489P/Denning
Lost for 130 years, recovered as asteroid 2007 HE4.
489P/Denning is a Jupiter-family comet that takes 9.4 years to orbit the Sun. It was first spotted by William Frederick Denning on 26 March 1894 from Bristol, England, while he was scanning the sky for comets. The object appeared faint, around magnitude 11, with a tail stretching about 5 arcminutes. By the time of its discovery, the comet had already passed its closest point to the Sun and its nearest approach to Earth, and it quickly faded. Observations continued into April, when it looked very faint and elongated, and a few more were made in May. The last detection came on 5 June 1894. An outburst may have occurred that year, producing an unusual tail that led some to believe the nucleus had disintegrated—until the comet’s recovery in 2024 proved it was still intact.
Initial orbit calculations by Lipót Schulhof, using positions from late March 1894, suggested a parabolic path with perihelion on 13 February. By mid-April, Schulhof noticed deviations and revised the orbit to an ellipse with a perihelion date of 9 February and a period of 6.74 years. Other astronomers produced different periods: Lewis Boss got 7.94 years, Schulhof later gave 6.79 years, and John Russell Hind calculated both 6.76 and 7.70 years. The comet was then lost.
In 2024, the Mars-crossing asteroid 2007 HE4—discovered by LINEAR on 17 April 2007 at magnitude 18.7–19.5—was flagged by a machine learning model as potentially active, having been observed only during one opposition. Maik Meyer noticed its orbit closely matched the proposed orbit of Denning’s comet. Re-examined images from the Great Shefford Observatory (18–19 April 2007) showed the object slightly elongated, with an aspect ratio of 3:2 compared to nearby stars, extending eastward. Stacked images from the 1.82-meter telescope at the Dominion Astrophysical Observatory on 19 April 2007 revealed a coma 5.3 arcseconds across and a faint, spike-like tail 8 arcseconds long. Apparitions between 1972 and 1997 were unfavorable, and no plates were found for the promising 1963–64 apparition.
For the 2025 apparition, the comet was predicted to reach magnitude 18. It received its permanent numerical designation, 489P, in July 2025.
As a Jupiter-family comet, 489P/Denning makes close approaches to Jupiter. One such approach occurred in July 1830 at 0.071 AU (10.6 million km).
Quick Facts
- Discovery Ref
- Denning_1894
- Discoverer
- William F. Denning
- Discovery Site
- Bristol, England
- Discovery Date
- 26 March 1894
- Mpc Name
- D/1894 F1, P/2007 HE · 4
- Designations
- 1894 I, 1894a
- Orbit Ref
- jpldata
- Epoch
- 18 August 2002 (JD 2452504.5)
- Obs
- 302
- Perihelion
- 1.575 AU
- Aphelion
- 7.312 AU
- Semimajor
- 4.443 AU
Facts from the source article.
Lore & Background
William Frederick Denning discovered the comet on 26 March 1894 from Bristol, England, as a faint object in Leo Minor. Its magnitude was about 11, with a tail 5 arcminutes long. The comet had already passed perihelion and closest approach to Earth and was fading; it was last detected on 5 June 1894. An outburst in 1894 produced an anomalous tail, leading to initial speculation that the nucleus was destroyed, but recovery in 2024 showed it intact.
Early orbit calculations varied: Lipót Schulhof first proposed a parabolic orbit with perihelion on 13 February 1894, then an elliptical orbit with a period of 6.74 years. Other orbits ranged from 6.76 to 7.94 years. The comet was lost until 2024, when Mars-crossing asteroid 2007 HE4 (discovered by LINEAR on 17 April 2007, magnitude 18.7–19.5) was identified as the comet by Maik Meyer. Re-examination of images from the Great Shefford Observatory showed elongation, and stacked images from the Dominion Astrophysical Observatory revealed a coma 5.3 arcseconds across and a faint tail 8 arcseconds long.
Reader's Guide
489P/Denning's significance lies in its long period of being lost and eventual recovery, demonstrating the value of linking historical comet observations with modern asteroid surveys. The recovery in 2024 resolved a 130-year mystery, confirming that the comet's nucleus survived an 1894 outburst that had been misinterpreted as its destruction. The object's orbit evolved due to close approaches to Jupiter: a 1925 approach at 0.24 AU raised its period to 8.00 years and perihelion to 1.29 AU, and a 1986 approach at 0.19 AU raised them to 9.31 years and 1.56 AU. The comet's 1894 apparition included an Earth approach of 0.27 AU on 12 February. Its recovery was aided by a machine learning model that flagged 2007 HE4 as likely active, and by Maik Meyer's orbital matching. The comet received its permanent numerical designation 489P in July 2025.
Did You Know?
- It was lost for 130 years until 2024, when asteroid 2007 HE4 was identified as the same object.
- An 1894 outburst led to initial speculation that the comet's nucleus was destroyed, but recovery showed it intact.
- The comet's orbital period changed from 7.42 years in 1894 to 9.31 years after a 1986 Jupiter approach.
More in Periodic Comets 1-24
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