C/2004 Q2 (Machholz)

Physical characteristics

During its closest approach to Earth in January 2005, ground observations from La Palma, Spain revealed that the comet may have a rotation period of 9.1±0.2 hours, although a 2012 study regarding the morphological structures in its coma suggested it may have a rotation period of 0.74 days instead.

Between December 2004 and January 2005, observations from Lulin and La Silla observatories spotted two jets of carbon and cyanogen (CN) compounds that form a spiral-like structure within the coma, which were caused by two active surface regions in its nucleus. In March 2005, the GALEX spacecraft observed the comet in far-ultraviolet (FUV) light, where it determined that ionized methane (CH₄) molecules dominated its coma, which explained the shorter than expected lifetime of carbon molecules detected. Spectral analysis of this comet's chemical composition suggest that it may have formed in a relatively inner region of the solar nebula than other typical Oort cloud comets, at a distance of at least 5.0 AU (750 million km) from the Sun. Additionally, the methane molecules frozen in the comet and its ratio relative to water indicate formation at relatively high temperatures compared to other comets.

Near-infrared measurements of the comet has indicated that its surface material generate an outflow in the form of fragments that separated into gas and dust under sublimation on time scales of the order of days.

Its nucleus has a mean diameter of 6.0 km (3.7 mi).