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.