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Parker Solar Probe: Implications for Sundiver

We’re going to be keeping a close eye on what is now called the Parker Solar Probe as work continues toward a July 2018 launch. This is a mission with serious interstellar implications because it takes us into the realm of so-called ‘sundiver’ maneuvers in which a solar sail could be brought as close as possible to the Sun (perhaps behind a protective occulter) and then unfurled to get maximum effect. Velocities well beyond Voyager’s can grow from this.

Throw in the prospect of beamed propulsion and such sails could receive an additional boost. To be sure, the Parker Solar Probe is not a solar sail but an unmanned, instrumented probe designed to explore a region as close as 10 solar radii from the Sun’s surface, where temperatures can be expected to reach about 1375° Celsius. But the sundiver implications are there, and we’ll gain priceless data about operations in this extreme environment.

Why a sundiver? Voyager 1 is exiting the neighborhood of the Solar System at 17.1 kilometers per second. And while New Horizons left Earth breaking all speed records for spacecraft launched into interplanetary space, it’s worth remembering that its velocity during the Pluto/Charon encounter had fallen to about 14 kilometers per second, a consequence of the long climb out of the gravity well. So finding ways to get spacecraft to two or three times this velocity is an obvious objective as we explore ways of moving faster still.


Image: Artist’s impression of NASA’s Solar Probe Plus spacecraft on approach to the sun. Set to launch in 2018, Solar Probe Plus will use Venus’ gravity during seven flybys over nearly seven years to gradually bring its orbit closer to the sun. The spacecraft will fly through the Sun’s atmosphere as close as 6.2 million kilometers to our star’s surface, well within the orbit of Mercury and more than seven times closer than any spacecraft has come before. Credit: NASA/Johns Hopkins University Applied Physics Laboratory.

The first use of the word ‘sundiver’ in a scientific paper that I am aware of is Greg and Jim Benford’s “Near-Term Beamed Sail Propulsion Missions: Cosmos-1 and Sun-Diver” (Beamed Energy Propulsion, AIP Conf. Proc. 664, pg. 358, A. Pakhomov, ed., 2003), though the word’s history goes back a bit further to David Brin’s 1980 novel Sundiver, which turned out to be the first book in his Uplift Trilogy. Greg Benford worked with Brin on some of the novel’s concepts, discussions that he recalled in a column in Fantasy & Science Fiction:

I called this craft the Sundiver. The term is old—I gave it to David Brin when he first came to see me, back when he was struggling with his first novel. (As he now recounts, I asked him how his craft that literally plunges into the Sun could survive. He answered that he would throw in some jargon, techtalk, whatever. I disdainfully replied, “Oh—magic.” So David went home and found a physically possible way to do it, confounding me.)

I read Sundiver not long after it came out and don’t recall anything like a close solar pass mission — instead (as Benford says above), Brin was looking for a way to actually get a craft into the Sun by way of exploring the novel’s unusual lifeforms, creatures that lived off magnetic fields in the chromosphere. But the Benford paper mentioned above (available here) discusses sail concepts using a close solar pass as well as desorption of heated embedded molecules from the hot side of the sail that can deliver a second propulsive ‘burn.’

Let’s pause on desorption, which turned out to be interesting because in their laboratory work at JPL pushing an ultra-light carbon sail with a microwave beam, the Benfords found that the beam alone could not account for the acceleration they observed. Subsequent investigation showed that embedded molecules — CO2, hydrocarbons and hydrogen that had been incorporated in the sail material lattice when it was made — could be ejected under high enough temperatures. The original sail material was left unharmed by this propulsive effect.

Incorporating that phenomenon into a sundiver mission, we get this: The sail approaches the Sun turned edge-on to minimize solar flux that would push against it. The spacecraft then turns at perihelion to get the full effect of both photons and related sail desorption, gaining velocity even as (because of the loss of some of the molecules in its fibers) it loses a bit of mass. When desorption is complete, the sail operates like any other solar sail, though one now moving fast enough to explore the outer Solar System in far less time than it took Voyager.

On the way to a sundiver, what the Parker Solar Probe gives us, among other things, is the ability to investigate the high energy particle environment that any future sundiver mission would have to cope with. An 11.43 cm carbon-composite shield will be used to protect the craft. We’ll learn much about the solar wind, findings that will also prove useful as we contemplate future magsail possibilities, in which a spacecraft might use the highly variable plasma flow to reach high velocities. More about that possibility tomorrow, when we’ll take a closer look at the conditions the Parker Solar Probe will face and ponder the insights it is certain to provide.


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  • Neil S June 5, 2017, 12:30

    As an off the wall proposal, how about carrying what amounts to a small squirt bottle of appropriate molecules to spray onto the sail before it enters the target system so as to be able use desorption to slow down more. I have no idea whether it would be worth the weight.

    • Alexander Tolley June 5, 2017, 15:17

      Desorption will only give you a moderate Isp AFAICS. The delta v probably won’t be anywhere good enough to make this a worthwhile approach even if the propellant was nearly massless.

      However, I do wonder if it makes some sort of sense at perihelion exiting sol. This isn’t so much to add velocity, but to accelerate the sail so that it gets away from Sol more quickly, reducing sail damage. This would happen before any outbound beaming might be used to accelerate the sail to take advantage of the Oberth effect. Either adding the material to the sail, or using the material in a small ion or electrostatic engine powered by any electricity generated by the sail at perihelion. It might be worth doing some BoE calculations to test the idea.

  • Alexander Tolley June 5, 2017, 12:50

    Some “sundiver” nissions in fiction.

    Perhaps Icarus flying too close to the sun was the first failed mission. If only Daedalus had used carbon fiber!

    There was Bradbury’s “The Golden Apples of the Sun”, a very poetic short story collected in his “R is for Rocket”.

    Gerry Anderson’s “Thunderbirds” has a rescue of the “Sunprobe” manned mission to capture a piece of the sun.

    In the movies, we had the abysmal “Solar Crisis” where the sun needs a bomb to be planted to reduce its intensity and stop a solar flare, and the later “Sunshine” where a similar mission is needed, but to restart the sun instead.

    Star Trek TNG had an episode where a new shield was invented to allow a spacecraft to enter a star’s photosphere.

    Our fascination with the sun and how to get closer to it must go back to sun worship. Our desire to get close, even embrace it is almost religious.
    A solar sail sundiver mission can be viewed in this context as a way to be given the some of the powers of a sun god to gain the speed necessary to escape the solar system at a decent clip.

    • Mark Zambelli June 13, 2017, 12:06

      In the game ‘Elite’, rather than pay for fuel you can scoop directly from the stellar surface while your ship’s temps climb.

      And there is always Spock’s calculation for ‘slingshot effect’ time travel.

      • Alexander Tolley June 13, 2017, 12:48

        There are quite a few sundiver references in fiction. Your reference reminds me that there are sundiver refuelings in Stargate: Universe as the ship travels across te galaxy.

  • Michael June 5, 2017, 14:17

    I once looked at an idea about using a long thin double sail configuration that would fly towards the sun edge on. When it was close enough it would open up with the sunlight helping the opening, it would get very close indeed.

  • Larry Kennedy June 5, 2017, 14:47

    Don’t forget ” The Mote in God’s Eye”.

  • George King June 5, 2017, 14:58

    In my non-rocket scientist mind, I wonder whether some additional useful velocity might be obtained by orienting the photonic sail more or less toward the Sun as the craft was passing closely by the Sun but was still prior to perihelion.

    Yes, the solar irradiance would start to apply a force more or less away from the center of the Sun. But perhaps the competing force of gravity could act as a de facto “keel” that resisted this lateral “side-slipping” – at least up to the point that the craft reached escape velocity at its particular solar altitude – such that the primary force transmitted to the craft was forward in its desired trajectory rather than straight outward from the Sun at that point.

    I get that photonic sails operate on different principles than modern boating sails. In particular, current photonic sails lack the aerodynamic lift component used by modern boating sail shapes moving within air flow to generate much of a sailboat’s forward momentum.

    (I saw some research a while back where they produced an analogous lifting force with light, but it was very nascent nano-level in-the-lab type stuff with no current practical application for propulsion.)

    Photonic sails operate instead more akin to the centuries old square rig type sails, which relied upon the simple pushing force of wind from more or less astern rather than aerodynamic lift to produce propulsion.

    So I’m not talking about tacking or trying to use photonic sails to do things that only modern boating sails can do by using aerodynamic lift in an airstream.

    That said, though, it would seem that – in a very rough analogy instead to a beam reach or close reach point of sail – it perhaps might be possible to use the Sun’s gravitational force to serve in the nature of that lateral-movement-resisting “keel” to allow a photonic sail craft to pick up some useful forward velocity pre-perihelion by orienting the sail more or less toward the Sun pre-perihelion.

    Sail materials and the near solar environment permitting of course.

    And the lateral “side-slipping” that nonetheless would occur even if this approach were to produce useful net additional forward velocity would need to be accounted for in the trajectory selected to reach the perihelion point desired.

    • Larry Kennedy June 5, 2017, 16:30

      Solar Sails can indeed apply thrust along, against, or at a divergent angle in relation to the direction of travel, not just outward.

    • Alexander Tolley June 5, 2017, 16:52

      A sail oriented at an angle will provide a forward velocity component that will increase velocity if traveling in the same direction as the orbit and vice versa. For a sundiver mission, the sail will be oriented so that the orbital velocity will be reduced so that the sail “falls” quickly towards the sun, then at perihelion reverse that orientation to accelerate the sail. Gravity assist is used to increase velocity. Actual trajectories are more complex. “Solar Sails” (Vulpetti et all) has some nice illustrations of various sundiver trajectories to maximize velocity. Wright’s “Space Sailing” has a nice section on trajectories and sail orientations.

      • George King June 6, 2017, 12:27

        Thanks, guys. I do have that Solar Sails volume on my shelf and I reviewed the sundiver discussions again before my post.

        Could be a classic “a little knowledge is . . .” issue from my end. Or maybe I’m being inartful in describing my point/question.

        I do understand that, as a general matter, you can orient the sail in different attitudes to achieve different trajectory results.

        And I do understand that, specifically, you can use the sail to decelerate in relation to the Sun out near Earth orbit in order to fall into an accelerating sundiver trajectory. (Of course, if the sail is going to remain furled until perihelion as discussed in Paul’s piece and in Solar Sails, one perhaps might have to use a heavy lift vehicle like they’re using for the Parker Solar Probe to get on that trajectory initially from Earth.)

        As backdrop, like Paul’s piece here, the initial discussion of the sundiver trajectory in Solar Sails states that – once proximate to the Sun – the sail “unfurls at perihelion.” G. Vulpetti, L. Johnson, & G. Matloff, Solar Sails: A Novel Approach to Interplanetary Travel, at 93, Illustration 9.3 (2d ed. 2015). See also id., at 238-44 (more detailed technical discussion, including of the possible use of the sail to decelerate out at Earth orbit and fall into a sundiver trajectory).

        Paul similarly states in his piece that “a solar sail could be brought as close as possible to the Sun . . . and then unfurled to get maximum effect.” Such that, again, while proximate to the Sun, the sail is not being used prior to perihelion. See also: “The sail approaches the Sun turned edge-on to minimize solar flux that would push against it. The spacecraft then turns at perihelion to get the full effect of both photons and related sail desorption, gaining velocity even as (because of the loss of some of the molecules in its fibers) it loses a bit of mass.”

        For a second, let’s put the desorption issue – an as-yet also relatively untested low technology readiness level strategy – to one side. I’ll come back to it, but for now I’m going to look at it from the perspective of a “plain” photonic sail not relying also on desorption. Many of the sundiver trajectory discussions that I’ve seen similarly discuss the maneuver in the context of such “plain” photonic sails.

        With a chemical rocket-propelled craft, one of course achieves the maximum effect from conducting an acceleration burn by waiting until perihelion.

        But this is a sail craft not a rocket. Regardless of how the craft got to being on a close solar pass on a sundiver trajectory (i.e., whether by using the sail for deceleration out at Earth orbit or by use of a heavy lift vehicle), that craft (again putting desorption to the side for the moment) is not relying on a limited chemical fuel reserve to accelerate. It’s not limited by fuel and accompanying mass constraints to only a limited burn or burns that must be carefully marshaled for maximum effect.

        That sail craft – while on that close approach – instead has an unlimited propulsion source freely available without any on-board mass/exhaustion concerns. There’s that steady torrent of photons being emitted by the Sun. But that propulsion source – in turn – has the limitation that it is stronger when one is closer to the Sun and weakens as one gets further away.

        It just seems arguable to me that – once you’ve made the long journey to the vicinity of the Sun, you can pick up free further speed on your way to perihelion by using the freely available photonic pressure available as you approach the Sun. The Sun’s giving you a free additional propulsion source on your way in during your close pass as well as on the way out.

        (Well, again, assuming a sufficiently resilient sail material, but that’s a distinct issue from possible flight plans. The sundiver discussions that I’ve seen all have discussed possible trajectories for craft that we do not yet have the type of material that we need to fly those trajectories – whether in terms of resilience, mass, and/or reflectance. Maybe we’ll ultimately find/develop something that has resilience like reinforced carbon carbon in an also ultralight material like graphene that in turn is highly reflective.)

        In sum, with a chemical rocket, you get your burn and then the craft acts like any other object in the solar system acted upon by gravity on a passive trajectory in relation to the sun. So it pays to wait to perihelion for maximum effect. With a sail, in contrast, you can add acceleration more or less continuously as long as you’re sufficiently close to the Sun. So it would appear to me that you would be able accelerate continuously both on the way in and the way out during the actual close pass itself. (Faster in and faster out also perhaps would reduce the overall close-in exposure of the sail craft material to the near solar environment.)

        Now, adding desorption back into the mix (if ultimately a viable strategy in practice once it’s been run through the TRL’s), does the Oberth effect advantage from waiting and unfurling to utilize desorption only at perihelion outweigh being able to reach a perihelion (perhaps not the same perihelion point) at a higher velocity in the first instance via accelerating continuously on the way in?

        See also Wikipedia’s “Oberth Effect” article: “[T]he Oberth maneuver is much more useful for high-thrust rocket engines like liquid-propellant rockets, and less useful for low-thrust reaction engines such as ion drives, which take a long time to gain speed.”

        Anyway, that’s my thought, such as it is. We obviously didn’t do much in the way of orbital dynamics when I was studying philosophy and history at LSU (well before Tabby Boyajian’s time). But the fact that it’s a sail rather than a rocket would seem to me to allow one to also utilize an essentially free (with a sufficiently resilient sail craft material) additional acceleration source prior to the perihelion point that one otherwise would wait for with a solely rocket-propelled (or otherwise propulsion mass-limited) craft.

        Thanks again for your replies.

        • Alexander Tolley June 6, 2017, 23:49

          I think the easiest way to show why this doesn’t work is to assume that the sail is at1 AU and in solar orbit to start and uses the propulsive effect to accelerate. What happens is that the sail will try to move to a higher orbit. In effect it will spiral outwards with the velocity gained by the propulsive effect reduced as it tries to climb to a higher orbit. As it’s orbit is raised, the propulsive force is reduced by distance. My interpretation of what you are saying is that the acceleration as the sail approaches the sun will not impact the orbit.

          Trying to accelerate towards the sun just keeps the sail moving to a higher orbit and slowing down to that orbital speed. So what you really want is to gain velocity by falling into a very close orbit of the sun and then using the very high photonic pressure to accelerate away again so that it will be traveling at a much faster velocity at the same level of the “gravitational hill” that it started from. The acceleration at perihelion gives you “more bang for the buck” in delta v for the energy through the Oberth effect, just as it would for a rocket.

          I hope this explanation clarifies it better for you. The only other suggestion I can offer is to find a simulator where you can test different trajectories by turning thrust on and off so that you can get a better feel of what happens.

          • George King June 7, 2017, 18:23

            Thanks for the patient explanations. Think I get it fully this time.

            Sigh . . . guess I’ll have to put off that mission design career to the next lifetime, where I stay on a math track.

        • Michael June 7, 2017, 2:59

          We may need two sails, one to slow the craft to fall towards the sun, cancel ~30km/s, laser may help, which is then ejected. And then the curled up sail which opens close to the sun.

  • Alberto June 6, 2017, 11:20

    “What,” said Trillian in a small quiet voice, “does sundive mean?”

    “It means,” said Marvin, “that the ship is going to dive into the sun. Sun … Dive. It’s very simple to understand.”

  • hiro June 6, 2017, 17:26

    This is only the 1st stage anyway, the process of mapping Sol’s magnetic field and then converting for energy in the form of beam is useful in several different ways, but the field of material science must advance further in order to do that. Lol, maybe this is the inception in theory of Sol defense system, a swarm of special satellites will use death ray zapping any ET mothership entering this system.

    • Alexander Tolley June 6, 2017, 23:57

      Your suggestion does make me wonder if it might not be easier to put superconducting coils to generate magnetic fields that would then harvest energy as the solar protons/electrons streamed by. Thus instead of building a Dyson swarm to extract energy which blocks the starlight, much smaller masses would extract energy, perhaps more efficiently for the mass used, but these would not occlude the light. Magnetodynamic energy extraction rather than photonic. It would harvest less of the star’s output, but perhaps at a lower cost per unit of electrical energy.

      • Michael June 7, 2017, 15:39

        The problem is that the energy from charged particles is 1/10000 that of phototonic energy from the sun.

        • Alexander Tolley June 7, 2017, 22:33

          It is only a problem if the aim is to harvest all the energy. It may not be if the harvesting of that amount is more economic.

          While not equivalent, I would point out that the electric sail uses those protons and gets a theoretical performance comparable to solar sails because the mass of the wire sails is so small and scales well. If, A BIG IF, something similar was possible to harvest the energy of the solar wind, then perhaps this might make sense and therefore provide power for a planetary civilization without blocking its light and coincidentally be signaling its presence.

          Full capture of the sun’s output is 10^9x that of Earth’s capture. If the solar wind was just 10^-4 the energy of the output, that would still produce 100,000 the earth’s output. Not bad for a Sol system civ.

      • hiro June 8, 2017, 0:51

        That’s antimatter mining, Sol does produce “small” amount of antimatter in the upper atmosphere region along the magnetic lines. It’s mentioned briefly in *Marooned in Real time*, the concept requires several important advances in the later half of this century or next century. It’s best to stay just right above Sunspots (cooler temperature regions & strong magnetic fields) collecting energy; well this probably won’t happen within our lifetimes.

  • Dmitry Novoseltsev June 6, 2017, 23:39

    Ideas for new solar probes: