January 1974, Mariner 10 made ultraviolet observations of Comet Kohoutek. Another mid-course correction was made on 21 January 1974. Venus flyby The spacecraft passed Venus on 5 February 1974, the closest approach being at 17:01 UTC. It was the twelfth spacecraft to reach Venus and the eighth to return data from the planet, as well as the first mission to succeed in broadcasting images of Venus back to Earth. Mariner 10 built upon observations made by Mariner 5 six years earlier; importantly, Mariner 10 had a camera whereas the prior mission lacked one. As Mariner 10 veered around Venus, from the planet's night side to daylight, the cameras snapped the probe's first image of Venus, showing an illuminated arc of clouds over the north pole emerging from darkness. Engineers initially feared that the star-tracker could mistake the much brighter Venus for Canopus, repeating the mishaps with flaking paint. However, the star-tracker did not malfunction. Earth occultation occurred between 17:07 and 17:11 UTC, during which the spacecraft transmitted X-band radio waves through Venus' atmosphere, gathering data on cloud structure and temperature. Although Venus's cloud cover is nearly featureless in visible light, it was discovered that extensive cloud detail could be seen through Mariner's ultraviolet camera filters. Earth-based ultra-violet observation had shown some indistinct blotching even before Mariner 10, but the detail seen by Mariner was a surprise to most researchers. The probe continued photographing Venus until 13 February 1974. Among the encounter's 4165 acquired photographs, one resulting series of images captured a thick and distinctly patterned atmosphere making a full revolution every four days, just as terrestrial observations had suggested. The mission revealed the composition and meteorological nature of the atmosphere of Venus. Data from the radio science experiment measured the extent to which radio waves passing through the atmosphere were refracted, which was used to calculate the density, pressure, and temperature of the atmosphere at any given altitude. Overall, atmospheric temperature is higher closer to the planet's surface, but Mariner 10 found four altitudes where the pattern was reversed, which could signify the presence of a layer of clouds. The inversions occurred at the levels, confirming previous observations made by the Mariner 5 encounter. The ultraviolet spectrometers identified the chemical substances that comprise Venus' atmosphere. The elevated concentration of atomic oxygen in the upper atmosphere showed that the atmosphere is stratified into upper and lower layers that do not mix