The SOHO spacecraft detected Comet Leonard in its Solar Wind Anisotropies (SWAN) instrument in Lyman-alpha. SWAN doesn’t look directly at the Sun, it rather maps the whole sky in ultraviolet light, and sees interstellar hydrogen that interacts with the solar wind. The maps can be used to reveal bright points in ultraviolet light indicating the presence of a comet, for example, as is the case here.
SWAN’s Lyman-alpha observations are routinely used to estimate the rate of production of water from comets, which is one of several important factors astronomers use to assess the health and behaviour of comets. Thus SOHO has been monitoring the hydrogen production of Comet Leonard since 15 November 2021 and through perihelion in early January 2022, which will enable a water production rate to be determined for this comet as well.
The animation was provided by Sungrazer project citizen scientist Worachate Boonplod.