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)