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First of all, the title is just an assumption based on a very small sample size of the Voyager spacecrafts and New Horizons. We have orbiters around the moon and Mars because we had/have plans to land humans there. Is that the only reason we would spend the money to establish long term observation satellites?

Naturally I expect the answer to be that the extra cost of fuel necessary to achieve orbit is unjustified, but is that the only factor? I would imagine that adding the capability to remain in orbit around a planet (specifically Jupiter and beyond) with the potential of accumulating decades of data is far more sensible than trying to maneuver a single probe to take quick shots of multiple planets on a one-way trip out of the solar system.

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    $\begingroup$ "I expect the answer to be that the extra cost of fuel necessary to achieve orbit is unjustified, but is that the only factor?" That is one huge factor. $\endgroup$
    – Andrew Thompson
    Jul 10, 2015 at 4:35
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    $\begingroup$ New Horizons launched 2006 is the first flyby since Voyager was launched in 1977. And NH costs only 1/4 of the Curiosity rover. They were popular during the 1960s' space race when it was politically prestigious to "do a first", be it a flyby or crash landing, with new planets. Although the Cassini orbiter flew by Jupiter and about a dozen asteroids have also been flewn by in later decades, often as secondary missions. $\endgroup$
    – LocalFluff
    Jul 10, 2015 at 4:52
  • $\begingroup$ @LocalFluff I guess in the case of NH an orbital rendezvous trajectory would have taken far too long compared to a direct intercept to be of any use to the current generation, and the fuel needed to slow down from the latter would be immense. A Jupiter orbiter would be more sensible, and I noticed there was a cancelled Jupiter Europa Orbiter mission, but I guess they'd rather put more money into Mars rovers and Orion, or Venus dirigibles. $\endgroup$
    – Bobe
    Jul 10, 2015 at 5:28
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    $\begingroup$ An orbiter can only gather information on a single target. A probe can potentially gather information on lots of targets. And Voyager is now gathering information from outside* the solar system. $\endgroup$
    – Bill Michell
    Jul 10, 2015 at 8:46
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    $\begingroup$ Don't forget Galileo (Jupiter orbiter), Cassini (Saturn orbiter), MESSENGER (Mercury orbiter), and several Venus orbiters. Flyby missions are much easier than orbiters -- and the data they gather is helpful in planning future orbiters. $\endgroup$
    – Keith Thompson
    Jul 10, 2015 at 17:30

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NASA likes to do things gradually, building knowledge as it goes along. Each step provides information used in building the next probe.
The evolution goes something like this:

  1. Flyby. Initial reconnaissance of a planet. Gather information on e.g. the local radiation environment, and other potential obstacles for an orbiter mission.
  2. Orbiter. Detailed surface mapping to help plan a lander mission.
  3. Lander. More details on surface composition to help plan a rover mission.
  4. Rover.

Each step costs more than the previous one. So only the most interesting targets have rovers sent to them.

There are other factors. Sending an orbiter to Pluto wasn't possible in the timeframe of the New Horizons launch: an orbiter would need a much heavier rocket than was available at the time. NH was hurried to launch because scientists had discovered Pluto has an atmosphere at the moment, and predictions were this atmosphere would collapse somewhere between 2014 and 2020, the probe had to arrive before this happened.

Because Pluto's so small and doesn't have a dense atmosphere, all braking must be done using rocket fuel. So either:

  • you launch at a low speed to reduce the delta-V for getting into Pluto orbit. This means the orbiter takes very long to get there (wild guess: at least 30 years instead of 9). Keeping a science team together for that long is difficult.

  • or you launch a probe with a huge fuel tank and engine, and you need two SLS or Saturn V to get that off the ground. This means 20 times the launch cost of New Horizons.

New Horizons goes through the Pluto system at a relative velocity of 11 km/s.

This Slashdot post contains an interesting calculation of launch weight for an orbiter:

The Space Shuttle Main Engines, one of our most efficient rocket engines, has an Isp of 4.436 km/s. By the rocket equation [this means that, to change velocity by 11 km/s using this engine, a spacecraft would need a ratio of wet mass to dry mass of exp(11/4.436) = 11.9. In other words, to stop the New Horizons probe at Pluto, we'd need to have sent along an extra 10.9 times its mass in fuel. And that's ignoring the mass of the engine and tankage, which makes things worse.

Fuel boil-off... is an additional problem: it means we couldn't use the liquid-hydrogen/liquid-oxygen propellant used by the SSMEs, but some more stable (and less efficient) propellant, which further increase the required fuel mass.

...New Horizons was launched on an Atlas V 551, which has a capacity of 19t to LEO. To send the probe plus 10.9 times its mass in fuel would therefore take an equivalent capacity of ~11.9*19 = 226t to LEO. The Saturn V, the most powerful launcher ever made, had a capacity of 118t to LEO. So you'd need two Saturn V launches, rendesvousing in orbit, to get a spacecraft with enough fuel to fly to Pluto and stop there. (Probably 3-4 launches, when you consider the other problems described above.)

The Pioneer and Voyager missions had similar constraints. No knowledge at all of space beyond Mars' orbit, budget limits because of the Apollo project.

Your initial premise is also off. We've had some high-profile flyby missions, but much more money has been spent on orbiters (Cassini, various Mars orbiters, Rosetta), landers (Viking) and rovers (Curiosity is several times more expensive than New Horizons).

Voyager and New Horizons are just the tip of the iceberg. 47 orbiters and landers, 43 flyby missions, with some overlap in that list (Rosetta is counted as both a flyby and an orbiter, for instance).

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  • $\begingroup$ Fair points, and I wasn't implying that Voyager and New Horizons were the only flyby missions, just to be clear. However, I was only comparing flybys to orbiters, not landers or rovers. I understand the principle of gradually progressive missions, but is it very unreasonable to imagine at least just one orbiter for each planet in the solar system? $\endgroup$
    – Bobe
    Jul 10, 2015 at 11:28
  • $\begingroup$ In my opinion, that's very reasonable, but I don't control the world's space budgets ;-) $\endgroup$
    – Hobbes
    Jul 10, 2015 at 15:46
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    $\begingroup$ The rationalization in the yellow box is a little silly. (Where is that from?) You don't have to go by Pluto at 11 km/s. They did that intentionally for this flyby to get there fast. If you wanted to orbit Pluto, you would accept taking longer to get there. Then you can do it with one existing launch vehicle. $\endgroup$
    – Mark Adler
    Jul 10, 2015 at 17:42
  • $\begingroup$ I've added a reference for that quote. It's a bit extreme, but I thought it'd be useful to put a number to at least one scenario (NH-based orbiter with the same arrival date), and it saved me going through the rocket equation myself. $\endgroup$
    – Hobbes
    Jul 10, 2015 at 18:16
  • $\begingroup$ There's of course a third possibility: you launch a probe with an ion engine, like the Dawn mission to Vesta & Ceres. But then (since solar power isn't workable at that distance) you'd have to supply a nuclear reactor. PS: add Dawn to your list of orbiters. $\endgroup$
    – jamesqf
    Jul 10, 2015 at 18:47
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There certainly were concepts developed for a Pluto orbiter. Some argued as you do that even though such a flagship mission would be much more expensive than a flyby, it would provide more science return per dollar.

However those concepts were never funded, so they never returned any science at all. Pluto was not a high enough science priority to demand that much of a small pie. So what made it there now is a flyby mission. A flyby provides a huge advancement over what we can get from Hubble (or eventually Webb), so it was deemed worth the cost.

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In addition to the fuel concerns, radiation becomes a much larger concern when orbiting certain bodies. Radiation impact can be minimized with fly-by's but is hard to avoid in orbit.

For a real-world example, NASA's upcoming mission to Europa will actually orbit Jupiter to avoid flying through Jupiter's powerful radiation belts.

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Pluto satellite would cost much more because it would need a lot of fuel for decelerating and orbit insertion. It is just the question of capabilities, building a flyby probe is much easier and cheaper.

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We (meaning the US) have (or had) dedicated orbiters around Mercury (MESSENGER), Venus (Magellan), the Moon (multiple), Mars (multiple), Jupiter (Galileo, Juno), and Saturn (Cassini). Where the cost in both time and money of getting an orbital spacecraft to the target is not unreasonable, we send orbiters.

Past Saturn, you start running into problems where it takes too much time or too much propellant to enter orbit, at least with current launchers and satellite propulsion systems, so we rely on flybys. Good news is that there's progress on that front, between SpaceX building their monster rocket and development of electrical propulsion drives like VASIMR. In another decade or two, you might be able to get a small orbiter to Pluto in just a little over the time it took New Horizons to get there.

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