103P/Hartley 2

Past, Present, and Future Orbits by Kazuo Kinoshita

EPOXI image of 103P exposed on 2010 November 4

This image from the High-Resolution Instrument on NASA's EPOXI mission spacecraft shows part of the nucleus of comet Hartley 2. The sun is illuminating the nucleus from the right. A distinct cloud of individual particles is visible. This image was obtained on Nov. 4, 2010, the day the EPOXI mission spacecraft made its closest approach to the comet.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the EPOXI mission for NASA's Science Mission Directorate, Washington. The University of Maryland, College Park, is home to the mission's principal investigator, Michael A'Hearn. The spacecraft was built for NASA by Ball Aerospace & Technologies Corp., Boulder, Colo.

Summary

Periodic comet 103P/Hartley 2 is classed as a young, dwarf comet, with a nucleus roughly 2.2 kilometers (1.4 mile) across that rotates once every 18.1 hours. It belongs to the Jupiter family of comets (comets with periods less than 20 years). The comet was discovered in 1986. Although it then had an orbital period of 6.3 years, an analysis of its orbit reveals the period had been longer in the recent past. During the early decades of the 20th century, the orbital period had been 9.3 years. A close approach to Jupiter in August 1947 (0.22 AU) reduced the period to 7.9 years, while another close approach during April 1971 (0.09 AU) reduced the period to 6.1 years. The comet has been seen at every return since its discovery. The 2010 return is exceptional, as the comet will pass 0.12 AU from Earth on October 20. The Deep Impact space craft passed about 1000 kilometers from the comet on November 4.

Discovery

This image is a digitized section of the actual photographic plate that the comet was discovered upon. The original plate was exposed on 1986 March 15.59 and, after it was developed, Malcolm Hartley (U. K. Schmidt Telescope Unit, Siding Spring Observatory, New South Wales, Australia) noted the diffuse trail of a comet. He estimated the magnitude as 17-18 and added that a faint tail was evident. Further photographic plates were exposed on March 17.68 and 20.61, whereupon Hartley annnounced his discovery to the Central Bureau for Astronomical Telegrams.

Historical Highlights

APPARITION OF 1985
[Perihelion Date=1985 June 4.87; Period=6.26 years] Upon the arrival of the three initial positions of this comet, D. W. E. Green (Central Bureau for Astronomical Telegrams) computed a "very uncertain" parabolic orbit which indicated the comet had been closest to the sun on 1985 June 20 at a distance of 0.076 AU. The inclination was given as 60 degrees. Green added, "A low-inclination, short-period orbit gives similar residuals." The discovery info, as well as the rough orbit, were published on March 24. On April 8, the Central Bureau published a revised orbit which included the original positions as well as more recent ones acquired on April 5. B. G. Marsden confirmed Green's suspected short-period orbit was the correct one. His calculations indicated the comet had passed perihelion on 1985 June 5 at a distance of 0.961 AU. He said the comet's angular distance from the sun would have been too small to have permitted observations during 1985 and added that the comet appeared to have made a close approach to Jupiter during 1982. The comet steadily faded as it moved away from both the Sun and Earth. J. Gibson (Palomar Observatory, California, USA) estimated the photographic magnitude as 18 on April 5. The comet was last detected on June 7 by astronomers at Steward Observatory (Kitt Peak, Arizona, USA).

The comet had passed 0.33 AU from Jupiter on 1982 November 2, which increased the perihelion distance from 0.90 AU to 0.95 AU and increased the period from 6.12 years to 6.26 years. Even more impressive, is the fact that the comet passed 0.085 AU from Jupiter on 1971 April 28. This decreased the perihelion distance from 1.62 AU to 0.90 AU and decreased the period from 7.92 years to 6.12 years.

APPARITION OF 1991
[Perihelion Date=1991 September 11; Period=6.27 years] A Siding Spring Observatory photograph of 103P obtained on 1992 January 15

A Siding Spring Observatory photograph of 103P obtained on 1992 January 15

This photograph was obtained by the U. K. Schmidt Telescope Unit, Siding Spring, Australia on 1992 January 15.

Nakano supplied a prediction for the comet's 1991 return, but searches were unsuccessful; however, the comet was accidentally rediscovered on July 9 of that year. G. R. Kastel' (Institute for Theoretical Astronomy) reported on July 11 that T. V. Kryachko (Majdanak) reported the discovery of a comet on images exposed on July 9 and confirmed the find on the 10th. He described it as magnitude 11, with a coma 15 arc minutes across. Nakano suggested this was Hartley 2, which indicated his prediction had been 5.6 days too late. R. E. McCrosky and C.-Y. Shao (Oak Ridge Observatory, Massachusetts, USA) confirmed Nakano's suggestion on July 12 when they photographed the comet with the 155-cm reflector. The comet passed 0.77 AU from Earth around mid-August and passed perihelion on September 11. The comet was brighter than magnitude 10 in late July and was brighter than magnitude 9 by early August. The comet reached a maximum magnitude of 8 during the first half of September and maintained that brightness into October. Thereafter, it faded from 9 in early November to 12 by late December. Thoughout December, the coma was generally 2-3 arc minutes across. The comet was last detected on 1992 May 4, when astronomers at Oak Ridge Observatory obtained two images.

APPARITION OF 1997
[Perihelion Date=1997 December 22; Period=6.39 years] H. Mikuz image of 103P exposed on 1997 December 28

This image was obtained on 1997 December 28.73 UT with 36-cm, f/6.7 Schmidt-Cassegrain telescope, V filter and CCD. Exposure time was 5 minutes. (Image reversed by Author)

[Click here for additional images from the 1997 apparition]

The comet was recovered on 1997 May 2 at Whipple Observatory (Mt. Hopkins). It attained a maximum brightness of about 8 at the end of December and the coma was then about 8 arc minutes across. The comet slowly faded as 1998 progressed. It passed closed to Earth (0.8177 AU) on January 8. Most observers estimated a magnitude near 8.5 on January 20, while the coma diameter was about 7 arc minutes. By mid-February, the magnitude was between 9.5 and 10, while the coma was about 4 arc minutes across. Only a few visual observers were still following the comet by mid-March. In general, the magnitude was then near 11, while the coma was 2 arc minutes across. The comet was last observed on 1999 April 12, when observers at the Lunar and Planetary Laboratory (Catalina Station, Arizona, USA) determined the total magnitude as 19.2.

APPARITION OF 2004
[Perihelion Date=2004 May 17; Period=6.40 years]

I. Ferrin image of 103P exposed on 2005 January 12

I. Ferrin image of 103P exposed on 2006 March 1

These two images were obtained by Ignacio Ferrin using the 1-m Schmidt telscope (f/3) at the National Observatory of Venezuela. The image on the left is a 21-minute exposure and was obtained on January 12.19, 2005. The image on the right is a 12 minute exposure and was obtained on March 1.20, 2006. (Ferrin granted the Author permission to use these images.)

The comet passed perihelion on 2004 May 17.98, but it was not well placed, as it was lost in the sun's glare when it would have been brightest. Photographic observers caught the comet between magnitude 16 and 17 during September and October of 2004. The comet was last detected on 2006 March 1, when I. Ferrin (National Observatory of Venezuela) was able to obtain an image using the 1-m Schmidt telescope. The comet was then 5.03 AU from the sun and was described as exhibiting an R magnitude of 20.2 with a faint coma.

APPARITION OF 2010
[Perihelion Date=2010 October 28; Period=6.47 years]      One of the earliest predictions for this apparition came from K. Kinoshita in January 2007. He took 564 positions from the comet's apparitions of 1985, 1991, 1997, and 2004, and predicted the comet would pass perihelion on 2010 October 28.27. A prediction by S. Nakano during April 2007 used 615 positions from the apparitions of 1991, 1997, and 2004, and gave the perihelion date as October 28.24. This apparition will be the most favorable since the comet's discovery, as the comet will pass only 0.12 AU (11 million miles) from Earth on 2010 October 20. The comet is expected to reach magnitude 5, which will enable it to be a naked-eye object for observers in very dark skies.
     The first observations during this apparition were obtained during 2008. Astronomers at Paranal Observatory (Chile) obtained 62 images of the comet on May 5. They acquired additional images on June 1 and 4. The comet was also observed by the Spitzer Space Telescope on August 12 and 13. The Spitzer observations revealed the comet's nucleus was probably around 1.2 km across.
     The comet was next observed on 2010 March 12, when the 8.1-m reflector at Gemini South Observatory at Cerro Pachon (Chile) obtained mid-infrared observations of the comet. Additional observations were also acquired by the 2.4-m reflector at Magdalena Ridge Observatory (New Mexico, USA) on March 13 and 17. These two observatories, as well as Observatoire Chante-Perdrix (Dauban, Haute-Provence, France), Mauna Kea Observatory (Hawaii, USA), and the Southern Astrophysical Research Telescope at Cerro Pachon made additional observations during April.
     Visual observations finally began in August. The first such observation was made by J. J. Gonzalez (Leon, Spain) on the 6th. Using a 20-cm reflector, he gave the magnitude as 13.2 and said the coma was 0.6' across. On the next night, S. Yoshida (Gunma, Japan) saw the comet with his 40-cm reflector. He said the magnitude was 13.4, while the coma was 0.6' across. As expected, the comet brightened during the month and was slightly fainter than 11 by the 31st.
     The comet continued to brighten during September, although the increasing size of the comet's coma caused some problems for observers. The comet began the month near magnitude 11. By the middle of the month, observers with telescopes were reporting the coma as 3' to 4' across and the magnitude as about 9.5, while observers using binoculars were seeing the coma as 10' to 15' across and the magnitude as about 8.5. By the end of the month, the magnitude was around 7.0, with a coma diameter near 20'.
     The comet officially became a naked-eye object early in October. The first such observation was made by Piotr Guzik (Poland) on October 3, while, Gonzalez made a similar observation the next night. Both of these observers are very experienced and made their observations from very dark sites. The comet never became a wide-spread naked-eye object, because the coma expanded considerably as it approached Earth. Around the time when the comet was closest to our planet on October 20, the coma was estimated as 30' to at least one degree across. Observers reporting the largest coma were either observing under very dark skies or were obtaining images optimized to detect the greenish glow of carbon molecules within the comet. Only observers under very dark skies were able to see the comet with the naked-eye, even though the maximum magnitude was then around 4.5.

Arecibo Radio Telescope: The Arecibo Radio Telescope observed the comet during the period of 2010 October 24-27 and obtained images of the rotating nucleus. The long axis measures at least 2.2 kilometers, while the rotation period appears to be near 18.1 hours.

Comet Rendezvous: The Deep Impact spacecraft, which studied periodic comet 9P/Tempel 1 in 2005, examined 103P/Hartley 2 from September 5 and through November 25, 2010, as part of the EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) mission. The closest approach occurred at 14:02 Universal Time on November 4, when the minimum distance to the comet was only 700 kilometers (434 miles). The spacecraft used two telescopes with digital color cameras and an infrared spectrometer to image the nucleus, which is now known to be shaped like a peanut.

Additional Images

Images from 2010 July and August

Images from 2010 September

G. W. Kronk image of 103P exposed on 2010 October 1

This image was obtained on 2010 October 1.07 using an Orion EON 80-mm ED Apo refractor (f/6.25) and a Canon T2i digital camera. The camera was set at 1600 ISO and two 2-minute exposures were obtained and subsequently stacked to produce this image. The field of view is 0.8 degree.

G. W. Kronk image of 103P exposed on 2010 October 6

This image was obtained on 2010 October 6.07 using an Orion EON 80-mm ED Apo refractor (f/6.25) and a Canon T2i digital camera. The camera was set at 1600 ISO and single 2-minute 30 second exposure was obtained. The field of view is 0.8 degree.

G. W. Kronk image of 103P exposed on 2010 October 8

This image was obtained on 2010 October 8.09, as the comet was approaching the Double Cluster in Perseus. I used an Orion EON 80-mm ED Apo refractor (f/6.25) and a Canon T2i digital camera. The camera was set at 1600 ISO and single 2-minute exposure was obtained. The field of view is 1.3 degree.

M. Jäger image of 103P exposed on 2010 October 8

This image was obtained on 2010 October 8.92 using a 14-cm reflector (f/2.9) and a KAF 6300 CCD camera.

G. W. Kronk image of 103P exposed on 2010 October 9

This image was obtained on 2010 October 9.17 using an Orion EON 80-mm ED Apo refractor (f/6.25) and a Canon T2i digital camera. The camera was set at 1600 ISO and single 2-minute 30 second exposure was obtained. The field of view is 0.8 degree.

M. Jäger image of 103P exposed on 2010 October 11

This image was obtained on 2010 October 11.82 using a 14-cm reflector (f/2.9) and a Sigma 6303 CCD camera. Michael obtained eleven 240-second exposures using various filters and combined them to produce this picture.

M. Jäger image of 103P exposed on 2010 October 14

This image was obtained on 2010 October 14 using a 14-cm reflector (f/2.9) and a KAF 6300 CCD camera. Twelve 230-second exposures using various filters were combined for this image.

M. Brown image of 103P exposed on 2010 October 20

This image was obtained on 2010 October 20.25 using a Celestron 20-cm SCT (f/6.3) and a Canon Digital Rebel digital camera. The camera was set at 400 ISO and fourteen 2-minute exposures were obtained and subsequently stacked using Registax 5. Bright moonlight was present.

R. Ligustri image of 103P exposed on 2010 November 3

M. P. Mobberly image of 103P exposed on 2010 November 11

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