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Cutting Through Interstellar Dust

When the British Interplanetary Society’s Daedalus designs were being created in the 1970s, the scientists and engineers involved quickly realized that interstellar dust would become a problem for a vehicle traveling at 12 percent of light speed. That led to shielding concepts involving materials like beryllium, boron and graphite. But what of concepts like Robert Forward’s vast lightsails? If dust posed a problem to Daedalus on its way to Barnard’s Star, surely a huge lightsail was even more threatened, there being no effective way to shield it.

Forward himself suggested an answer in a 1986 letter to the Society’s journal. His optimum sail materials (still far beyond our capabilities) would be much thinner than the diameter of the interstellar grains the starship would likely encounter. The result: such materials would pass right through the sail, creating a hole about as big as themselves. For work within the range of nearby stars, Forward believed, interstellar dust would not pose a significant problem, an assumption that, as far as I am aware, has yet to be modeled in any laboratory experiments.

But we’re learning more about interstellar dust through space-based observatories like the Spitzer Space Telescope. Space dust is believed to be composed of the same essentials from which the Earth is made: carbon, silicon, magnesium, iron and oxygen, and a further assumption has long held that this material was produced by red giants as they aged. That assumption is now challenged by observations of a supernova in the spiral galaxy NGC 628, findings that begin to fill in our knowledge of interstellar materials. They suggest that supernovae are significant contributors to dust clouds between the stars.

In the case of supernova 2003gd, infrared measurements taken 500-700 days after the supernova event by the Spitzer Space Telescope show that solid dust particles in an amount equal to 7000 Earth masses had formed. “2003gd is, quite literally, the smoking gun,” says Doug Welch (McMaster University). “These carbon and silicon dust particles which form from the supernovae blast make possible the many generations of high-mass stars and all the heavy elements they produce. These are elements which make up the bulk of everything around us on Earth, including you and me.”

Observational work like this will one day have to be supplanted by missions to take in situ readings of what spacecraft encounter outside the heliosphere. The Innovative Interstellar Explorer (IIE), a NASA Vision Mission study now being developed, is one possibility, although payload constraints make dust measurements in its earliest designs problematic. Another possibility: a second-generation mission like Claudio Maccone’s FOCAL, which might one day piggyback a suite of instruments aboard an observatory designed to make use of the lensing properties at the Sun’s gravitational focus. Missions to nearby stars are in our future, but there is a vast amount of science to be done just outside the Solar System.

For those interested, Forward’s correspondence on lightsail shielding appears in the Journal of the British interplanetary Society 39, p. 328 (1986). A. R. Martin discussed shielding for Daedalus in “Bombardment by Interstellar Material and Its Effects on the Vehicle,” Project Daedalus Final Report (JBIS, 1978), pp. S116–S121. The recent findings on supernovae appear in Sugerman, Ercolano, Barlow et al., “Massive-Star Supernovae as Major Dust Factories,” published in the June 8 Science Express edition of the journal Science.