Quick! How many planetary probes has the US sent out? Would you believe that we have sent out some 76 probes since the Pioneer 1 was launched toward the Moon in 1958? That’s a pretty impressive result, yes? No.
If you look at the chart showing the total number of probes over the years, you should see two major problems. First and most obviously, there is a large gap in probe launches between 1978 and 1990. Why would we stop sending out probes for more than a decade? The answer is the Space Transportation System (STS), a.k.a., the Space Shuttle . As the STS developed in the 70s, its current dollar costs steadily and inexorably rose . As a result, NASA management was forced to choose between funding planetary probes or the Shuttle. The Shuttle won.
But that only explains the gap up to 1981, right? Wrong. Even after the STS became operational, the costs were much higher than predicted and so planetary science was drained to keep the “Buck Rodgers” group flying. Worse yet, because the STS was supposed to handle all launches, the planetary group had to redesign their planned probes to fit into the Orbiter’s cargo bay. In addition, using the Shuttle was more expensive than using an unmanned Titan for the launch – and guess who paid the extra? But the worst was the hiatus in launches caused by the Challenger explosion in 1986. That disaster  caused both a 30 month stoppage in all launches and a new requirement that no new probes could be launched from the cargo bay ; it also contributed to the near-failure of the Galileo probe . The remainder of the gap was caused by the need to re-redesign the probes following the change in STS protocols.
The second problem that should jump out at you is the change in the rate at which probes were launched. Before the Shuttle, probes were launched at a rate of one every five months or so; after the Shuttle, that rate dropped to one every ten months. The reason for that is the STS was much ore expensive to run than had been expected. As a result, NASA had to “find” extra money to keep the Shuttle flying and, once again, that money came from teh basic science and planetary probe programs. So it is clear that all of our space exploration problems were caused by the STS and now that it is gone, we should return to that speedy rate, right? Wrong.
So here are we today? The STS has closed down, and its follow-on, variously known as Constellation and “Apollo on steroids”, has been rejected for being too risky and too costly . In theory, that should free up funding for more planetary probes; however, as one of my professors used to say “That is necessary but not sufficient”. In other words, simply getting rid of the big money sink isn’t enough to put us back on the fast track of exploration.
You see, the STS was just one of the factors in this problem. The other was national pride (or, to put a slightly more honest face on it, fear of the Communists). If we look at the size of probe that was launched each year, a clear progression is seen in probe size up until the STS hiatus happened. The probes became gradually larger and went further out, up until the “Fall of Communism” (FoC)  when suddenly the will to send big probes out was lost. The exceptions to this (Galileo and Cassini-Huygens) were both built and ready to launch before the fall; only the STS accident kept them from flying earlier and giving us a perfect chart.
After the FoC, we were left with little will to spend the “peace dividend” on exploration. Instead, we squandered it on tax cuts and ill-advised military hardware purchases, while NASA was forced to adopt an ethic of “Faster, Better, Cheaper” . The idea was to send out smaller probes more often for less money; the practice was that we sent out one-offs, which drove up the costs and spread out the timelines.
As a result, we have visited fewer places and seen less in the two decades of planetary probes send after the FoC than we saw in the two decades before . Before the FoC, we sent 48 probes to 9 bodies; after the FoC, we sent just 28 probes to 10 bodies . In addition, the tendency to make one-offs has now become institutionalized at NASA. There are no more probe series; you will never see a “Mars Global Surveyor 7” or a “Clementine 4”. This means that we will never see the cost savings associated with making multiple copies of one probe .
Fortunately (?), there is a new “Red Dawn” on the horizon. The Chinese have already launched the first component of their new space station (no capitalists need apply) and begun an ambitious lunar exploration program. Though China is a signatory of the Space Treaty, they have been notorious in their interpretation of the various elements of that treaty . As a result, we may now be back where we were forty years ago, with an aggressive Communist country taking the lead in planetary exploration and an underpowered domestic space program. As history has shown, that is exactly the sort of situation that brings out the best in us (and US). May that be true once more.
 Technically, the Shuttle is the Orbiter, whereas the Space Transportation System is the Orbiter, External Tank, and Solid Rocket Boosters, plus all of the ground support needed to make them work together.
 The constant dollar costs remained very close to the original budget [i]; unfortunately, the 70s were a period of high inflation, which made every long-term project appear to jump in cost.
 “Disaster” is used in the precise form here, as it means “bad star” in ancient Greek.
 Due to the additional danger posed by the propellants. There were exceptions made for Magellan, Ulysses, and Galileo (because they were already built), but they were the last.
 Space nerds [ii] will recall the early difficulties with the Galileo probe, caused by a balky radio antenna. Ultimately, the problem was found to have been caused by Galileo’s three cross-country trips (JPL to KSC for planned launch, KSC to JPL for storage after Challenger, JPL to KSC for actual launch); all of the vibration caused one of the ribs to slip and get caught.
 How costly? It is estimated that Constellation would have forced us to close down the ISS in order to pay for it; it did force us to stop research into ion propulsion and other basic science matters. The Augustine Commission put the price tag at $1.2 billion per flight with four astronauts (up from $450 million for the Shuttle with seven astronauts). We currently pay the Russians $60 million per seat to use the Soyuz.
 Which happened in the spring of 1989 and didn’t end Communism so much as force a Chapter 11 bankruptcy on it [iii].
 Or, as the common NASA in-joke had it “Pick two”.
 Right now, some smart-alec is saying “Hah! 2011-1958 is five decades, not four!” Obviously, the smart-alec has forgotten the lost decade caused by the development of and accident with the STS.
 Counting all comets as one “body” and all asteroids as another. From a planetological standpoint, that is reasonably correct.
 Those cost savings affect more than just the one series of probe. Magellan was famously made out of the spare parts from other planetary probes; had each of the earlier probes been a one-off, making it would have been nigh-well impossible and our most informative tool for exploring Venus would never have launched.
 Most ominously, in their test of an anti-satellite weapon on one of their own satellites (which they claimed was the loophole allowing the test). Though it did not set off the Kessler Syndrome that that some feared, it nevertheless measurably increased the risk to other things in orbit (including the ISS).
[i] 50% over budget is “very close to the original budget”, right?
[ii] Sounds like a Japanese B-Movie doesn’t it?
[iii] The finance wonks out there are now rolling in the aisles. What we wanted was a Chapter 7 (complete liquidation). What we got is a Chapter 11 (re-organization with the same firm in charge).