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9P/Tempel 1

Past, Present, and Future Orbits by Kazuo Kinoshita

M. Jäger image of 9P exposed on 2005 May 1
Copyright © 2005 by Michael Jäger (Austria)

This image was obtained on 2005 May 1.87 UT with the 8-inch Celestron Schmidt camera and a SXV H9 CCD camera. The image was a combination of 8 separate images and had a total exposure time of 28 minutes.


     Ernst Wilhelm Liebrecht Tempel (Marseille, France) discovered this faint, diffuse comet in Libra on 1867 April 3.90. Later calculations revealed it was then situated 0.71 AU from Earth and 1.64 AU from the sun. Tempel noted the comet had an apparent diameter of 4-5 arc minutes and noted "several little stars" pulsating in the middle. The comet was only slightly condensed.

Historical Highlights

  • The comet was very well placed during the 1867 discovery apparition thanks to the closest approach to Earth (0.568 AU) and the perihelion (1.562 AU) occurring on May 15 and 24, respectively. Subsequently, there were numerous observations which ultimately covered nearly 5 full months. The comet was last detected on August 27.8 by J. F. J. Schmidt (Athens), at which time it was too faint for position measurements. The comet was then 1.30 AU from Earth and 1.81 AU from the sun. The comet was still about 4 arc minutes across in early May. Total magnitude estimates were not typically made during this period in history, but estimates of the magnitude of the nucleus, or at least the most brilliant, condensed portion of the coma, reached a maximum of 10.5 shortly before mid-May.
  • The comet was first recognised as periodic early in May when K. C. Bruhns (Leipzig, Germany) determined the orbital period as 5.74 years. By the time of the final observations, the orbital period had been revised to 5.68 years.
  • Conditions for the comet's recovery in 1873 were very good, with the circumstances being only marginally worse than in 1867. The comet was recovered on April 4, by E. J. M. Stephan (Marseille, France). The comet remained under observation until July 1.
  • Predictions were made for the 1879 return, with the most ambitious being that of Raoul Gautier who computed definitive orbits for the previous two appearances before making his prediction for the upcoming return. This prediction enabled Tempel to recover his comet on April 25. Conditions for this apparition were nearly identical to those of 1873, but a full moon early in April 1879 caused the later recovery. The comet was last detected on July 8.
  • The comet passed 0.55 AU from Jupiter during 1881 and the orbital period was increased to 6.5 years. This situation caused the comet to be observable only at every other return. In addition, the perihelion distance was increased from 1.8 AU to 2.1 AU, making the comet an even fainter object. Subsequently, the comet was not seen at its next returns. Photographic attempts during 1898 and 1905 also failed to recover the comet.
  • An investigation as to why this comet became lost was conducted by B. G. Marsden during 1963. He found that further close approaches to Jupiter in 1941 (0.41 AU) and 1953 (0.77 AU) had actually decreased both the perihelion distance and orbital period to values smaller than when the comet was initially discovered in 1873. Subsequently, predictions were published for the 1967 and 1972 returns, the former of which was not particularly favorable, while the latter was expected to be exceptional.
  • During the early days of 1967, J. Schubart (Astronomisches Rechen-Institut) began his investigation with an unpublished orbit by Guntrum Schrutka von Rechtenstamm (Vienna University Observatory) and integrated the orbit through the upcoming apparition. He predicted the perihelion date would be January 12.55. Schubart remarked, "on account of possible non-gravitational effects, the uncertainty in the predictions may be larger than the difference between them."
  • Despite the comet's unfavorable placement during the 1967 apparition, Elizabeth Roemer (Catalina Observatory) obtained a 30-minute exposure on 1967 June 8.42 using the 154-cm Catalina reflector. Her initial inspection revealed a "suspect object" which, although moving at the expected speed, seemed farther from the predicted area than expected. She estimated the magnitude as 18-18.5. Roemer had planned to obtain follow-up photographs of the predicted position during early July and early August, but clouds from the summer rainy season thwarted her efforts. Roemer's next observing run was in late September, but since the comet would have faded, no search attempts were made. Since the single image did not provide definite proof of the comet's existence, confirmation had to await the 1972 return. Interestingly, the precise position of the 1967 object was very close to Schubart's prediction and inticated a correction in the perihelion date of only +0.13 day.
  • The 1972 return was very favorable. Schrutka von Rechtenstamm's orbit was now published and Schubart provided ephemerides in a couple of major publications. Roemer and L. M. Vaughn recovered the comet on January 11 from Steward Observatory. The comet became widely observed and reached a maximum magnitude of 11 during late May. The comet was last seen on July 10. This apparition proved that the single image found in 1967 was indeed 9P/Tempel 1.
  • The comet has been recovered at every apparition since 1972, but its period of 5.5 years has caused it to alternate between unfavorable and favorable returns. It remained a faint object during the 1977-1978 apparition and was only observed at a couple of observatories; however, it reached a magnitude of 9.5 as it passed 0.74 AU from Earth on 1983 May 1 and was an easy target for amateur astronomers. After another faint return during 1987-1989, the comet again reached a magnitude of 9.5 when it passed 0.69 AU from Earth on 1994 May 5. The 2000 return was again a faint one, while the present 2004-2005 apparition has been very favorable for observers. The comet will pass 0.71 AU from Earth on 2005 May 3.
  • This comet was selected as the target of the Deep Impact mission. First proposed back in 1999, the mission was launched on 2005 January 12. Deep Impact released the "Impactor" on 2005 July 3 and this crashed into the comet on July 4. The predicted crater was expected to be the size of a typical sports stadium, but the debris which sprayed out from the impact effectively blocked the crater from the spacecraft's view. Nearly every major observatory on our planet observed the event, as did amateur astronomers. The event was also studied by several satellites and space probes, including the Hubble Space Telescope, the Chandra X-ray Observatory, and Rosetta, the later of which was heading toward a rendezvous with another comet and provided valuable observations of 9P/Tempel 1 from a different observing angle. Another satellite, the Submillimeter Wave Astronomy Satellite, which had been put to sleep on 2004 July 21 after completing its mission, was reactivated to monitor water output during the Deep Impact event.
  • The pattern of alternating between unfavorable and favorable returns will continue through 2022, with that of 2016 being the next favorable one. The comet will pass 0.55 AU from Jupiter on 2024 May 26, which will increase the perihelion distance from 1.54 AU to 1.77 AU and increase the period to 6.0 years. Unfortunately, this locks the comet into unfavorable returns for 2028 and 2034. Another close approach to Jupiter (0.91 AU) on 2036 April 7 will increase the perihelion distance to 1.93 AU and the period to 6.3 years. Although this again allows for some favorable returns, the comet's much larger perihelion distance will prevent it from becoming as bright as it did in 1983, 1994, and 2005.
  • Additional Images

    G. Rhemann image of 9P exposed on 1994 April 3
    Copyright © 1994 by Gerald Rhemann

    This image was obtained on 1994 April 3.95 UT with the 171/200/257mm Schmidt camera. Exposure time was 6 minutes and the photographic emulsion was hypered Technical Pan 2415. The comet's total magnitude was then about 10.8. (The image has been cropped by the webmaster to save space.)

    G. Sostero image of 9P exposed on 2004 December 10
    Copyright © 2004 by G. Sostero (Remanzacco Observatory, Italy)

    G. Sostero image of 9P exposed on 2005 January 20
    Copyright © 2005 by G. Sostero (Remanzacco Observatory, Italy)

    G. Sostero image of 9P exposed on 2005 May 9
    Copyright © 2005 by G. Sostero (Remanzacco Observatory, Italy)

    G. Sostero image of 9P exposed on 2005 June 9
    Copyright © 2005 by G. Sostero (Remanzacco Observatory, Italy)

    E. Guido image of 9P exposed on 2005 June 22
    Copyright © 2005 by E. Guido ("NM Skies" Observatory, New Mexico, USA)