (SN: 8/26/16). That possibility of past, and perhaps current, life on Venus is one reason scientists are anxious to get back (SN: 10/28/20).
In one of the proposed plans discussed at the AGU meeting, scientists have ridged, folded mountainous terrain on Venus called tessera in their sights. “Safely landing in tessera terrain is absolutely necessary to satisfy our science objectives,” said planetary scientist Joshua Knicely of the University of Alaska Fairbanks in a talk recorded for the meeting. “We have to do it.”
Knicely is part of a study led by geologist Martha Gilmore of Wesleyan University in Middletown, Conn., to design a hypothetical mission to Venus that could launch in the 2030s. The mission would include three orbiters, an aerobot to float in the clouds and a lander that could drill and analyze samples of tessera rocks. This terrain is thought to have formed where edges of continents slid over and under each other long ago, bringing new rock up to the surface in what might have been some version of plate tectonics. On Earth, this sort of resurfacing may have been important in making the planet hospitable to life (SN: 4/22/20).
Ridged, folded mountainous terrain on Venus called tessera (bright region in this false-color image from NASA’s Magellan spacecraft) might have formed through long-ago tectonic activity.JPL-Caltech/NASA
But landing in these areas on Venus could be especially difficult. Unfortunately, the best maps of the planet — from NASA’s Magellan orbiter in the 1990s — can’t tell engineers how steep the slopes are in tessera terrain. Those maps suggest that most are less than 30 degrees, which the lander could handle with four telescoping legs. But some could be up to 60 degrees, leaving the spacecraft vulnerable to toppling over.
“We have a very poor understanding of what the surface is like,” Gilmore said in a talk recorded for the meeting. “What’s the boulder size? What’s the rock size distribution? Is it fluffy?”
So the lander will need some kind of intelligent navigation system to pick out the best places to land and steer there. But that need for steering brings up another problem: Unlike landers on Mars, a Venus lander can’t use small rocket engines to slow down as it descends.
The shape of a rocket is tailored to the density of air that it will push against. That’s why rockets that launch spacecraft from Earth have two sections: one for Earth’s atmosphere and one for the near-vacuum of space. Venus’ atmosphere changes density and pressure so quickly between space and the planet’s surface that “dropping a kilometer would go from the rocket working perfectly, to it’s going to misfire and possibly blow itself apart,” Knicely says.
Instead of rockets, the proposed lander would use fans to push itself around, almost like a submarine, turning the disadvantage of the dense atmosphere into an advantage.
The planet’s atmosphere also presents the biggest challenge of all: seeing the ground. Venus’ dense atmosphere scatters light more than Earth’s or Mars’ does, blurring the view of the surface until the last few kilometers of descent.
Worse, the scattered light makes it seem like illumination is coming from all directions at once, like shining a flashlight into fog. There are no shadows to help show steep slopes or reveal big boulders that the lander could crash into. That’s a major issue, according to Knicely, because all of the existing navigation software assumes that light comes from just one direction.
“If we can’t see the ground, we can’t find out where the safe stuff is,” Knicely says. “And we also can’t find out where the science is.” While proposed solutions to the other challenges of landing on Venus are close to doable, he says, this one remains the biggest hurdle.
Sticking the landing on Europa
Jupiter’s icy moon Europa, on the other hand, has no air to blur the surface or break rockets. A hypothetical future Europa lander, also discussed at the AGU meeting, would be able to use the “sky crane” technique (SN: 8/6/12). That method, in which a platform hovers above the surface using rockets and drops a spacecraft to the ground, was used to land the Curiosity rover on Mars in 2012 and will be used for the Perseverance lander in February 2021.
“The engineers are very excited about not having to deal with an atmosphere on the way down,” said spacecraft engineer Jo Pitesky of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., in a recorded talk for the meeting.
Still, there’s a lot that scientists don’t know about Europa’s surface, which could have implications for any lander that touches down, said planetary scientist Marissa Cameron of the Jet Propulsion Laboratory in another talk.
The best views of the moon’s landscape are from the Galileo orbiter in the 1990s, and the smallest features it could see were half a kilometer across. Some scientists have suggested that Europa could sport jagged ice spikes called penitentes, similar to ice features in the Chilean Andes Mountains that are named for their resemblance to hooded monks with bowed heads — although more recent work shows Europa’s lack of atmosphere should keep penitentes from forming.
Another mission, the Europa Clipper, that’s already underway will take higher-resolution images when the orbiter visits the Jovian moon later this decade, which should help clarify the issue.
In the meantime, scientists and engineers are running elaborate dress rehearsals for a Europa landing, from simulating ices with different chemical compositions in vacuum chambers to dropping a dummy lander named Olaf from a crane to see how it holds up.
“We have a requirement that says the terrain can have any configuration — jagged, potholes, you name it — and we have to be able to conform to that surface and be stable at it,” says John Gallon, an engineer at the Jet Propulsion Laboratory. (The dummy lander was named for his 4-year-old daughter’s favorite character in the movie Frozen.)
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Olaf, a scale model of a possible Europa lander, is helping NASA engineers test different strategies for landing on the icy moon of Jupiter. The rover is named for the snowman in the movie Frozen.
Over the last two years, Gallon and colleagues have tested different lander feet, legs and configurations in a lab by suspending the lander from the ceiling like a marionette. That suspension helps simulate Europa’s gravity, which is one-seventh that of Earth’s.
Without much gravity, a massive lander could easily bounce around and damage itself when trying to land. “You’re not going to stick the landing like a gymnast coming off the bars,” Gallon says. His team has tried sticky feet, bowl-shaped feet, springs that compress and push into the surface and legs that lock to help the lander stay put on various terrains. The lander might crouch like a frog or stand stiff like a table, depending on what type of surface it lands on.
Although Olaf is hard at work helping scientists figure out what it will take to build a successful Europa lander, the mission itself, like its Venusian counterpart, remains only on some planetary scientists’ wish lists for now. Meanwhile, other researchers dream about voyages to entirely different worlds, including Saturn’s geyser moon Enceladus.
“Some people will pick favorites,” Cameron says. “I just want to land someplace we’ve never been to that’s not Mars. I’d love that.” More

For the first time, scientists are about to get their (carefully gloved) hands on asteroid dirt so old it may contain clues to how our solar system formed and how water got to Earth.
A capsule containing two smidgens of dirt from asteroid Ryugu arrived in Japan on December 7, where researchers will finally get a chance to measure how much was collected. The goal of Japan’s Hayabusa2 mission was to collect at least 100 milligrams of both surface and subsurface material, and send it back to Earth.
“Hayabusa2 is home,” said project manager Yuichi Tsuda of the Japanese Aerospace Exploration Agency, or JAXA, at a news conference December 6, hours after the sample return capsule landed successfully in Woomera, Australia. “We collected the treasure box.”
Ryugu is an ancient, carbon-rich asteroid with the texture of freeze-dried coffee (SN: 3/16/20). Planetary scientists think it contains some of the earliest solids to form in the solar system, making it a time capsule of solar system history.
Hayabusa2 explored Ryugu from June 2018 to November 2019, and grabbed two samples of the asteroid (SN: 2/22/19). One came from inside an artificial crater that Hayabusa2 blasted into the asteroid’s surface, giving the spacecraft access to the asteroid’s interior (SN: 4/5/19). On December 4, the spacecraft released the sample return capsule from about 220,000 kilometers above Earth’s surface. The capsule created a brilliant fireball as it streaked through Earth’s atmosphere.
The sample return capsule left a brilliant fireball as it blazed through Earth’s atmosphere, but its heat shield prevented it from disintegrating.JAXA
At a “quick look facility” in Woomera, gases the asteroid material may have emitted were initially analyzed. But the capsule won’t be opened until after it reaches the JAXA center in Sagamihara, Japan.
Hayabusa2 is the second mission to successfully return an asteroid sample to Earth. The first Hayabusa mission visited stony asteroid Itokawa and returned to Earth in 2010. Engineering and logistical problems meant that its return was years later than planned, and it grabbed only 1,534 grains of asteroid material (SN: 6/14/10).
For Hayabusa2, though, everything seems to have gone according to plan. The spacecraft itself still has enough fuel to visit another asteroid, 1998 KY26, which is smaller and spins faster than Ryugu. It will study how such asteroids might have formed, how they hold themselves together, and what might happen if one collided with Earth. The spacecraft will reach that asteroid in July 2031, although it won’t take any more samples. More

The sun has set on the iconic Arecibo telescope.
Since 1963, this behemoth radio telescope in Puerto Rico has observed everything from space rocks whizzing past Earth to mysterious blasts of radio waves from distant galaxies. But on December 1, the 900-metric-ton platform of scientific instruments above the dish came crashing down, demolishing the telescope and spelling the end of Arecibo’s observing days.
Arecibo has made too many discoveries to include in a Top 10 list, so some of its greatest hits didn’t make the cut — like a strange class of stars that appear to turn on and off (SN: 1/6/17), and ingredients for life in a distant galaxy. But in honor of Arecibo’s 57-year tenure as one of the world’s premier observatories, here are 10 of the telescope’s coolest accomplishments, presented in roughly reverse order of coolness.
10. Clocking the Crab Nebula pulsar
Astronomers originally thought that apparently blinking stars called pulsars, discovered in 1967, might be pulsating white dwarf stars (SN: 4/27/68). But in 1968, Arecibo saw the pulsar at the center of the Crab Nebula flashing every 33 milliseconds — faster than white dwarfs can pulsate. (SN: 12/7/68). That discovery strengthened the idea that pulsars are actually rapidly spinning neutron stars, stellar corpses that sweep beams of radio waves around in space like celestial lighthouses (SN: 1/3/20).
Arecibo observations of the frequency of radio flashes from the pulsar at the center of the Crab Nebula (red star in the middle) gave support to the idea that pulsars are rapidly spinning neutron stars.Optical: NASA, HST, ASU, J. Hester et al.; X-ray: NASA, CXC, ASU, J. Hester et al.
9. Reborn pulsars
In 1982, Arecibo clocked a pulsar, dubbed PSR 1937+21, flashing every 1.6 milliseconds, unseating the Crab Nebula neutron star as the fastest known pulsar (SN: 12/4/82). That find was puzzling at first because PSR 1937+21 is older than the Crab Nebula pulsar, and pulsars were thought to rotate more slowly with age.
Then, astronomers realized that old pulsars can “spin-up” by siphoning mass from a companion star, and flash every one to 10 milliseconds. The NANOGrav project now uses such rapid-fire radio beacons as extremely precise cosmic clocks to search for the ripples in spacetime known as gravitational waves (SN: 2/11/16).
Pulsars typically rotate more slowly as they age. But data from Arecibo showed that pulsars can ‘spin-up’ to rotate hundreds of times per second by siphoning material off a neighboring star (as seen in this artist’s impression; pulsar in blue).ESA, Francesco Ferraro/Bologna Astronomical Observatory
8. Ice on Mercury
Mercury seems like it would be an unlikely place to find water ice because the planet is so close to the sun. But Arecibo observations in the early 1990s hinted that ice lurked in permanently shadowed craters at Mercury’s poles (SN: 11/9/91). NASA’s MESSENGER spacecraft later confirmed those observations (SN: 11/30/12). Finding ice on Mercury raised the question of whether ice might exist in shadowed craters on the moon, too — and recent spacecraft observations indicate that it does (SN: 5/9/16).
Images of Mercury taken by NASA’s MESSENGER spacecraft in 2011 and 2012 confirmed that hints of water ice (yellow) seen on the planet by Arecibo reside in shadowy regions at Mercury’s poles (north pole, shown; two craters labeled).NASA, JHUAPL, Carnegie Institution of Washington, Arecibo Observatory
7. Unveiling Venus
Venus is shrouded in a thick layer of clouds, but Arecibo’s radar beams could cut through that haze and bounce off of the rocky planet’s surface, allowing researchers to map the terrain. In the 1970s, Arecibo’s radar vision got the first large-scale views of Venus’ surface (SN: 11/3/79). Its radar images revealed evidence of past tectonic and volcanic activity on the planet, such as ridges and valleys (SN: 4/22/89) and ancient lava flows (SN: 9/18/76).

Arecibo provided this early view of Venus’ surface using radar in 1971.D.B. Campbell/Cornell University
Technological advances have allowed Arecibo to get crisper views of Venus. This 2015 image showcases the planet’s northern hemisphere.Smithsonian Institution, NASA GFSC, Arecibo Observatory, NAIC

6. Mercury’s revolution
In 1965, Arecibo radar measurements revealed that Mercury spins on its axis once every 59 days, rather than every 88 days (SN: 5/1/65). That observation cleared up a long-standing mystery about the planet’s temperature. If Mercury had turned on its axis once every 88 days, as previously thought, then the same side of the planet would always face the sun. That’s because it also takes 88 days for the planet to complete one orbit around the sun.
As a result, that side would be much hotter than the planet’s dark side. The 59-day rotation better matched the observation that Mercury’s temperature is fairly even across its surface.
Arecibo’s early radar observations measured the 59-day rotation rate of Mercury (shown in this false-color image of MESSENGER spacecraft data, which highlights chemical and mineralogical features on the planet’s surface).NASA, JHUAPL, Carnegie Institution of Washington
5. Mapping asteroids
Arecibo has cataloged the features of many near-Earth asteroids (SN: 5/7/10). In 1989, the observatory created a radar image of the asteroid 4769 Castalia, revealing the first double-lobed rock known in the solar system (SN: 11/25/89). Arecibo has since found space rocks orbiting each other in pairs (SN: 10/29/03) and trios (SN: 7/17/08).
Other odd finds have included a space rock whose shadows made it look to Arecibo like a skull, and an asteroid with the improbable shape of a dog bone (SN: 7/24/01). Understanding the characteristics and motion of near-Earth asteroids helps determine which ones might pose a danger to Earth — and how they could be safely deflected.
Arecibo radar images in 2000 revealed the strange dog bone shape of an asteroid named 216 Kleopatra (shown from multiple angles).WSU, NAIC, JPL/NASA
4. Phoning E.T.
The Arecibo Observatory broadcast the first radio message intended for an alien audience in November 1974 (SN: 11/23/74). That famous message was the most powerful signal ever sent from Earth, meant in part to demonstrate the capabilities of the observatory’s new high-power radio transmitter.
The message, beamed toward a cluster of about 300,000 stars roughly 25,000 light-years away, consisted of 1,679 bits of information. That string of binary code detailed the chemical formulas for components of DNA, a stick figure sketch of a human, a schematic of the solar system and other scientific data. 

3. Repeating radio blasts
Fast radio bursts, or FRBs, are brief, brilliant blasts of radio waves with unknown origins. The first FRB known to give off multiple bursts was FRB 121102, which Arecibo first spotted in 2012 and again in 2015 (SN: 3/2/16). Finding a repeating FRB ruled out the possibility that these bursts were generated by one-off cataclysmic events, such as stellar collisions. And because FRB 121102 kept recurring, astronomers were able to trace it back to its home: a dwarf galaxy about 2.5 billion light-years away (SN: 1/4/17). This confirmed the decade-long suspicion that FRBs come from beyond the Milky Way.
A repeating source of radio waves discovered by Arecibo (radio image, left) was the first fast radio burst traced back to its home galaxy. The burst originated in a dwarf galaxy about 2.5 billion light-years away (visible light image, right).H. Falcke/Nature 2017
2. Making waves
Gravitational waves were first directly detected in 2015 (SN: 2/11/16), but astronomers saw the first indirect evidence of ripples in spacetime decades ago. That evidence came from the first pulsar found orbiting another star, PSR 1913+16, first sighted by Arecibo in 1974 (SN: 10/19/74).
By tracking the arrival time of radio bursts from that pulsar over several years, astronomers were able to map its orbit, and found that PSR 1913+16 was spiraling toward its companion. As the orbits of the two stars contract, the binary system loses energy at the rate that would be expected if they were whipping up gravitational waves (SN: 2/24/79). This indirect observation of gravitational waves won the 1993 Nobel Prize in physics (SN: 10/23/93).
The first pulsar found orbiting another star, sighted by Arecibo in 1974, provided indirect evidence for the existence of ripples in spacetime called gravitational waves (illustrated).ESO, L. Calçada
1. Pulsar planets
The first planets discovered around another star were three small, rocky worlds orbiting the pulsar PSR B1257+12 (SN: 1/11/92). The find was somewhat serendipitous. In 1990, Arecibo was being repaired, and so it was stuck staring at one spot on the sky. During its observations, Earth’s rotation swept PSR B1257+12 across the telescope’s field of view. Small fluctuations in the arrival time of radio bursts from the pulsar indicated that the star was wobbling as a result of the gravitational tug of unseen planets (SN: 3/5/94).
Thousands of exoplanets have since been discovered orbiting other stars, including sunlike stars (SN: 10/8/19). Recent exoplanet surveys, however, suggest that pulsar-orbiting planets are rare (SN: 9/3/15).
The first worlds ever spotted beyond the solar system were three rocky planets (seen in this artist’s illustration) orbiting the pulsar PSR B1257+12.NASA, JPL-Caltech, R. Hurt/SSC More

Edgard Rivera-Valentín first visited the Arecibo Observatory as a little kid.
“I definitely remember this feeling of just being awestruck,” Rivera-Valentín says. “Looking at this gigantic telescope … getting to hear about all this neat work that was being done … it definitely leaves an impression.” Important science was happening right in the backyard of Rivera-Valentín’s hometown of Arecibo, Puerto Rico — and someday, Rivera-Valentín wanted to be a part of it.
As an adult, Rivera-Valentín returned to the observatory to work as a planetary scientist, using Arecibo to map the shapes and motions of potentially dangerous near-Earth asteroids. Now at the Lunar and Planetary Institute in Houston, Rivera-Valentín continues to use Arecibo data to study planetary surfaces. So the recent news that the Arecibo Observatory would shut down was “heartbreaking.”
In August and November, two cables supporting a 900-metric-ton platform of scientific instruments above Arecibo’s dish unexpectedly broke. After assessing the damage, the National Science Foundation, which funds Arecibo, announced that the telescope could not be safely repaired and would be torn down (SN: 11/19/20). But before the telescope could be dismantled, the entire instrument platform crashed down into the dish on December 1.
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After suffering damage in recent months, the Arecibo Observatory radio telescope in Puerto Rico collapsed on December 1. Cables that suspended a platform of scientific instruments above the dish snapped, causing the platform to fall into the dish.
For Puerto Rico, losing Arecibo is like New York losing the Empire State Building, or San Francisco losing the Golden Gate Bridge, Rivera-Valentín says — but with the added tragedy that Arecibo was not just a cultural and historic icon, but a prolific research facility.
“The loss of Arecibo is a big loss for the community,” says Tony Beasley, director of the National Radio Astronomy Observatory in Charlottesville, Va. “The life cycle of Arecibo was really quite remarkable, and it did some amazing science.”
The observatory’s radar maps of the moon and Mars, for example, helped NASA pick landing sites for the Apollo (SN: 5/1/65) and Viking missions (SN: 7/17/76). And observations of the asteroid Bennu helped NASA plan its OSIRIS-REx mission to snag a sample from the space rock (SN: 10/21/20). Arecibo views of Saturn’s moon Titan have revealed hydrocarbon lakes on its surface (SN: 10/1/03).
Beyond the solar system, Arecibo has observed mysterious flashes of radio waves from deep space, called fast radio bursts (SN: 2/7/20), and the distribution of galaxies in the universe. Arecibo has also been used for decades in the search for extraterrestrial intelligence (SN: 11/7/92), and it beamed the first radio message to aliens into space in 1974 (SN: 11/23/74).
In the wake of Arecibo’s collapse, the radio astronomy community is “going to have to look at what was going on at Arecibo and figure out how to replace as best we can some of those capabilities with other instruments,” Beasley says.
In its 57-year lifetime, the huge radio telescope at the Arecibo Observatory in Puerto Rico (shown) made important discoveries in planetary science and astronomy.University of Central Florida
But many of Arecibo’s capabilities can’t be easily replaced.
“Arecibo was unique in several ways,” says Donald Campbell, an astronomer at Cornell University and a former director of the observatory. For starters, Arecibo was enormous. At 305 meters across — covering some 20 acres — Arecibo was the world’s largest radio dish from the time it was built in 1963 (SN: 11/23/63) until 2016, when China completed its Five-Hundred-Meter Aperture Spherical Telescope, or FAST. With such a huge dish to collect radio waves, Arecibo could see very faint objects and phenomena.
That incredible sensitivity made Arecibo particularly good at detecting hard-to-spot objects such as rapidly spinning neutron stars called pulsars (SN: 1/3/20). As a pulsar rotates, it sweeps a beam of radio waves around in space like a lighthouse, which appears to Earth as a radio beacon flickering on and off.
“Arecibo was the king” of spotting the fickle light of pulsars, Beasley says. “There’s not going to be a simple solution to regenerating that level of collecting area.” The next biggest radio dish in the United States is the 100-meter-wide Green Bank Telescope in West Virginia. Smaller telescopes may require several hours of observing a target to collect enough radio waves for analysis, whereas Arecibo took only minutes.
Besides its mammoth size, Arecibo could also transmit radio waves. “Most radio astronomy telescopes do not have transmitters,” Campbell says. “They’re just receiving radio waves from space.” Radar transmitters allowed Arecibo to bounce radio waves off of gases in the atmosphere (SN: 1/31/70), or the surfaces of asteroids and planets. The reflected signals that came back contained information about the target such as size, shape and motion.

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“The high-powered transmitters allowed what was the original primary purpose of the telescope — the study of the Earth’s ionosphere,” Campbell says. The U.S. military, which funded the construction of Arecibo, wanted to better understand Earth’s atmosphere to help develop missile defenses (SN: 2/10/68). But Arecibo’s radar transmitters “were also used to study solar system bodies — the planets, the moons, including our own moon,” Campbell says. “More recently, the emphasis has been on studying near-Earth asteroids” that could be on a collision course with Earth.
Other big radio dishes, such as China’s FAST or the Green Bank Telescope, are not outfitted with radar transmitters. NASA’s Goldstone Deep Space Communications Complex in the Mojave Desert has a 70-meter dish with radar capabilities. But Goldstone “is used both as a military installation and also as part of the Deep Space Network that talks to spacecraft, so it doesn’t have a lot of time,” Rivera-Valentín says. “And it’s not as sensitive as Arecibo,” so it can’t see as many asteroids.
Even at the time of its demise, the Arecibo Observatory still had “a bright scientific future,” says Joan Schmelz, an astronomer at the Universities Space Research Association in Mountain View, Calif., and a former deputy director of the observatory. “It wasn’t just resting on its laurels.” For instance, Arecibo was a key facility for the ongoing NANOGrav project, which uses observations of pulsars to search for ripples in spacetime kicked up by supermassive black holes (SN: 9/24/15).
Arecibo’s observing days may be over, but that doesn’t mean data from the telescope won’t make any more contributions to science, Schmelz says. Some of radio astronomy’s most exciting discoveries have emerged from the reanalysis of old telescope data (SN: 7/25/14). “People will be continuing to analyze Arecibo data for some time,” she says, “and we’ll hopefully be seeing new scientific results as those data get analyzed and published.” More

On Sunday night, December 13, countless meteors will shoot across the sky as space particles burn up in our atmosphere and meet a fiery end. Most meteor showers occur when Earth slams into debris left behind by a comet.
But not this meteor shower, which is likely to be the most spectacular of the year. Known as the Geminid shower, it strikes every December and arises not from a flamboyant comet but from an ordinary asteroid — the first, but not the last, linked to a meteor shower.
Although both comets and asteroids are small objects orbiting the sun, icy comets sprout beautiful tails when their ice vaporizes in the heat of the sun. In contrast, asteroids have earned the name “vermin of the skies” for streaking through and ruining photographs of celestial vistas by reflecting the sun’s light.
So how can a mere asteroid outdo all of the glamorous comets and spawn a meteor shower that surpasses its rivals? “It remains a mystery,” says David Jewitt, an astronomer at UCLA. It’s akin to an ugly duckling’s offspring usurping the beautiful swan’s to win first place in a beauty contest.
Astronomers still don’t know the secret to the asteroid’s success in creating a shower that at its peak normally produces more meteors per hour than any other shower of the year. Three years ago, however, the asteroid swung extra close to Earth and gave scientists their best chance to study the humble space rock. They now look forward to the launch of a spacecraft that will image the asteroid’s surface.

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Cosmic connections
Astronomers first linked a meteor shower to a comet in 1866. They connected the well-known Perseid meteors, visible to most of the world every August, with a comet named Swift-Tuttle that had passed Earth four years earlier. Astronomers later matched most major meteor showers with one comet or another.
When a comet’s ice vaporizes in sunlight, dust grains also fly off the comet. These dust particles, called meteoroids, sprinkle along the comet’s orbit like a dandelion gone to seed. If Earth plows into this long dust stream, we see a fiery shower as the particles hit our atmosphere. The typical meteoroid is no larger than a grain of sand, but it travels so fast that it energizes electrons both in its own atoms as it disintegrates and in atmospheric atoms and molecules. As these electrons lose energy, they emit the streak of light — the meteor — that looks as though a star has fallen from the sky.
Still, as comet after comet was linked to different meteor showers, the Geminids remained apart; no one knew their source.
The Geminid meteors stood out in other ways, too. Unlike the Perseid meteors, which people have been observing for nearly 2,000 years, the Geminids are relatively new. First reports of their existence came from England and the United States in 1862. The shower in those days was weak, producing at most only one or two dozen meteors an hour. During the 20th century, however, the shower strengthened. Nowadays, at the shower’s peak, a single observer under a dark sky can see more than 100 meteors an hour. That’s better than most Perseid performances.
On top of that, the Geminid meteoroid stream, the ribbon of dust that traces the asteroid’s orbit around the sun, is newer than many other streams. Over time, streams spread out, but this one is so narrow it must have formed less than 2,000 years ago and maybe only a few hundred years ago. And based on how little the meteoroids slow down when they hit the air, astronomers deduced that Geminid meteoroids are fairly dense, about three times as dense as water and twice as dense as the Perseid meteoroids.

In 1983, astronomers finally found the Geminids’ parent. Jewitt, then a graduate student at Caltech, remembers walking home one January evening when he happened to see a rocket lift off from a military base. “I assumed it was an ICBM or something that the Air Force was launching to test,” he says. Instead, it was a heat-seeking spacecraft named the Infrared Astronomical Satellite.
In October of that year, the satellite discovered a small asteroid. To Harvard astronomer Fred Whipple, best known for his “dirty snowball” model of comets (SN: 3/14/92, p. 170), that small object stood out. It followed the same path around the sun as the particles in the Geminid meteoroid stream. Half a century earlier, Whipple himself had determined the orbit of the meteoroids by photographing the paths of the meteors against the sky. The newfound asteroid, Whipple declared, must be their long-sought source. The find also explained why the meteoroids were so dense: They come 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, cutting well inside the orbit of Mercury, the innermost planet. Astronomers therefore christened the asteroid Phaethon, a son of Helios the sun god in Greek mythology. At its farthest, Phaethon ventures beyond the orbit of Mars and reaches the asteroid belt, home of the largest space rocks, between the paths of Mars and Jupiter.
For a quarter century after Phaethon’s discovery, though, no one saw it shedding any dust particles or pebbles that could account for the many meteors that make up December’s show. Because of the sun’s glare, astronomers couldn’t observe Phaethon when it was closest to the sun. Observing during a close pass might be especially interesting because calculations indicated that the intense sunlight caused Phaethon’s surface temperature to soar to roughly 1,000 kelvins (1,340° Fahrenheit), hotter than any planet in the solar system. The torrid temperature might cause the asteroid to shoot particles into space.
A lucky break came about because Jewitt married an astrophysicist who studies the sun. “Really, the key was talking to my wife about this,” he says. Jing Li, also at UCLA, and Jewitt realized that a solar spacecraft might be able to pick up details about the asteroid when it’s nearest the sun and thus offer clues to why the space rock is such a fertile meteor-maker.
Sure enough, in 2009 and again in 2012, images taken by a NASA solar spacecraft named STEREO A caught Phaethon brightening when near the sun, which suggested the asteroid was throwing off dust particles. Then, in 2013, Jewitt and Li noticed a short dust tail in that data. The tail lasted only two days. “It’s really, really faint in basically the world’s crappiest data,” Jewitt says. The bright background sky makes the tail hard to see.
The researchers attribute Phaethon’s dust production to the extreme heat, which breaks rocks on the asteroid’s surface and sends particles aloft. Phaethon has so little gravity that those particles can escape into space. Additional dust may result from desiccation, Jewitt says: In the presence of such heat, hydrated minerals on the asteroid may dry out and crack, the way empty lake beds do on Earth, releasing more particles.
As seen from California’s Mojave Desert in 2009, a Geminid meteor streaks past Orion’s Belt. Walter Pacholka, Astropics/Science Source
Phaethon’s fast spin causes further stress. The asteroid makes a full turn every three hours and 36 minutes. Such rapid rotation is typical of small asteroids, and it means the surface freezes and then fries over a short period of time. The spin also creates a centrifugal force that might help lift particles into space.
Yet these findings don’t solve the mystery of how a modest asteroid produces such a stunning meteor shower, Jewitt says. For one thing, as he and colleagues noted in 2013 in the Astrophysical Journal Letters, the particles in Phaethon’s temporary tail are much too small.
Most of the Geminid meteors we see come from particles roughly a millimeter across. But the particles in the tail are even tinier, spanning only about one one-thousandth of a millimeter. Jewitt and Li deduced the small size because sunlight exerts radiation pressure, which is weak, that pushes the tail straight back away from the sun; if the particles were larger, they would resist the weak pressure and the tail would be curved.
Plus, Phaethon’s close passages to the sun don’t eject nearly enough particles to populate the Geminid stream. This suggests that some catastrophe hit the asteroid in the recent past and made 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 meteoroid stream itself. He had reanalyzed old data from a spacecraft whose chief mission had nothing to do with the solar system: the Cosmic Background Explorer, which NASA had launched a quarter century earlier to study the Big Bang’s afterglow and probe the universe’s birth.
“They didn’t really have the tools to look at the data in the right way back then,” Arendt says. With modern computers, he made movies of the data and glimpsed glowing strands of dust threading the solar system that emit infrared light as the sun heats them. He used this approach to view the never-before-seen dust trail along the orbit of Halley’s comet, as well as Phaethon’s dust trail: the Geminid meteoroid stream, 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 the stream (SN: 1/18/20, p. 6). “This is the first time it’s been 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 the light to the probe. The observations put the stream’s mass at roughly 1 percent that of Phaethon itself. This is much more material than the asteroid produces when closest to the sun, which Battams says again favors the idea that the bulk of the Geminid meteoroid stream owes its existence to some past catastrophe.
Phaethon visits Earth
In December 2017, the asteroid helped astronomers by flying only 10 million kilometers from Earth, the closest the rock will come until 2093. “This was a great opportunity to look at Phaethon,” says Patrick Taylor, an astronomer then at Arecibo Observatory in Puerto Rico.
Hurricane Maria had devastated the island and damaged the radio telescope just three months earlier, yet the observations succeeded. “That was the result of a tremendous amount of effort by the observatory staff, the community and the local government,” Taylor says. The telescope was repaired, and commercial power was restored to the observatory by clearing roads and replacing downed poles and cables to the site. “Everyone was aware how important this observation was going to be,” he says.
Over a period of five days, his team bounced radar signals off the asteroid, watching different features come into view as the rock rotated. As published in 2019 in Planetary and Space Science, the observations indicate that Phaethon’s equatorial diameter is about 6.25 kilometers, which means the asteroid is a bit more than half the size of the one that hit Earth and did in the dinosaurs (SN: 2/15/20, p. 7). The images show what may be craters, one more than a kilometer across, on Phaethon’s surface. There’s also a possible boulder 300 meters wide.
The radar images suggest Phaethon isn’t perfectly round. Instead, it may resemble a spinning top, like Bennu and Ryugu, two even smaller asteroids that spacecraft have recently visited. Both of those asteroids have equatorial diameters larger than their polar diameters. More than a thousand Bennus could fit inside Phaethon, but the two asteroids have similar shapes, Taylor notes. He thinks Phaethon may owe its shape to its rapid spin.

Jewitt also tried to take advantage of Phaethon’s close visit. “It was a bit of a letdown,” he says, laughing. “We saw absolutely nothing at all.” Neither the Hubble Space Telescope nor the Very Large Telescope in Chile discerned any dust or rocks coming off the asteroid.
But the future should hold much better views. In 2024, Japan will launch the DESTINY+ spacecraft, which will fly past Phaethon several years later. Japan has already sent spacecraft to two other small asteroids, and the new mission promises sharp images that should reveal Phaethon’s shape, structure, geologic features and dust trail. The spacecraft may even see the asteroid emit particles in real time, as NASA’s OSIRIS-REx mission did for Bennu (SN: 4/13/19, p. 10).
The DESTINY+ spacecraft will search for signs of a recent catastrophe that could have excavated enough material to create the Geminid meteoroid stream. 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 had to occur less than 2,000 years ago. Nevertheless, if such an impact did happen, it surely carved a fresh scar, which a spacecraft might pick up.
Perhaps some other catastrophe made the meteoroids. Maybe the asteroid was once a larger object that broke apart, because sunlight stressed it or it spun too fast. 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 by Phaethon, it may visit one of these other asteroids to investigate.
There’s another question the spacecraft might address: The Geminids come from Phaethon, all right, but where did Phaethon come from? It wasn’t born where it is, because it crosses the paths of four planets. Within just a few tens of millions of years, it will either crash into one of them or else their gravity will hurl the rock into the sun or far away from it.
Some astronomers have proposed that Phaethon is really a chunk kicked off of the large asteroid Pallas, a resident of the asteroid belt. “Could Phaethon be a piece of Pallas? Yes,” Jewitt says. “Is it likely to be a piece of Pallas? I’m not really sure about that.” The two asteroids resemble each other in composition, but there are also differences. Those distinctions may merely mean that strong sunlight has altered Phaethon’s surface. Or they may indicate the two asteroids have nothing to do with each other.
Whatever the case, this month’s show should be especially good because moonlight won’t interfere. Any astronomers watching may make a wish on the falling stars for greater insight into how those meteors and their unlikely parent came to be. More