I just finished reading the excellent and entertaining book Voyager by Stephen J. Pyne, about the missions of the Voyager 1 and Voyager 2 spacecraft past Mars, Jupiter, Saturn, Uranus, and Neptune (reached in 1989). Launched a few weeks apart in 1977, they have now passed the edge of our solar system, still collecting data and photos, and communicating their findings even as late as 1990 (the mission's official end) despite many internal failures and wear-out.
[To give you some perspective, some vital numbers for Voyager 1 and 2 (as of August 2009) were, respectively: distance from Earth of 16/13 billion km; velocity relative to Earth of 131,000/107,000 km/hr; round-trip signal time of 30/24 hrs; propellant remaining 27/28 kg; and electrical output of 277/278 watts.]
Beyond the incredible tangible, technical challenges of such a mission—the deep cold, and the finite life expectancies of the various internal electronic and mechanical subsystems (amazingly crude on-board computer, inertial and star-based navigation systems, various instrumentation packages, thermoelectric power source, and the RF receiver/transmitter, to cite a few); tracking, communicating, and guidance—the book also brought up two critical issues that may affect the practicality and viability of similar deep-space missions to the edges of our solar system (or beyond).The issues are:
First, in our hyped-up, super-speeded world, with instant "everything", and where we expect a response immediately, can engineers live with a mission which has inherent time-lags of single- and double-digit hours between sending out an instruction or command and receiving a response? (For most of us, if that web page doesn't load in under a second, we go elsewhere!) Where problems must be solved by "try this, watch the result, try that, watch the result"? Can engineers deal with a system whose data rates are in the tens of kilobytes/sec range, due to incredibly weak signals and abysmal SNR?
Second, can we even manage missions which will have lifetimes of decades, and therefore where new generations of project leaders and their teams must be trained and sequenced? Where subtleties of the design's history, along with any implicit, critical institutional memory, may be lost? (Think of the movie Space Cowboys (2000), but aggravated by a factor of ten or twenty.) Will incoming engineers even want to be associated with projects whose technology is, by definition, grossly obsolete by the time they get involved with it?
The good news is that the capabilities of these unmanned, robotic explorer spacecraft has improved by orders of magnitude since the Voyager launches. But even if the space vehicle is increasingly autonomous and self-managing, and adapts itself to changes in its health and status, we'll still need skilled, experienced mission teams. And they will have to deal with the frustrations of the link lag and the length of the mission itself. The laws of physics won't yield on those points. ?