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P/2006 HR30 (Siding Spring)

This image, taken at a higher resolution (1.1"/pixel) than most of the images on this web site shows a bright 'jet' of material pointing almost due north, about 12" long and 6" wide as well as a fainter and shorter fan of material to the west. A very faint tail may be visible to the southwest (almost in the opposite direction to the Sun as would normally be expected).

Discovered on 20 April 2006 by Robert McNaught and Gordon Garrard at Siding Spring Observatory it was originally announced as a minor planet in MPEC 2006-H40 and given designation 2006 HR30.

The discovery MPEC included pre-discovery Siding Spring positions from 2005 July and from 2005 October from the NEAT survey from Palomar Mountain, so the orbit was well determined from the outset. Perihelion distance was just outside the Earths orbit at 1.22 AU and the period was just under 22 years. The absolute brightness of the object (the brightness it would have at a distance of 1 AU from both Sun and Earth) was 11.9, which in normal minor planet terms would indicate a diameter in the range 7-24 Km. The object was likely to be a comet, but no cometary features were indicated in the initial observations.

Several amateur astronomers followed the object after discovery to check whether, as it approached perihelion in 2007 January it would show signs of being a comet. However, two professional teams were first to announce cometary features had actually been observed, Lowry and Fitzsimmons using the 3.5-m New Technology Telescope at the European Southern Observatory (ESO), La Silla on 2006 July 29.3 detected a coma extending to 5.5" in p.a. 31 in a stack of 3x10 second exposures, while Hicks and Lawrence using the Palomar 5-m Hale telescope detected a faint coma extending approximately 10" on 2006 August 3. IAUC 8735 (subscription required) was issued on 2006 August 3rd announcing the comet and changing its asteroidal designation to the periodic comet designation P/2006 HR30.

Subsequently, on 2006 August 4th, Giovanni Sostero and Ernesto Guido noted on the Comets Mailing List (message 10375) that on 2006 July 20 UT they could not detect any cometary features on stacks of images taken with a 0.45-m reflector totalling 42 minutes exposure. Juan Lacruz also noted on the Comets Mailing list (message 10382) that on 2006 July 29, although it looked stellar,  he had measured the FWHM to be nearly 40% larger than nearby stars in images taken with a 0.3-m, with a total exposure of 62.5 minutes.

It appears that the object 'turned on' to show cometary features at the end of 2006 July and by 2006 Aug 4th (see image above) significant activity was evident.

The comet may reach 10th magnitude or brighter around perihelion time in 2007 January. However, it  was poorly placed when at perihelion at its two previous returns:

  • 1985 March (but it was well placed in 1984 August and may have then been mag 14-15). It may have been recorded on a single 2 hour exposure at the ESO from 1986 March 5, when its distance from Earth was 3.5 AU and from the Sun 4.2AU
  • 1963 June (but it was well placed in 1962 Aug and 1964 March and may have been mag +17 on both occasions, though at southerly declinations).


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