(53319) 1999 JM8 is an asteroid, slow rotator and tumbler, classified as a near-Earth object and potentially hazardous asteroid (PHA) of the Apollo group, approximately 7 kilometers (4 miles) in diameter, making it the largest PHA known to exist.

[1][9] It has also been characterized as a carbonaceous C-type asteroid, which seems more likely due to its exceptionally low albedo (see below).

[10] Radar imaging by Goldstone and Arecibo observatories revealed that 1999 JM8 has an unusually slow and possibly chaotic rotation period, similar to that of asteroid 4179 Toutatis.

It gave a period of 136±2 hours with a brightness amplitude of 0.7 magnitude (U=2), and suggested that the body is in a non-principal axis rotation, commonly known as tumbling.

[3][4][6][7] The Collaborative Asteroid Lightcurve Link derives an albedo of 0.03 and adopts a diameter of 7 kilometers based on an absolute magnitude of 15.2.