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Bouncing radar off of Venus' surface has yielded a rich body of data about the planet. Based on Venus' aphelion and Earth's perihelion, we know then that the answer is at least 38 million kilometers with a round trip of over four minutes.

Have radar reflections from Venus or any other natural solar system body been measured over distances larger than that?

Here I am primarily interested in reflections from the natural solar system body itself, so I've asked Farthest distance to a solar system object that's been determined by spacecraft transponder? separately.

For more about Venus and radar see:

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According to the list at Wikipedia's article on Radio Astronomy, the furthest object that's been measured was probably Saturn's rings; Titan may have been further, depending on where in its orbit it was. Other radar observations of note have involved making elevation maps of Mercury and Venus, and since you asked about distance, high-precision distance measurements of Mercury as a test of General Relativity.

Out beyond Saturn, it's unlikely that anything is a suitable target for radio astronomy. The gas giants don't have well-defined radar echos, while nothing else is very big: the radar reflection from Titania would be roughly 1/150th that of Titan, that of Triton, 1/300th, and that of Pluto, 1/500th at best.

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