by Paul Gilster | Aug 11, 2005 | Outer Solar System
Asteroid 87 Sylvia, known since 2001 to be part of a double system, has just gotten more interesting. A team of astronomers at the University of California at Berkeley and the Observatoire de Paris have now established that the asteroid is actually part of a triple system, the first ever discovered. Using data from the European Southern Observatory’s Very Large Telescope array in Chile, the team found two small moons in nearly circular orbits.
Astronomical names are always a challenge, but in this case naming comes easy. 87 Sylvia was named for Rhea Sylvia, the mother of the founders of Rome. Thus it made perfect sense for Berkeley’s Franck Marchis to propose Romulus and Remus as the names of the moons. The suggestion has already been approved by the International Astronomical Union.
Image: Artist’s conception shows twin moonlets, Romulus and Remus, orbiting the large main-belt asteroid 87 Sylvia. Credit: European Southern Observatory.
Calling them ‘moons’ should not obscure the fact that these are tiny objects. 87 Sylvia, which orbits between Jupiter and Mars roughly 3.5 AU from the Sun, is potato-shaped and measures some 380 x 260 x 230 kilometers. Romulus orbits about 1360 kilometers from the asteroid and measures a scant 18 kilometers across, while Remus is a tiny 7 kilometers in diameter.
The observations of these tiny satellites help astronomers calculate the mass and density of 87 Sylvia. It seems to be composed of water ice and rubble from an ancient asteroid and could, according to co-discoverer Daniel Hestroffer at the Observatoire de Paris, be composed of up to 60 percent empty space. From an ESO news release:
“It is most probably a “rubble-pile” asteroid”, Marchis added. These asteroids are loose aggregations of rock, presumably the result of a collision. Two asteroids smacked into each other and got disrupted. The new rubble-pile asteroid formed later by accumulation of large fragments while the moonlets are probably debris left over from the collision that were captured by the newly formed asteroid and eventually settled into orbits around it. “Because of the way they form, we expect to see more multiple asteroid systems like this.”
Centauri Dreams‘ note: The Romulus and Remus story comes from the Roman historian Livy (d. 17 A.D.). The complete background is nicely summarized here, but the charm of the story comes from the abandoned twins being nursed by a she-wolf after being set adrift on the Tiber. Taken in by a shepherd, they eventually built their settlement on the Palatine Hill. Romulus, having killed Remus in a dispute, made himself king of the city. Marchis says he chose Romulus for the larger moon because of the name’s derivation from the Greek word for strength, whereas Remus (which is related to Latin remorari, meaning to linger or delay) seemed appropriate for the small, inner moon.
The paper is Marchis, F., Descamps, P., Hestroffer, D. et al., “Discovery of the first triple asteroidal system 87 Sylvia,” Nature 436 (August 11, 2005), pp. 822-824. The Observatoire de Paris press release is also available, and so is this release from UC-Berkeley, which contains links to some interesting animations of the triple system.
by Paul Gilster | Aug 10, 2005 | Outer Solar System
The recent images from Cassini’s flyby of Mimas remind us how violent the history of the early Solar System was. Now a study at the Australian National University shows that three huge asteroids — between 20 and 50 kilometers across and traveling as a cluster — collided with the Earth some 3.2 billion years ago. The result: volcanic eruptions, the creation of major fault lines, earthquakes and a variety of disruptive effects deep inside Earth’s crust.
This work grows out of evidence of extraterrestrial impact deposits from this era discovered in South Africa and extends the effects of those impacts to the Pilbara region of Western Australia. According to an ANU news release, the materials studied in the eastern Transvaal had indicated that impact craters several hundreds of kilometers in diameter had been created in oceanic parts of the Earth. In the Australian studies, the impacts coincide with the formation of fault escarpments and fault troughs and a major volcanic episode.
Image: Artist’s concept of a catastrophic asteroid impact with the Earth. Life near the impact would be instantly wiped out from the effects of high temperatures and pressures. Injection of huge masses of dust (and gases) into the atmosphere would effectively block out sunlight for long periods of time to the point that most life could not be sustained (“Nuclear Winter”). Credit: Don Davis/NASA.
“The precise coincidence of the faulting and igneous activity with the impact deposits, coupled with the sharp break between basaltic crust and continental formations, throws a new light on the role of asteroid impacts in terrestrial evolution,” Dr. Andrew Glikson said. Glickson has been working on these studies with John Vickers from the Department of Earth and Marine Sciences at ANU.
Centauri Dreams‘ note: Another result of this investigation is the indication that the Moon was affected at roughly the same time by major asteroid impacts and resurgent vulcanism. Any way you look at it, a forming planetary system seems to be a place of chaos, as witness the growing evidence that our Moon was caused by a collision between the Earth and a body perhaps the size of Mars. We should expect the same situation to be occurring in young extrasolar planetary systems. Possible corroboration is found in the intense dust cloud around the Sun-like star BD +20 307 in Aries (discussed earlier in Centauri Dreams), where major collisions between planetoids seem to have occurred.
by Paul Gilster | Aug 9, 2005 | Exoplanetary Science
A star in the constellation Serpens may be a close match for our own Sun. HD 143436 (also known as HIP 78399, from its listing in the Hipparcos survey) is an 8th magnitude star that’s 140 light years from Earth and visible with binoculars. According to a recent article on the star by astronomer Ken Croswell, both its spectral type and its absolute magnitude are closely similar to Sol, and the star appears to be equally hot, with a temperature of 5768 Kelvin vs. the Sun’s 5777 K. In terms of mass, HD 143436 is the Sun’s twin.
Finds like this are intriguing because they raise the possibility that similar stars have similar solar systems, within which may lurk a terrestrial world. No one knows whether this star has a planet like Earth around it — or any planets at all, for that matter — but we do know that the close stellar match calls for further work. Exactly how old HD 143436 is remains conjectural, with an uncertainty either way of 2.9 billion years. The star may be as old as the Sun, or considerably younger, with potential ramifications on the development of intelligent life on any planets around it.
The work on HD 143436 is by Jeremy King and Simon Schuler, both of Clemson University, and Ann Boesgaard at the University of Hawaii; their paper “Keck/HIRES Spectroscopy of Four Candidate Solar Twins,” which will be published in The Astronomical Journal in November, can be found online at the arXiv site. King and colleagues studied four solar twin candidates, two that turned out to be older and metal-poor compared to the Sun, one which is significantly older, and HD 143436 itself, which they say “…may be equivalent to the ‘closest ever solar twin.'”
The astronomers’ work points out that solar analogs like this one give us the ability to learn more about the evolution of planets. From the paper:
“…it has been suggested that solar-type stars may be subject to highly energetic superflare outbursts, perhaps induced by orbiting planets, that would have dramatic effects on atmospheres surrounding and lifeforms inhabiting orbiting planets. It also seems clear that the nominal non-stochastic gradual evolution of solar-type chromospheres has important implications for a diversity of planetary physics (in our own solar system and others): the structure and chemistry of planetary atmospheres, the water budget on Mars, and even the evolution of planetary surfaces; such issues are critical ones to understand in the development and evolution of life.”
Centauri Dreams‘ note: Serpens is actually a divided constellation, consisting of Serpens Caput (the Head of the Serpent, and the segment in which HD 143436 is found) and Serpens Cauda (the Tail of the Serpent). The two parts are divided by the constellation Ophiuchus (the Serpent Holder).
by Paul Gilster | Aug 8, 2005 | Outer Solar System
Can any other surface in the Solar System be this battered? The twin views below show Saturn’s moon Mimas as imaged by Cassini on August 2. Note how the false-color brings out the variation across this tortured surface — at left is an enhanced clear-filter image, at right a color composite of ultraviolet, infrared and clear-filter images. The most prominent feature in both is the 140-kilometer wide Herschel Crater, an ancient strike that is today filled with landslide material. Close study reveals numerous other craters and long grooves similar to those found on asteroids.
Are these grooves related to the enormous impact that created the Herschel crater? No one knows, but study of this moon’s turbulent history may help scientists understand how many impactors have moved through the Saturn system, a reminder of how dangerous a place the Solar System can be when you’re in the crosshairs of an approaching piece of space debris.
Mimas is a tiny place, measuring just 397 kilometers across. Its value to us at present is its durability, for it appears that the moon’s surface features date all the way back to its creation. That gives us a superb geological record of what can happen to any celestial object over time. Be sure to look at the Cassini team’s ‘Flying Over Mimas’ animation, taken during a period of seven hours during the recent encounter.
Image: False color images of Saturn’s moon, Mimas, reveal variation in either the composition or texture across its surface. During its approach to Mimas on Aug. 2, 2005, the Cassini spacecraft narrow-angle camera obtained multi-spectral views of the moon from a range of 228,000 kilometers (142,500 miles). Credit: NASA/JPL/Space Science Institute.
Centauri Dreams‘ note: The B612 Foundation continues to promote an attempt to significantly alter the orbit of an Earth-crossing asteroid in a controlled manner by 2015. Citing the probability of an ‘unacceptable collision’ in this century of two percent, the group’s project comes into vivid focus when we examine objects like Mimas. The era of massive cratering may be over, but impacts still occur, and a sensible program to identify and tag Earth-crossing objects while developing the technologies to adjust their orbits should be a high priority of the space program.
by Paul Gilster | Aug 5, 2005 | Astrobiology and SETI, Culture and Society |
A continuing preoccupation at Centauri Dreams is long-term thinking. What can we as a species do to extend our time-frame beyond the infuriating short-term outlook of today, so that we can start thinking realistically about shaping a future beyond our own lifetimes? This kind of thinking will be necessary when we build our first interstellar probes, traveling journeys that will surely take decades and may involve centuries. What will drive us to think and plan within the millennial time frames that would allow humans to expand into and throughout the galaxy?
Novelist Stephen Baxter addresses this question in a recent paper in the Journal of the British Interplanetary Society. Baxter points out the enormity of the time challenge: Voyager 1, the fastest human object ever built, travels at some 17.3 kilometers per second. It would reach Alpha Centauri (if headed in that direction) in 73,000 years. But starships that can reach 0.1c are not beyond possibility. If we can develop them, it’s possible to imagine a wave of colonization that slows to half that speed in the time needed to colonize and industrialize new worlds.
What you wind up with is human penetration of the entire galaxy in a time frame of perhaps a million years. In discussing these matters, Baxter draws on the work of Michael Hart (citation below). And he notes that a 100-year mission to Alpha Centauri seems within the reach of what he calls ‘conscious time.’ That is to say, we’ve already seen examples of programs that last several centuries, including the British Empire (growth and dissolution) and the establishment of the USA. Such projects are not fully defined by their originators, “…but they do show a continuity of vision, cultural values and purpose,” Baxter writes, “and they may originate in conscious decision-making.”
Image: The Milky Way as seen by an approaching traveler. Will humanity’s migration into the Galaxy be a matter of will, or a playing out of instinctive imperatives? Art copyright Jon Lomberg, www.jonlomberg.com. Lomberg’s superb work is well known; among other projects, he was chief artist for Carl Sagan’s series ‘Cosmos,’ and this image is part of the animation sequence that opened the series.
While not as sanguine as Baxter (I think a century-long project would require a drastic — and long overdue — reorientation of priorities in today’s society), I am cheered by the prospect that “…in the long term, it will only take a few refugees, for whatever motive, to flee from one crowded star system to the next to sustain an onward expansion.” The most significant settlers of North America tended to come, Baxter notes, for ideological, not economic, reasons, and a few refugees is all it takes to keep interstellar migration going.
Beyond ‘conscious time,’ in Baxter’s notion, is ‘dreaming time.’ Here we are talking of thousand year time frames, with expansion beyond the early outposts to a large number of nearby stars. A human institution that covers such a time frame: the Roman empire, which dominated the West for a thousand years. Baxter believes an idea of themselves that went beyond conscious planning enabled the Romans to create such a culture, and sees a continuity in such cultures that goes beyond intellect into the subconscious. And here is the crux:
What is important in this context is that ‘dream’-induced motivations appear to work themselves out at timescales longer than the ‘conscious’ – over millennia rather than centuries. And therefore what may unite us with our much-transformed starfaring descendants, over the epochs of the second Hart timescale, will not be abstractinos of the intellect but more mystical ties. Perhaps we will build starships as we build cathedrals, as repositories of faith sailing into the future.
Finally, Baxter discerns what he calls ‘genetic time,’ driven by the interests of biology and human genes. If the working out of genetic imperatives drives our species, it ultimately becomes inappropriate to talk of ‘purpose’ on the largest time scales. Our movement across the Galaxy may take place in the same way species move into ecological niches, driven by an expansion that the species seems unable to curb consciously. Such a movement ‘…may be carried aboard starships maintained with the unthinking accuracy with which a bower bird builds her nest — and for similar purposes.”
Note several key Baxter assumptions: first, that we will remain restricted by known physics; a warp drive, needless to say, changes everything. Second, that while we may find life on other worlds, it will probably not be intelligent. The discovery of sentient aliens along the route of expansion clearly creates a whole new set of possibilities!
Baxter’s paper is “A Human Galaxy: A Prehistory of the Future,” in JBIS Vol. 58, pp. 138-142. The Michael Hart paper he draws from is “An Explanation for the Absence of Extraterrestrials on Earth,” in Quarterly Journal of the Royal Astronomical Society 16, pp. 128-135, a study that deserves (and will soon get) a review in these pages.
