161P/Hartley–IRAS

Lore & Background

Malcolm Hartley first detected the trail of this comet on a photographic plate exposed from the Siding Spring Observatory on 4 November 1983, when it was a 15th-magnitude object in Capricornus. Before he could obtain a confirmation plate, John K. Davies and Simon F. Green reported that IRAS detected a comet on 10 November 1983. Kenneth S. Russell confirmed it was the same object by 23 November, and the discovery was announced on 25 November 1983. Follow-up observations in December 1983 reported a weak condensation and short tail. John E. Bortle reported that the comet reached peak brightness at magnitude 7.8 on 24 February 1984, indicating an outburst that brightened it by two magnitudes. It faded gradually, and by 27 May 1984 had a total visual magnitude of 12.2, last observed on 4 June 1984.

Robert H. McNaught recovered the comet on 3 November 2004 ahead of its 20 June 2005 perihelion, and it received the official numerical designation 161P on 24 November 2004. It reached magnitude 11.3 on 16 June 2005. For the 2026 apparition, Hirohisa Sato recovered it from Siding Spring Observatory on 18 July 2026 at nearly 2–3 times fainter than predicted. As of September 2026, it is around magnitude 13, reaches opposition on 27 September 2026, and makes its closest approach to Earth on 2 October 2026 at 0.505 AU. Perihelion is on 27 November 2026 at a magnitude of approximately 11.

In March 1984, Stephen M. Larson measured the comet's spectroscopy, detecting emissions of OH, NH, CN, C3 and C2 from its coma, noted to be comparable to that of 27P/Crommelin. Brian G. Marsden confirmed it as a periodic comet in December 1983, suspecting a near-Jupiter encounter in 1957, later refuted by Shuichi Nakano and Kazuo Kinoshita in 1984 and 1998. The comet has a retrograde, near-polar orbit, with only a few close encounters with Jupiter between 1921 and 2171. Non-gravitational accelerations will likely perturb its orbit enough to cause close encounters with Venus, Earth, and Mars in the next 5,000 years.

Reader's Guide

161P/Hartley–IRAS holds significance as the sixth and final comet discovered by the Infrared Astronomical Satellite (IRAS), marking a milestone in space-based comet detection. It is also the third of twelve comets discovered by Malcolm Hartley, underscoring his prolific contributions. The comet's 1983 discovery involved a collaborative effort: Hartley's initial photographic detection, IRAS's independent infrared detection, and Kenneth S. Russell's confirmation, highlighting the synergy between ground-based and space-based astronomy. Its 1984 outburst, brightening by two magnitudes to magnitude 7.8, demonstrates the dynamic activity of Halley-type comets. The recovery by Robert H. McNaught in 2004 led to its official numerical designation, and the 2026 apparition recovery by Hirohisa Sato, at fainter-than-predicted brightness, illustrates the challenges of predicting comet behavior. Spectroscopic analysis by Stephen M. Larson revealed typical cometary emissions (OH, NH, CN, C3, C2), comparable to 27P/Crommelin. Orbital studies show a retrograde, near-polar orbit with a nominal Lyapunov time of 650 years, but non-gravitational forces may drive future close encounters with inner planets. The comet's legacy includes its role in demonstrating the value of infrared surveys and the ongoing need for ground-based follow-up.

Did You Know?

• 161P/Hartley–IRAS is the sixth and final comet discovered by the infrared space telescope IRAS.
• The comet experienced an outburst in February 1984 that brightened it by two magnitudes to a peak of magnitude 7.8.
• Its 2026 recovery by Hirohisa Sato found it 2–3 times fainter than predicted from its 2005 apparition.