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The Snowbank Orbit, Redux

We haven’t yet found Planet Nine, but the evidence for its existence is solid enough that we can start thinking about its possibilities as a mission target. That work falls in this essay to Adam Crowl, a Centauri Dreams regular whose comments on articles here began not long after I started the site. An active member of the Project Icarus attempt to re-design the 1970s Project Daedalus starship, Adam is also the author of Crowlspace, where his insights are a frequently consulted resource. Today he harkens back to a 1960s science fiction story that has given him notions about a way not only to reach Planet Nine but to establish orbit around it.

by Adam Crowl


Fritz Leiber is better known for his fantasy and SF-fantasy, but he could write hard-SF too. A fine example is his 1962 story, “The Snowbank Orbit”, the title of which alludes to World War II tales of pilots surviving bailouts without parachutes by plunging into snow-drifts. Five spacecraft, racing towards Uranus at 100 miles per second with empty tanks, intend a fiery plunge through the planet’s atmosphere to brake into orbit. The rest of that story I will leave to the interested reader [available here] but the idea of aerobraking into orbit around a distant Planet Nine is worth discussing.

Presently we know very little about Telisto – the mellifluous name suggested for Planet Nine by physicist Lorenzo Iorio [1] which I’ll use for convenience. Brown & Batygin [2] suggest an orbit averaging about 700 AU and a mass of at least 10 Earth masses. The mass could be somewhat higher, though certainly not of the order of a Saturn-mass as its infra-red glow would’ve been seen by earlier surveys. Modelling [3] suggests a 10 Earth-mass planet, with a substantial hydrogen-helium envelope, could be as ‘warm’ as 50 K – about 40 degrees warmer than the ~10 K from sunlight alone. A range of compositions were modelled. A Super-Earth, an Ice-Giant (like Uranus/Neptune) and a miniature version of Jupiter/Saturn are all possible. Some cosmogonic simulations [4] suggest a Neptune like object is likely to have been flung from amongst the other giant planets during their formation, so it seems the most likely option.

A Neptune-like Telisto would then be an ice-wrapped rocky core wrapped in a layer of captured hydrogen/helium mixture. It’s likely that hydrogen will be depleted from its atmosphere by some fraction being chemically bound and mixed with its core, so helium will be a higher fraction of the atmosphere, as appears to be the case for Neptune. If the atmosphere is a small fraction of Telisto’s mass, then it’s possible it will have an icy surface or even a liquid water ocean under a hydrogen atmosphere via its greenhouse effect trapping the planet’s internal heat. In that case Telisto will be very interesting from an astrobiological perspective, though the energy sources available to sustain life are impossible to quantify at present.

Telisto, at 700 AU, would be in interstellar space, well beyond the moving boundary of the Sun’s magnetosphere, so its intrinsic magnetic field would dominate over a vast volume of space. The raw flux of impinging cosmic rays might allow enhanced creation and trapping of antimatter, as suggested occurs around the planet Saturn and the Earth. Any moons of Telisto would also provide a ready source of materials, if we chose to build starships there.


Image: Artist’s impression of Planet Nine as an ice giant eclipsing the central Milky Way, with a star-like Sun in the distance. Neptune’s orbit is shown as a small ellipse around the Sun. The sky view and appearance are based on the conjectures of its co-proposer, Mike Brown. Credit: Tomruen, nagualdesign; background taken from File:ESO – Milky Way.jpg (Own work) CC BY-SA 4.0, via Wikimedia Commons.

Deep Space Propulsion

Given sufficient motivation we’ll send a probe and eventually follow in person. Getting there will be a challenge. At the present 3.5 AU/year of “Voyager 1” the journey would take 200 years. Leiber’s 100 miles per second would get a probe there in 20 years, which might be acceptable if the probe has a compelling secondary mission it can pursue during the long cruise phase. Long baseline telescopic observations might be sufficiently attractive to combine the two. A flyby at 100 miles per second is probably too quick to provide sufficient science return for the investment, so stopping will be required.

Conventional propulsion, such as nuclear powered ion drives, are unlikely to be up to the task. In 1987 the Thousand AU (TAU) probe to was studied as a first interstellar mission [6]. The eventual design chosen used a nuclear reactor that was technically not far removed from the SNAP reactors that had been tested in the 1960s. The ion-drive would run for a decade and the probe would take 50 years to reach 1,000 AU – without stopping. Ion drives have improved significantly since then and could bring the mission time down to 20 years. The chief performance limitation is power supply. Fissioning a kilogram of uranium produces about 90 trillion joules of energy, but the rate at which it can be released is limited by the maximum temperature at which the reactor can operate. Typically a power reactor runs at less than the melting point of the fuel elements and its components, especially when required to operate reliably for years at a time. Then waste heat has to be ejected into space, which requires heavy radiators. Minimising radiator size means the reactor’s power production cycle must convert raw heat into power at less than 25% efficiency, so 75% of the energy of fissioning uranium has to be dumped to space. Due to these limitations solid-core power reactors can supply power with a specific power of at most 50 to 100 watts per kilogram (W/kg) of reactor power-system.

To reach 700 AU in 20 years requires increasing the vehicle’s kinetic energy at the rate of about 470 watts per kilogram of vehicle. If the reactor power-system is a hefty 75% of the vehicle’s mass, then it must supply power at over 600 W/kg of total vehicle mass. Advanced ion drives typically can convert raw electricity into kinetic energy with an efficiency of between 75%-85%, so the total power supply from the reactor needs to be over 700 W/kg. No solid core reactor can run hot enough to achieve this. Reactors, in theory, can run hotter – much hotter. Liquid, gas or plasma core reactors have been researched, but require several decades of development to bring to operational readiness. As yet theoretical, Fission fragment reactors might also push performance beyond this level, though with similar development times.

An incredible nuclear fusion power source already exists in space – the Sun. Two torrents of momentum and energy stream out from the Sun, in the form of photons and the Solar-Wind, with a total power of 400 trillion trillion watts. Tapping just a tiny fraction of that torrent would allow a quick trip to Telisto – and the stars beyond. Doing so is the challenge. Solar-sail propulsion taps the photon torrent and is the option being vigorously tested by the Planetary Society, NASA and JAXA. Another option is the Electric-Sail, or E-Sail, which uses a multitude of long, thin wires that are charged so they reflect the charged particles of the Solar-Wind. E-Sails are being developed by a Finnish team led by Pekka Janhunen [5], with some NASA involvement.

A less well-developed option is the Magnetic-Sail (Mag-Sail), which uses a loop of superconducting wire to form a miniature magnetosphere to ride the Solar-Wind. Unfortunately the need for cooling systems for the superconducting wire makes the Magnetic-Sail less attractive for operation near the Sun. The Solar Wind itself is quite turbulent magnetically on the size-and-time scales relevant to interplanetary Mag-Sail applications, so considerable applied research into Mag-Sailing the Solar-Wind is needed before it can be used with confidence.

All Sail types will require a probe to closely approach the Sun to intercept sufficient photons or Solar-Wind (mostly protons and alpha particles). If the probe were to drop to 0.2 AU – half the orbit of Mercury – it would need to be pushed outwards with a force about 7 times greater than the Sun’s gravitational attraction at that distance to reach a final speed of 35 AU per year (i.e. 100 miles per second.) For a total mass of 500 kilograms the probe would need an E-Sail about 48 km across or a Solar-Sail about 700 metres across. Increasing the total mass will require proportionally larger sails of both kinds, though the exact final mass will depend on power sources and payloads chosen.

The Challenge of Deceleration


After cruising for 20 years, the probe then needs to stop, a non-trivial task. Telisto certainly isn’t radiating enough photons to slow a Solar-Sail, but the Interstellar Medium that it is embedded in might provide some drag for an E-Sail. Past the orbit of Saturn, the Mag-Sail’s superconducting wires would no longer need active cooling and the Interstellar Medium (ISM) is a calmer medium for Mag-Sailing. Thus a combination of E-Sail and Mag-Sail can be used. A Mag-Sail could form the outer ring to support the E-Sail and would only be powered up once the ambient temperature was low enough. For years there have been hints of high temperature superconductivity materials, so there might be a breakthrough at any time which would allow a purely Mag-Sail system, but it’s unnecessary at present. Before Aero-Capture at Telisto – the probe’s “Snowbank Orbit” maneuver – the Mag-Sail/E-Sail will need to be packed away or detached.

Image: Fritz Leiber’s “The Snowbank Orbit” involved aerobraking in the atmosphere of Uranus. Can we adapt these methods to Planet Nine, achieving a stable orbit for our probe?

Some speed will need to be shed in the atmosphere, but how much? The fastest re-entry ever survived was by the Galileo mission’s Descent Probe in 1996, which re-entered at a speed over 47 kilometres per second, surviving more than 200 gees of peak deceleration. That probe’s Thermal Protection System – an aerodynamic cone of material designed to ablate away absorbed re-entry heat – lost almost half its mass during its fiery plunge. An Orbiter doesn’t want to plunge all the way into Telisto, but shed sufficient speed to go into orbit. An initial capture can be a highly elliptical almost-escape orbit, with a short burn needed at its highest point (apoapsis) to raise the low-point (periapsis) of the orbit away from the planet into something more circular that doesn’t plunge back into the atmosphere.

A 10 Earth-mass Super-Earth Telisto with a relatively thin atmosphere – less than 0.1% of its mass – would need a capture speed of about 25 km/s. If a probe arrived at 50 km/s it would need to shed 75% of its kinetic energy to brake to 25 km/s. If the planet is rotating relatively quickly the relative arrival velocity will be reduced if the maneuver happens close to the equator – Galileo’s descent probe arrived at nearly 60 km/s, but Jupiter’s cloud tops rotate at 12.5 km/s, thus reducing the relative velocity to a more manageable 47.3 km/s. If Telisto were a mini-Neptune (mostly ices) or mini-Jupiter (mostly hydrogen/helium) then the planet will be significantly larger. Both compositions in this mass-range have been modelled. A Neptune-like Telisto would have a radius of about 4 times Earth, while a Jupiter-like Telisto would be more like 8 Earth radii. Required capture orbital velocity would be significantly lower – 18 km/s and 12 km/s respectively – but would have higher cloud-top rotation speeds for the same rotational period. A top re-entry speed of 50 km/s seems likely, but there is some research into magnetic braking in an outer atmosphere environment that may change that figure once matured.

A journey to Telisto is a stretch-goal for our aerospace technologies. Any suite of techniques that can place an orbiter there in 20 years, or less, is a breakthrough for all missions to the Outer Planets and beyond. In the wider historical context, we can liken Telisto to an over the horizon island that’s hinted at by flights of birds, a trick that ancient mariners used to guide them successfully to new lands. Star formation models suggest that the ISM is home to thousands of planet-like objects that formed from tidally disrupted proto-stars, a prospect strongly supported by microlensing data, though as yet only from the further reaches of the Galaxy. As microlensing surveys and techniques become more refined we will discover closer islands in the Dark Ocean between the stars. Getting beyond the noisy, obscuring Heliosphere will give us new means of detecting such worlds via their radio emissions and other disturbances of the ISM. Even long-range gravitational detectors could be deployed, away from perturbations from the flotsam and jetsam of the solar system. Colonizing Telisto’s likely system of moons would thus be the first outpost in a cosmic Ocean dotted by interstellar planets.

Reference Links

[1] http://adsabs.harvard.edu/abs/2015arXiv151205288I

[2] http://adsabs.harvard.edu/abs/2016AJ….151…22B

[3] http://adsabs.harvard.edu/abs/2016arXiv160207465L

[4] http://adsabs.harvard.edu/abs/2012ApJ…744L…3B

[5] http://www.electric-sailing.com/

[6] https://en.wikipedia.org/wiki/TAU_(spacecraft)


Comments on this entry are closed.

  • James Stilwell April 8, 2016, 10:55

    “A cosmic Ocean dotted by interstellar planets…”
    A depressing thought…That would imply first writing correct navigation charts in this arm of the galaxy…and that would imply centuries of work by artilects staffing roving outposts beyond the heliosphere to be completed before sending human crews in hibernation to the stars…if ever…five hundred years in hibernation seems a bit extreme…a five century flight managed by artilects also seems too hopeful…Obviously there has to be a way around the FTL barrier…

    • kamal ali April 8, 2016, 23:32

      to me it’s very hopeful : way points are good to have! Assuming no FTL, an unbroken 200 year journey is much less palatable than 4×50-year journeys , speeding by 3 interesting objects. Although, in reality, I doubt the density of wandering planets is enough to give us a non-negligible probability that one of them would be a short detour to any interstellar trip.
      In any case, I don’t see interstellar occurring until human lifetimes are significantly increased (and until we have self-repairing autonomous probes).

  • Andrew LePage April 8, 2016, 12:43

    I *loved* this piece on how to reach the purported “Planet Nine” (a question that has crossed my mind but I haven’t had a chance to address in any detail). However, I did recently address the question of the shortest practical transit times to the various planets given the physical limitations of most cargo and human passengers:


    If at some point in the future we have a propulsion system capable of prolonged high-g flight (e.g. the sort that would be used for interstellar mission like a high-thrust fusion propulsion system), human passengers could reach “Telisto” at 700 AU and slip into orbit using a 1-g ship in about 76 days. Short of some sort of FTL propulsion technology or other scientific breakthrough, “Planet Nine” will be a difficult target to reach even with propulsion technology far more advanced than what we have today.

    • kamal ali April 8, 2016, 23:36

      Andrew, I had been hoping for a long time that we had an intermediate target such as this O(10^2) to 10^3 AU.

      It offers some reason to develop our propulsion systems to 10x and 100x of what they currently are at. Interstellar is O(10^5) AU : too many orders of magnitude.

  • WLM April 8, 2016, 13:32

    This looks like something a fission-fragment rocket would be a good match for. At say 1% C it would get there in a little over a year.

  • Tom Mazanec April 8, 2016, 13:39
  • Michael April 8, 2016, 14:12

    ‘but the idea of aerobraking into orbit around a distant Planet Nine is worth discussing.’

    This will be tough at 100 miles/sec the Jupiter probe impacted at around ~50 km/sec and lost a lot of the heat shield! getting the sweet narrow atmospheric zone, cryogenically cold, on an object we know little about would be a tall order requiring a semi-powerful on-board local control system.

    ‘Leiber’s 100 miles per second would get a probe there in 20 years, which might be acceptable if the probe has a compelling secondary mission it can pursue during the long cruise phase.’

    Maybe two or three probes could be used to form a higher resolution platform and be used as a gravity wave detector as well, and it should go through the solar focus point as well.

    ‘As yet theoretical, Fission fragment reactors might also push performance beyond this level, though with similar development times.’

    Pu 238 ionised and sprayed thinly with an ion gun onto a sail would give tremendous ISP, a lump of it could act as a more localised power source. Give it a stand off magnetic nozzle (doubles up as a magsail) behind the sail and we are laughing.

    ‘Unfortunately the need for cooling systems for the superconducting wire makes the Magnetic-Sail less attractive for operation near the Sun.’

    A small cooling gas (helium) can be circulated or expelled (not needed that greatly) in the superconducting wire, by shallow angling the ‘ribbon’ the wire and with low contact stand off reflectors it can approach the sun quite very closely no problem.

    ‘For years there have been hints of high temperature superconductivity materials’

    No need the ones we have are fine ;)

    ‘Before Aero-Capture at Telisto – the probe’s “Snowbank Orbit” maneuver – the Mag-Sail/E-Sail will need to be packed away or detached.’

    Magsails are very, very easy to deploy, and there is a neat trick to boost their area of influence.

    ‘A top re-entry speed of 50 km/s seems likely, but there is some research into magnetic braking in an outer atmosphere environment that may change that figure once matured.’

    If an ionised gas is released inside a magsail it could act as a large drag chute in the planets magnetic field and thin upper atmosphere, there is also the possibility of the magnetic field of the planet cutting the coil to generate power (generating its own magnetic field) and slow it.

    • Ashley Baldwin April 9, 2016, 17:50

      Thanks Michael . Great as ever. A poser for you . Thinking “conventional” with what is currently available or at most within the decade or so to allow for planning How quick to Telisto if it’s approaching aphelion at say 1000 AU ?
      Assume maximum power from a Block II SLS , a maximal Earth and Jupiter gravity assist with a plus 35 KW enhanced solar cell array powered triple NEXT ion drive engine acceleration out to a maximum usable distance of say 4 AU ( discussed concept from a 2007 paper on NEXT and its potential for outer solar system missions ) . What sort of velocity could be attained and could sufficient braking propellant be carried to allow “TOI” , with or without aero capture for a maximum five tonne Probe ? ( aero braking not yet done obviously but theoretically achievable so allowed given the modest degree of technological extrapolation assumed with the selection of the Block II SLS )

      • Michael April 10, 2016, 3:13

        @Ashley Baldwin April 9, 2016 at 17:50

        Thanks Ashley, a nice easy one ;) there are a lot of unknowns here so mathematically speaking if it were an equation it would get messy.

        ‘Thinking “conventional” with what is currently available or at most within the decade or so to allow for planning How quick to Telisto if it’s approaching aphelion at say 1000 AU ?’

        Currently conventionally in no time at all, it would simply cost too much for the science return/timescale and shelved, new technologies will be needed.

        Apart from that there is also the time versus energy decision, conventionally with chemical engines we could not hope to achieve a 100 miles/sec velocity so we will have to use as many tricks as ever to reduce the energy need but that increases the time of flight.

        There are also a few showstopper unknowns like its potential orbital inclination and aphelion position which could create a huge spanner in the works. If they are very high it would make gravity assistance much less effective and there may be none at all in the timeframe we are looking for.

        ‘Assume maximum power from a Block II SLS’

        130 tons to orbit makes for a very large nuclear powered rocket (near term tech) but that could be dangerous, good luck getting that one passed.

        ‘a maximal Earth (moon) and Jupiter gravity assist’

        At best a few km/s each more for Jupiter, but Jupiter can alter the inclination of the probe quite well which is very useful.

        ‘Jupiter gravity assist with a plus 35 KW enhanced solar cell array powered triple NEXT ion drive engine acceleration out to a maximum usable distance of say 4 AU ( discussed concept from a 2007 paper on NEXT and its potential for outer solar system missions ).’

        Do you have a link to the paper, I would like to have a look at it please?
        The issue with solar power is that inverse sqr effect on light, if we could have thin film reflectors to enhance the sunlight it would allow for a smaller probe.

        ‘What sort of velocity could be attained and could sufficient braking propellant be carried to allow “TOI” , with or without aero capture for a maximum five tonne Probe ?’

        I could not answer the question because there is a lot of unknowns and orbital mechanics is not my strong point.

        ‘(aero braking not yet done obviously but theoretically achievable so allowed given the modest degree of technological extrapolation assumed with the selection of the Block II SLS )’

        Aero braking has been done around Mars and around Venus for slowing down and orbital alterations, they can be complex as the atmospheric properties are quite changeable especially around Mars. Around a more massive world it would be even more complex as the mass of the planet pulls the blanket of gases more tightly around it leaving a very narrow window of aero braking opportunity.

        I am thinking along the lines of say using a heavy lift capacity rocket to drop a probe towards the Sun with a Venus aero brake direction change and then using solar light tacking techinics to drop closer and then a solar/mag/E sail or combination to accelerate outwards again.

    • J. Jason Wentworth April 12, 2016, 0:39

      The Pioneer Venus atmospheric probes were designed to withstand calculated entry deceleration forces of up to 565 g (for the three small probes rather than the large [sounder] probe, if memory serves). Hughes Aircraft, the prime contractor for the Pioneer Venus Multiprobe and Orbiter spacecraft, was only able to test the probes at up to 400 g on Earth, but all four probes survived the savage deceleration at Venus, so a Telisto (or Uranus) orbiter that utilized aerocapture could be built to survive such deceleration forces (and possibly more–the electronics of the Copperhead gun-launched guided projectile withstand a firing acceleration of 9,000 g!).

      • Michael April 12, 2016, 8:39

        I don’t think the deceleration is the problem, artillery shell electronics can handle 30 000g easily. It is the friction that ablates the material in contact with the atmosphere, 100 miles/sec is a serious amount of velocity and kinetic energy.

  • djlactin April 8, 2016, 14:50

    Let’s confirm it exists (i.e., FIND it) before planning missions!

    • Andrew LePage April 8, 2016, 18:22

      Who’s planning a mission??? Adam’s article is simply a healthy intellectual exercise to see what might be possible given the mix of technologies and techniques that are currently available or expected to be available in the near future. It addresses a question I have seen asked repeatedly in a range of forums across the web (including queries about whether New Horizons or the Voyagers could be diverted to investigate this purported planet – which of course they can not). Naturally any real planning won’t take place until this planet is located and characterized (assuming it exists, of course). In the mean time, considering how we could reach it in no way takes absolutely nothing away from the effort to find it.

  • Alex Tolley April 8, 2016, 15:28

    Fascinating article. At 700 AU, could a pilot FOCAL mission be tried?
    A fail seems like a good way to go, but the sail area suggested seems unlikely to achieved anytime soon. Would adding beaming help, and if so, what sort of facility would be needed that could carry out the mission.

    Miniaturization seems worth exploring to reduce power requirements. I saw that femto satellites are now being designed. If some useful instrumentation can be packaged into these devices, that would make this mission much more achievable and provide reconnaissance for a larger payload mission.

    Slowing down the satellite for aerobraking is going to be a challenge. Are there approaches that could be used that could manage this, e.g. a mesh parachute?

  • djlactin April 8, 2016, 15:30

    That said, I return to an idea that I have posted here before (as if anyone cares): nanoprobes. Here I take issue with the (approximate) statement “100 mi/s is too fast to do meaningful science.” Massive probes carrying fuel? The ideas are too ’21st century’.
    Instead of sending one probe and attempting to achieve orbit a few decades later by aerobraking through an atmosphere of unknown attributes, I suggest sending a fleet of tiny probes.
    The basic idea is to use a laser cannon to fire a large number of nanogram-sized probes at a substantial fraction of c. Moore’s law almost assures that within a decade or so, a nanogram-sized device will have remarkable computing power. I omit details of my idea, but a flotilla of intelligent probes could establish a powerful comm net and make detailed observations during flyby.
    A series of flotillas could be launched, cheaply, with each programmed remotely (after the fact) to follow up on the observations of the previous.
    Big heavy stuff could follow later. Maybe.

    • Michael April 9, 2016, 14:41

      The annoying thing about nanoprobes is that they are small and radiation will have larger impact on the electronics as there is less physical protection for them.

    • Eniac April 9, 2016, 20:09

      Also, there isn’t really such thing as a nano-antenna, nano-laser, or nano-camera. All these things need to be of a certain size compared to the wavelength they are dealing with. Worse, as both power and gain decrease with size, communication range decreases rapidly. By a power of 3 with size, at least, would be my guess. Similarly, there are not a lot of good propulsion options as you scale down far enough.

      I do like the idea of small probes, but I think the practical limit is a penny-sized, extremely thin chip that uses its body as a solar sail and has antennas printed on it for communication. Microprobes, not nanoprobes.

  • Charlie April 8, 2016, 15:42

    I’m not really quite sure why, but I’m not all that fond of the idea of sending the space probe to this hypothetical planet 700 astronomical units beyond Earth. Why? I suppose it boils down to the fact that these are extremely costly projects, and I’m not really certain as to why this is something that is a NEEDED project.
    How does such a project influence and enhance interstellar exploration?
    This is not being negative for negative sake but asking the question as to what benefit such exploration actually goes to meeting interstellar progress as this is where the emphasis is here

    • George King April 8, 2016, 20:42

      For me at least, a mission to Planet Nine serves the goal of interstellar exploration the same way that Mercury and Gemini set the stage for Apollo – serving as test beds for the technologies required to reach the more ambitious goal.

      Development of the technologies required for getting to 700 AU – faster propulsion, followed then by deceleration, long distance communication, and autonomous spacecraft operation given the long communication delays – will move us closer to a realistic, non-theoretical capacity for fully interstellar flight.

      If the mission also can serve as a test bed for telescopy using the Sun’s gravitational focus – a technology allowing us to learn more about our closest interstellar neighbors and possibly to discover nearby exoplanets – that also will move us toward being an interstellar species.

      The capacity for deep space flight into the neighboring interstellar medium, well beyond the Voyagers, is not that far beyond our reach. What we need is a plausibly attainable target in the interstellar medium that will inspire the public enough to support the expenditure of public funds for a mission.

      Planet Nine holds the promise of being such a plausibly attainable target that will prompt us to reach further faster than we might otherwise – serving both as interplanetary and interstellar precursor mission.

  • Brett Bellmore April 8, 2016, 16:52

    I’m not so sure a magsail actually requires active cooling at this distance from the Sun. The sky, so long as you keep nearby bodies out of view, is very cold. Heating is due to radiation from the Sun. Or the Earth, if you’re close by.

    A well designed sun screen between the superconducting loop and the sun, maybe several aluminum foil chevrons, would likely permit the loop to radiatively cool to the point of superconducting. A challenge would be keeping the loop precisely oriented relative to the Sun, of course.

    After all, it gets cold enough for superconductors during the Lunar night, and that’s with a substantial hunk of rock to bleed heat out of, and only a couple weeks to do it in. In the perpetually shaded areas near the Lunar poles, it gets positively cryogenic.

    • Eniac April 9, 2016, 20:17

      I agree, a properly shaded superconductor could stay cool passively even in close proximity to the sun. Which makes me wonder: As you get closer to the sun, the magnetic field should get stronger, too. Maybe, given a coil that can be turned on and off, you could get a much better boost from riding magnetic activity than from photon or proton pressure.

      • Michael April 10, 2016, 8:57

        Magnetic activity rotates with the sun as does the solar wind which controls its direction. Photon pressure is by far the largest of the momentum givers from the sun but a nice thing about magsails is the quick deployment and the on/off option, you could wait for a CME and get a really high boost ~1000 to 1500 km/s.

        Thats to AC in less than 1000 years and ~2.5 years to planet nine.

      • Project Studio April 11, 2016, 9:04

        @Michael “you could wait for a CME and get a really high boost ~1000 to 1500 km/s.”

        That would be ‘running before the storm’ for sure. We’d need a way to predict coronal mass ejections and incredibly intelligent systems to trim the magsail for rapid and drastic fluctuations. I can see a sci fi scenario here.

        • Michael April 11, 2016, 15:13

          Getting the timing right will be needed, only an onboard intellegent system could do it effectively. As for the velocity it is higher over higher latitudes than at the equator. I was looking at a magsail design a few months back, looked relativity easy to construct, they are very easy to deploy and control. I sent an e-mail off to the Planetary society to see if a magsail could be used as competitor against the light sail but no reply.

  • Phil April 8, 2016, 19:28

    Surely trying to aerobrake without knowing the characteristics of the atmosphere well is a recipe for disaster? For this reason maybe a fast flyby precursor might be a good idea? Either that or hoping for a bunch of (rare, given the tiny angle the planet will subtend at multiple 100s of AU) stellar occultations that will presumable need to be observed by the new generation of extremely large telescopes.


    • kamal ali April 8, 2016, 23:48

      I suspect by the time we launch, we’ll have quite a good idea of the atmosphere. I bet for aerobraking from 100km/s, if you mis-estimate the atmosphere density by even a few percent, you’re toast.

    • Michael April 9, 2016, 15:58

      The beauty of it been so ‘close’ is that we can move the observation craft to create an occultation, this could be done to determine the atmosphere more precisely.

    • Eniac April 9, 2016, 20:23

      Then again, you could program the device to measure atmospheric density as it goes and steer accordingly, without human intervention. There are not that many parameters involved.

  • Joe G April 8, 2016, 22:54

    Very interesting article… As a side comment, Fritz Leiber was an amazing writer…such a wide range of interests… “A Pail of Air” comes to mind (I think there was a discussion here about rogue planets not too long ago) …in the story, a wandering “dark star” flings the earth out of its orbit… And things start cooling down. Scarey thought…
    Back to main issue, Initially it seems like it might be a stepping stone to the stars, but the mass of the planet would make landing and resource extraction difficult. What worries me is that there may be thousands of moon sized objects (and smaller) out there… Navigation hazards. I can see a phase of exploration that maps all of these objects before interstellar ships exit the solar system.

  • david lewis April 8, 2016, 23:12

    It would be fascinating.

    Going through the sun’s focal point maybe a probe would have an interesting target (star) to study there, but more likely not. Still, at the very least, it could view the background radiation of the universe through the advantage of using the sun as a lens.

    Such a large planet so far from the inner system might have, over the eons, been bombarded by comets and such from other star systems. In effect it migt have acted as a large scoop gathering such material and concentrating it. Assuming the atmosphere isn’t too dense a lander could explore for it.

    Chemicals rockets aren’t going to cut it, and will force us to move up to something nonchemical. I hope even the most zealous proponents of solar power doesn ‘t expect the proble to use solar energy out there to power its systems and to maneuver. Hopefully that something will also be of use to us in getting future probes to the closer planets faster.

    If it has liquid water then ….

    (sigh) I really hate how long these missions take. I don’t expect even a crewed mission to mars in the next thirty years, let alone finding the ninth planet, planning a mission to it, getting the probe there, and getting any information back. That it takes over 9.5 years for a flyby mission of Pluto is a daunting wait, and this planet is 20x as far. Heck, we still know very little about Mars, relatively speaking, and it’s next door. Or the moon.

  • Eniac April 8, 2016, 23:47

    According to the Stefan Boltzmann equation, a black body radiator will be good for ~5600 W per square meter at 1000K, 16 times that at 2000K. To achieve the 700 W/kg you mention at 20% conversion efficiency, the reactor needs to have a surface to weight ratio of 0.625 m^2/kg at 1000K, or 390 cm^2/kg at 2000K. That seems easily achievable. The key is that the radiator has to be really hot, white hot, ideally, and the reactor plus radiator have to be made entirely from highly refractory materials. Uranium carbide as the fuel, for example, has a melting point above 2000K, and tungsten is a good structural material with a melting point above 3000 K. Carbon, likewise.

  • Eniac April 9, 2016, 0:05

    Such a reactor might resemble a giant light bulb filament, with a uranium carbide core, a cesium vapor filled gap for thermoelectric conversion, and a tungsten envelope to radiate the heat. It would be coiled up tightly, just like the filament, to achieve critical mass for the nuclear reaction while maintaining a large surface area for radiating the waste energy in the form of really bright light.

    • Michael April 10, 2016, 9:37

      The radiated heat could also acts as thrust, very little admittedly, if the decay products could be allowed to evaporate onto a holding film and then after subsequently decays directionally into space they to could act as thrust particles. I am sure there is a way to directly convert the alpha particle emissions directly into electrical power.

  • Eniac April 9, 2016, 0:07

    That was supposed to be “thermionic conversion”, not “thermoelectric”.

  • Michael Spencer April 9, 2016, 7:27

    An interesting read, thanks for taking the time.

    I wonder why when we speak, often fancifully, about any sort of trip to these distant spots that the NERVA systems aren’t part of the picture? Certainly it’s a tech well-understood?

    • Eniac April 9, 2016, 20:35

      The ISP of NERVA isn’t really that much better than that of chemical rockets, and the H2 propellant is hard to keep contained for a long time. Nuclear electric is a much better option for extended missions like this. Unless and until fission fragment rockets are feasible, that is.

  • Singe April 9, 2016, 7:56

    The phrase that has inspired the most sadfaces in all of hard sci-fi is: “which requires heavy radiators”

  • Ashley Baldwin April 9, 2016, 9:42

    The biggest possible launcher , be it the Space Launch system or better still even a much bigger future Mars Colonial Transport launcher , in combination with a next generation solar powered ion propulsion accelerating constantly as far as the largest arrays possible can power it ( NEXT can already work in theory with three engines powered by a realistic solar array out to 3.5AU) then a close in Jupiter gravity assist . As fast as I can think of . The extra power provides the enormous payload capacity needed for the propellant required for deceleration nearer its destination. The Sun is the ultimate gravity assist body of course but would need a lot of thermal protection and prevent the use of large unprotected solar arrays .

    • Michael April 9, 2016, 16:04

      The Sun can’t gravity assist to any great degree because we are moving with it, we could however use the solar light and wind to dive towards it gaining momentum and increase our directional velocity substantially.

  • Ashley Baldwin April 9, 2016, 9:46

    Sorry. Forgot to add an Earth Gravity assist too at the beginning of the journey . Every little bit counts .

  • Tom Mazanec April 9, 2016, 11:29
    • Ashley Baldwin April 9, 2016, 17:26

      There is a very readable preprint of the brief simulation based article this came from available on arXiv

  • Ashley Baldwin April 9, 2016, 16:45

    There are actually many other reasons for exploring space as far out as any presumed Planet Nine. The Kuiper belt , Oort Cloud , the Sun’s gravitational lens point to name but a few. And to start are perigrinations into interstellar space too ! So I think it’s reasonable to consider propulsion technology that can achieve exploration of the area in a reasonable time.

    NERVA was indeed a very promising concept that was developed to a high level before being abandoned. A bit controversial at present as a direct nuclear option but technology like Prometheus , the small space nuclear reactor sadly recently abandoned , might offer a more realistic shorter term bet .

    Though it looks as if the VASIMR ion engine concept is going nowhere fast , the ion propulsion principle is robust as is shown by upcoming NEXT ( now available as a freeby on the New Frontiers 4 mission) . Not uber powerful but still capable of continuous acceleration over significant times with a suitable high power source and several engines in combination . In the first instance 25 KW plus solar arrays ( within 4 AUs of the Sun or so) but longer term even a “small” reactor like Prometheus would provide several hundred kilowatts , enough power to provide continuous acceleration from some next generation ion propulsion variant or other ( there are many theoretical options) over the years required to reach the nether regions of the solar system in reasonable time and with facility for significant braking rather than fast flyby. Especially with the kick start and generous de acceleration propellant mass offered by a potent “conventional ” launcher in combination with inner solar system gravity assists . With enough thermal protection such a system might even be able to use the Sun as the ultimate gravity assist body ( with several heliophysics missions planned over the next decade that can cope with very close approaches the technology required is already developing rapidly )

  • Ashley Baldwin April 9, 2016, 18:00

    In terms of moons Telisto should be interesting. If it is indeed a fifth ejected ice/gas giant core , it is likely to have lost most if not all of its “regular” moons . So it should have a good collection of variably sized KBO “irregular” moons . Any as large as Triton or larger ? It’s travelled a long way and possibly in the company of a whole host of objects ejected by the same process , some of which could even be planetissmals from the inner solar system or displaced “regular” moons from the other Giants . A veritable “coalition of the unwilling” .

  • Charlie April 11, 2016, 16:55

    One other thing, if this body does exist out there and it is considered to be the ninth planet of our solar system I sure hope that they don’t intend to name it “Telisto”. I don’t even know how to pronounce the name of that object; but aside from that, it doesn’t seem to fit in with the scheme that we name the planetary bodies (or have named the planetary bodies) since time immemorial.

    What would be a good name for the body? I don’t know. But I bet a much better name can be found than that…

  • Alex Tolley April 12, 2016, 0:47

    I see Nasa is now testing the electric sail concept.


    Definitely a possibility for a fast 700 AU mission.

  • Adam April 12, 2016, 4:12

    Hi Charlie

    The suggested name is from the Greek: τηλιστος …which means “farthest, most remote”.

  • J. Jason Wentworth April 12, 2016, 21:43

    Hafnium carbide (which can stand up to 7,500 degrees F. if memory serves–Arthur C. Clarke wrote that “We could send a hafnium carbide nose cone to graze the Sun’s surface and get it back intact”) could be used for a Telisto aerobraking orbiter.