15440 Eioneus, provisional designation: 1998 WX4, is a dark Jupiter trojan from the Greek camp, approximately 66 kilometers (41 miles) in diameter.
[5] Since January 2013, a large number of a rotational lightcurve of Eioneus have been obtained from photometric observations by Robert Stephens at the Center for Solar System Studies in California.
[14][15][11][a] Analysis of the best-rated lightcurve from June 2017 gave a longer-than-average rotation period of 21.43±0.02 h hours with a low brightness amplitude of 0.09±0.02 magnitude (U=2+), indicative of a rather spherical shape.
[5][11] According to the surveys carried out by the Japanese Akari satellite, the Infrared Astronomical Satellite IRAS and the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer, Eioneus measures between 62.52 and 71.88 kilometers in diameter and its surface has an albedo between 0.072 and 0.092.
[8][9][10] The Collaborative Asteroid Lightcurve Link derives an albedo of 0.0585 and a diameter of 66.04 kilometers based on an absolute magnitude of 9.6.