by Paul Gilster | Aug 22, 2005 | Sail Concepts
The theories behind solar sailing go back a long way — some would trace them to the days of Kepler. But papers on actual mission design began to emerge only in the 1950s, when ‘Russell Saunders’ (the pseudonym of aeronautical engineer Carl Wiley) wrote a sail description for a 1951 issue of John Campbell’s Astounding Science Fiction. That paper was followed by Richard Garwin’s more technical analysis in a 1958 issue of the journal Jet Propulsion.
Now another paper from that era has surfaced and is available online. T.P. Cotter’s “An Encomium on Solar Sailing” was produced in 1958 as an informal report for Los Alamos (originally no more than an office memorandum), and was issued in more comprehensive form in 1973. Cotter described his intent as not to break new ground but to fill in the details of the solar sail idea through an actual design, an unmanned sail mission to Mars.
The Cotter sail is a flat, circular disk some 10-4 cm thick and 500 meters in diameter. It is made of a plastic film coated on one side with 20 or 30 micrograms per square centimeter of aluminum. The sail is made in panels with reinforced seams to prevent tears. Pulling a 250 kg payload, the sail would be set spinning shortly after deployment. Here’s Cotter’s description of that process:
The capsule consists of two parts connected through a universal joint, and provided with a motor which now causes the sail package and the main part of the load to start counterrotating. At the appropriate times the collapsed structural backbone is extended, the capsule cases are jettisoned and the vehicle blossoms forth under centrifugal force… The sail is spinning at the rate of one revolution in two minutes. The load, two-thirds of which is suspended in three pods by wires at 50 meters from the axis of spin, is counter-rotating at a rate of 10 revolutions per minute. The vehicle as a whole has zero net angular momentum.
Image: The artwork for Carl Wiley’s article “Clipper Ships of Space,” which ran in 1951 in Astounding Science Fiction. Wiley’s was the first scientific paper on the topic of solar sailing and mission design.
Cotter notes that the ‘indefinitely sustained thrust’ of the sail simplifies interplanetary navigation enormously, for course correction capability is always present using a sail. “The penalty for a non-optimum trajectory or an actual navigational error is only to the flight time, which is hardly the case for a rocket operating with a small fuel margin.”
The Cotter paper is “An Encomium on Solar Sailing,” Report No. LA-5231-MS, May 1973. As I understand it, this was once a restricted access paper at Los Alamos, now declassified and made available through the Web site of the Federation of American Scientists. The two other papers mentioned above are Carl Wiley (Russell Saunders), “Clipper Ships of Space,” Astounding Science Fiction, May 1951, pp. 135ff and “Richard Garwin, “Solar Sailing: A Practical Method of Propulsion within the Solar System,” Jet Propulsion 28 (March 1958), pp. 188-90.
by Paul Gilster | Aug 20, 2005 | Outer Solar System |
Just how young is the average meteorite? One way to study the question is through the chondrules that make up stony meteorites. Chondrules are mineral deposits formed by rapid cooling; they give the appearance of tiny, spherical bits of glassy rock. Stony meteorites are generally called chondrites because they contain such chondrules. And it’s generally assumed that chondrites formed in the early Solar System in the condensation of the first solid materials.
But University of Toronto geologist Yuri Amelin and Alexander Krot (University of Hawaii) now have data that call that conclusion into question. Their paper in an August issue of Nature reports on chondrules that are the youngest ever found. The researchers used meteorites named Gujba and Hammadah al Hamra, studying their minerological structure and fixing an approximate isotopic age. “It soon became clear that these particular chondrules were not of a nebular origin,” says Amelin. “And the ages were quite different from what was expected. It was exciting.”
In fact, the chondrules Amelin, Krot and their colleagues found post-date the oldest asteroids. Amelin again: “We think these chondrules were formed by a giant plume of vapour produced when two planetary embryos, somewhere between moon-size and Mars-size, collided.”
Perhaps the formation of the Solar System wasn’t as cut and dried as some theories suggest. Indeed, this finding is the first suggestion that some chondrules and the meteorites that contain them formed at considerably different times than others, which implies that the planets — at least in early form — were already there when some chondrules were made. Another suggestion that our understanding of planetary formation is far from comprehensive.
The paper is Alexander N. Krot, Yuri Amelin et al., “Young chondrules in CB chondrites from a giant impact in the early Solar System,” Nature 436, (18 August 2005), pp. 989-992.
by Paul Gilster | Aug 19, 2005 | Administrative
Unless I am traveling, Centauri Dreams publishes Monday through Saturday. Entries are usually available by early afternoon EST, with the exception of the Friday and Saturday postings, which may appear later in the day. This schedule will occasionally be modified as events warrant. A lack of posts for several days running simply means I am on the road (I don’t carry a computer when I travel).
by Paul Gilster | Aug 19, 2005 | Administrative
Of the many contributions of Robert L. Forward to interstellar studies, the bibliography he produced with Eugene Mallove was one of the most useful to theorists in the field. The last appearance of the Forward/Mallove collaboration was in the Journal of the British Interplanetary Society in 1980, including 2700 items in seventy subject categories; since then, anyone hoping to track ongoing research has been forced to do so one journal at a time, or else use online database sources that are in many cases incomplete and often do not include older materials.
Image: The extraordinary Robert Forward, wearing one of the trademark vests created by his wife Martha. Forward chose this photograph to appear on his own Web site.
One of my goals is to restore the interstellar bibliography to regular publication under the auspices of the Interstellar Flight Foundation. Many scientists recall Forward’s antimatter newsletter, which circulated among a few hundred physicists for a brief period. In both his bibliography and newsletter, Forward recognized the value of keeping researchers informed about the work of their peers. The fragmentary nature of today’s bibliographies is an unnecessary impediment to progress. IFF’s eventual restoration of the Forward bibliography will be a contribution to furthering interstellar studies.
But the Foundation is in its early stages (expect more news as things continue to develop this fall). Until such time as a full-scale bibliography becomes possible, I will continue to review recent articles dealing with interstellar flight, aiming at a more regular survey of the leading journals with short commentaries on studies that stand out in terms of insight and innovation. These will be interspersed with Centauri Dreams‘ usual mix of news and analysis on all subjects pertaining to interstellar flight.
by Paul Gilster | Aug 19, 2005 | Exoplanetary Science
Response to the August 12 post about Fermi’s Paradox was heavy, an indication that the physicist’s famous question — Where are they? — will not go away. A number of readers asked for background on my statement (drawn from Milan ?irkovi?’s paper) that the average age of Earth-like planets in the Milky Way is now thought to be 6.4 billion years, an indication that there should be planets that have had a two billion year head start on Earth in terms of evolving intelligent life.
The number 6.4 billion is a broad estimate, to be sure, but it has been the subject of intense investigation by Charles H. Lineweaver and colleagues. Lineweaver’s key paper “An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect” ran in Icarus Vol. 151, No. 2 (2001), pp. 307-313 (available here in PDF form), and was followed by a paper in collaboration with Yeshe Fenner and Brad Gibson titled “The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way.” The latter ran in Science Vol. 303 (2004), pp. 59-62, and is also available as a PDF online.
Both papers, and particularly the earlier study, are worthy of more time than I could give them in the August 12 post. Look for a fuller discussion of each on Centauri Dreams in coming weeks.
