- Jan 27, 2020
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By Sharmila Kuthunur
The two feats almost did not happen.
The most comprehensive plans need a sprinkle of luck, even in space.
In October 2022, the James Webb Space Telescope (JWST or Webb) watched as Chariklo, a tiny ringed asteroid, eclipsed a star. This event, called an occultation, marked a first for Webb. At the month's end, Webb turned toward Chariklo again and notched another victory: For the first time, astronomers analyzing the telescope's data spotted clear signs of water ice, the presence of which was only hinted at until now. These observations will guide astronomers to better understand the nature and behavior of tiny bodies in the outer reaches of our solar system.
But the two feats almost did not happen.
This illustration shows what the Centaur Chariklo and its rings could look like, based on our current understanding. (Image credit: NASA, ESA, CSA, Leah Hustak (STScI)
Although it is the largest of its kind, Chariklo is still too small and too far for even the mighty Webb to photograph directly. Instead, astronomers decided to study it through occultation, which is an indirect but powerful method to study small bodies like Chariklo. But the team did not know if and when a star — without which an occultation would not occur — would fall into Webb's field of view. This made Chariklo part of Webb's target of opportunity(opens in new tab) program: If the asteroid happened to cross in front of a star, the program would allow astronomers to temporarily interrupt the telescope's schedule to observe the event.
The team calculated only a 50% chance that Webb would spot a star bright enough with an interesting object like Chariklo crossing in front. After its launch in 2021, as Webb went through routine course corrections to hold it steady in its parking spot in space, the team continued predicting and revising its list of possible occultations. Late last year, astronomers ended up on the favorable side of that 50% when they discovered "by remarkable good luck" that Chariklo was on track to occult a star that also fell into Webb's view.
"This was the first stellar occultation attempted with Webb," the team wrote in a NASA statement(opens in new tab) published Wednesday (Jan. 25). "A lot of hard work went into identifying and refining the predictions for this unusual event."
On Oct. 18, 2022, Chariklo and its system of two rings crossed in front of a star. Using Webb's near-infrared camera (NIRCam), astronomers monitored the star's brightness for an hour. Resulting data showed two dips in the star's brightness as expected: When the asteroid's rings first hid the star as the eclipse began, and again when the last of its rings wrapped up the occultation.
"The shadows produced by Chariklo's rings were clearly detected," the team wrote in the statement, "demonstrating a new way of using Webb to explore solar system objects."
Graphic showing the dimming effects of Chariklo's rings on a background star. (Image credit: IMAGE: NASA, ESA, CSA, Leah Hustak (STScI) SCIENCE: Pablo Santos-Sanz (IAA-CSIC), Nicolás Morales (IAA-CSIC), Bruno Morgado (UFRJ, ON/MCTI, LIneA))
Objects like Chariklo are called centaurs*, thanks to their hybrid nature. (Centaurs are mythological horse-human hybrids.) They look like asteroids but behave like comets — complete with visible tails. Their home, an unstable orbit between Jupiter and Neptune, hosts thousands of centaurs of varying shapes and sizes. As interesting as they are, their small size and vast distance make them difficult to study. The composition of even the biggest centaur, Chariklo — which is still tiny at just 160 miles (250 km) in diameter and distant at a whooping 2 billion miles (3.2 billion km) from us — is poorly understood. Also, past research hinted at water ice somewhere in Chariklo's system, but had yet to conclusively detect it.
In this latest research, astronomers pointed Webb at Chariklo again. This time, they used the telescope's Near-infrared Spectrograph (NIRSpec) instrument to measure the sunlight reflected by Chariklo and its two rings. The resulting spectrum showed three absorption bands of water ice, marking the first clear indication of crystalline ice.
The presence of crystalline ice likely indicates that Chariklo is subject to constant bombardment, according to Dean Hines, an astronomer at the Space Telescope Science Institute in Maryland. "Because high-energy particles transform ice from crystalline into amorphous states, detection of crystalline ice indicates that the Chariklo system experiences continuous micro-collisions that either expose pristine material or trigger crystallization processes," Hines said in NASA's statement.
Reflectance spectrum of the double-ringed centaur 10199 Chariklo, captured by Webb's Near-Infrared Spectrograph (NIRSpec) on Oct. 31, 2022. This spectrum shows clear evidence for crystalline water ice on Chariklo's surface. (Image credit: IMAGE: NASA, ESA, CSA, Leah Hustak (STScI) SCIENCE: Noemí Pinilla-Alonso (FSI/UCF), Ian Wong (STScI), Javier Licandro (IAC))
Astronomers have gotten one step closer to studying the Chariklo system, but there is still much that remains unknown about the centaur. The spectrum analyzed in the latest research includes information about the system as a whole, but at the moment, it is difficult to distinguish the data between Chariklo and its two rings.
For example, although astronomers spotted the first clear signs for crystalline water ice, they do not yet know for sure where in the asteroid's system the ice is present. In the coming months, researchers hope to use Webb's high sensitivity to dig up individual features of Chariklo and its two rings, Pablo Santos-Sanz, an astronomer at the Instituto de Astrofísica de Andalucía in Spain who took part in this research, said in the statement.
"We hope [to] gain insight into why this small body even has rings at all, and perhaps detect new fainter rings," Santos-Sanz said.
Follow Sharmila Kuthunur on Twitter @Sharmilakg(opens in new tab). Follow us @Spacedotcom(opens in new tab), or on Facebook(opens in new tab) and Instagram(opens in new tab).
See: https://www.space.com/james-webb-space-telescope-chariklo-water-ice?utm_term=D97F3BCF-719F-4D6F-9A52-D93368EC5062&utm_campaign=58E4DE65-C57F-4CD3-9A5A-609994E2C5A9&utm_medium=email&utm_content=C134B5FF-9444-49BF-B7E1-B80305AA2A4E&utm_source=SmartBrief
* Centaurs - Trojans and Centaurs are primitive, peculiar objects orbiting in the middle solar system. Both groups have characteristically low albedos and red colors. Physical observations of Trojans reveal featureless reddish spectra, implying surfaces probably rich in complex organic solid materials. The interiors are expected to be rich in H2O ice and other volatile material. Centaurs have surfaces showing dramatically different spectral reflectances, from neutral to very red. Some spectra are featureless, while others show signatures of water ice, methanol, or other light hydro- carbons. Trojans were formed near Jupiter’s orbit, while Centaurs were formed far beyond Jupiter’s orbit, but both were formed at low temperatures at which water exists as solid ice.
Reflectance spectra of three Centaurs and Trojan asteroid 624 Hektor. The spectrum of 2060 Chiron is from Luu et al. (2000) and shows a model consisting of H2O ice and olivine. The scaling to geometric albedo is approximate. Data for 10199 Chariklo (orig- inally 1997 CU26) are from Brown et al. (1998); the ordinate for the spectrum of Chariklo is given on the right side of the figure. The 5145 Pholus data and model come from Cruikshank et al. (1998), while the 624 Hektor data and model come from Cruikshank et al. (2001). The ordinate for both Hektor and Pholus is given on the left side of the figure. All three Centaurs show the prominent 2-μm H2O ice band, with indications of the weaker 1.5-μm band of H2O ice. There is no spectral evidence for ice on Hektor.
Centaurs are minor planets having unstable orbits with semimajor axes between those of Jupiter (5.2 AU) and Neptune (30 AU). Their planet-crossing orbits imply a short dynamical lifetime (106–107 yr) compared to the age of the solar system (Hahn and Bailey, 1990; Asher and Steel, 1993). The origin of the Centaurs is uncertain, but they are thought to have been ejected from the transneptunian belt by planetary perturbations or mutual collisions (Duncan et al., 1995). Levison and Duncan (1997) suggest that these objects could be the source of short-period comets. Later, Levison et al. (2001), on the basis of their numerical model, predict that some Centaurs could have originated in the Oort Cloud. Centaurs seem to be located on a boundary between many solar system populations, and they are important for understanding the dynamical evolution of the outer solar system.
To date, 25 such objects have been discovered, following the continuously updated list from the Minor Planet Center (Marsden, 2001), and the discoveries continue. Although no formal definition exists, Centaurs have been identified as asteroids at the times of discovery, even though 2060 Chiron was subsequently shown to have cometary activity. Jewitt and Kalas (1998) add to the list of Centaurs the comets P/ Oterma and P/Schwassmann-Wachmann 1 because their orbits lie inside the orbits of Jupiter and Neptune. Recently, Marsden (2001) includes Centaurs in a common list with scattered transneptunian objects, arguing that there are no dynamical reasons to make a distinction, but in this paper we will consider the two populations to be well separated.
From the paper: Physical Properties of Trojan and Centaur Asteroids
by M.A. Barucci, D.P. Cruikshank, S. Mottola and M. Lazzarin
See: https://www.lpi.usra.edu/books/AsteroidsIII/pdf/3001.pdf
Chariklo is so small and so far away that even the mighty Webb cannot photograph it directly. Instead, astronomers are studying it through occultation**, which is an indirect but powerful method to study small bodies like Chariklo. As a result astronomers have spotted the first clear signs for crystalline water ice on this small body and have been able to flesh out its unique rings.
Hartmann352
** Occultation occurs when a solar-system body passes in front of a more distant object (e.g. a star or another solar system body), partially or totally hiding the more distant object and momentarily blocking its light. Each occultation can be seen only at the proper limited time and from a limited part of the Earth.
For asteroid occultations the star is usually the brightest component of the occultation. The asteroid is usually several magnitudes fainter than the star and often too faint to be detected in a small telescope. In an asteroid occultation, the observer must find the star to be occulted and monitor the star to watch for any drop in brightness that would signal an occultation. Asteroid occultation events typically last several seconds but may observers may record much shorter or much longer events in rare cases. As the asteroid moves in its orbit, a shadow is created from light cast by the star about to be occulted. The shadow (equal in size to the asteroid) then moves across the Earth (diagram not to scale). An observer will only see an event (drop in the brightness of the star) if they are located inside the path of the asteroid’s shadow. Since asteroids are generally much smaller than the moon, choosing a location for observing an asteroid occultation is more important than location in lunar occultations because of the small diameters of the asteroids. In addition, asteroid subtend a much smaller angular size on the sky and this leads to more uncertainty in the actual location of the asteroid’s shadow. Asteroid occultation predictions posted by IOTA*** provide information on the expected location of the shadow path, expected time of the occultation, the level of drop in the star’s light and the expected duration of the occultation event. An observer can expect to see a single disappearance (or drop in starlight) and a single reappearance though it is possible to see step events.
For asteroid occultations the star is usually the brightest component of the occultation. The asteroid is usually several magnitudes fainter than the star and often too faint to be detected in a small telescope. In an asteroid occultation, the observer must find the star to be occulted and monitor the star to watch for any drop in brightness that would signal an occultation. Asteroid occultation events typically last several seconds but may observers may record much shorter or much longer events in rare cases. In the following diagram of an asteroid occultation: As the asteroid moves in its orbit, a shadow is created from light cast by the star about to be occulted. The shadow (equal in size to the asteroid) then moves across the Earth (diagram not to scale). An observer will only see an event (drop in the brightness of the star) if they are located inside the path of the asteroid’s shadow. Since asteroids are generally much smaller than the moon, choosing a location for observing an asteroid occultation is more important than location in lunar occultations. In addition, asteroid subtend a much smaller angular size on the sky and this leads to more uncertainty in the actual location of the asteroid’s shadow. Asteroid occultation predictions posted by IOTA provide information on the expected location of the shadow path, expected time of the occultation, the level of drop in the star’s light and the expected duration of the occultation event. An observer can expect to see a single disappearance (or drop in starlight) and a single reappearance though it is possible to see step events.
Note in the diagram above that multiple observers span the shadow of the asteroid. In this way an outline of the actual shape of the asteroid can be determined – information that would be difficult or impossible to obtain in any way other than by visiting the asteroid itself!
See: https://occultations.org/occultations/what-is-an-occultation/
*** IOTA - International Occultation Timing Association which primarily observes two basic areas of occultation astronomy: lunar occultations and asteroid occultations. IOTA is a volunteer science and research organization born in 1983. They gather data from timings of astronomical occultations and provide a variety of educational resources to promote and encourage observations of astronomical occultations.
See: https://occultations.org
The two feats almost did not happen.
The most comprehensive plans need a sprinkle of luck, even in space.
In October 2022, the James Webb Space Telescope (JWST or Webb) watched as Chariklo, a tiny ringed asteroid, eclipsed a star. This event, called an occultation, marked a first for Webb. At the month's end, Webb turned toward Chariklo again and notched another victory: For the first time, astronomers analyzing the telescope's data spotted clear signs of water ice, the presence of which was only hinted at until now. These observations will guide astronomers to better understand the nature and behavior of tiny bodies in the outer reaches of our solar system.
But the two feats almost did not happen.
This illustration shows what the Centaur Chariklo and its rings could look like, based on our current understanding. (Image credit: NASA, ESA, CSA, Leah Hustak (STScI)
Although it is the largest of its kind, Chariklo is still too small and too far for even the mighty Webb to photograph directly. Instead, astronomers decided to study it through occultation, which is an indirect but powerful method to study small bodies like Chariklo. But the team did not know if and when a star — without which an occultation would not occur — would fall into Webb's field of view. This made Chariklo part of Webb's target of opportunity(opens in new tab) program: If the asteroid happened to cross in front of a star, the program would allow astronomers to temporarily interrupt the telescope's schedule to observe the event.
The team calculated only a 50% chance that Webb would spot a star bright enough with an interesting object like Chariklo crossing in front. After its launch in 2021, as Webb went through routine course corrections to hold it steady in its parking spot in space, the team continued predicting and revising its list of possible occultations. Late last year, astronomers ended up on the favorable side of that 50% when they discovered "by remarkable good luck" that Chariklo was on track to occult a star that also fell into Webb's view.
"This was the first stellar occultation attempted with Webb," the team wrote in a NASA statement(opens in new tab) published Wednesday (Jan. 25). "A lot of hard work went into identifying and refining the predictions for this unusual event."
On Oct. 18, 2022, Chariklo and its system of two rings crossed in front of a star. Using Webb's near-infrared camera (NIRCam), astronomers monitored the star's brightness for an hour. Resulting data showed two dips in the star's brightness as expected: When the asteroid's rings first hid the star as the eclipse began, and again when the last of its rings wrapped up the occultation.
"The shadows produced by Chariklo's rings were clearly detected," the team wrote in the statement, "demonstrating a new way of using Webb to explore solar system objects."
Graphic showing the dimming effects of Chariklo's rings on a background star. (Image credit: IMAGE: NASA, ESA, CSA, Leah Hustak (STScI) SCIENCE: Pablo Santos-Sanz (IAA-CSIC), Nicolás Morales (IAA-CSIC), Bruno Morgado (UFRJ, ON/MCTI, LIneA))
Objects like Chariklo are called centaurs*, thanks to their hybrid nature. (Centaurs are mythological horse-human hybrids.) They look like asteroids but behave like comets — complete with visible tails. Their home, an unstable orbit between Jupiter and Neptune, hosts thousands of centaurs of varying shapes and sizes. As interesting as they are, their small size and vast distance make them difficult to study. The composition of even the biggest centaur, Chariklo — which is still tiny at just 160 miles (250 km) in diameter and distant at a whooping 2 billion miles (3.2 billion km) from us — is poorly understood. Also, past research hinted at water ice somewhere in Chariklo's system, but had yet to conclusively detect it.
In this latest research, astronomers pointed Webb at Chariklo again. This time, they used the telescope's Near-infrared Spectrograph (NIRSpec) instrument to measure the sunlight reflected by Chariklo and its two rings. The resulting spectrum showed three absorption bands of water ice, marking the first clear indication of crystalline ice.
The presence of crystalline ice likely indicates that Chariklo is subject to constant bombardment, according to Dean Hines, an astronomer at the Space Telescope Science Institute in Maryland. "Because high-energy particles transform ice from crystalline into amorphous states, detection of crystalline ice indicates that the Chariklo system experiences continuous micro-collisions that either expose pristine material or trigger crystallization processes," Hines said in NASA's statement.
Reflectance spectrum of the double-ringed centaur 10199 Chariklo, captured by Webb's Near-Infrared Spectrograph (NIRSpec) on Oct. 31, 2022. This spectrum shows clear evidence for crystalline water ice on Chariklo's surface. (Image credit: IMAGE: NASA, ESA, CSA, Leah Hustak (STScI) SCIENCE: Noemí Pinilla-Alonso (FSI/UCF), Ian Wong (STScI), Javier Licandro (IAC))
Astronomers have gotten one step closer to studying the Chariklo system, but there is still much that remains unknown about the centaur. The spectrum analyzed in the latest research includes information about the system as a whole, but at the moment, it is difficult to distinguish the data between Chariklo and its two rings.
For example, although astronomers spotted the first clear signs for crystalline water ice, they do not yet know for sure where in the asteroid's system the ice is present. In the coming months, researchers hope to use Webb's high sensitivity to dig up individual features of Chariklo and its two rings, Pablo Santos-Sanz, an astronomer at the Instituto de Astrofísica de Andalucía in Spain who took part in this research, said in the statement.
"We hope [to] gain insight into why this small body even has rings at all, and perhaps detect new fainter rings," Santos-Sanz said.
Follow Sharmila Kuthunur on Twitter @Sharmilakg(opens in new tab). Follow us @Spacedotcom(opens in new tab), or on Facebook(opens in new tab) and Instagram(opens in new tab).
See: https://www.space.com/james-webb-space-telescope-chariklo-water-ice?utm_term=D97F3BCF-719F-4D6F-9A52-D93368EC5062&utm_campaign=58E4DE65-C57F-4CD3-9A5A-609994E2C5A9&utm_medium=email&utm_content=C134B5FF-9444-49BF-B7E1-B80305AA2A4E&utm_source=SmartBrief
* Centaurs - Trojans and Centaurs are primitive, peculiar objects orbiting in the middle solar system. Both groups have characteristically low albedos and red colors. Physical observations of Trojans reveal featureless reddish spectra, implying surfaces probably rich in complex organic solid materials. The interiors are expected to be rich in H2O ice and other volatile material. Centaurs have surfaces showing dramatically different spectral reflectances, from neutral to very red. Some spectra are featureless, while others show signatures of water ice, methanol, or other light hydro- carbons. Trojans were formed near Jupiter’s orbit, while Centaurs were formed far beyond Jupiter’s orbit, but both were formed at low temperatures at which water exists as solid ice.
Reflectance spectra of three Centaurs and Trojan asteroid 624 Hektor. The spectrum of 2060 Chiron is from Luu et al. (2000) and shows a model consisting of H2O ice and olivine. The scaling to geometric albedo is approximate. Data for 10199 Chariklo (orig- inally 1997 CU26) are from Brown et al. (1998); the ordinate for the spectrum of Chariklo is given on the right side of the figure. The 5145 Pholus data and model come from Cruikshank et al. (1998), while the 624 Hektor data and model come from Cruikshank et al. (2001). The ordinate for both Hektor and Pholus is given on the left side of the figure. All three Centaurs show the prominent 2-μm H2O ice band, with indications of the weaker 1.5-μm band of H2O ice. There is no spectral evidence for ice on Hektor.
Centaurs are minor planets having unstable orbits with semimajor axes between those of Jupiter (5.2 AU) and Neptune (30 AU). Their planet-crossing orbits imply a short dynamical lifetime (106–107 yr) compared to the age of the solar system (Hahn and Bailey, 1990; Asher and Steel, 1993). The origin of the Centaurs is uncertain, but they are thought to have been ejected from the transneptunian belt by planetary perturbations or mutual collisions (Duncan et al., 1995). Levison and Duncan (1997) suggest that these objects could be the source of short-period comets. Later, Levison et al. (2001), on the basis of their numerical model, predict that some Centaurs could have originated in the Oort Cloud. Centaurs seem to be located on a boundary between many solar system populations, and they are important for understanding the dynamical evolution of the outer solar system.
To date, 25 such objects have been discovered, following the continuously updated list from the Minor Planet Center (Marsden, 2001), and the discoveries continue. Although no formal definition exists, Centaurs have been identified as asteroids at the times of discovery, even though 2060 Chiron was subsequently shown to have cometary activity. Jewitt and Kalas (1998) add to the list of Centaurs the comets P/ Oterma and P/Schwassmann-Wachmann 1 because their orbits lie inside the orbits of Jupiter and Neptune. Recently, Marsden (2001) includes Centaurs in a common list with scattered transneptunian objects, arguing that there are no dynamical reasons to make a distinction, but in this paper we will consider the two populations to be well separated.
From the paper: Physical Properties of Trojan and Centaur Asteroids
by M.A. Barucci, D.P. Cruikshank, S. Mottola and M. Lazzarin
See: https://www.lpi.usra.edu/books/AsteroidsIII/pdf/3001.pdf
Chariklo is so small and so far away that even the mighty Webb cannot photograph it directly. Instead, astronomers are studying it through occultation**, which is an indirect but powerful method to study small bodies like Chariklo. As a result astronomers have spotted the first clear signs for crystalline water ice on this small body and have been able to flesh out its unique rings.
Hartmann352
** Occultation occurs when a solar-system body passes in front of a more distant object (e.g. a star or another solar system body), partially or totally hiding the more distant object and momentarily blocking its light. Each occultation can be seen only at the proper limited time and from a limited part of the Earth.
For asteroid occultations the star is usually the brightest component of the occultation. The asteroid is usually several magnitudes fainter than the star and often too faint to be detected in a small telescope. In an asteroid occultation, the observer must find the star to be occulted and monitor the star to watch for any drop in brightness that would signal an occultation. Asteroid occultation events typically last several seconds but may observers may record much shorter or much longer events in rare cases. As the asteroid moves in its orbit, a shadow is created from light cast by the star about to be occulted. The shadow (equal in size to the asteroid) then moves across the Earth (diagram not to scale). An observer will only see an event (drop in the brightness of the star) if they are located inside the path of the asteroid’s shadow. Since asteroids are generally much smaller than the moon, choosing a location for observing an asteroid occultation is more important than location in lunar occultations because of the small diameters of the asteroids. In addition, asteroid subtend a much smaller angular size on the sky and this leads to more uncertainty in the actual location of the asteroid’s shadow. Asteroid occultation predictions posted by IOTA*** provide information on the expected location of the shadow path, expected time of the occultation, the level of drop in the star’s light and the expected duration of the occultation event. An observer can expect to see a single disappearance (or drop in starlight) and a single reappearance though it is possible to see step events.
For asteroid occultations the star is usually the brightest component of the occultation. The asteroid is usually several magnitudes fainter than the star and often too faint to be detected in a small telescope. In an asteroid occultation, the observer must find the star to be occulted and monitor the star to watch for any drop in brightness that would signal an occultation. Asteroid occultation events typically last several seconds but may observers may record much shorter or much longer events in rare cases. In the following diagram of an asteroid occultation: As the asteroid moves in its orbit, a shadow is created from light cast by the star about to be occulted. The shadow (equal in size to the asteroid) then moves across the Earth (diagram not to scale). An observer will only see an event (drop in the brightness of the star) if they are located inside the path of the asteroid’s shadow. Since asteroids are generally much smaller than the moon, choosing a location for observing an asteroid occultation is more important than location in lunar occultations. In addition, asteroid subtend a much smaller angular size on the sky and this leads to more uncertainty in the actual location of the asteroid’s shadow. Asteroid occultation predictions posted by IOTA provide information on the expected location of the shadow path, expected time of the occultation, the level of drop in the star’s light and the expected duration of the occultation event. An observer can expect to see a single disappearance (or drop in starlight) and a single reappearance though it is possible to see step events.
Note in the diagram above that multiple observers span the shadow of the asteroid. In this way an outline of the actual shape of the asteroid can be determined – information that would be difficult or impossible to obtain in any way other than by visiting the asteroid itself!
See: https://occultations.org/occultations/what-is-an-occultation/
*** IOTA - International Occultation Timing Association which primarily observes two basic areas of occultation astronomy: lunar occultations and asteroid occultations. IOTA is a volunteer science and research organization born in 1983. They gather data from timings of astronomical occultations and provide a variety of educational resources to promote and encourage observations of astronomical occultations.
See: https://occultations.org
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