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Rough Sailing on the Solar Wind

All of nature is a kind of laboratory, which is why good propulsion ideas can flow from astronomical observations that show us how things work. Recent news about the solar wind is a case in point. An analysis of data from the Ulysses spacecraft shows that the solar wind is now lower than at any time previously measured. That has implications for the heliopause, that region where the solar wind encounters true interstellar space, for this region plays a role in shielding the Solar System from the effects of galactic cosmic rays.

“Galactic cosmic rays carry with them radiation from other parts of our galaxy,” says Ed Smith, NASA’s Ulysses project scientist at the Jet Propulsion Laboratory in Pasadena, Calif. “With the solar wind at an all-time low, there is an excellent chance the heliosphere will diminish in size and strength. If that occurs, more galactic cosmic rays will make it into the inner part of our solar system.”

Image: The Ulysses spacecraft. Credit: Jet Propulsion Laboratory.

Cosmic rays can have ramifications on spacecraft engineering, especially when human crews are involved. As to propulsion, we have to be careful when talking about the solar wind. Sailing it is not similar to working with conventional solar sails, which use the momentum imparted by photons to move the spacecraft. The solar wind involves a much different medium, the stream of charged particles that flows at high speed from the Sun, attaining speeds anywhere from 350 to 800 kilometers per second. In this news release, David J. McComas (SwRI), principal investigator of the Solar Wind Observations Over the Poles of the Sun (SWOOPS) experiment on board Ulysses, confirms what Smith said:

“The heliosphere is a big bubble that’s inflated from the inside by the million-mile-per-hour solar wind blowing out in all directions. The size of the bubble is determined by the balance of pressure of the solar wind pushing from the inside out and the pressure of interstellar space pushing from the outside in. If the solar wind is blowing out a quarter less hard, that means the outer boundaries of the heliosphere must be shrinking. The entire heliosphere must be getting smaller.”

This may sound like a more unusual story than it is. Earlier observations of the solar wind have made it clear that it can weaken and regain strength. We’re learning how the strength of the wind varies, with data from both Ulysses and the Advanced Composition Explorer spacecraft showing that the wind has weakened at all solar latitudes. But remember that Ulysses, which circles the Sun in a polar orbit, is only on the third such orbit since its 1990 launch, so we have a long way to go in compiling the kind of data that will give us a true perspective on wind strength.

Currently, the speed of the solar wind is roughly what it was at the previous minimum phase of the solar activity cycle, but its density and pressure are significantly lower. The findings have a bearing on several propulsion concepts. While we have ingenious proposals for solar wind-driven spacecraft, exactly how to make them work is more problematic than ever. A vessel using a self-generated magnetic field (possibly with plasma trapped within) as a sail could theoretically ride the wind at great speed to the outer Solar System, an idea that researchers like Robert Winglee have done much to investigate.

But the very inconstancy of the solar wind points to the question of how well we could control such a spacecraft. In their new book Solar Sails: A Novel Approach to Interplanetary Travel, Gregory Matloff, Les Johnson and Giovanni Vulpetti note that this variability makes sailing such a craft something like tossing a bottle with a note inside it into the surf at high tide, hoping the ocean will carry it to a specific destination. Because we don’t know how to control the result directionally, it’s clear that we have much work to do in sorting out the magsail concept as it relates to the solar wind. It remains promising, but beamed magsail designs seem to offer a far more straightforward deep space solution.

Comments on this entry are closed.

  • John Hunt September 26, 2008, 14:36

    If we could design an electric sailing or otherwise solar wind sailing craft that travels at 350 to 800 km/sec then, by my calculations, it would take us only 1,645 to 3,760 years to make it to Alpha Centauri. I feel like a bit of a broken record here but that timeframe won’t work for a science probe but it would make sense for a seeding mission to establish a second human civilization. Of course we would need to solve the cosmic ray problem and perhaps use pre-gamete tissue cultures as a way of ensuring viable embryos at destination.

    Now Dr. Pekka Janhunen tells me that we cannot reach the full speed of the solar wind. He estimates that we could reach about 25-30% of that. Not exactly sure why. Also, he points out here the following:

    The injected plasma cannot remain in the vicinity of the spacecraft and it eventually escapes with a rather high velocity. And this force cannot come from anywhere else but from the solar wind. When we analysed this, we found that a large fraction of the solar wind force will not be used in accelerating the spacecraft but in accelerating the plasma. It is the same physical phenomena as in a comet

    Well, I hope he’s wrong because M2P2 is a great idea. But I’m afraid he might be right.

  • John Hunt September 26, 2008, 14:51

    But the very inconstancy of the solar wind points to the question of how well we could control such a spacecraft.

    But the advantage of electric sailing is that we can turn on and off it’s field at will. I imagine that we would launch our craft in essentially at a tangential direction to the Earth’s orbit. Turn on the power and the craft begins accelerating radially in an ever enlarging spiral. Use any number of methods to get the right inclination. Once the craft’s vector points to destination, turn off the field. Hopefully with the right calculations this can be done after acceleration from the solar wind has been maximally exploitated. From there on small course adjustments would be needed.

  • John Hunt September 26, 2008, 14:55

    Cosmic rays can have ramifications on spacecraft engineering, especially when human crews are involved.

    Does anyone know how much worse the cosmic radiation is outside our solar system than inside? Sure hope it’s not much.

  • andy September 26, 2008, 17:38

    In the interstellar context, this also has implications for using a solar sail or a magsail for braking into a target system… would be unfortunate if you arrived at the target star only to find it has gone into an inactive state while you were travelling and you can’t stop.

  • Adam September 27, 2008, 6:29

    Hi John & andy

    John, AFAIK Galactic Cosmic Rays (GCRs) aren’t much affected by the heliopause. The ISM’s energies are about 1-10 eV, the Solar Wind ~1 KeV, but GCRs are ~ 1-10 GeV or so. Thus GCRs should cut through pretty easily. However on a galactic scale the ambient field deflection is significant and how much GCR exposure the Solar System experiences varies with the Sun’s position above and below the Galactic plane.

    andy, the interstellar mag-sails will be ploughing through the ISM and not a star’s stellar wind. Thus no problem. And, of course, solar-sails have no problems at all with the stellar wind, since they’re riding photons. Flare stars might cause momentary fluctuations, but that’s a different issue to the stellar wind.

  • Mark September 27, 2008, 14:30

    Has anyone noticed the rather obvious and timely connection between the increased deployment of solar power cells on this planet and the decline of the solar wind in space?

    It’s humans who are causing the solar wind depletion by our unbridled and increasingly reckless use of solar power. Sure it was billed as limitless clean energy, but until now we simply did not know the drastic effects this would have on our nearest star. With all these solar cells collecting the output of the sun, we’re sucking our poor sun dry, and the depleted solar wind is just the beginning, it’s the canary in the coal mine, a harbinger of really bad things to come. I shudder to think what’s in store for our sun if we don’t change our evil solar powered ways.

    We’ve got to do something, we’ve got to preserve the solar wind for future generations. I hope you’ll join with me as I write my legislators for an immediate moratorium on all use of solar power until this effect can be studied further.

  • philw1776 September 28, 2008, 10:43

    Mark Says: Has anyone noticed the rather obvious and timely connection between the increased deployment of solar power cells on this planet and the decline of the solar wind in space?

    Also note that Dark Energy was ‘discovered’ soon after we started using solar power. Coincidence? I think not! It seems as if the Dark Side of the Force is now manifesting itself as we parasitic vampires suck the Sun dry of photons.

  • ljk August 13, 2009, 11:13

    Three years of Ulysses dust data: 2005 to 2007

    Authors: Harald Krueger, V. Dikarev, B. Anweiler, S. F. Dermott, A. L. Graps, E. Gruen, B. A. Gustafson, D. P. Hamilton, M. S. Hanner, M. Horanyi, J. Kissel, D. Linkert, G. Linkert, I. Mann, J. A. M. McDonnell, G. E. Morfill, C. Polanskey, G. Schwehm, R. Srama

    (Submitted on 10 Aug 2009)

    Abstract: The Ulysses spacecraft has been orbiting the Sun on a highly inclined ellipse since it encountered Jupiter in February 1992. Since then it made almost three revolutions about the Sun.

    Here we report on the final three years of data taken by the on-board dust detector. During this time, the dust detector recorded 609 dust impacts of particles with masses 10^-16 g <= m <= 10^-7 g, bringing the mission total to 6719 dust data sets.

    The impact rate varied from a low value of 0.3 per day at high ecliptic latitudes to 1.5 per day in the inner solar system. The impact direction of the majority of impacts between 2005 and 2007 is compatible with particles of interstellar origin, the rest are most likely interplanetary particles.

    We compare the interstellar dust measurements from 2005/2006 with the data obtained during earlier periods (1993/1994) and (1999/2000) when Ulysses was traversing the same spatial region at southern ecliptic latitudes but the solar cycle was at a different phase.

    During these three intervals the impact rate of interstellar grains varied by more than a factor of two. Furthermore, in the two earlier periods the grain impact direction was in agreement with the flow direction of the interstellar helium while in 2005/2006 we observed a shift in the approach direction of the grains by approximately 30 deg away from the ecliptic plane.

    The reason for this shift remains unclear but may be connected with the configuration of the interplanetary magnetic field during solar maximum. We also find that the dust measurements are in agreement with the interplanetary flux model of Staubach et al. (1997) which was developed to fit a 5-year span of Ulysses data.

    Comments: 50 pages, 9 b/w Figures, 1 colour figure, 4 Tables; submitted to Planetary and Space Science

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0908.1279v1 [astro-ph.EP]

    Submission history

    From: Harald Krueger [view email]

    [v1] Mon, 10 Aug 2009 08:04:29 GMT (726kb,D)