C/2015 ER61 (PanSTARRS)

C/2015 ER₆₁ (PanSTARRS)

Discovery
Discovered by	Pan-STARRS 1 (F51)
Discovery date	15 March 2015
Orbital characteristics
Epoch	7 May 2017 (JD 2457880.5)
Observation arc	1.96 yr
Aphelion	2456±62 AU 1425/854 AU[1]
Perihelion	1.0421 AU
Semi-major axis	1229±31 AU 715/428 AU[2]
Eccentricity	0.99730
Orbital period	7591 years[1] 9000 years (epoch 2050)
Inclination	6.3490°
Longitude of ascending node	235.21°
Argument of periapsis	68.197°
Earth MOID	0.1016 AU[2][3]
Mars MOID	<0.00005 AU[3]
Jupiter MOID	0.0794 AU[2][3]
Saturn MOID	0.2869 AU[3]
Physical characteristics
Dimensions	8–20 km[4] 20 km[3]
Geometric albedo	~0.05 (assumed)
Comet total magnitude (M1)	20.5 (January 2016) 14.74 (peak 1 May 2017)

C/2015 ER₆₁ (PanSTARRS) is a comet and inner Oort cloud object. When classified as a minor planet, it had the fourth-largest aphelion of any known minor planet in the Solar System, after 2005 VX3, 2012 DR30, and 2013 BL76. It additionally had the most eccentric orbit of any known minor planet, with its distance from the Sun varying by about 99.9% during the course of its orbit, followed by 2005 VX3 with an eccentricity of 0.9973. On 30 January 2016, it was classified as a comet when it was 5.7 AU from the Sun.^[5] It comes close to Jupiter, and a close approach in the past threw it on the distant orbit it is on now.

Though the comet nucleus was probably mildly active, early asteroidal estimates gave an absolute magnitude (H) of 12.3,^[6] which would suggest a nucleus as large as 8–20 km in diameter. But it could easily be half that size due to activity brightening the nucleus.