Approximately 129,390 km
It was not actually an astronaut that discovered the planet Uranus. It was recorded a few times before official credit was given. It was recorded first by John Flamsteed in 1690. He believed it was part of the Taurus constellation.
Pierre Lemonnier observed the planet several times before 1750 and 1769, still not recognizing it for what it was.
It was William Herschel, who on March 13, 1781, observed the planet and came to think something more. At first Herschel described what he saw as a comet. After more study, he came to realize that it was a planet.
Uranus has a gravity of 8.69 m/s2 or 0.886 g
Compared to the Earth of 9.780327 m/s² or 0.99732 g
Uranus is a gas giant planet. There is no oxygen there and northing to stand on. Where atmospheric pressure is in a range that humans could survive, it is extremely cold.