The puzzle of Asteroid 3200 Phaethon<\/strong><\/h2>\u201cThe puzzle started a long time ago with the Geminids meteor stream,\u201d says Professor David Jewitt \u2013 a planetary scientist and expert on Phaethon \u2013 from the University of California, Los Angeles<\/a>.<\/p>\u201cWhereas most meteor streams consist of particles released from comets, the Geminids had no known parent, despite people having looked for one.\u201d<\/p>
In the early 1980s the Infrared Astronomical Satellite (IRAS)<\/a> spotted a new asteroid<\/a>, 3200 Phaethon.<\/p>Its path around the Sun appeared to align with the clouds of interplanetary material responsible for the Geminid meteor shower<\/a>.<\/p>The IRAS discovery brought with it a slew of fresh questions, however.<\/p>
\u201cThe new puzzle, and the one we are still working on, is that Phaethon is an asteroid not a comet. How can an asteroid produce the Geminid meteor stream?\u201d says Jewitt.<\/p>
Early studies tried to catch Phaethon behaving like a comet, ejecting dusty material into space. Yet the asteroid seemed inactive.<\/p>
\u201cThen, one day my wife, Jing Li, suggested that we should try to look at Phaethon near perihelion,\u201d says Jewitt.<\/p>![\"Hot](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2019\/11\/Phaethon_orbit-cd40449.jpg\" "\\"\\"\/")
Hot pursuit: scientists observed Asteroid 3200 Phaethon as its orbit took it close to the Sun\u2019s searing heat. Planets\/Phaethon not to scale. Credit: BBC Sky at Night Magazine<\/figcaption><\/figure>Perihelion is the point in an object\u2019s orbit where it\u2019s nearest the Sun, and for Phaethon that is a blisteringly close 21 million km from our star.<\/p>
\u201cMost telescopes can\u2019t look there, but Jing is a solar astronomer and she knew all about the telescopes on the STEREO<\/a> spacecraft that are designed to point into the Sun.\u201d<\/p>When STEREO\u2019s images were analysed they showed the little asteroid increasing in brightness as it faced the onslaught of the Sun\u2019s heat.<\/p>
This was not the behaviour of an icy object \u2013 akin to a comet \u2013 warming as it approached our star.<\/p>
Phaethon was being baked at more than 700\u00b0C. At those temperatures any ice would be long gone.<\/p>
Instead, astronomers argued the surface of Phaethon was emitting grains into space as it dried and cracked in the searing heat.<\/p>![\"Illustration](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2019\/11\/asteroid-3200-phaethon-1024x683.jpg?fit=800%2C534\")
Illustration showing asteroid Phaethon being heated by the Sun. Credit: NASA\/JPL-Caltech\/IPAC<\/figcaption><\/figure>In this unusual process lay another mystery.<\/p>
Researchers were able to work out the quantity of dust being shed by Phaethon during its close swoop into the Sun\u2019s inner realm.<\/p>
It was nowhere near what\u2019s required to explain the amount of dust in the Geminid stream.<\/p>
The astronomers used the Hubble Space Telescope<\/a> to observe Phaethon as it was passing by Earth in 2017.<\/p>\u201cWe wondered if Phaethon could be active away from perihelion as well, but just launching big particles that would be hard to see in most telescopes,\u201d recalls Jewitt. <\/p>
Hubble didn\u2019t see anything. That in itself said something important about the way Phaethon may have produced the Geminid meteoroid stream.<\/p>
\u201cWhat it means is that the mass loss from Phaethon cannot be in steady state,\u201d explains Jewitt.<\/p>
\u201cWe need about 700kg\/s to supply the Geminids but the STEREO observations gives us only 3kg\/s. We need more than one hundred times as much, but [Hubble] shows that this is not happening.<\/p>
“So we conclude that Asteroid 3200 Phaethon didn\u2019t produce the Geminids in a steady way, but perhaps impulsively as a result of some disruptive event in the last few thousand years.\u201d<\/p>
A \u2018disruptive event\u2019 might be a collision with another object or an explosion on Phaethon caused by buried ice sublimating.<\/p>
However, Jewitt suggests these scenarios are unlikely due to the emptiness of space and the anticipated high temperature of the asteroid.<\/p>
\u201cOther people, and I\u2019m one of them, note that the rotation period of Phaethon (3.6 hours) is very close to the period at which gravity would be unable to hold the body together,\u201d says Jewitt. \u201cSo maybe it shed some material because it was spinning too fast.\u201d<\/p>
That theory has its pitfalls too, though.<\/p>
\u201cThe problem is that we need this event to have occurred recently and the implication is that we are just lucky to be on-scene at the right time to see the consequences,\u201d explains Jewitt.<\/p>
\u201cReasonable people argue that appealing to this kind of luck is a step too far. I tend to agree with that.\u201d<\/p>![\"A](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2019\/03\/Geminds-004-846fca2.jpg\" "\\"\\"\/")
A Geminid meteor streaking across the December night sky. Credit: Pete Lawrence<\/figcaption><\/figure>Cracking the Geminid code<\/h3>
While it remains unclear what exactly went on to scatter the Geminid meteorid stream into space, researchers continue to examine what Phaethon is made of and where it might have originated \u2013 information that could illuminate the enduring enigma of the Geminids.<\/p>
For Teddy Kareta, a graduate student at the University of Arizona<\/a>, trying to understand Phaethon\u2019s make-up has been a key aspect of his PhD research.<\/p>He has used telescopes in Arizona and Hawaii to capture spectra of Phaethon \u2013 essentially a series of chemical fingerprints that are hidden in the light from the asteroid.<\/p>
Those fingerprints need to be decoded, and the way Kareta and his fellow researchers intend to do that is by baking meteorites to examine how their spectra match up to the signatures in Phaethon\u2019s glow.<\/p>
“When we started trying to interpret our observations of Phaethon, we realised that most previous experiments that heated meteorites to high temperatures, to compare to telescopic measurements, simply didn\u2019t get hot enough to be a great comparison to Phaethon,” says Kareta.<\/p>
“The options were to give up and say that we need better data, or build a vacuum heating chamber in the basement of our building.”<\/p>
The researchers did the latter and, at the time of writing, they are now preparing the apparatus for its first experimental run.<\/p>
“The general idea is that we grind up a meteorite, put it inside the heating chamber, pump all the air out, and then heat it up and cool it down over and over to replicate what being in an eccentric near-Sun orbit might be like,” explains Kareta.<\/p>
“We\u2019re planning on heating up organic and water rich meteorites as the most likely analogues for Phaethon,” he adds.<\/p>
Research like Kareta\u2019s provides context for the understanding of Phaethon\u2019s origin.<\/p>
Some scientists think the asteroid could be a piece hewn off the much larger asteroid Pallas. Kareta isn\u2019t so sure.<\/p>
While Pallas\u2019s and Phaethon\u2019s spectra \u2013 the chemical signature of their surface in their light \u2013 are alike, the amounts of sunlight they reflect (a characteristic known as an object\u2019s \u2018albedo\u2019) are not.<\/p>
\u201cWhen Phaethon moved into its current orbit and started getting really hot, its surface properties were cooked and changed as a result,\u201d explains Kareta.<\/p>
\u201cWhen you cook a meteorite in the lab, the spectra and albedo both change, so we think it\u2019s hard to explain Phaethon as just being a cooked fragment of Pallas.\u201d<\/p>
While Kareta and his colleagues\u2019 research works to build a picture of Phaethon\u2019s composition, other teams have been collecting actual images of the asteroid to examine its shape and size.<\/p>
As Phaethon was swinging by Earth in December 2017, astronomers using the Arecibo radio telescope<\/a> in Puerto Rico were bouncing radio waves off the asteroid to create a radar image of it.<\/p>![\"Radar](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2019\/11\/Phaethon_Arecibo-6ec6154.jpg\" "\\"\\"\/")
Radar images of Asteroid 3200 Phaethon generated by astronomers at the National Science Foundation\u2019s Arecibo Observatory on Dec. 17, 2017. Credits: Arecibo Observatory\/NASA\/NSF<\/figcaption><\/figure>\u201cWe find that Phaethon is roughly spherical in shape and rather non-descript,\u201d says Dr Patrick Taylor, from the Lunar and Planetary Institute in Texas<\/a>, who led the team using Arecibo.<\/p>\u201cFrom the variation in signal strength in the radar images, we can hypothesise that some of the features are a large crater at the low latitudes of Phaethon and possibly a large boulder,\u201d he explains.<\/p>
\u201cWe believe Phaethon has a shape very similar to asteroids Bennu and Ryugu, currently being visited by the OSIRIS-REx<\/a> and Hayabusa2<\/a> spacecraft, but much, much larger.\u201d<\/p>Following these radar glimpses, scientists should soon get even better pictures of Phaethon.<\/p>
The Japanese space agency, JAXA, hopes to launch a mission \u2013 named DESTINY \u2013 to the asteroid in 2022.<\/p>
It\u2019ll be equipped with a range of imaging cameras and scientific instruments to examine Phaethon, and it could help answer some of the big questions that remain about this intriguing object.<\/p>
For Teddy Kareta, the opportunity to finally see the asteroid\u2019s surface up-close is a thrilling prospect.<\/p>
\u201cFor decades, Phaethon has been emblematic of everything we don\u2019t know about asteroids, comets, and everything in between,\u201d he says.<\/p>
\u201c[It] is at the centre of one of the great mysteries of the inner Solar System, and I\u2019m just stoked that we\u2019re gonna go and take a look.\u201d<\/p>![\"A](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2019\/11\/Asteroid_Bennu-b364f0f.jpg\" "\\"\\"\/")
A view of asteroid Bennu captured by NASA\u2019s OSIRIS-REx spacecraft on 29 October 2018 from a distance of 330km.<\/figcaption><\/figure>How big is Asteroid 3200 Phaethon?<\/strong><\/h2>We put Asteroid 3200 Phaethon in context by comparing its size to some well-known objects, both terrestrial and celestial.<\/p>
Phaethon:<\/strong> 6km (diameter)<\/p>Bennu, the asteroid investigated by NASA\u2019s OSIRIS-REx sample-return mission:<\/strong> 500m (diameter)<\/p>
\u201cWhereas most meteor streams consist of particles released from comets, the Geminids had no known parent, despite people having looked for one.\u201d<\/p>
In the early 1980s the Infrared Astronomical Satellite (IRAS)<\/a> spotted a new asteroid<\/a>, 3200 Phaethon.<\/p> Its path around the Sun appeared to align with the clouds of interplanetary material responsible for the Geminid meteor shower<\/a>.<\/p> The IRAS discovery brought with it a slew of fresh questions, however.<\/p> \u201cThe new puzzle, and the one we are still working on, is that Phaethon is an asteroid not a comet. How can an asteroid produce the Geminid meteor stream?\u201d says Jewitt.<\/p> Early studies tried to catch Phaethon behaving like a comet, ejecting dusty material into space. Yet the asteroid seemed inactive.<\/p> \u201cThen, one day my wife, Jing Li, suggested that we should try to look at Phaethon near perihelion,\u201d says Jewitt.<\/p> Perihelion is the point in an object\u2019s orbit where it\u2019s nearest the Sun, and for Phaethon that is a blisteringly close 21 million km from our star.<\/p> \u201cMost telescopes can\u2019t look there, but Jing is a solar astronomer and she knew all about the telescopes on the STEREO<\/a> spacecraft that are designed to point into the Sun.\u201d<\/p> When STEREO\u2019s images were analysed they showed the little asteroid increasing in brightness as it faced the onslaught of the Sun\u2019s heat.<\/p> This was not the behaviour of an icy object \u2013 akin to a comet \u2013 warming as it approached our star.<\/p> Phaethon was being baked at more than 700\u00b0C. At those temperatures any ice would be long gone.<\/p> Instead, astronomers argued the surface of Phaethon was emitting grains into space as it dried and cracked in the searing heat.<\/p> In this unusual process lay another mystery.<\/p> Researchers were able to work out the quantity of dust being shed by Phaethon during its close swoop into the Sun\u2019s inner realm.<\/p> It was nowhere near what\u2019s required to explain the amount of dust in the Geminid stream.<\/p> The astronomers used the Hubble Space Telescope<\/a> to observe Phaethon as it was passing by Earth in 2017.<\/p> \u201cWe wondered if Phaethon could be active away from perihelion as well, but just launching big particles that would be hard to see in most telescopes,\u201d recalls Jewitt. <\/p> Hubble didn\u2019t see anything. That in itself said something important about the way Phaethon may have produced the Geminid meteoroid stream.<\/p> \u201cWhat it means is that the mass loss from Phaethon cannot be in steady state,\u201d explains Jewitt.<\/p> \u201cWe need about 700kg\/s to supply the Geminids but the STEREO observations gives us only 3kg\/s. We need more than one hundred times as much, but [Hubble] shows that this is not happening.<\/p> “So we conclude that Asteroid 3200 Phaethon didn\u2019t produce the Geminids in a steady way, but perhaps impulsively as a result of some disruptive event in the last few thousand years.\u201d<\/p> A \u2018disruptive event\u2019 might be a collision with another object or an explosion on Phaethon caused by buried ice sublimating.<\/p> However, Jewitt suggests these scenarios are unlikely due to the emptiness of space and the anticipated high temperature of the asteroid.<\/p> \u201cOther people, and I\u2019m one of them, note that the rotation period of Phaethon (3.6 hours) is very close to the period at which gravity would be unable to hold the body together,\u201d says Jewitt. \u201cSo maybe it shed some material because it was spinning too fast.\u201d<\/p> That theory has its pitfalls too, though.<\/p> \u201cThe problem is that we need this event to have occurred recently and the implication is that we are just lucky to be on-scene at the right time to see the consequences,\u201d explains Jewitt.<\/p> \u201cReasonable people argue that appealing to this kind of luck is a step too far. I tend to agree with that.\u201d<\/p> While it remains unclear what exactly went on to scatter the Geminid meteorid stream into space, researchers continue to examine what Phaethon is made of and where it might have originated \u2013 information that could illuminate the enduring enigma of the Geminids.<\/p> For Teddy Kareta, a graduate student at the University of Arizona<\/a>, trying to understand Phaethon\u2019s make-up has been a key aspect of his PhD research.<\/p> He has used telescopes in Arizona and Hawaii to capture spectra of Phaethon \u2013 essentially a series of chemical fingerprints that are hidden in the light from the asteroid.<\/p> Those fingerprints need to be decoded, and the way Kareta and his fellow researchers intend to do that is by baking meteorites to examine how their spectra match up to the signatures in Phaethon\u2019s glow.<\/p> “When we started trying to interpret our observations of Phaethon, we realised that most previous experiments that heated meteorites to high temperatures, to compare to telescopic measurements, simply didn\u2019t get hot enough to be a great comparison to Phaethon,” says Kareta.<\/p> “The options were to give up and say that we need better data, or build a vacuum heating chamber in the basement of our building.”<\/p> The researchers did the latter and, at the time of writing, they are now preparing the apparatus for its first experimental run.<\/p> “The general idea is that we grind up a meteorite, put it inside the heating chamber, pump all the air out, and then heat it up and cool it down over and over to replicate what being in an eccentric near-Sun orbit might be like,” explains Kareta.<\/p> “We\u2019re planning on heating up organic and water rich meteorites as the most likely analogues for Phaethon,” he adds.<\/p> Research like Kareta\u2019s provides context for the understanding of Phaethon\u2019s origin.<\/p> Some scientists think the asteroid could be a piece hewn off the much larger asteroid Pallas. Kareta isn\u2019t so sure.<\/p> While Pallas\u2019s and Phaethon\u2019s spectra \u2013 the chemical signature of their surface in their light \u2013 are alike, the amounts of sunlight they reflect (a characteristic known as an object\u2019s \u2018albedo\u2019) are not.<\/p> \u201cWhen Phaethon moved into its current orbit and started getting really hot, its surface properties were cooked and changed as a result,\u201d explains Kareta.<\/p> \u201cWhen you cook a meteorite in the lab, the spectra and albedo both change, so we think it\u2019s hard to explain Phaethon as just being a cooked fragment of Pallas.\u201d<\/p> While Kareta and his colleagues\u2019 research works to build a picture of Phaethon\u2019s composition, other teams have been collecting actual images of the asteroid to examine its shape and size.<\/p> As Phaethon was swinging by Earth in December 2017, astronomers using the Arecibo radio telescope<\/a> in Puerto Rico were bouncing radio waves off the asteroid to create a radar image of it.<\/p> \u201cWe find that Phaethon is roughly spherical in shape and rather non-descript,\u201d says Dr Patrick Taylor, from the Lunar and Planetary Institute in Texas<\/a>, who led the team using Arecibo.<\/p> \u201cFrom the variation in signal strength in the radar images, we can hypothesise that some of the features are a large crater at the low latitudes of Phaethon and possibly a large boulder,\u201d he explains.<\/p> \u201cWe believe Phaethon has a shape very similar to asteroids Bennu and Ryugu, currently being visited by the OSIRIS-REx<\/a> and Hayabusa2<\/a> spacecraft, but much, much larger.\u201d<\/p> Following these radar glimpses, scientists should soon get even better pictures of Phaethon.<\/p> The Japanese space agency, JAXA, hopes to launch a mission \u2013 named DESTINY \u2013 to the asteroid in 2022.<\/p> It\u2019ll be equipped with a range of imaging cameras and scientific instruments to examine Phaethon, and it could help answer some of the big questions that remain about this intriguing object.<\/p> For Teddy Kareta, the opportunity to finally see the asteroid\u2019s surface up-close is a thrilling prospect.<\/p> \u201cFor decades, Phaethon has been emblematic of everything we don\u2019t know about asteroids, comets, and everything in between,\u201d he says.<\/p> \u201c[It] is at the centre of one of the great mysteries of the inner Solar System, and I\u2019m just stoked that we\u2019re gonna go and take a look.\u201d<\/p> We put Asteroid 3200 Phaethon in context by comparing its size to some well-known objects, both terrestrial and celestial.<\/p> Phaethon:<\/strong> 6km (diameter)<\/p> Bennu, the asteroid investigated by NASA\u2019s OSIRIS-REx sample-return mission:<\/strong> 500m (diameter)<\/p>Cracking the Geminid code<\/h3>
How big is Asteroid 3200 Phaethon?<\/strong><\/h2>