[HN Gopher] Feasibility study of a mission to Sedna - Nuclear pr...
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Feasibility study of a mission to Sedna - Nuclear propulsion and
solar sailing
Author : speckx
Score : 157 points
Date : 2025-07-01 14:08 UTC (8 hours ago)
(HTM) web link (arxiv.org)
(TXT) w3m dump (arxiv.org)
| pavel_lishin wrote:
| > _Sedna is expected to pass through the perihelion of its orbit
| in 2075--2076 and then move again away from the Sun. Considering
| the distances involved, a mission targeting the object would need
| to be launched "relatively" soon, especially if using
| conventional propulsion systems, which could require up to 30
| years of deep-space travel._
|
| Sedna's perihelion is ~76 AU - more than twice as far as Pluto,
| which took New Horizons nearly a decade to reach.
|
| Sedna's apehelion is over _500_ AU.
|
| > _The Direct Fusion Drive rocket engine is under development at
| Princeton University Plasma Physics Laboratory_
|
| Is it ... is it actually working? How close are they? And even if
| they get it to work next year, will it be something well-
| engineered & reliable enough to send it into space for 10 years
| and expect it to work?
| pfdietz wrote:
| It's a scheme based on rotating magnetic field drive (RMF) of
| field reversed configurations. The _claim_ is that they can
| preferentially accelerate and recover energy from 3He ions,
| greatly reducing DD fusion and associated neutrons. I question
| the recovery part (how is the entropy that is introduced by
| ion-ion collisions removed?), but do not have the expertise to
| fully evaluate the claim.
|
| In any case, it certainly cannot be ready next year, and would
| require large amounts of 3He.
| nicktelford wrote:
| There's also Pulsar Fusion, a UK company currently building a
| Dual Direct Fusion Drive (DDFD). They claim:
|
| > Modelling shows that this technology can potentially propel a
| spacecraft with a mass of about 1,000 kg (2,200 lb) to Pluto in
| 4 years.
|
| They're apparently targeting an in-orbit test in 2027. Even if
| this were to slip to 2030, and becomes commercially available
| in 2040, I expect that would be plenty of time for a rendezvous
| with Sedna's perihelion
| moffkalast wrote:
| When it comes to the UK space industry all I can think of is
| Skylon and Reaction Engines Ltd. Or more how they spent 20
| years working on an engine that never left the ground until
| going bankrupt.
|
| Hopefully this time round it goes a bit better than that.
| ortusdux wrote:
| I really wanted that thing to fly. Anyone know the fate of
| the IP?
| nicktelford wrote:
| Yeah, the British space industry has struggled; principally
| with investment. Reaction Engines largely went under
| because they ran out of money and their investors declined
| to put more money in.
|
| My hope with Pulsar Fusion is that their existing thruster
| business provides the necessary revenue to both keep them
| solvent, and attract continued investment, until they're
| able to get their Fusion Drive off the ground.
| JumpCrisscross wrote:
| I remember when Reaction turned down relocating to
| America in favour of some minor support from London. It
| was around 2014 and we all figured it was D. O. A.
| PaulHoule wrote:
| Frankly it seemed like an idea that made no sense for
| multiple reasons. For one thing the density of
| atmospheric oxygen is a fraction of the density of liquid
| oxygen so it's hard to picture getting enough oxygen in
| the thing to make a difference. If you're liquifying it
| you're going to slow your rocket down by bringing O2 as
| well as 4 times as much N2 on board, then there is the
| weight of the liquification plant. Investing in Skylon is
| like investing in cold fusion.
|
| It was bad enough that Richard Branson discredited
| private orbital spaceflight with the overly long
| development process for a vehicle that made the Space
| Shuttle look like a paragon of safety and low costs --
| Skylon was so much worse.
| pfdietz wrote:
| Henry Spencer on air breathing launchers (New Scientist,
| 2009):
|
| https://www.newscientist.com/blogs/shortsharpscience/2009
| /03...
|
| 'Trying to build a spaceship by making aeroplane fly
| faster and higher is like trying to build an aeroplane by
| making locomotives faster and lighter - with a lot of
| effort, perhaps you could get something that more or less
| works, but it really isn't the right way to proceed. The
| problems are fundamentally different, and so are the best
| solutions.
|
| As Mitch Burnside Clapp, former US Air Force test pilot
| and designer of innovative launcher concepts, once
| commented: "Air breathing is a privilege that should be
| reserved for the crew".'
| JumpCrisscross wrote:
| > _How close are they?_
|
| Not very. That said, DFD is a technology with tremendous
| moonshot potential.
|
| Fusion propulsion is inherently easier than fusion power on
| Earth because you don't have to worry about converting heat to
| electricity and the breakeven threshold is far lower; depending
| on the mission, even Q < 1 could be fine.
| sigmoid10 wrote:
| "Easier" in this context is still ridiculously hard. Fusion
| rocket designs were first seriously researched 50 years ago
| and not a single one of the countless designs proposed since
| then has reached readiness for in-space use.
| PaulHoule wrote:
| Note the economics might be better than for terrestrial
| fusion energy because you're not paying for watts you're
| paying for thrust and something like D-He3 has a great
| exhaust velocity.
| JumpCrisscross wrote:
| > _" Easier" in this context is still ridiculously hard_
|
| Absolutely. I've just noticed that a lot of people think,
| correctly, that fusion power is hard and space is hard so
| doing them together is stupidly difficult. Not so in this
| application--the relaxation of requirements on fusion
| outweigh the difficulties of doing it in space.
|
| Put another way, the dollars going into fusion power might
| be better spent on DFD.
| imglorp wrote:
| And tragically, nuclear propulsion at NASA has been
| aggressively singled out for the axe so humanity will be
| counting on more advanced countries to finish that research.
|
| Was that the fossil fuel lobby's doing?
| ajford wrote:
| I always figured it was from Nuclear pearl-clutching and
| genuine fear about launch disasters. Especially after the
| various Apollo and shuttle disasters.
|
| Though with how SpaceX has been blowing up rockets left and
| right, probably a good idea to not have nuclear materials
| launching until that's been resolved entirely.
|
| Boca Chica beach is a mess now, I can only imagine what new
| Fallout installment we'd get if South Texas became irradiated
| from a failed launch.
| perihelions wrote:
| > _" probably a good idea to not have nuclear materials
| launching until that's been resolved entirely"_
|
| This isn't an issue at all: fission reactors aren't
| hazardous until after they first start up (go critical),
| which in the space electric-propulsion context means after
| (if) they've successfully launched, and are no longer in
| the vicinity of Earth.
|
| At any rate, China is apparently[0] moving in this
| direction, regardless of what the US does.
|
| [0] https://www.scmp.com/news/china/science/article/3255889
| /star... ( _" Starship rival: Chinese scientists build
| prototype engine for nuclear-powered spaceship to Mars"_
| (2024)) (mirror: https://archive.is/sGUJr )
| GolfPopper wrote:
| > _fission reactors aren 't hazardous until after they
| first start up (go critical)_
|
| This is only true if the fission reactor's fuel isn't
| scattered over square kilometers after a launch failure.
| perihelions wrote:
| It's not radioactive enough to matter.
| Symmetry wrote:
| Generally the sort of lightweight reactors NASA is
| looking at for space power use highly enriched uranium.
| U234 isn't particularly radioactive (it's lasted since
| the Earth was formed) and far less toxic than the
| hydrazine propellant our ships carry but it's a
| significant proliferation risk if it should all into the
| wrong hands.
|
| But yeah, it's not dangerous like the P238 in a
| radioisotope thermal generator (RTG). To put off enough
| heat to power a spacecraft just through natural decay you
| need something ferociously radioactive.
| XorNot wrote:
| SpaceX let rockets explode because they're using chemical
| propellants and the consequences of that are low provided
| no one gets hit by debris.
|
| It's bizarre to suggest that the same strategy would be
| used with nuclear materials onboard. Developing the "can
| not fail" rocket is the sort of thing NASA does well, and
| kind of highlights how we've squandered them.
| wombatpm wrote:
| So 75-76 for closest approach. How far away will it be in 2100?
| Given that orbit size I think we have some slack in the launch
| date.
| chuckledog wrote:
| https://en.m.wikipedia.org/wiki/Sedna_(dwarf_planet)
| imglorp wrote:
| They know its radius is ~1000km but interestingly, there is no
| way to determine its mass without a flyby or other
| gravitational interaction. I guess you could swag it by using
| the lunar density, which gives ~~~ 10^22 kg.
|
| https://www.wolframalpha.com/input?i=4.189%C3%9710%5E9+km%5E...
| _joel wrote:
| Wouldn't it's composition be of more ice and rock (like
| pluto), therefore lower density than the moon?
| sapiogram wrote:
| That's a reasonable assumption, but given Sedna's unusual
| orbit, its origin could also be quite different from
| Pluto's.
| benbayard wrote:
| In the wikipedia post you are replying to has the
| chemical composition of the surface of the planet,
| obviously we can't know what is beneath that, but to me,
| indicates this is closer to Pluto than it is to our Moon.
|
| > Detailed spectroscopic analysis has revealed Sedna's
| surface to be a mixture of the solid ices of water
| (H2O),[15] carbon dioxide (CO2), and ethane (C2H6), along
| with occasional sedimentary deposits of methane
| (CH4)-derived,[16] vividly reddish-colored organic
| tholins,[15] a surface chemical makeup somewhat similar
| to those of other trans-Neptunian objects.[17]
| myrmidon wrote:
| This direct fusion drive is a really interesting concept. Maybe
| something like this could be used for interstellar travel in a
| century (or five), it is very encouraging that there is active
| research on it. ~5kg of thrust is not a lot, but over time...
|
| This sounds significantly more feasible than nuclear pulse
| propulsion ("project orion" style) which I used to think was the
| _only_ feasible approach to get to another star.
|
| One thing that was unclear from the paper to me: How does the
| fusion drive "pick" D/He3 fusion over D/D? Can this be "forced"
| by just cranking the plasma temperature way up? Or do you still
| just have to deal with a bunch of neutrons from undesired D/D
| fusion?
| floxy wrote:
| Roundtrip Interstellar Travel Using Laser-Pushed Lightsails
|
| https://ia800108.us.archive.org/view_archive.php?archive=/24...
| myrmidon wrote:
| This is very interesting. Apparently beam collimation is much
| less of a show-stopper than I would have assumed.
|
| But I don't see us putting a a 1000 kilometer lens into orbit
| anytime soon, and that multi-terawatt (sustained!) laser
| system sounds like a bit of a headache, too...
| cnity wrote:
| See: https://en.wikipedia.org/wiki/Breakthrough_Starshot
| pavel_lishin wrote:
| > _This paper discusses the use of solar system-based lasers
| to push large lightsail spacecraft over interstellar
| distances. The laser power system uses a 1000-km-diam.
| lightweight Fresnel zone lens that is capable of focusing
| laser light over interstellar distances. A one-way
| interstellar flyby probe mission uses a 1000 kg (1-metric-
| ton), 3.6-km-diam. lightsail accelerated at 0.36 m /s2 by a
| 65-GW laser system to 11% of the speed of light (0.11 c),
| flying by a Centauri after 40 years of travel. A rendezvous
| mission uses a 71-metric-ton, 30-km diam. payload sail
| surrounded by a 710-metric-ton, ring-shaped decelerator sail
| with a 100-km outer diam. The two are launched together at an
| acceleration of 0.05 m/s2 by a 7.2-TW laser system until they
| reach a coast velocity of 0.21 c. As they approach a
| Centauri, the inner payload sail detaches from the ring sail
| and turns its reflective surface to face the ring sail. A
| 26-TW laser beam from the solar system, focused by the
| Fresnel lens, strikes the heavier ring sail, accelerating it
| past a Centauri. The curved surface of the ring sail focuses
| the laser light back onto the payload sail, slowing it to a
| halt in the a Centauri system after a mission time of 41
| years. The third mission uses a three-stage sail for a
| roundtrip manned exploration of e Eridani at 10.8 light years
| distance._
|
| Very cool.
| foobiekr wrote:
| The Mote in God's Eye
|
| I guess this will be the Niven-Pournelle thread.
| topynate wrote:
| The easiest way (perhaps the only practical way) to favour the
| aneutronic reaction is to run a helium-rich mixture. The trade-
| off is lower power density.
| MadnessASAP wrote:
| > This sounds significantly more feasible than nuclear pulse
| propulsion ("project orion" style) which I used to think was
| the only feasible approach to get to another star.
|
| I still carry a torch for project Orion, it's impossible to not
| love.
|
| * Feasible 50 years ago, not 50 years from now.
|
| * No ultra lightweight fancy space age materials, steel and
| lots of it.
|
| * Seriously, lots of it, let's launch a battleship to to Mars,
|
| * or Jupiter,
|
| * or Alpha Centauri.
|
| * Gives everyone something way better to do with all those
| nuclear bombs they have laying around.
| randallsquared wrote:
| > _Gives everyone something way better to do with all those
| nuclear bombs_
|
| The counterpoint there is it gives lots of reasons to make so
| many more, increasing proliferation worries.
|
| However, there's an SF novel that just came out that features
| nuclear pulse: Fenrir, by Ryk Spoor and (posthumously) Eric
| Flint. I enjoyed it.
| pavel_lishin wrote:
| I should re-read Footfall, by Larry Niven. Quite a few
| banger lines in there.
| eszed wrote:
| My favorite SF along those lines is _King David 's
| Spaceship_, by Jerry Pournelle.
|
| https://en.m.wikipedia.org/wiki/King_David%27s_Spaceship
| MadnessASAP wrote:
| Yeah, if I'm being really honest, I don't want to give
| anyone an excuse to put a 1000+ nuclear bombs in orbit.
| Plus the few dozen you'd have to detonate in quick
| succession to even get it above the karman line.
| PaulHoule wrote:
| The electron beam ignition they talked about doesn't work.
| Heavy ion probably does
|
| https://en.wikipedia.org/wiki/Heavy_ion_fusion
|
| but the accelerator needs like 100 barrels that are each 1
| km. Maybe you can build a generation starship with that but
| whatever it is it's going to be big.
| hermitcrab wrote:
| I once spoke to Freeman Dyson at a book signing and asked him
| if Orion would work. He said he thought it would. And I asked
| him if it should be launched. He said probably not (IIRC due
| to the amount of radiation that would be put into the
| atmosphere).
| jerf wrote:
| It is almost the epitome of steampunk romance. Launch an
| entire mid-20th century city and economy into space! And it
| might even work!
|
| But, yeah, you probably don't want to be launching these
| routinely. People generally _badly_ underestimate the
| number of nuclear explosions that have been set off on
| Earth and overestimate the badness of nuclear explosions.
| Putting one or two of these into orbit might be
| justifiable. It 's certainly not a bad emergency plan to
| have in your pocket in case of emergencies. But you still
| certainly wouldn't want an entire industry routinely
| lighting these things off.
|
| Still... the romance of it all...!
| hermitcrab wrote:
| He also made the interesting point that pretty much every
| big engineering project kills people.
| voxleone wrote:
| Project Orion was the promise of my youth [70/80s]. It speaks
| to both the technological courage and the philosophical
| optimism that once characterized space exploration -- and how
| that momentum seems to have faded. By all accounts, it was
| technically feasible. And yet...
|
| Of course there was 'the shadow of the Bomb'. From bold, almost
| reckless experimentation (Mercury, Gemini, early Apollo, things
| shifted to safety-optimized, cost-constrained engineering. And
| there was Cost and Politics; the post-Apollo world didn't want
| to colonize the solar system. It wanted low Earth orbit, and
| safe returns. Budgets followed.
|
| Kinda sad.
| accrual wrote:
| Very fascinating mission idea. Given how Sedna reaches so far
| away (>500AU), I wonder if the flyby would also reveal some
| details about conditions that distant. Maybe the surface contains
| some unexpected molecules that could shed light on its origin and
| what it's like that far out.
| pier25 wrote:
| > relatively soon
|
| If the DFD takes 10 years to get there it means it would need to
| be launched in 40 years. That's quite a timeline.
|
| Amazing that an organization can keep budgeting and planning for
| such a long project.
| ananddtyagi wrote:
| Sound like something out of 3 Body Problem
| mikewarot wrote:
| Already in orbit is OTP-2, which has 2 novel drive systems, one
| based on non-Newtonian thrusters, and the other based on an ION
| drive.[1]
|
| Edit: The latter is "Fusion enhanced"[3] The
| company's the FireStar Drive uses is a water-fueled pulsed plasma
| thruster that uses a form of aneutronic nuclear fusion to boost
| its performance.
|
| I watch the orbital observations closely to see if any altitude
| is being gained.[2] This is their second satellite in orbit, the
| first one had high voltage power supply issues so they never got
| to try the thruster.
|
| [1] https://www.nanosats.eu/sat/otp-2
|
| [2] https://celestrak.org/NORAD/elements/graph-orbit-
| data.php?CA...
|
| [3]
| https://www.aerospacetestinginternational.com/news/space/roc...
| WaxProlix wrote:
| Already in orbit around Earth, notably. Not Sedna.
| mikewarot wrote:
| Non-Newtonian drives have to prove they work outside the
| influences of a laboratory, if they work in low earth orbit,
| they should work anywhere. The Semi-Major Axis Altitude
| (SMAA) is a great proxy for orbital energy, and if they can
| make that number go way up, we should all take note, and
| start looking for new physics.
| lgats wrote:
| *IVO - Quantum Drive Propellantless Thruster - *The objective
| of the IVO Quantum Drive is to test the system in the LEO
| environment and qualify the drive's ability to provide thrust
| utilizing proprietary quantum technology with no required
| propellant. Estimated Thrust: 1.75mN.
| VonTum wrote:
| Why does this give me EM-Drive vibes? Haven't we established
| that some kind of propellant is required for conservation of
| momentum?
| mikewarot wrote:
| I was surprised there were no references to past nuclear
| (fission) efforts, including a long test (more than 12.5 minutes)
| at 4000 megawatts of Pheobus 2A.[1]
|
| Perhaps there are some solid or non-cryogenic liquid fuels that
| could take place of the liquid hydrogen and make fission based
| systems far more feasible in the near term.
|
| [1] https://en.wikipedia.org/wiki/Project_Rover#Phoebus
| fusionadvocate wrote:
| Based on my experiences with Kerbal Space Program, this object
| seem to be almost being pushed off from solar orbit. Given its
| 'small' size, how much energy would be required to push it off
| the solar system?
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