Last Friday, as I write this, the space shuttle Atlantis had returned from its final mission, the final mission of the Space Shuttles. This brings to a close the Shuttle era of US spaceflight, an era which has been most disappointing. The idea of the Shuttle was a legitimate one, to try to reuse as much as possible of one’s rocket to avoid having to build one each time. However, it required so much maintenance that it was not much cheaper than traditional expendable rockets, if not more expensive. Instead of costing $50 million per flight, it cost $450 million. The Shuttles also had two failures which shut down the program for 2 1/2 years each time, and while they were flying, they only had 5 launches/year.
Automated spacecraft have repeatedly proven their worth. They can easily tolerate vacuum, ionizing radiation, and extreme heat and cold, they can be powered by sunlight or radioactive decay, they can keep going for years, and we do not have to try to bring them back. In fact, some spacecraft have been deliberately crashed into celestial objects as part of their missions, like the later Ranger spacecraft on the Moon. Spacecraft have now visited all the planets and their larger satellites, and also some asteroids and comets.
They do have some important deficiencies, however. They have hardly any onboard intelligence, and they usually have no manipulative ability. That means that they must receive very explicit instructions, and that they usually have little or no self-repair capability. For many applications, needing explicit instructions is not much a problem, but that cannot be said of planetary-surface exploration.
Real-time teleoperation is only feasible when the communication round-trip time is less than a few seconds. The Moon’s round-trip time is 2 1/2 seconds, making it borderline. Mars is much worse, with round-trip times ranging from 6 to 45 minutes, and averaging at 30 minutes. The operators of the Mars rovers plan those vehicles’ next actions and then upload their instructions to those vehicles. Instructions like move or turn or take pictures or move the arm or operate various instruments.The outer planets are even worse. Jupiter’s round-trip time is 1 1/2 hours, Saturn’s is 2 1/2 hours, Uranus’s 5 hours, Neptune’s 8 hours, and Pluto’s 11 hours — nearly half a day.
What next for NASA’s astronauts? Cadging rides on Russia’s Soyuz spacecraft until NASA has an alternative ready. An alternative much like NASA’s earlier Apollo spacecraft and the Soyuz spacecraft itself — a small spacecraft atop an expendable rocket. Back to square one. But at least NASA will not have to support the Shuttles any longer.