29P/Schwassmann–Wachmann
Periodic comet with 14 year orbit
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Through Comet and Magnitude (astronomy)
Discovery
It was discovered photographically, when the comet was in outburst and the magnitude was about 13. Precovery images of the comet from March 4, 1902, were found in 1931 and showed the comet at 12th magnitude.
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Orbit and physical properties
The comet reached its most recent perihelion on March 7, 2019. It spends its entire orbit just beyond Jupiter at 5.8–6.3 AU from the Sun where the comet remains active with a coma as it is close to the snow line. It will reach aphelion on 30 September 2026.
The comet is a member of a class of objects called "centaurs", of which at least 1000 are known. These are small icy bodies with orbits between those of Jupiter and Neptune. The centaurs have been recently perturbed inward from the Kuiper belt, a disk of trans-Neptunian objects occupying a region extending from the orbit of Neptune to approximately 50 AU from the Sun. Frequent perturbations by Jupiter will likely accumulate and cause the comet to migrate either inward or outward by the year 4000. A number of centaurs appear to be dynamically and perhaps even physically related to 29P; such objects may traverse the coma of 29P when in outburst.
The comet nucleus is estimated to be 60.4±7.4 kilometers in diameter. As of 2026, the comet has a rotation period of about 58 days.
Outbursts
The comet is unusual in that while normally hovering at around 16th magnitude, it suddenly undergoes an outburst. This causes the comet to brighten by 1 to 5 magnitudes. This happens with a frequency of 7.3 outbursts per year, fading within a week or two. The magnitude of the comet has been known to vary from 18th magnitude to 10th magnitude, a more than thousand-fold increase in brightness, during its brightest outbursts. Between 2014 and 2026, there have been 120 outbursts observed or about 10 outbursts per year. Outbursts are very sudden, rising to maximum in about 2 hours, which is indicative of their cryovolcanic origin; and with the times of outburst modulated by an underlying 58-day periodicity possibly suggesting that its large nucleus is an extremely slow rotator.
During 2025, there were 8 notable outbursts occurring as four pairs of twin events: January 2/6, February 1/2, May 13/25, and December 4/11. The December outburst reached about magnitude 13.
In 2026, there was an outburst on February 9/15 to magnitude 12.6V. On April 15 there was outburst from magnitude 16 to 13. Another outburst occurred on May 29. The next outburst is expected to occur around November 12‒18 when it will have a solar elongation of about 40 degrees.