On 25 February 1979, when the Voyager 1 spacecraft was 9,200,000 km (5,700,000 mi) from Jupiter, it transmitted the first detailed image of the Great Red Spot. Cloud details as small as 160 km (99 mi) across were visible. The spot is a persistent high-pressure region in the atmosphere of Jupiter, producing an anticyclonic storm that is the largest in the Solar System. Located 22 degrees south of the planet's equator, it produces wind-speeds up to 432 km/h (268 mph). Observations from 1665 to 1713 are believed to be of the same storm; if this is correct, it has existed for at least 357 years. The first sighting of the is often credited to Robert Hooke, although it appears that he more likely recorded the shadow of a transiting moon instead. A more likely accreditation for the discovery is accorded to Giovanni Cassini, who first noted its existence the following year in 1665, and continued to track it's progress until 1713. It was next observed on 5 September 1831, with 60 recorded observations between then and 1878, when continuous observations began

In the 21st century, the Great Red Spot has been observed to be shrinking in size. At the start of 2004, its longitudinal extent was approximately half that of a century earlier, when it reached a size of 40,000 km (25,000 mi), about three times the diameter of Earth. At the present rate of reduction, it would become circular by 2040.