London - Jul 03, 2003 The next generation of spacecraft propulsion systems could be dead in the water before they are even launched. A physicist is claiming that solar sailing- the idea of using sunlight to blow spacecraft across the solar system- is at odds with the laws of thermal physics reports New Scientist this Saturday.

Both NASA and the European Space Agency are developing solar sails and, although never tested, the concept is quite simple. A solar sail is essentially a giant mirror that reflects photons of sunlight back in the direction they came from.

Although photons do not have mass, they are considered to have momentum, so according to the law of conservation of momentum, the photon loses some of its energy to the sail as it bounces off, giving the sail a shove in the opposite direction.

But Thomas Gold from Cornell University in New York says the proponents of solar sailing have forgotten about thermodynamics, the branch of physics governing heat transfer. Solar sails are designed to be perfect mirrors, meaning that they reflect all the photons that strike them.

Gold argues that when photons are reflected by a perfect mirror, they do not suffer a drop in temperature.

That brings in a thermodynamic law called the Carnot rule, which basically states that you never get something for nothing: if there is no temperature change when the photons are reflected, it is impossible to extract any free energy from them to push the sail along.

"Carnot's rule says there must be a degradation of energy in any machine that turns out free energy," Gold says in the New Scientist report. "A mirror does not have any degradation."

This does not mean sunlight cannot exert a force- comet tails point away from the sun, and are often cited as evidence in favour of solar sails. But Gold says this is because a comet tail is not a perfect mirror: it absorbs some of the light.

In this scenario Carnot's rule says some energy can be extracted, so long as the object absorbing the light remains cooler than the radiation itself. A solar sail that absorbed photons would heat up within seconds, Gold argues.

The claim has been greeted with scepticism. "There may be limits on how much solar radiation can be turned into work, but I do not think these are thermodynamic limits," says Jeffrey Lewins, a thermodynamics expert at the University of Cambridge.

But Gold insists that thermodynamics does have to be taken into account. "It's no good saying, 'I cannot turn heat into free energy, but I can if I turn the heat into radiation first'," he says. "That's obviously nonsense."

Steven Soter, an astronomer at the Hayden Planetarium in New York, is open to Gold's idea. He says applying conservation of momentum to photons could be a mistake.

"Light is very different from matter, and one may wonder if the momentum rules are also different." There may also be evidence to support Gold's theory, in the form of a quirky device called a Crookes radiometer.

It consists of four paddles attached to the arms of a rotor, inside a vacuum jar. Each paddle is silvered on one side and coated with a black absorber on the other.

When placed in sunlight, the rotor spins. If the theory of solar sailing is right, the rotor should spin with the reflecting silver surfaces moving away from the light. But it actually spins the other way, just as Gold predicts.

The dispute could be settled in September, when the Pasadena-based Planetary Society hopes to launch Cosmos 1, the world's first solar sail. The 100-kilogram craft will be sent into orbit around the Earth, before unfurling a set of reflecting blades in an attempt to boost its altitude. Louis Friedman, the project's director, is undaunted by Gold's criticism. "Solar sailing is possible," he insists.

This article is based upon a report by Paul Parsons which appears in the July 5 issue of New Scientist

Quite a controversy with added potential for more embarrassment for NASA.

Gold sounds correct to me. Without a temperature change, how is work performed?

IIRC, a Crookes radiometer spins because its dark side absorbs solar energy, heats up, and then emits ir at a higher temperature than its silvered reverse which merely reflects the photons striking it. If the analogy holds, it seems it would make more sense to have a dark solar wing rather than one that's a "perfect" mirror. And what's the calculated effect of that absorption, thermal or momentum transfer?

We know it's human nature to want something for nothing (ask any huckster). But one would expect scientists were above that level of "faith." Maybe it comes from the contemporary culture at NASA where funds seem to arrive like manna from heaven (despite a growing reputation for failure) simply for having faith in government?

Ah yes, one of the miracles of our Age of Incompetence. HOSANA is, afterall, a jumble of OH, NASA.

#18. The Solar Wind

The first indication that the sun might be emitting a "wind" came from comet tails, observed to point away from the Sun, whether the comet was approaching the Sun or whether it was moving away. Kepler in the early 1600s guessed that those tails were driven by the pressure of sunlight, and his guess still holds true for the many comet tails which consist of dust.

Comet Halley, with
plasma tail kink

ion

Comet Hale-Bopp

Sunlight pressure cannot explain such behavior, but in 1943 Cuno Hoffmeister in Germany, and later Ludwig Biermann, proposed that apart from sunlight, the Sun also emitted a steady stream of particles, a "solar corpuscular radiation" which pushed the ions. Variations in the speed of the particles would explain the accelerations, and the tail did not point straight away from the Sun because the flow velocity of the particles was not too many times larger than the velocity of the comet itself.

Parker's Theory

No one gave a good reason why this "particle radiation" should exist, until Eugene Parker of the University of Chicago in 1958 tried to derive the equilibrium structure of the corona. One expects the corona at great distances to dwindle away to zero pressure and density, but Parker found that the conduction of heat interfered with such an equilibrium and instead another solution suggested itself, in which the topmost layers of the corona flowed away from the Sun at a velocity like that of Biermann's "corpuscular radiation." The flow was named "solar wind" and its existence was later confirmed by instruments aboard spacecraft.

The solar wind shapes the Earth's magnetosphere and supplies energy to its many processes. Its density at the Earth's orbit is around 6 ions per cubic centimeter--far, far less than that of the "best vacuum" obtainable in labs on Earth. The distribution of ions in the solar wind generally resembles the distribution of elements on the Sun-- mostly protons, with 5% helium and smaller fractions of oxygen and other elements. (There are electrons too, of course, counteracting the positive charge of the ions and keeping the plasma electrically neutral.) All this flows away from the Sun with a mean speed of about 400 km/sec, and as shown by the Voyager 2 space probe, this flow extends past the outermost planets, more than 30 times more distant from the Sun than Earth, and it probably continues much further than that.

The Interplanetary Magnetic Field

The regions where the solar wind starts are immersed in the Sun's magnetic field (though perhaps in regions where that field is relatively weak). However, plasma outflows from regions of magnetic fields can spread those fields to wherever they arrive. This happens by "field line preservation," a property derived from the equations of an ideal plasma. By those equations, in an ideal plasma ions and electrons which start out sharing the same magnetic field line continue to do so later on, as if the line were a (deformable) wire and the particles beads threaded by it.

If the energy of the magnetic field is dominant, its field lines keep their shapes and particle motion must conform to them; that is what happens in the radiation belts. On the other hand, if the energy of the particles is dominant--that is, if the field is weak and the particles dense--the motion of the particles is only slightly affected, whereas the field lines are bent and dragged to follow that motion. That is the case with the solar wind.

Imagine a field line extending from the bulk of the Sun to the upper corona. The particles at its "roots" stay with the Sun, but those in the high corona flow out with the solar wind, to the Earth's orbit and far beyond. All that time (under ideal conditions--a fair approximation) the same field line continues to link both groups. Thus some solar field lines will extend to the Earth and further out, producing the interplanetary magnetic field (IMF). It is the IMF that allows the solar wind to "pick up" the ions in a comet's ion tail, as it also did to an "artificial comet" produced in a 1985 experiment (see positive ions, "clouds of barium ions"). As will be seen, the IMF plays a major role in linking the magnetosphere to the solar wind.

Further Exploring

The sister-site "From Stargazers to Starships" which also discusses the Sun and the solar wind, includes an optional section for deriving and drawing the shape of interplanetary magnetic field lines, using the concept of field-line preservation. That section was added to "Exploration" and it is linked here.

Next Stop: #18H. The Solar Wind--History

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