Space is big
It takes a long time to get to the outer planets.
Our astronomer recently read that NASA is considering sending a space probe to Uranus in the next few years. It would be the first since Voyager 2, which examined the planet from close range in 1986.
We remember the excitement of the Pioneer and then Voyager missions, the first to reach the outer planets. Recall that everything we knew about them up to that time had been inferred from far away, mostly using chemical photography (electronic detectors were new) through a turbulent atmosphere. That set severe limits on the fineness of detail one could see. It is instructive to look at planet photographs in an astronomy textbook from, say, the 1970s or before: at best, rather fuzzy, and Uranus and Neptune were no more than tiny blobs. And there was no getting beyond the atmosphere with a telescope of any size: the Hubble Space Telescope didn’t appear until 1990. So the shock of seeing these former blobs in exquisite detail may well be imagined.
Nowadays, when the electronic detectors and image processing available even to amateurs yield such fantastic results, the products of the old space probes may even seem a bit quaint. Technology has moved on in the past four decades. Which prompts the question: why has it been so long? Voyager 2 met Uranus well before our tutor’s students were born, and even their parents may be too young to remember it. Has NASA been asleep all this time?
Well, no. There have been dedicated missions to Jupiter and Saturn, and the former has one in operation now; to say nothing of a small herd of rovers on Mars. The delay comes from the fact that space is big. Really big. And there’s a lot of space in the outer Solar System. In round numbers, Saturn is a billion miles from the Sun, and takes thirty years to make one orbit; Mars revolves in about two. Uranus is about twice as far away as Saturn, and none of our consultants is even close to seeing one complete revolution of the former planet. To send a space probe there is a program of decades. It takes time just to cover the mileage. We think this fact is not very well appreciated.
And a machine designed to function that long, and to do things that far away, is a major undertaking. For instance, the data link must be four times as strong as one going only to Saturn, or it must accept a lower data rate, or some such work-around. Any signal going out or coming back will take several hours, even at the speed of light. Up to this time, it has seemed that missions closer to home would give more scientific return.
But we’re very happy that a trip to Uranus is being considered. Apart from the new things to be learned, it just might underline how big space actually is.
