Transition Comets -- UV Search for OH Emissions in Asteroids
Encyclopedia
Hubble search for transition comets (Transition Comets — UV Search for OH Emissions in Asteroids) was a study involving amateur astronomer
s and the use of the Hubble Space Telescope
. This was one of only six studies involving amateur astronomers that was approved by NASA.
(HST) project, NASA
and Congress were interested in finding ways for amateur astronomers to participate in HST research. The director of the Space Telescope Science Institute (STScI), Riccardo Giacconi
, decided to allocate some of his "Director's Discretionary" time to amateur observing programs. In December 1985, the leaders of seven national amateur astronomy organizations met at STScI in Baltimore to discuss the participation of amateur astronomers in the HST project.
to perform a spectroscopic search for OH emission from five asteroids. OH emission would indicate that the asteroids were once comets. 944 Hidalgo
and 2201 Oljato
move in elliptical, comet-like orbits. 182 Elsa
, 224 Oceana
, and 899 Jokaste
are main-belt asteroids. The last three have been observed with coma (Kresak, 1977). Concurrently with the spectroscopic study, ground-based visual observations were carried out by 80 amateur astronomers in 22 countries.
Scientists suspect that some asteroids were once comets. A comet
loses part of its mass with each passage around the Sun. It would appear that some would eventually use all of their volatiles, or perhaps cover these under a blanket of dust after repeated passages around the Sun. Such an object might then have an asteroid appearance.
The asteroid 944 Hidalgo
is most frequently discussed as being in a comet-like orbit. In fact, Kresak (1977) identified it as an "extinct comet nucleus". In addition, comets tend to approach Jupiter
closely while asteroids do not. Hidalgo approaches Jupiter at nearly the same distance as 3 comets: Comet Encke
, Comet Arend-Rigaux, and Comet Neujmin I, all of which exhibit the low levels of activity expected of comets before they become extinct.
The Pioneer Venus Orbiter detected magnetic field disturbances that are correlated with 2201 Oljato
. This could be caused by an outgassing of H at a rate of about 10-4 that for an active comet. Oljato's possible comet-like nature is supported by its unusual UV reflectance. It has been interpreted as Rayleigh scattering from a cloud of fine particles around it.
On December 13, 1923, the astronomer Josep Comas Solá
observed the asteroid 224 Oceana
with a coma
. The asteroid was photographed with a faint halo 30 arc-seconds across. The asteroid's magnitude
was determined to be 11.6, and at the asteroid's heliocentric distance of 167 million miles, this made the coma about 24,000 miles across.
The existence of volatiles on asteroids would be of great significance to future miners of the asteroid belt. Volatiles could supply water, fuel and oxygen for missions.
For the 1993 study involving amateur astronomers and the Hubble Space Telescope
, 944 Hidalgo
and 2201 Oljato
were examined with the Hubble Space telescope's Faint Object Spectrograph
for 3085 A emission of OH. These two asteroids were selected because of the nature of their orbits, their meteor-shower associations, and their other characteristics; see Weissman et al. (1989) for a full discussion of their comet-like features. The amber detector was used in the accumulation mode with spectral element G270H. This element covers wavelengths 2325-3225 A. The aperture was 1 arc-second.
The asteroids 182 Elsa
, 224 Oceana
, and 899 Jokaste
were also observed with the Faint Object Spectrograph
, and with the same spectral element G270H.
The team leaders placed advertisements in amateur astronomy publications for amateur observers who would be willing to observe these objects in the visible spectrum from the ground, while the HST was making studies in the UV. The ground-based observers were asked to check for evidence of a dust coma.
, and 2201 Oljato
are essentially identical to the solar spectrum. No OH emission, or other emission lines were apparent.
Ground-based observations of these two objects were limited to several CCD
images of the area in which 2201 Oljato was calculated to appear. Both 944 Hidalgo, and 2201 Oljato were near 19th magnitude. This placed them beyond the limit of most amateur telescopes.
None of the other three asteroids showed signs of the cometary emission that would be expected from a weakly active cometary nucleus. This should not be surprising, since the targets lie in stable main-belt orbits.
Ground-based observations of 182 Elsa, 224 Oceana, and 899 Jokaste were conducted visually, photographically, with VHS, and with CCD equipment. 224 Oceana was at 12th magnitude during the HST observations. 182 Elsa was also at 12th magnitude, and 899 Jokaste appeared at 15th magnitude.
All visual observations showed point images, with no sign of a dust coma. Photographic images gave the same result. A number of observers were able to accumulate CCD images in order to reach fainter magnitudes. Using this method, two observers reported signs of a possible short tail on the asteroid 899 Jokaste. This appeared around 17th magnitude. Studies of nearby stars on the original image suggest that this was due to a small amount of trailing during guiding. There was no further evidence of a coma around any of the three asteroids.
Observations of 944 Hidalgo and 2201 Oljato had to be conducted before the Hubble Space Telescope servicing mission. As a result, the two asteroids were not in convenient positions to detect OH. Hidalgo was imaged at 5 Astronomical Units from the sun, and moving away. This is approximately the same distance as SL-9 was before it impacted on Jupiter. The Hubble Space Telescope, using the same Faint Object Spectrograph and spectral element G270H, also failed to find any evidence of OH from SL-9.
2201 Oljato was near aphelion when it was observed. From the distance of the asteroid belt, any OH signature may have been hidden by noise in the data of the 19th magnitude asteroid. Ideally, both objects should have been studied shortly after perihelion passage.
The main-belt asteroids 224 Oceana, 182 Elsa, and 899 Jokaste were observed near opposition
, but did not show any evidence of OH emission. The last two were observed when the Hubble Space Telescope was having guidance problems, and had to present special orientation to the sun due to the loss of one solar panel. The reports of previous coma may have been due to impacts.
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...
s and the use of the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
. This was one of only six studies involving amateur astronomers that was approved by NASA.
History
In the beginning years of the Hubble Space TelescopeHubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
(HST) project, NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
and Congress were interested in finding ways for amateur astronomers to participate in HST research. The director of the Space Telescope Science Institute (STScI), Riccardo Giacconi
Riccardo Giacconi
Riccardo Giacconi is an Italian/American Nobel Prize-winning astrophysicist who laid the foundations of X-ray astronomy. He is currently a professor at the Johns Hopkins University.- Biography :...
, decided to allocate some of his "Director's Discretionary" time to amateur observing programs. In December 1985, the leaders of seven national amateur astronomy organizations met at STScI in Baltimore to discuss the participation of amateur astronomers in the HST project.
Research and study
The team used the Hubble Space TelescopeHubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
to perform a spectroscopic search for OH emission from five asteroids. OH emission would indicate that the asteroids were once comets. 944 Hidalgo
944 Hidalgo
944 Hidalgo is a small Solar System body with a semi-major axis beyond Jupiter's and an orbital period of 13.77 years. This makes it a centaur, but it was discovered in 1920 and has hence traditionally been considered an asteroid....
and 2201 Oljato
2201 Oljato
2201 Oljato is a Apollo asteroid discovered on December 12, 1947 by H. L. Giclas at Flagstaff .2201 was a target of Hubble search for transition comets .- External links :*...
move in elliptical, comet-like orbits. 182 Elsa
182 Elsa
182 Elsa is a Main belt asteroid. It is an S-type asteroid.It was discovered by Johann Palisa on February 7, 1878. The origin of the name is uncertain; it may be named after the character in the legend of Lohengrin perpetuated by Richard Wagner's opera of the same name.It rotates very slowly; its...
, 224 Oceana
224 Oceana
224 Oceana is a sizeable Main belt asteroid. It is an M-type, but is not metallic.It was discovered by Johann Palisa on March 30, 1882 in Vienna.It was named after the Pacific Ocean....
, and 899 Jokaste
899 Jokaste
899 Jokaste is a minor planet orbiting the Sun. It was one of five minor planets included in the 1993 study, Transition Comets -- UV Search for OH Emissions in Asteroids, which was research involving amateur astronomers who were permitted to make use of the Hubble Space Telescope. Not to be...
are main-belt asteroids. The last three have been observed with coma (Kresak, 1977). Concurrently with the spectroscopic study, ground-based visual observations were carried out by 80 amateur astronomers in 22 countries.
Scientists suspect that some asteroids were once comets. A comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
loses part of its mass with each passage around the Sun. It would appear that some would eventually use all of their volatiles, or perhaps cover these under a blanket of dust after repeated passages around the Sun. Such an object might then have an asteroid appearance.
The asteroid 944 Hidalgo
944 Hidalgo
944 Hidalgo is a small Solar System body with a semi-major axis beyond Jupiter's and an orbital period of 13.77 years. This makes it a centaur, but it was discovered in 1920 and has hence traditionally been considered an asteroid....
is most frequently discussed as being in a comet-like orbit. In fact, Kresak (1977) identified it as an "extinct comet nucleus". In addition, comets tend to approach Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
closely while asteroids do not. Hidalgo approaches Jupiter at nearly the same distance as 3 comets: Comet Encke
Comet Encke
Comet Encke or Encke's Comet is a periodic comet that completes an orbit of the Sun once every three years — the shortest period of any known comet...
, Comet Arend-Rigaux, and Comet Neujmin I, all of which exhibit the low levels of activity expected of comets before they become extinct.
The Pioneer Venus Orbiter detected magnetic field disturbances that are correlated with 2201 Oljato
2201 Oljato
2201 Oljato is a Apollo asteroid discovered on December 12, 1947 by H. L. Giclas at Flagstaff .2201 was a target of Hubble search for transition comets .- External links :*...
. This could be caused by an outgassing of H at a rate of about 10-4 that for an active comet. Oljato's possible comet-like nature is supported by its unusual UV reflectance. It has been interpreted as Rayleigh scattering from a cloud of fine particles around it.
On December 13, 1923, the astronomer Josep Comas Solá
Josep Comas Solá
Josep Comas i Solà was a Catalan Spanish astronomer.He observed planets including Mars and Saturn, measuring the period of rotation of the latter...
observed the asteroid 224 Oceana
224 Oceana
224 Oceana is a sizeable Main belt asteroid. It is an M-type, but is not metallic.It was discovered by Johann Palisa on March 30, 1882 in Vienna.It was named after the Pacific Ocean....
with a coma
Coma (cometary)
frame|right|The [[153P/Ikeya-Zhang|comet Ikeya-Zhang]] exhibiting a bright, condensed coma In astronomy, a coma is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate...
. The asteroid was photographed with a faint halo 30 arc-seconds across. The asteroid's magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
was determined to be 11.6, and at the asteroid's heliocentric distance of 167 million miles, this made the coma about 24,000 miles across.
The existence of volatiles on asteroids would be of great significance to future miners of the asteroid belt. Volatiles could supply water, fuel and oxygen for missions.
For the 1993 study involving amateur astronomers and the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
, 944 Hidalgo
944 Hidalgo
944 Hidalgo is a small Solar System body with a semi-major axis beyond Jupiter's and an orbital period of 13.77 years. This makes it a centaur, but it was discovered in 1920 and has hence traditionally been considered an asteroid....
and 2201 Oljato
2201 Oljato
2201 Oljato is a Apollo asteroid discovered on December 12, 1947 by H. L. Giclas at Flagstaff .2201 was a target of Hubble search for transition comets .- External links :*...
were examined with the Hubble Space telescope's Faint Object Spectrograph
Faint Object Spectrograph
The Faint Object Spectrograph was a spectrograph installed on the Hubble Space Telescope. It was replaced by the Space Telescope Imaging Spectrograph in 1997, and is now on display in the National Air and Space Museum in Washington DC. -FOS facts:...
for 3085 A emission of OH. These two asteroids were selected because of the nature of their orbits, their meteor-shower associations, and their other characteristics; see Weissman et al. (1989) for a full discussion of their comet-like features. The amber detector was used in the accumulation mode with spectral element G270H. This element covers wavelengths 2325-3225 A. The aperture was 1 arc-second.
The asteroids 182 Elsa
182 Elsa
182 Elsa is a Main belt asteroid. It is an S-type asteroid.It was discovered by Johann Palisa on February 7, 1878. The origin of the name is uncertain; it may be named after the character in the legend of Lohengrin perpetuated by Richard Wagner's opera of the same name.It rotates very slowly; its...
, 224 Oceana
224 Oceana
224 Oceana is a sizeable Main belt asteroid. It is an M-type, but is not metallic.It was discovered by Johann Palisa on March 30, 1882 in Vienna.It was named after the Pacific Ocean....
, and 899 Jokaste
899 Jokaste
899 Jokaste is a minor planet orbiting the Sun. It was one of five minor planets included in the 1993 study, Transition Comets -- UV Search for OH Emissions in Asteroids, which was research involving amateur astronomers who were permitted to make use of the Hubble Space Telescope. Not to be...
were also observed with the Faint Object Spectrograph
Faint Object Spectrograph
The Faint Object Spectrograph was a spectrograph installed on the Hubble Space Telescope. It was replaced by the Space Telescope Imaging Spectrograph in 1997, and is now on display in the National Air and Space Museum in Washington DC. -FOS facts:...
, and with the same spectral element G270H.
The team leaders placed advertisements in amateur astronomy publications for amateur observers who would be willing to observe these objects in the visible spectrum from the ground, while the HST was making studies in the UV. The ground-based observers were asked to check for evidence of a dust coma.
Results
The spectra for 944 Hidalgo944 Hidalgo
944 Hidalgo is a small Solar System body with a semi-major axis beyond Jupiter's and an orbital period of 13.77 years. This makes it a centaur, but it was discovered in 1920 and has hence traditionally been considered an asteroid....
, and 2201 Oljato
2201 Oljato
2201 Oljato is a Apollo asteroid discovered on December 12, 1947 by H. L. Giclas at Flagstaff .2201 was a target of Hubble search for transition comets .- External links :*...
are essentially identical to the solar spectrum. No OH emission, or other emission lines were apparent.
Ground-based observations of these two objects were limited to several CCD
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
images of the area in which 2201 Oljato was calculated to appear. Both 944 Hidalgo, and 2201 Oljato were near 19th magnitude. This placed them beyond the limit of most amateur telescopes.
None of the other three asteroids showed signs of the cometary emission that would be expected from a weakly active cometary nucleus. This should not be surprising, since the targets lie in stable main-belt orbits.
Ground-based observations of 182 Elsa, 224 Oceana, and 899 Jokaste were conducted visually, photographically, with VHS, and with CCD equipment. 224 Oceana was at 12th magnitude during the HST observations. 182 Elsa was also at 12th magnitude, and 899 Jokaste appeared at 15th magnitude.
All visual observations showed point images, with no sign of a dust coma. Photographic images gave the same result. A number of observers were able to accumulate CCD images in order to reach fainter magnitudes. Using this method, two observers reported signs of a possible short tail on the asteroid 899 Jokaste. This appeared around 17th magnitude. Studies of nearby stars on the original image suggest that this was due to a small amount of trailing during guiding. There was no further evidence of a coma around any of the three asteroids.
Observations of 944 Hidalgo and 2201 Oljato had to be conducted before the Hubble Space Telescope servicing mission. As a result, the two asteroids were not in convenient positions to detect OH. Hidalgo was imaged at 5 Astronomical Units from the sun, and moving away. This is approximately the same distance as SL-9 was before it impacted on Jupiter. The Hubble Space Telescope, using the same Faint Object Spectrograph and spectral element G270H, also failed to find any evidence of OH from SL-9.
2201 Oljato was near aphelion when it was observed. From the distance of the asteroid belt, any OH signature may have been hidden by noise in the data of the 19th magnitude asteroid. Ideally, both objects should have been studied shortly after perihelion passage.
The main-belt asteroids 224 Oceana, 182 Elsa, and 899 Jokaste were observed near opposition
Opposition (astronomy)
In positional astronomy, two celestial bodies are said to be in opposition when they are on opposite sides of the sky, viewed from a given place . In particular, two planets are in opposition to each other when their ecliptic longitudes differ by 180°.The astronomical symbol for opposition is ☍...
, but did not show any evidence of OH emission. The last two were observed when the Hubble Space Telescope was having guidance problems, and had to present special orientation to the sun due to the loss of one solar panel. The reports of previous coma may have been due to impacts.