C/1997 O1 (Tilbrook)

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C/1997 O1 (Tilbrook)[ i ]
Properties of the orbit (animation)

Epoch: September 24, 1997 (JD 2,450,715.5)
Orbit type long-period
Numerical eccentricity 0,984
Perihelion 1,37 AE
Aphelion 172.7 AU
Large semi-axis 87.0 AU
Sidereal period ~812 a
Inclination of the orbital plane 115,8°
Perihelion 13. July 1997
Orbital velocity at perihelion 35.8 km/s
Discoverer J. Tilbrook
Discovery date 22. July 1997
Source: Unless individually noted otherwise, data are from JPL Small-Body Database Browser. Please also note the reference to comet articles.

C/1997 O1 (Tilbrook) is a comet which could be observed in 1997.


The comet was discovered on the evening of 22 July 1997 (local time) by amateur astronomer Justin Tilbrook at Clare, north of Adelaide in South Australia, using a 200 mm f/6 reflector.[1] It was his first comet discovery, and an accidental one at that. Tilbrook, an experienced observer of variable stars, was not actually looking for comets. When he went to observe the dwarf nova TV Corvi on the evening of July 22, he noticed a washed-out spot nearby. Being familiar with the star field, he knew he hadn’t seen anything there before. He noted the position and after only 1 ½ hours was able to determine that the spot had moved. Tilbrook described the comet as diffuse and estimated the brightness to be about 10 mag.[2] The very next day, the discovery of the new comet was officially confirmed by the IAU.[3]

If the comet had been discovered three months earlier, it could already have been seen with binoculars. But although it has always had a greater elongation from the Sun than 60° since March, at that time it was moving through areas of the southern sky that are very rich in stars, star clusters and nebulae. In addition, its discovery was complicated by the fact that during the winter in the southern hemisphere, bad weather prevailed for a long time.

The comet was initially observable only from the southern hemisphere and southern areas of the northern hemisphere. At the end of July its brightness still reached between 10 and 11 mag, by the end of August it had dropped to 12 mag. Only from September on the comet could be observed from northern latitudes.[4] The last position determination was made on 23. March 1998 at a brightness of only about 18 mag at the Astronomical and Geophysical Observatory in Modra, Slovakia.[5]

Scientific evaluation

Using the Solar Wind Anisotropies (SWAN) instrument aboard the SOHO spacecraft, maps of the entire sky were made from late 1996 to mid-1998 in the ultraviolet light of the Lyman-α line of hydrogen at 121.5 nm. This also allowed several comets to be detected by their very extended hydrogen envelopes. Comet Tilbrook was subsequently detected in this way in 41 images taken between 20 May and 26 August 1997, two months before its actual discovery. Since the hydrogen envelope of comets is essentially formed by photodissociation of water, it was also possible to determine the production rate of water for the comet.[6]


A relatively accurate elliptical orbit has been determined for the comet from 115 observations over a period of 243 days, which is inclined at about 116° to the ecliptic.[7] It thus runs through its orbit in the opposite sense (retrograde) to the planets. At the closest point to the Sun in its orbit (perihelion), which the comet last passed on 13 July 1997, it was located between the orbits of Earth and Mars at a distance of about 205.2 million km from the Sun. It had already come within 0.91 AU/136.7 million km of Earth on June 24. On July 31 it approached Mars up to about 41.3 million km and on August 12 Venus up to about 107.9 million km.

The comet moves in an extremely elongated elliptical orbit around the Sun. According to the orbital elements, which are subject to some uncertainty, its orbit had an eccentricity of about 0.9843 and a major semimajor axis of about 87.5 AU some time before its passage of the inner solar system in 1997, so its orbital period was about 819 years. Thus, the comet may have last appeared in 1178 or 1179 (uncertainty ±11 months), apparently going undetected due to its low brightness. Planetary gravitational attraction, especially close passes of Saturn on 23 April 1996 at 6 ¾ AU and of Jupiter on 9 May 1997 at a distance of 4 ½ AU, reduced its orbital eccentricity to about 0.9833 and its major semimajor axis to about 82.1 AU, shortening its orbital period to about 744 years. When it reaches the farthest point (aphelion) from the Sun in its orbit in 2369 (uncertainty ±4 months), it will be about 24.4 billion km from the Sun, over 160 times as far as Earth and nearly 5 ½ times as far as Neptune. Its orbital velocity at aphelion is only about 0.30 km/s. The comet’s next perihelion transit is expected to occur around the year 2741 (uncertainty ±9 months).[8]

See also

  • List of comets

Web links

Individual references

  1. Comets Discovered from South Australia.Astronomical Society of South Australia, retrieved 2 June 2016.
  2. A Tale of Discovery.Astronomical Society of South Australia, retrieved 2 June 2016 (English, with image of J. Tilbrook and the comet).
  3. D. W. E. Green:IAUC 6705: 1997 O1; BL Lac.IAU Central Bureau for Astronomical Telegrams, retrieved 2 June 2016 (English).
  4. J. Shanklin: The comets of 1997. in Journal of the British Astronomical Association. Vol. 112, 2002, pp. 130-150 (bibcode:2002JBAA..112..130S).
  5. C/1997 O1 (Tilbrook).IAU Minor Planet Center, retrieved 3 June 2016 (English).
  6. J. T. T. Mäkinen, J.-L. Bertaux, T. I. Pulkkinen, W. Schmidt, E. Kyrölä, T. Summanen, E. Quémerais, R. Lallement: Comets in full sky Lα maps of the SWAN instrument. I. Survey from 1996 to 1998. in Astronomy & Astrophysics. Vol. 368, 2001, pp. 292-297 doi:10.1051/0004-6361:20000545(PDF; 250 kB).
  7. NASA JPL Small-Body Database Browser: C/1997 O1 (Tilbrook).Retrieved 1 June 2016 (English).
  8. A. Vitagliano:SOLEX 11.0.(No longer available online.) Archived from.Original18September 2015; retrieved 2 May 2014 (English).