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Solar Probe Plus: Prelude to ‘Sundiver’?

‘Sundiver’ maneuvers are surely the most extreme events to which we could subject a solar sail. To my knowledge, it was Gregory Benford who first came up with the term — he mentions in Fantasy & Science Fiction that he passed the coinage on to David Brin when Brin was working on the book that would bear its name (Sundiver, published in 1985, would be the first volume in Brin’s Uplift Saga). But Benford credits Brin with the actual concept, which he needed to make his plot work, so it seems best to give credit to both writers for an idea both went on to explore, Benford not only in fiction but in scientific papers as well.

The maneuver is straightforward if breathtaking. Benford explains it in terms of a carbon sail being deployed in low Earth orbit and then launched into deep space by microwave beam:

Consider the sundiving sail. Approaching the Sun turned edge-on (to prevent the increasing flux of sunlight from pushing against its fall), the carbon sail heats up. At closest approach, the craft could turn to absorb the full glare of the intense Sun, gaining a high velocity as it accelerates strongly, under desorption. It exhausts the store of molecules lodged in its fibers, losing mass while gaining velocity. It then sails away as a conventional, reflecting solar sail. Its final speed could be high enough to take it beyond Pluto within five years. There it could do a high velocity mapping of the outer solar system, the heliopause and beyond, to the interstellar medium—the precursor to true interstellar exploration.

So there you are, a fast, propellantless way to do missions to the outer Solar System. But how likely is it that a craft like this would survive a close approach to the solar furnace? To find out just what the parameters would have to be, we need more data from this extreme environment. It’s interesting to note, then, that the Johns Hopkins University Applied Physics Laboratory (APL) is engaged in the advanced stages of design, development and testing of Solar Probe Plus now that its work has received a thumbs up from an independent assessment board.

With a launch set for 2018, the spacecraft is intended to orbit the sun 24 times, assisted by seven flybys of Venus along the way. The craft is going to be moving at extraordinary speeds at its closest approaches, some 190 kilometers per second. Contrast that with Voyager 1’s 17.1 kilometers per second, or the previous record-holder, the two Helios probes, that reached up to 70 kilometers per second. In terms of distance, Solar Probe Plus will take its ten scientific instruments a little more than 6 million kilometers from the Sun’s surface.


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 orbit the sun 24 times, closing in with the help of seven Venus flybys. The spacecraft will carry 10 science instruments specifically designed to solve two key puzzles of solar physics: why the sun’s outer atmosphere is so much hotter than the sun’s visible surface, and what accelerates the solar wind that affects Earth and our solar system. The Johns Hopkins University Applied Physics Laboratory manages the Solar Probe Plus mission for NASA and leads the spacecraft fabrication, integration and testing effort. Credit: NASA/Johns Hopkins University Applied Physics Laboratory.

An extreme environment indeed, with temperatures exceeding 1370 degrees Celsius (2500 degrees Fahrenheit). Solar Probe Plus is equipped with a carbon-carbon composite heat shield designed to withstand these temperatures, not to mention the impacts of hypervelocity dust particles, and the spacecraft’s liquid-cooled system should keep its solar arrays at survivable temperatures through all 24 solar passes. We’ll learn much about the Sun’s outer atmosphere and the solar wind from all this, but I like what NASA’s Lika Guhathakurta threw into the mix:

“Solar Probe Plus is a pathfinder for voyages to other stars and will explore one of the last unexplored regions of the solar system, the solar corona, where space weather is born.”

Guhathakurta is a program scientist at NASA headquarters in Washington who is aware of just how challenging this mission is going to be. As this APL news release notes, we’re talking about going ten times closer to the Sun than the planet Mercury. Amidst everything else we learn, we will have data that can assist in any future sundiver missions. In their book Solar Sails: A Novel Approach to Interplanetary Travel (Copernicus, 2008), Greg Matloff, Les Johnson and Giovanni Vulpetti make the case that a sundiver could reach outbound speeds of at least 120 kilometers per second.

Solar Probe Plus will achieve high speeds as well, of course, but only within the context of operations near the Sun. A true sundiver that used the Sun for a massive gravity assist would attain speeds going outward that could allow it not only to explore the outer planets but reach the Sun’s gravitational focus. “[W]e may view these early efforts as humanity’s first true starships,” the authors write, the beginning of what we can hope is an extended era of exploration.


Comments on this entry are closed.

  • Alex Tolley March 20, 2014, 10:33

    This is why we use robotic probes and not crewed missions. In Gerry Anderson’s “Thunderbirds”, the crewed Sun Probe mission goes wrong and Thunderbird 3 is dispatched buy International Rescue to get them home safely. Even Bradbury’s “The Golden Apples of the Sun” isn’t a cakewalk for the crew getting samples of the sun in a refrigerated spaceship.

    Is there any sense in using a specialized craft like a Sun Probe Plus to launch a high velocity solar sail at close approach? I could see such a craft launching a flotilla of small sails with basic sensors or a single large sail. Would the complexity of deployment be more problematic than trying to keep a sail edge on to the sun for its voyage sunward, outweighing its benefits? Would the same reasoning apply to an electric sail?

    At 6 million km, is the sun’s magnetic field strong enough to use for propulsion with a electric current in a tether, either on its own, or as a boost for a sail?

  • Gregory Benford March 20, 2014, 12:41

    Alex: Yes to your questions. The quoted passage deals with desorption of materials “painted” on the sail, to act like a rocket when near the sun.
    Worth noting that the F&SF column described the Cosmos-1 experiment that failed to attain orbit, launched from a Russian submarine in ICBM fashion. The second attempt failed, too, dooming the whole Planetary Society experiment. So the first trial of beamed power to accelerate sails lies in our future, still.

  • James Benford March 20, 2014, 13:45

    Centauri-Dreams readers who want to go deeper into the Sun-Diver concept can find four papers by Greg & myself, with worked examples of Sun-Divers, including an interstellar Sun-Diver, on my jamesbenford.com website:


    The papers are:

    1. “Desorption Assisted Sun Diver Missions”
    2. “Near-Term Beamed Sail Propulsion Missions: Cosmos-1 and Sun-Diver”
    3. “Acceleration Of Sails By Thermal Desorption Of Coatings”
    4. “Power-Beaming Concepts for Future Deep Space Exploration”

    James Benford

  • Michael March 20, 2014, 14:02

    I once was working on a similar concept, it too was also called ‘Sun diver’ as well! Instead of using an edge on disc it used thinly woven mesh on spools (paired) that unwound at 90 degrees to the suns glare, which kept it at 90 degrees by solar pressure. At closest approach and after been unwound the heat shield would be ejected and rocket motors starting opening the sails, the sunlight would also aid the opening as well as the now superheated rocket motor exhaust.
    Once the sail was fully open a gas containing a reflective material would be atomised and blown onto the sail (to stick) which allows a very thin reflective atomic layering to be deposited. The modules could be attached in a line in near earth space to make a much larger sail area. Gave up in the end.

    Paul if you have an e-mail address I will e-mail the basic concept to you, it is an unfinished project I might add.

  • Eniac March 20, 2014, 15:55

    So there you are, a fast, propellantless way to do missions to the outer Solar System.

    Technically, the desorption sail Benford describes is not propellantless, although it becomes so pretty quickly after its propellant is exhausted.

  • NS March 21, 2014, 0:21

    Glad to see a sundiver mission being discussed, partly for the selfish reason that it might be the only interstellar mission I’ll have a chance of seeing. If the proposed Innovative Interstellar Explorer launches in 2026 (the next available launch window) I’d be 100 when it reaches its goal after 30 years!

    IIRC some time ago there was a guest poster here who didn’t think a sundiver was practical though…

  • Lionel March 21, 2014, 2:18

    Nicely done post linking the planned Solar Plus Probe with Benford’s sun diving sails, always a pleasure to read Centauri Dreams

  • Geoffrey Hillend March 21, 2014, 15:35

    Maybe the same thing that accelerates the solar wind is what makes the Sun’s outer atmosphere so much hotter than the surface: We can make a few hypothesis based on quantum physics to see in advance what the reason might be.

    Maybe the radiation pressure from solar radiation such as ultra-violet, x-rays and gamma rays transfer some of their kinetic energy to the vibrational energy of the atoms of the solar wind; electrons and protons tapped in the magnetic field of the Sun as well which happens in atomic bomb and hydrogen bomb explosions to the atoms in our atmosphere.

  • Geoffrey Hillend March 21, 2014, 15:36

    It’s called inelastic scattering

  • David W March 21, 2014, 19:26

    What kind of laser power would it take to get it going 100 times faster?
    Anyway its nice to see a speed mission actually taking off

  • Michael March 22, 2014, 12:32

    @David W March 21, 2014 at 19:26

    ‘What kind of laser power would it take to get it going 100 times faster?
    Anyway its nice to see a speed mission actually taking off’

    The problem with increasing the laser/microwave power is that the material may not be able to handle the thermal and g force stresses.

    Just had a thought about using the ultra thin sail material in small discs (cm -to- meters in diameter) which could be blown out of the solar system as ‘technology’ messages to the stars on the solar wind. Perhaps we could pay a small fee for our names to be woven into the fabric, could generate a little cash for a space mission if enough people where interested.

    @Geoffrey Hillend March 21, 2014 at 15:35

    ‘Maybe the same thing that accelerates the solar wind is what makes the Sun’s outer atmosphere so much hotter than the surface. Maybe the radiation pressure from solar radiation such as ultra-violet, x-rays and gamma rays transfer some of their kinetic energy to the vibrational energy of the atoms of the solar wind…’

    I believe that the solar wind is mostly transparent to solar radiation, it is most likely the rotational energy input from the sun that is driving the velocity increase to a point via the moving magnetic field.

  • Michael March 22, 2014, 15:37

    @James Benford March 20, 2014 at 13:45

    ‘Centauri-Dreams readers who want to go deeper into the Sun-Diver concept can find four papers by Greg & myself, with worked examples of Sun-Divers, including an interstellar Sun-Diver, on my jamesbenford.com website…’

    Thanks very much for the link to the papers, very informative and a good read.

    Now desorption would give quite a boost to the sail but I was wondering if you factored in the possible effect of the desorbed gas after giving a kick to the sail that it picks up energy/momentum for the beamed radiation and sunlight turns around and re-impacts the sail. If that is the case a smaller amount of material ‘mass’ could be used, say in the centre of the disc and it slowly bleeds away outwards been used over and over.

    Apologies if I missed it in the articles.

  • Gregory Benford March 23, 2014, 16:48

    Michael: You’re right, the beam can propell desorbed particles back onto the sail, accelerating it further. This is a variation on the idea of bouncing photons back and forth in a piston, one end being a laser, the other the moving wall. In principle you can get a lot of pushes, and the limits are set by how the desrobed particles speed out of the beam’s path. I looked at this but didn’t do a full calculation. Might be fun to try!

  • Geoffrey Hillend March 24, 2014, 15:26

    @Michael I agree with you on the moving magnetic field is probably accelerating the solar wind. The temperature difference between the visible surface of the Sun and it’s the outer atmosphere is probably affected by the solar radiation x-rays etc. which would be a different cause than the acceleration of the solar wind.

  • Geoffrey Hillend March 24, 2014, 15:45

    My hypothesis is wrong. Solar scientists believe that magnetohydrodynamic (MHD) waves transfer the energy from beneath the surface through the chromosphere and into the corona which results in a much higher temperature than the surface. http://www.sciencedaily.com/releases/2013/02/130204094608.htm

  • ljk April 18, 2014, 9:47

    Here is a detailed NASA report from 2008 on Solar Probe Plus:


  • ljk August 6, 2014, 10:13

    To Navigate The Seas Of The Sun: NASA’s Solar Probe Plus Mission To The Nearest Star

    By Leonidas Papadopoulos

    “The scorching rays of the nearer Sun
    softened the fragrant wax which held his wings.
    The wax melted; his arms were bare as he beat them up and down,
    but, lacking wings, they took no hold on the air.”

    – Ovid, ‘Metamorphoses’, Book VIII (8th century AD)

    One of the most well-known myths of ancient Greek mythology recounts the story of Icarus, who in his quest for freedom and the thrill of adventure, flies perilously close to the Sun only to meet his doom after the latter’s intense heat melts the young man’s wax wings. In real life, NASA is aiming to conduct a similar daring close approach to the Sun with the Solar Probe Plus mission which is scheduled to launch in July 2018.

    Currently under development, Solar Probe Plus will ultimately approach the Sun closer than any other spacecraft in history by flying into its outer atmosphere, or corona, in its quest to answer a series of fundamental questions in the science of heliophysics. Unlike the mythological Icarus however, the well-shielded spacecraft will be designed to brace the extreme conditions near the vicinity of the Sun, leading to breakthrough insights into the mechanisms that drive the activity of our home star.

    Being the most luminous object in the sky, the Sun had been studied extensively by many different cultures throughout history. Yet, the realisation of the Sun’s true nature as a typical star among many, arose slowly in the last few centuries with the advent of modern science. The 20th century in particular, saw many major breakthroughs in heliophysics research.

    Notable scientific findings include the discovery of the solar wind, the continuous flow of charged particles that emanates at supersonic speeds from the Sun’s atmosphere, the corona, and the unexpected discovery that the latter is actually millions of degrees hotter than the solar surface below it which has a mean temperature of only 5,800 K. The advent of the space age has further revolutionized our understanding of the Sun’s activity and its effects throughout the entire Solar System.

    In the last 50 years, a plethora of space missions has observed our home star in multiple wavelengths throughout the entire electromagnetic spectrum, allowing us to study the properties and overall structure of the solar wind and the solar magnetic field in great detail. These missions have allowed us to map many of the physical processes that govern the Sun’s activity from far below its surface to the outer reaches of its atmosphere which extends all the way to the heliopause, well beyond the orbit of Pluto.

    Yet, for all the advances made in the field of heliophysics during the last decades, a series of puzzling mysteries have remained unsolved. In particular, the exact mechanism by which the solar wind is generated inside the Sun’s atmosphere and then propelled outwards at supersonic speeds, has remained unknown to this date. Furthermore, the solution to the famous “coronal heating probelm” regarding the corona’s inexplicably high temperatures of several million Kelvins has equally eluded scientists.

    Observational evidence by various space-based observatories in recent years have suggested that the corona’s inexplicable heating may be caused by magnetic waves that travel along the lines of the solar magnetic field that emanates from deep within the Sun and extends all the way through the corona into interplanetary space, while carrying energy in the form of heat from the solar interior to the atmosphere in the process. Despite these tantalising evidence however, current solar observatory missions lack the imaging resolution needed to provide a definite answer to this puzzle.

    Full article here:


    To quote:

    As outlined by its science definition team, Solar Probe Plus will aim to address four primary science objectives:

    Determine the structure and dynamics of the magnetic fields at the sources of both fast and slow solar wind.

    Trace the flow of energy that heats the corona and accelerates the solar wind.

    Determine what mechanisms accelerate and transport energetic particles.

    Explore dusty plasma phenomena near the Sun and its influence on the solar wind and energetic particle formation.