John Roger’s excellent book has the history of the GRS: it its likely that it’s not the same spot as observed in the 17th century by Cassini, but the first definitive observations of ‘our’ GRS came in the 19th century - observers started to draw the ‘hollow’ around the GRS in 1831. The vortex within the hollow seemed to come and go with time, until the 1870s when observers first started to draw a large red oval within the hollow. It covered about 34 degrees of longitude between 1879 and 1882.
2. What are its dimensions today?
Hubble images in 2014 showed the GRS to be about 10,000 miles in east-west distance. See the press release by Simon et al. : http://www.nasa.gov/press/2014/may/nasas-hubble-shows-jupiters-great-red-spot-is-smaller-than-ever-measured/#.VCz_Ri5dVR4
3. Would it be fair to say that the GRS consists mostly of hydrogen gas with a colossal cloud – mainly ammonia ice, plus something, probably phosphorus, making it reddish?
Well, hydrogen and helium are everywhere on the giant planet, so that does’t really distinguish it from the rest of the atmosphere. Instead, the GRS is a region of unusual clouds and chemicals, entrained by a peripheral collar of winds rotating anticlockwise. The composition of those clouds are largely unknown, but likely to be a combination of nitrogen, sulphur and phosphorus compounds and ices, possibly coated in hydrocarbons (and possibly nitriles) raining down from above. The source of the red colour remains a mystery - the compound has to be strongly blue absorbing, but it’s identify is unknown. Read my blog about it here: http://planetaryweather.blogspot.co.uk/2014/01/what-gives-jupiter-its-colourful-stripes.html
4. Would it also be correct to say that the revolving storm is stirred by the planet’s rotation and that it has raged for more than three centuries?
The formation of vortices is certainly related to the rapid rotation of the planet, as eddies are spawned by the unstable jets, interact and merge to provide energy to continually power the GRS. It’s longevity is unclear - it’s been there since at least the nineteenth century, and there’s the suggestion that large storms might be commonplace in this region of Jupiter’s atmosphere. I.e., if our GRS eventually dissipates, maybe another will form at a similar latitude.
5. Has the GRS always been oval shaped? And has it always been wider across (ie from east to west) rather than "higher" (ie from north to south)?
The early observations show a very elongated oval, which is shrinking steadily in east west extent. The shrinkage has been known for years, but amateur and professional data suggest that it’s now accelerating rapidly.
6. Is the GRS showing signs of becoming more circular?
That’s certainly what it looks like!
7. There have been some suggestions that the anticyclone storm is shrinking at the rate of about 1000 kilometres a year. Is this correct?
Not according to the Hubble and Voyager measurements, which show a decrease from 14,500 miles in 1979 to 10,250 miles in 2014.
8. If the shrinkage continued at this rate, does this imply that the GRS would have disappeared by the year 2030, or thereabouts, please?
That’s very hard to say, as it depends on the reason why the GRS is shrinking, and whether any particular aspect ratios (i.e., ovals or circles) are more stable than others. We certainly live in an interesting time.
9. Lastly, is it conceivable that the shrinkage will stop at some stage and perhaps start increasing in size again?
Absolutely. If the GRS is maintained by swallowing up smaller storms and eddies that have the misfortune to be at the same latitude, then large storms (such as those seen in 2010-11 when Jupiter’s faded SEB revived) could feed more energy into the GRS and help it to grow again. We’ll have to watch what’s going on very carefully.