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This dwarf planet reaches perihelion in the summer of 2076. At a distance of 76 au, is it feasible to launch a probe that would reach Sedna in time? With an orbital period of over 11,000 years, this may be humanity's (as we know it) only opportunity. Repeated internet searches have failed to reveal any nation or group making plans for this mission yet, but is this because of lack of vision, or is the task itself impossible?

It appears that Neptune may be in a position to help provide a gravity assist around 2056. If at all possible, such a mission should aim for an orbital capture and/or landing.

Besides the scientific goals & boasting rights, such a mission would also serve as an interstellar precursor. Any orbiter/lander, logically, would need to be designed to last as long as we can make it last, centuries if not millenia. Imagine having an interstellar beacon at the edge of our solar system!

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Great vision! But in that very long time frame, I think that one would rather wait a while and see if there are not any more easily accessible Sedna type objects to go to instead. I think its discovery is still too young to become a century mission plan. –  LocalFluff Feb 1 at 23:03
On a similar line of thought, you might want to wait 40 years for better propulsion and power systems, instead of launching one now to get there in 60 years. –  Mark Adler Feb 2 at 1:22
Sending one now should be feasible, even. It might be nice to have the before and during recorded by a probe. But funding would be a nightmare. –  aramis Feb 2 at 16:35
@LocalFluff: Why Sedna in particular? Check out the illustration of its orbit (the red line in the Celestia view above). If a probe can "hitch a ride," it will eventually be taken out to a distance of 937 AU (0.0148 ly). Also, because of the extremely long time frame involved, such a mission would provide an ideal engineering testbed for ultra-long-life equipment, which will be needed when humanity targets probes to other stellar systems. –  Jerard Puckett Feb 3 at 17:08
@MarkAdler: In my mind, designing a Sedna mission could very well serve as the impetus to develop better propulsion and power systems. –  Jerard Puckett Feb 3 at 17:14
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2 Answers

up vote 13 down vote accepted

First order analysis

Given that we have practical ion thrusters, it's time to look at them.

Deep Space 1

The DS1 probe massed 387kg, had 83kg of fuel, operated for 162 days, and generated 92mN. So, it generated about 0.2mm/s^2.

The craft is not tanks-dry, either. It has approximately 6 months (180 days) of fuel per design. That's a roughly 20% fuel design, and my estimate on the mass of the thruster itself is 10kg - about 0.01N per kg, and linearly scaling, with about 16x thruster mass in fuel per year. (These numbers are rough, but provide a baseline)

Powering a 2 kW thruster...

In the inner solar system, solar power is viable for an electric thruster; out past the asteroids, it becomes pretty much non-viable.

Radio-thermal generators, likewise, are measured in kilograms per watt... one of the most efficient was on the voyagers, at around 40W of electricity out per kilogram... to get a reasonable 0.2mm/s^2 acceleration, they become impractical.

Which pushes us into the range of nuclear fission reactors. Which also means large masses - the SNAP-10A was 290kg and 30kW.

Into the hypothesizing

We need a multi-ton spacecraft. There is a design for a 100kW Electrical output, ~520kg nuclear reactor. This would be adequate to power 50-some NSTAR units at 91mN each; assuming only 20 such units, and 80kg each per 6 months in reaction mass, and 10kg each, plus a 200kg science payload, we can get a good first order hypothesis. I will assume for now a 5 year plant duration, since the SAFE400 has been in testing for several years, and I cannot find documentation for its fuel use.

  kg    kW   Item
 200    40   NSTAR x20, giving 2N
 520  (100)  SAFE-400 400kW/100kWe nuclear reactor.
6400     0   2 years NSTAR fuel for 20 units.
 200    10   science package comparable to a mars orbiter.
7400    --   mission mass.

This would give a mission thrust at launch of 0.00027m/s^2. Almost directly comparable to DS1... and a 720 day thrust, using a turn and flip, is roughly 3.4 AU covered, and peak speed of 8.3km/s, or 17861396s per AU or about 209 days per AU ... and 71 AU to cover. This would mean about 41 additional years.

However, the actual acceleration would increase over the mission, and the mass of fuel being the largest proportion, we can use the average mass of around 4000kg for figuring overall - nearly doubling the engine-off speed, and cutting the coast time to about 20 years. The remaining issues are fuel for the power plant, which I lack the data to calculate.

A larger fuel mass could be used, increasing duration, but decreasing initial acceleration. A 4 year fuel duration, for example,

   kg    kW   Item
  200    40   NSTAR x20, giving 2N
  520  (100)  SAFE-400 400kW/100kWe nuclear reactor.
12800     0   2 years NSTAR fuel for 20 units.
  200    10   science package comparable to a mars orbiter.
21800    --   mission mass. (probably about 1050kg tanks dry)

Initial would be about 0.00009m/s^2, with a peak of about 0.0019m/s^2, and an average of about 0.001m/s^2... and would cover about 51 AU under thrust, and peak velocity of about 62km/s... or about 28 days per AU, for about 2 years coasting time.

This would put a rough mission travel time on the order of 6 years, and about 1/2 of it thrusting outbound, 1/3 coasting, and 1/6 decelerating into orbit.

Unfortunately, the technologies are not all fully proven. By not fully proven, I mean (1) we don't know that they actually will survive a 4-year constant "burn"... tho' we know they will last at least 160 days, and (2) the fission system hasn't been in existence long enough to establish that it will in fact last the 4-10 years needed for a mission

Speculative answer

Yes, a first order analysis indicates it is plausible that a mission could be made, and with a flight time of under 10 years.

There are a number of vagaries, however, in the available data. Structural mass is simply estimated; fuel mass may be insufficient for the indicated duration, etc.

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For the first example, I get $\Delta v$ of $\approx \text{64.77 km/s}$ using quoted NSTAR $I_\text{sp} = \text{3,300 s}$ and Tsiolkovsky rocket equation $\Delta v = v_\text{e} \ln \frac {m_0} {m_1}$. At half $\Delta v$ turn, that's then at $\approx \text{32.386 km/s}$ with roughly $\text{27%}$ reaction mass left for deceleration, but that's not really necessary for a flyby. Calculating time is a bit more tricky tho, since it depends on $\Delta v$ achieved at launch, gravity assists, trajectory, and so on. Thrust by NSTAR's itself would not be enough to escape heliocentric velocity tho. –  TildalWave Feb 2 at 22:32
Great answer. Learned a lot from reading this! –  Stu Feb 3 at 14:54
@aramis: Excellent analysis. Question: you say "the technologies are not all fully proven." Can you gestimate the Technology Readiness Levels of the unproven technologies? –  Jerard Puckett Feb 3 at 17:21
@TidalWave: thanks for the delta v calculations. Regarding NSTAR thrust: sufficient to match the orbital speed of Sedna at close approach for a capture/landing mission? –  Jerard Puckett Feb 3 at 17:26
By not fully proven, I mean (1) we don't know that they actually will survive a 4-year constant "burn"... tho' we know they will last at least 160 days. (2) the fission system hasn't been in existence long enough to establish that it will in fact last the 4-10 years needed for a mission. –  aramis Feb 3 at 22:17
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Considering that Voyager 1 is already 126 AU from the Sun 36 years since launch, there should be no reason that it would not be possible energetically using a normal launch, small maneuvers, and planetary flybys. Just a Jupiter flyby should be sufficient. Jupiter will also provide the necessary change in inclination.

Designing a probe that is assured to operate for that long would however be quite difficult. (The Voyager spacecraft were not assured to work past Saturn.)

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But (correct me if i'm wrong) the Voyagers have been reporting in almost since launch. Could a probe to Sedna, in contrast, could do it the Rosetta way with long periods of hibernation? –  Everyone Feb 2 at 19:10
Yes the Voyagers have been working, but they weren't designed and tested to do that. To design and test something to live that long is hard. (By that I mean, I don't know how to do it.) –  Mark Adler Feb 2 at 23:17
Rosetta's hibernation was to save money on Earth in operations. It does very little in the way of extending the life of the hardware. –  Mark Adler Feb 2 at 23:18
@Everyone the wattage drop of thermocouples is crippling the voyagers already, and has crippled the pioneers, as well. Voyager has been unable to operate the full science package effectively for a decade, and is now at the point where it can barely operate the radio, and not at the same time as the science instruments. Mind you, the radiothermal pile is still cooking away - but the thermocouples needed to turn heat into electricity have corroded and no longer are efficient enough to work well. –  aramis Feb 3 at 22:22
@Everyone: if you use an RTG, it will degrade whether the power is used or not. The heat degrades the thermocouples, and there is no way for an RTG to regulate the amount of heat it produces. –  Hobbes Feb 4 at 15:30
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