by Paul Gilster | Sep 27, 2004 | Exoplanetary Science
The Hubble Space Telescope cost far more to build and launch than the twin telescopes of Hawaii’s W.M. Keck Observatory, but Keck gathers twenty times the light and offers four to five times better resolution. Assuming, of course, that we can find a way to cancel out the effects of atmospheric blurring on its images. That’s where the science of adaptive optics comes in. By using a bright reference beacon nearby, an observatory like Keck can analyze atmospheric effects even as its observations are being made.
That this technique can be used in the hunt for extrasolar planets is clear. The infrared image to the right (credit: Michael Liu, IFA-Hawaii/W. M. Keck Observatory) shows a dust disk surrounding the star AU Microscopii. The image is 100 AU wide, roughly the size of our Solar System; Keck’s images are the sharpest ever obtained of a circumstellar disk, with an angular resolution of 1/25 of an arcsecond, about 1/500,000 the diameter of the full moon.
But adaptive optics has a catch: the small number of sufficiently bright nearby stars limits these methods to about one percent of the sky. Now, according to Keck scientists at the Mauna Kea site, a way around this problem has been discovered. Called the Laser Guide Star Adaptive Optics system, the new method removes the bright star limitation by placing a laser guide star as a reference in front of any target. The result: wide-field imaging with adaptive optic techniques, and unprecedented resolution for an Earth-based telescope.
From the observatory’s news release on the laser system:
By using a laser to create a “virtual star,” astronomers can study any object in the vicinity of much fainter (up to 19th magnitude) objects with adaptive optics and reduce its dependence on bright, naturally occurring guide stars. Doing so will increase sky coverage for the Keck adaptive optics system from an estimated one percent of all objects in the sky, to more than 80 percent.
“This new capability of using a laser guide star with a large telescope has invited astronomers to start exploring the night sky in a much more comprehensive manner,” said Adam Contos, optics engineer at the W.M. Keck Observatory.”In the future, I would expect most major observatories to be installing similar systems to take advantage of this incredible enhancement to their AO capabilities.”
Keck has already been a major player in the hunt for extraterrestrial planets; witness its involvement in the recent discoveries around 55 Cancri, which used Keck data. The new laser methods promise even more remarkable results. For more on the whole field of adaptive optics, check the Center for Adaptive Optics Web site.
by Paul Gilster | Sep 24, 2004 | Exoplanetary Science
At 12 light years away, Tau Ceti is the nearest Sun-like star, and has long been of high biological interest among possible interstellar probe targets. But a British team using the James Clerk Maxwell Telescope in Hawaii (and aided by the world’s most sensitive sub-millimeter camera, called SCUBA) has found a disk of cold dust around the star that bodes ill for stability among any planets that may be orbiting there.
Says Jane Greaves, lead scientist on the study:
“Tau Ceti has more than ten times the number of comets and asteroids that there are in our Solar System. We don’t yet know whether there are any planets orbiting Tau Ceti, but if there are, it is likely that they will experience constant bombardment from asteroids of the kind that is believed to have wiped out the dinosaurs. It is likely that with so many large impacts life would not have the opportunity to evolve.”
Image: Bombardment of a hypothetical planet around Tau Ceti: bad news for life? Credit: David Hardy.
Frank Drake made Tau Ceti a target in his Project Ozma investigations in 1960, the first attempt at radio-based SETI. The recent findings hardly resolve the issue, at least until we’ve learned more about the nature of the dust cloud around Tau Ceti and its dynamics, but Drake’s other target, Epsilon Eridani, begins to look a little more attractive.
Source: Particle Physics and Astronomy Research Council. PPARC is the UK’s strategic science investment agency. See also Greaves et al, “The debris disc around τ Ceti: a massive analogue to the Kuiper Belt,” Monthly Notices of the Royal Astronomical Society, Volume 351, Issue 3, pp. L54-L58. There is an abstract here. A quick but thorough backgrounder on Tau Ceti can be found at the SolStation site.
by Paul Gilster | Sep 23, 2004 | Breakthrough Propulsion, Culture and Society, Sail Concepts
On the left is the cover of the first paperback edition of Poul Anderson’s Tau Zero, published in 1970 (a shorter version called “To Outlive Eternity” appeared in 1967 in Galaxy Science Fiction, though unseen by me, as I was getting ready to leave for college). The first hardcover edition is below.
Many of the scientists I talked to in doing the research for Centauri Dreams told me they read science fiction, and most favored the ‘hard’ SF, scrupulously accurate to science as understood at the time, favored by writers like Anderson. And several said that it had been Tau Zero that got them into physics or engineering in the first place.
Here’s Anderson’s look at a Bussard ramjet as it consumes interstellar hydrogen on a runaway journey that will never end:
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…
The Bussard ramjet is today considered unworkable, producing more drag that the thrust it might generate. But the idea of absorbing your fuel from the medium through which you fly is so provocative that if there is any way to save this concept, we will certainly try to find it. If not, the ramjet has still led to productive ideas like the magsail that grew out of Dana Andrews’ and Robert Zubrin’s work.
Recommended reading (in addition to Tau Zero itself):
D. G. Andrews and R. M. Zubrin, “Magnetic Sails and Interstellar Travel,” International Astronautical Federation Paper IAF-88-5533, Bangalore, India, October 1988.
Robert Zubrin, Entering Space: Creating a Spacefaring Civilization (New York: Tarcher/Putnam, 1999.
by Paul Gilster | Sep 22, 2004 | Missions, Outer Solar System
NASA has just announced that it has selected Northrop Grumman Space Technology as the contractor for co-designing its proposed Jupiter Icy Moons Orbiter. JIMO will be designed to orbit and explore three of the most interesting Jovian moons: Callisto, Ganymede and Europa. All three may possess water, organic material and a source of energy, leading to the possibility of some form of life evolving there.
Image: The surface of Europa as seen by the Galileo orbiter. Note the crustal blocks on the left that seem to have once broken apart, and then ‘rafted’ into their current positions. They’re evidence of what may be a sub-surface ocean. Credit: Planetary Image Research Laboratory, University of Arizona.
Studying these moons closely will involve long periods in orbit around each before moving on to the next target. The propulsion system envisioned here is nuclear electric. NASA’s Deep Space 1 spacecraft has already demonstrated the principle, in which electrically charged particles are expelled to produce thrust. In contrast to Deep Space 1, which used solar arrays to power the system, JIMO will carry a small nuclear reactor, and would also tap the power of the reactor for its science investigations, with significant benefits to the mission.
NASA’s Prometheus Project is charged with developing the nuclear power options for JIMO. According to the agency’s FAQ on Prometheus: “The amount of power available from a nuclear reactor – potentially hundreds of times greater than that available to current interplanetary spacecraft — would enable delivery of larger payloads with vastly more capable instruments and faster data transmission back to Earth than such missions as Voyager, Galileo, and Cassini. In addition, because extremely fuel-efficient electric thrusters would propel the spacecraft, mission planners could make course adjustments throughout the mission in response to real-time discoveries.”
by Paul Gilster | Sep 22, 2004 | Missions, Outer Solar System
Worth noting in relation to the JIMO story above (and for the broader issue of generating power for deep space probes): “A Power Conversion Concept for the Jupiter Icy Moons Orbiter,” by Lee S. Mason (Journal of Propulsion and Power Vol. 20 No. 5, 1 September 2004, pp. 902-910). From the abstract: “An analytical study was performed to compare design options for a reactor power system that could be utilized on a Jupiter Icy Moons Orbiter mission employing nuclear electric propulsion.”
