by Paul Gilster | Oct 9, 2007 | Culture and Society |
A note from James Benford, soon to be followed by e-mail from other interstellar advocates, tells me of the death of Robert Bussard. The creator of the Bussard ramjet concept, Bussard (1928-2007) died of cancer in Santa Fe just a few days ago. Benford, who knew Bussard for forty years, recalls his open attitudes and deep technical insight, adding “He was still sharp as a pin into old age.” We should all be so lucky.
Recently we’ve seen a lot of discussion about Bussard’s fusion ideas, but it’s the ramjet that I return to as I think about him. If you collect classic papers, as I do, here’s one for you: Bussard’s “Galactic Matter and Interstellar Spaceflight” in Acta Astronautica 6 (1960), pp. 179-94. Imagine a scoop created by a magnetic field that sucks in interstellar hydrogen ionized by a forward-firing laser. The result is fed into a fusion reactor. Get the vehicle up to about six percent of light speed and you could light that engine, with presumably amazing results.
At least, that was the idea. Assume one hydrogen atom per cubic centimeter as the density of the interstellar medium and a one-ton probe would demand a scoop 6,000 kilometers in size, but to be realistic about interstellar densities, you’d probably need one as large as 60,000 kilometers to account for hydrogen density variations. The concept was indeed breathtaking even if later work made its success dubious (drag is a major problem, leading to the idea of using magnetic sails for deceleration into a target system).
But imagine going at one g continuously. When Carl Sagan went to work on that one, he got to Alpha Centauri in three years (ship time, obviously), but as the vessel picked up speed, things got more and more interesting. The Andromeda galaxy could theoretically be reached in about 25 years, assuming you’re willing to live with the two million years that passed in the interim back on Earth. Poul Anderson’s starship Leonora Christine in the novel Tau Zero grew directly out of Bussard’s idea.
Image: A Bussard ramjet could theoretically get a crew to Andromeda. Credit: ESA/ITSF/Mancu.
Anderson’s description of the Bussard ramjet at work is superb:
The ship was not small. Yet she was the barest glint of metal in that vast web of forces which surrounded her. She herself no longer generated them. She had initiated the process when she attained minimum ramjet speed; but it became too huge, too swift, until it could only be created and sustained by itself … Starlike burned the hydrogen fusion, aft of the Bussard module that focused the electromagnetism which contained it. A titanic gas-laser effect aimed photons themselves in a beam whose reaction pushed the ship forward—and which would have vaporized any solid body it struck. The process was not 100 per cent efficient. But most of the stray energy went to ionize the hydrogen which escaped nuclear combustion. These protons and electrons, together with the fusion products, were also hurled backward by the force fields, a gale of plasma adding its own increment of momentum . . . The process was not steady. Rather, it shared the instability of living metabolism and danced always on the same edge of disaster…
Suffice it to say that if you haven’t read Tau Zero (and yes, we did name the Tau Zero Foundation after the book, and Poul’s wife Karen is on our board of directors), then you should make haste to your nearest bookseller or library. I think Bussard must have taken great pleasure from reading Tau Zero, as well as knowing that his stunning starship concept would galvanize theoretical research into interstellar propulsion for decades to come. He’s been part of my thinking on interstellar matters for the last forty years, a loss everyone involved in our deep space future will take to heart. Our thoughts and condolences go out to his family.
by Paul Gilster | Oct 9, 2007 | Deep Sky Astronomy & Telescopes |
Following up on our recent discussion of interstellar distances and how they are determined comes word of a reassessment of the distance to the Orion Nebula. The star forming region is famous not only for its beauty but for the opportunity it gives us to assess young stars as they emerge from the interstellar gases around them. Their distance tells us something about their intrinsic brightness and thus their ages.
The change in distance revealed in the new studies is considerable. Whereas the previous best estimate to the Nebula was 1565 light years, the new one, drawn with an uncertainty of six percent, is 1270 light years, a twenty percent adjustment. The Very Long Baseline Array was behind this work, using familiar parallax methods to observe a star called GMR A from opposite sides of Earth’s orbit.
“This measurement is four times more precise than previous distance estimates,” says Geoff Bower (UC-Berkeley). “Because our measurement reduces the distance to this region, it tells us that the stars there are less bright than thought before, and changes the estimates of their ages.”
And what a change. These stars are twice as old as once thought.
What we’re still doing — and we’re early in the process — is getting an approximation of the three-dimensional structure of nearby interstellar space. VLBA is ideal for this work because its ten 25-meter radiotelescope dishes stretch from the Pacific (Hawaii) to the Caribbean (Virgin Islands), allowing it to produce images of remarkably high resolution. Huge amounts of work remain to be done as we adjust distances to various targets. It’s amazing to consider that if we somehow found a way to reach the stars tomorrow, we’d still be faced with the same conundrum experienced by sailors in the 16th Century, the absence of reliable maps.
VLBA has also made observations of star-forming regions in Taurus and Ophiuchus as well as examinations of the Milky Way’s spiral arms and pulsars. With operations managed from Socorro, New Mexico, it’s the world’s largest dedicated, full-time astronomical instrument. The new findings appear as Sandstrom et al., “A Parallactic Distance of 389 +24/-21 parsecs to the Orion Nebula Cluster from Very Long Baseline Array Observations,” accepted by The Astrophysical Journal (abstract).
by Paul Gilster | Oct 8, 2007 | Advanced Rocketry |
Please use this post for further comments in the above thread, which originally appeared under the title “A Practical Positron Rocket?”
by Paul Gilster | Oct 8, 2007 | Exoplanetary Science |
The news from Transitsearch couldn’t be better. Long a champion of amateur involvement in the exoplanet hunt, I was delighted to see, via Greg Laughlin’s systemic site, that this globally dispersed team of amateur astronomers is behind the confirmed transit observation of the planet HD 17156 b. Amateurs in Italy, the Canary Islands and California made key observations in early September, with confirmatory data coming in from Massachusetts and California on the night of September 30/October 1 as observers heeded Laughlin’s online call to participate.
Greg has the details and more about the individual observers at his site. The Transitsearch mode is to look at known planet-bearing stars during those times transits might conceivably occur. And it makes stunningly good sense because of two facts: 1) The tools available to dedicated amateurs today are fully capable of this kind of high-quality work; and 2) Telescope time at the major observatories around the world is obviously limited. The Internet allows this network of amateurs to collaborate, making serious contributions to our exoplanetary knowledge.
HD 17156 b is quite an interesting place. Its radius seems to be a bit larger than Jupiter’s, while its orbital period is 21.2 days. Note that this is almost four times longer than any other known transiting exoplanet. Note too that HD 17156 b’s orbit is highly eccentric, so much so that the planet experiences a 26-fold variation in the amount of flux it receives from its star. Interesting weather patterns doubtless emerge, as Laughlin notes, with the night side glowing with its own radiation. I poached the image below from the systemic site, but have a look at the full animation, showing Jonathan Langton’s hydrodynamical study of this world.
So there’s the method: Use Doppler radial velocity data to find exoplanets and check for transits with an ad hoc network of amateurs coordinating their work over the Internet. Can the Spitzer space telescope catch HD 17156 b in secondary transit? If so, says the discovery paper, that will enable “…a much-improved constraint on the still-uncertain radius of the parent star. In the event that secondary transits can be observed, a direct measurement of the excess tidally generated luminosity from the planet is a distinct possibility…”
The paper is Barbieri, Alonso et al., “HD 17156b: A Transiting Planet with a 21.2 Day Period and an Eccentric Orbit,” available online. Nice work!
by Paul Gilster | Oct 8, 2007 | Asteroid and Comet Deflection |
A possible impact in the Laurentide ice sheet in northeastern North America some 13,000 years ago is the subject of a new National Geographic special. Called “Mammoth Mystery,” the show ran yesterday and will replay multiple times this week. A clip from the show is available online. This is the impact (discussed in these pages in late May) that is implicated, some believe, in the extinction of the mammoth and mastodon, with presumably devastating effects on local human populations.
A press conference on this event is now available on YouTube, while National Public Radio’s Science Friday show offers its coverage here. The paper is Firestone et al., “Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas,” Proceedings of the National Academy of Sciences 10.1073/pnas.0706977104 (27 September, 2007). Abstract online.
