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I have seen this question posed often recently, without a satisfactory answer in my opinion:

Why won’t New Horizons orbit Pluto?

The answers I read are always along this line:

  • too much fuel
  • too expensive
  • probe too large

I would like to know the approximate scale of these factors, for example:

  • how much more fuel would it take?
  • how much more would it cost?
  • how much larger would the probe need to be?
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This mission study came up with a 900 kg nuclear-electric-propulsion spacecraft launched on an Ariane V with a C3 of 100 and a Jupiter gravity assist along the way. 1.05 kW electrical power at Pluto from RTGs is required. That would be four "classic" NASA RTGs, or about nine MMRTGs. It has a 20 kg science payload. (New Horizons has ~30 kg of instruments, one classic RTG, with a launch mass of 478 kg.)

There are some optimistic assumptions (only 20% dry mass margin on most components?!), but this gives a general idea of the scale of such a mission. It is not out of the question, but it would definitely be in the flagship cost class, comparable to Cassini. Cassini has three classic RTGs, but uses chemical propulsion.

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  • $\begingroup$ Congress being Congress, there's no flagship-class program in the near future is there... Is there? ;) $\endgroup$ – Vedant Chandra Jul 11 '15 at 7:22
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    $\begingroup$ Right now there are actually two flagship planetary science missions in an early phase of development. They are Mars 2020 and Europa Clipper. $\endgroup$ – Mark Adler Jul 11 '15 at 15:38
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    $\begingroup$ A short-lived child satellite would be the worst of both worlds - adding complexity and mass (the solid rocket would need to be I think 80 times the mass of the payload?) without getting a significant increase in mission time. $\endgroup$ – Russell Borogove Jul 11 '15 at 15:53
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    $\begingroup$ Note that the proposed mission involves an 18-year flight, in comparison to New Horizons' nine. $\endgroup$ – Mark Jul 11 '15 at 20:51
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    $\begingroup$ Which is exactly what's wrong with TildalWave's answer. You would never go there as fast as New Horizons if you needed to go into orbit. You need to go there slowly to arrive slowly. $\endgroup$ – Mark Adler Jul 11 '15 at 22:00
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This is probably the easiest to answer if we take the same mission profile and just track from the Pluto flyby backwards. I'll make some rather broad assumptions and first order approximations, like that NASA had their NEXT ion thruster developed to the highest technology readiness level (they did exist when New Horizons launched, but not at TRL required to actually launch on a deep space voyage of discovery).

I'll also spare you with pesky details and just assume that if we can bring New Horizons close to a dead stop somewhere at Pluto and relative to it from ~ 13.79 km/s, that would do. Delta-v to insert into Pluto's orbit from then on won't be that much compared to everything else we need to change, part of the job would simply be falling towards it on its own gravity, then circularize at some desired altitude. I'll also assume that the orbital space is free of debris. Brace yourself, it's going to be a dirty, Mos Eisley Spaceport type of a job;

  • Hew Horizons launch mass: ~ 478 kg
  • Mass of a single NEXT thruster: ~ 100 kg (guesstimating)
  • NEXT thruster power envelope: ~ 6.9 kW
  • 6.9 kW @ 16 years operation (~ 7.9 kW BoF) GPHS-RTG's fuel mass: 1,503.4 kg
  • 108 kW RTG's excess thermal power radiator mass: ~ 12,000 kg (est.)
  • Additional truss / structure mass: ~ 4,000 kg (est.)
  • Total dry mass: ~ 18,080 kg
  • NEXT thruster xenon reaction mass for Δv of 13.79 km/s @ Isp of 4,190 s: ~ 8,220 kg
  • Total wet mass: ~ 26,300 kg

So we got with propulsion that's great for the job but wasn't really available at the time, with 205.7 kg of plutonium for our RTG that nobody really had available at such quantity, and a few rather generously small estimates for the mass of additional systems and xenon fuel needed from probe's launch mass of 478 kg to 26,300 kg. And that's just the beginning of the story of our makeshift Pluto orbiter that now launches $(22,190\text{ kg} \times 13.79\text{ km/s} / 236\text{ mN})^{(1/2)}$ or about 6 years and 4 months sooner to get to Pluto at the same time as New Horizons will, assuming nothing breaks and our NEXT thruster's performance doesn't degrade during this additional time of continuous 236 mN thrust and consuming 6.9 kW of our RTG generated power.

Again, this is all generously underestimating the problem and throwing at it technology that wasn't yet ready, requiring parts and consumables that weren't available and would have to operate for over six years longer without degradation in performance (but I did account for Pu-238 decay rate with half-life of 87.7 years), i.e. if New Horizons was made in Mos Eisley Spaceport into an orbiter, it would be 55 times its launch mass as a flyby probe.

I'm not sure how or on what you could hurl such mass off the Earth and give it such a kick to still get there in time, but that's a different matter. Note that over six years longer mission also means that the Earth and Pluto don't align as nice and that comes with even more problems. But you now have an orbiter, and with enough power for the upcoming centuries to even transmit science and telemetry data back at a much faster rate. Oh, you might want a better transceiver antenna too, bigger and perhaps one that can gimbal independently of the orbiter itself, unlike the flyby version of New Horizons, so you can receive and transmit towards the Earth without having to take your eyes off the target of your observations. I hope Pluto is worth it, chances are that it you made it into its orbit, you'll be there a long, long time. ;)

Others will give you different estimates, depending on their choice of propulsion and power systems, but that's my view on the matter. I didn't read suggestions made in your link, because I wanted to see how that plays out on my own. This is now an updated edit with recalculation for required thrust time (not sure where I got those numbers I plugged in before, but they were suspiciously enough off to warrant a second look). Hope this helps.

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  • $\begingroup$ Try it with NSTAR instead of NEXT -- thruster and power controller 26kg, 2.3kW draw. $\endgroup$ – Russell Borogove Jul 11 '15 at 3:38
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    $\begingroup$ It's not nearly that bad. You would not pick a New Horizons fast flyby trajectory if you planned to go into orbit. You would take longer to get there, and have a much lower arrival velocity. $\endgroup$ – Mark Adler Jul 11 '15 at 3:45
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    $\begingroup$ @MarkAdler All that is true, or course. But now that I recalculated to correct a glaring error on my part, it would actually be over 6 years longer mission, not just a year. That means that all its parts have to function for that much longer without a major glitch. Going there slower wouldn't solve that, but there might be fewer parts to fail. $\endgroup$ – TildalWave Jul 11 '15 at 3:51
  • $\begingroup$ The Voyagers have been working for almost 37 years. Just don't put a scan platform on it. $\endgroup$ – Mark Adler Jul 11 '15 at 3:54
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    $\begingroup$ @MarkAdler Heh yes, well a small fraction of what Voyagers used to be at launch at least. Most of their science, heaters,... are powered down by now. $\endgroup$ – TildalWave Jul 11 '15 at 3:57
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To bottom-line TildalWave's answer:

  • Matching speed with Pluto at the intercept requires a very large amount of ∆v, necessitating an ion thruster rather than a chemical rocket;
  • Ion thrusters draw a lot of electrical power;
  • Solar power is very hard to come by that far from the sun, necessitating a very heavy radioisotope-thermal generator rather than solar panels.
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  • $\begingroup$ ion thrusters are only capable of achieving large ∆v by applying very small thrust for immense periods of time. $\endgroup$ – Octopus Jul 17 '15 at 22:27
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    $\begingroup$ Are we in a hurry? $\endgroup$ – Russell Borogove Jul 17 '15 at 23:07
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    $\begingroup$ Well the mission will be much longer than 9 years if it's what you're trying to achieve. Plus, what effect will the large outer planets have on the vehicle's trajectory if you are trying to slow down to orbit speed at that range? $\endgroup$ – Octopus Jul 17 '15 at 23:08
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This Slashdot post contains a calculation of launch weight for an orbiter, using these constraints:

  • the same probe as New Horizons
  • the same flight time as New Horizons, transiting at the same speed as NH (11 km/s)

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.)

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    $\begingroup$ Using chemical rockets for the insertion burn would be silly. $\endgroup$ – Russell Borogove Jul 11 '15 at 15:50

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