C/2015 ER61 (PanSTARRS)

Lore & Background

C/2015 ER61 was discovered on 15 March 2015 when it was 8.44 AU from the Sun and magnitude 21.5. By early February 2016, it reached magnitude 20. On 30 January 2016, it was classified as a comet when it was 5.7 AU from the Sun. The comet made a close approach to Jupiter on 28 March 2016 of 0.9245 AU, which significantly altered its orbit, decreasing its aphelion distance from 1430 AU to about 1200 AU; as it passed through the inner Solar System, its aphelion further decreased to 770 AU, and by 2020 it had an aphelion of 854 AU. The barycentric orbital period decreased from 19000 years (epoch 1950) to 9000 years (epoch 2050).

As of January 2017, the comet was magnitude 13 and increasing in brightness. On 4 April 2017, it was detected outbursting to magnitude 6.5. On 19 April 2017, it reached its closest point to Earth at about 1.2 AU, with an apparent magnitude of about 8 and, assuming a size of 20 km, an apparent size of 19 mas. It came to perihelion on 10 May 2017. The comet will not be 50 AU from the Sun until 2045. Though the nucleus was probably mildly active, early asteroidal estimates gave an absolute magnitude (H) of 12.3, suggesting a nucleus as large as 8–20 km in diameter, but it could easily be half that size due to activity brightening the nucleus.

Reader's Guide

C/2015 ER61 (PanSTARRS) holds significance as an object that bridges the classification of minor planet and comet, having been reclassified as a comet in January 2016 when it was 5.7 AU from the Sun. Its extreme orbital eccentricity—varying by about 99.9% during its orbit—and its former status as having the fourth-largest aphelion among known minor planets highlight its origin in the inner Oort cloud. The comet's close approach to Jupiter in March 2016 dramatically reshaped its orbit, reducing its aphelion from 1430 AU to roughly 1200 AU, with further reductions as it traversed the inner Solar System. This gravitational interaction provides a clear example of how Jupiter influences long-period comets. The comet's legacy includes its observed outburst in April 2017, reaching magnitude 6.5, and its perihelion passage in May 2017. The uncertainty in its nucleus size—estimated at 8–20 km from asteroidal data but possibly half that due to cometary activity—underscores the challenge of characterizing active comets. Its barycentric orbital period will decrease from 19000 years to 9000 years, reflecting its permanent orbital change.

Did You Know?

• It was reclassified as a comet on 30 January 2016 when it was 5.7 AU from the Sun.
• On 4 April 2017, the comet was detected outbursting to magnitude 6.5.