The Middle Zone is the region between the Asteroid Belt and the Kuiper Belt[...]
The second zone, the Jovian planets or gas giants, have enamored all astronomers from the time that Galileo turned the first telescope on them. Little wonder that as soon as the opportunity permitted, spacecraft began to visit these. The most stunning of these missions was the Voyager 1 and 2 probes to the outer solar system. Launched in 1977, these spacecraft visited their primary targets of Jupiter and Saturn and then went on to all the giant outer planets. The two Voyagers took well over 100,000 images of the outer planets, rings, and satellites, as well as millions of magnetic, chemical spectra, and radiation measurements. The Voyagers’ windshield tour of the Jovian planets in the second zone returned information to Earth that revolutionized the science of planetary astronomy.
Source: https://launiusr.wordpress.com/2016/03/18/three-zones-of-the-solar-system/