by Paul Gilster | Mar 13, 2008 | Outer Solar System |
One question jumps out at me from the blog entries that Cassini team members have been posting on the probe’s dazzling close pass by Enceladus. It’s from deputy project scientist Linda Spilker, who says: “I am thinking about the two Voyager flybys of the Saturn system that took place over 25 years ago. How in the world did we miss the Enceladus plumes back then???” Indeed, but that’s the nature of exploration, to learn something new each time you revisit a place, especially one that’s fully 10 AU out. The process is addictive, and breathtaking.
Do be aware of the flyby blog, offering an inside view of one of the most interesting of Cassini’s encounters thus far (also be aware that the entries are oddly out of order, a problem apparently being fixed). With the data downlink now started (as of about 0201 UTC today) via the Deep Space Network’s Goldstone station, we can ponder the chutzpah of taking a spacecraft so close to the huge geysers erupting out of the south pole of Enceladus. Cassini came within 200 kilometers of the surface as it flew through the outer edge of the plumes, and closed to a mere fifty kilometers at closest approach.
Image: Not from the latest flyby (images from that later), this false-color view of Enceladus has been enhanced to identify individual jets making up the plume. The images combined to create this view were acquired with the Cassini spacecraft narrow-angle camera on Nov. 27, 2005 at a distance of approximately 148,000 kilometers (92,000 miles) from Enceladus and at a sun-Enceladus-spacecraft, or phase, angle of 161 degrees. Credit: NASA/JPL/Space Science Institute.
Raw images are hitting the Web as I write (click the ‘Browse Latest 500 Raw Images’ arrow, but be patient; the server is apparently being slammed). And don’t miss this flyby video.
In terms of danger to the spacecraft, the threat is smaller than it at first appears, especially when you consider that Cassini deals with dust-size particles routinely as it orbits around Saturn. And the science return should be rich, with information on the density, size, composition and speed of the gas and particles collected. Sascha Kempf (Max Planck Institute, Heidelberg), deputy principal investigator for Cassini’s Cosmic Dust Analyzer, notes that various theories rise or fall on the outcome:
“There are two types of particles coming from Enceladus, one pure water-ice, the other water-ice mixed with other stuff. We think the clean water-ice particles are being bounced off the surface and the dirty water-ice particles are coming from inside the moon. This flyby will show us whether this concept is right or wrong.”
The halo of ice dust around Enceladus supplies Saturn’s E-ring with material from the apparently continuous eruptions. It’s known to be composed of water vapor, carbon dioxide, methane, possibly ammonia and other trace gases, but Cassini will now find out whether the gases coming out of the plume are identical with those surrounding the moon. The result may give us insights into the processes driving the eruptions. With four more Enceladus flybys planned for this year, the success of this encounter may determine whether the next one is even closer.
by Paul Gilster | Mar 12, 2008 | Asteroid and Comet Deflection |
The ever reliable Dennis Overbye gives us a look at the Earth’s fate in his most recent story for the New York Times. Citing the work of Klaus-Peter Schroeder (University of Guanajuato, Mexico) and Robert Connon Smith (University of Sussex), Overbye describes our planet’s eventual engulfment by a red giant Sun. Earlier studies had questioned whether the Earth might survive this phase, but Smith and Schroeder say no. Their calculations show a red giant Sun 256 times as wide as today’s star, and fully 2730 times more luminous. And it will swallow the Earth.
I was interested to see Overbye’s reference to a 2001 paper that, in the spirit of speculative jeu d’esprit familiar in good science fiction, looks at a way to save the planet. But first, let’s run through where our star is heading. Burning through its hydrogen on the main sequence, the Sun should keep getting hotter and larger. Figure 1.1 billion years until you reach the point where it is 11 percent brighter than today, creating a greenhouse effect that devastates the biosphere. By the time you’re 3.5 billion years out, the Sun is forty percent more luminous than today and any surviving vestiges of life as we know it should be gone.
But ponder this: We are at present less than halfway through the main sequence life of the Sun. In six billion years, even though the Sun’s luminosity should be a factor of 2.2 greater than its current value, a planet just 1.5 AU out would receive about the same amount of solar energy that Earth now receives. Thus the notion of increasing the radius of the Earth’s orbit, which Don Korycansky and our friend Greg Laughlin (both at UC-Santa Cruz), working with Fred Adams (University of Michigan), describe. Do this right and the lifespan of the surface biosphere gets a five billlion year reprieve. Here’s the basic idea, as drawn from the paper:
An attractive scenario for gradually increasing the Earth’s orbital radius is to successively de?ect a large object or objects from the outer regions of the solar system (the Oort Cloud or the Kuiper Belt) onto trajectories which pass close to the Earth. By analogy to the gravity-assisted ?ight paths employed by spacecraft directed to outer solar-system targets… the close passage of such an object to the Earth can result in a decrease in the orbital energy of the object and a concomitant increase of the Earth’s orbital energy.
A science fiction scenario par excellence! And one that recalls Stanley Schmidt’s Sins of the Fathers, which I read long ago (1973-74) when it ran as an Analog serial. There the idea was to move a threatened Earth (the galactic core has exploded) with the help of ingenious alien technologies to a new and safer home. A more recent example of world moving is Robert Metzger’s Cusp (2005), where mysterious alien technologies are activated, the Singularity arrives and stars and planets fly (not to mention ideas galore). I had the good fortune to discuss Cusp with Metzger over lunch in Chapel Hill a couple of years back — if you’re into hard SF that doesn’t let up, be sure to read him.
But back to the paper under discussion. Just how would we move the Earth? The authors work out a typical mass for large Edgeworth/Kuiper Belt objects (1022 grams) and calculate that some 106 passages involving a ‘cumulative flyby mass’ of about 1.5 Earth masses would do the trick, moving the Earth to 1.5 AU. An average of one pass every 6000 years over the remaining lifetime of the Sun ought to do the job, which doesn’t sound as onerous as one might have expected.
And, of course, we have an Edgeworth/Kuiper Belt stuffed with objects larger than 100 kilometers in diameter. Here we’re on speculative ground, but the authors figure the belt should hold as many as 105 such bodies, with a total of perhaps 10 percent of the Earth’s mass. And then there’s the Oort Cloud, thought to hold 1011 objects totaling thirty or more Earth masses. Small trajectory changes would bring many an Oort object into an Earth crossing orbit (Laughlin makes sure that Overbye understands, in the Times article, that he’s not advocating such a dangerous move — one mistake takes out our planet — but simply sketching out the boundaries of the possible).
What’s happening in the rescue scheme is that repeated gravity assists in effect transfer orbital energy from Jupiter to the Earth. A close pass by the Earth by an object in a highly elliptical orbit transfers energy from the object to the Earth. Outbound, the object crosses Jupiter’s orbit, timed to encounter the planet and pick up the energy lost to Earth. The paper spells out the procedure in details that point to a long-term and ‘almost alarmingly feasible’ scenario, one that would use technologies that, although beyond our current capabilities, are by no means beyond the powers of a more advanced civilization. The long-term result seems promising:
Due to the acceleration of the Sun’s luminosity increase, the encounters must be more frequent as the Sun approaches the end of its main-sequence life. In order to use the same secondary body for many encounters, modest adjustments in its orbit are necessary. However, by scheduling the secondary body to encounter additional planets (e.g., Jupiter and/or Saturn) in addition to the primary Earth encounter, the energy requirements for orbital adjustment at the object’s aphelion can be substantially reduced. In particular, the energy consumed by such course corrections is not likely to dominate the energy budget.
And note this interesting fact: The energy required to move the Earth is modest compared to that needed for interstellar travel. Working out the numbers, the team finds that the scheme is actually highly efficient when compared to interstellar migration and compares favorably with various terraforming projects that have been examined in the past. Usefully for more contemporary concerns, the basic methods might also be utilised to move hazardous asteroids, or to set up delivery mechanisms for useful Edgeworth/Kuiper Belt materials whose resources could be exploited.
The paper is Korycansky, Laughlin and Adams, “Astronomical engineering: a strategy for modifying planetary orbits,” Astrophysics and Space Science 275 (2001), pp. 349-366 (abstract). It’s one you don’t want to miss.
by Paul Gilster | Mar 11, 2008 | Exotic Physics |
How things change over time has never been as strikingly demonstrated as in recent findings. If you go back to the distant era when the universe was only 380,000 years old, you find that neutrinos made up fully ten percent of the universe. Given that these sub-atomic particles moving at nearly the speed of light are so abundant today that millions of them pass through us every second, you’d think they compose a substantial portion of today’s universe, but they actually account for less than one percent. And the change in neutrino ratio is only the beginning.
For according to five years of recently released data from the Wilkinson Microwave Anisotropy Probe (WMAP), the early universe was composed of 12 percent atoms, 15 percent photons, almost no dark energy, and 63 percent dark matter. The contrast is stark, given WMAP estimates of the current cosmos: 4.6 percent atoms, 23 percent dark matter, 72 percent dark energy. And, of course, those greatly diminished neutrinos.
Image: WMAP data reveal that its contents include 4.6% atoms, the building blocks of stars and planets. Dark matter comprises 23% of the universe. This matter, different from atoms, does not emit or absorb light. It has only been detected indirectly by its gravity. 72% of the universe, is composed of “dark energy”, which acts as a sort of an anti-gravity. This energy, distinct from dark matter, is responsible for the present-day acceleration of the universal expansion. WMAP data is accurate to two digits, so the total of these numbers is not 100%. This reflects the current limits of WMAP’s ability to define Dark Matter and Dark Energy. Credit: NASA / WMAP Science Team
Usefully, the neutrino information jibes with theories based on the amount of helium we see today, which predict a cosmic neutrino background present when the helium was made. That, in turn, fits with measurements of neutrino properties made in particle accelerators here on Earth. The WMAP data release thus carries us forward, using observation that confirms theories and helps us stake out evidence for an era long vanished. The seven papers just submitted to the Astrophysical Journal about these data further enhance WMAP’s position as a breakthrough instrument.
Other WMAP results of interest: While the process that drives cosmic ‘inflation’ is still problematic (a rather serious understatement on my part), the new data make it possible to eliminate particular versions of inflation while leaving others viable. WMAP principal investigator Charles Bennett (Johns Hopkins) puts it this way: “The new WMAP data rule out many mainstream ideas that seek to describe the growth burst in the early universe.” The new constraints will power future investigations.
Image (click to enlarge): A representation of the evolution of the universe over 13.7 billion years. The far left depicts the earliest moment we can now probe, when a period of “inflation” produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP was emitted ~400,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light; it also forms a backlight for later developments of the universe. Credit: NASA / WMAP Science Team
WMAP also indicates that the ‘cosmic fog’ produced when the first generation of stars began to shine first appeared when the universe was some 400,000 years old. What an era that would have been — the first stars put an end to the darkness, creating a fog of electrons in the surrounding gas that scattered microwaves. The microwaves WMAP looked at to help scientists make these assessments represent light that has lost energy over the subsequent 13.7 billion years of the universe’s expansion, but is still robust enough to tell an extraordinary tale.
For more, see this news release on the WMAP data and its implications. More on the papers in The Astrophysical Journal when they become available.
by Paul Gilster | Mar 10, 2008 | Exoplanetary Science |
A new observing program designed to study planets around small, cool stars is in the works. TEDI, the TripleSpec – Exoplanet Discovery Instrument, saw first light on the 200-inch Hale Telescope just before Christmas, and is now in its commissioning phase, with an observing program scheduled to begin this spring. And for those who occasionally wonder why we seldom discuss stars like Barnard’s Star or Proxima Centauri in terms of the planet hunt, read on. For TEDI is the kind of program that should be able to survey not just M dwarfs but L and T class stars as well, opening exciting possibilities for discovery.
Planets around Proxima Centauri? Perhaps, and extending all the way down to T-class brown dwarfs makes things interesting as well. But finding such planets is a challenge with conventional radial velocity methods. Here’s why: Radial velocity searches are generally conducted in the optical band, and work well with stars, like the Sun, that are bright at these wavelengths. The small, cool stars TEDI is going after are much brighter in the near infrared than the optical. TEDI is the first instrument specifically designed to study the spectral shifts that flag radial velocity changes — the effects caused by an orbiting planet — in the infrared.
The TEDI program leverages the power of Externally Dispersed Interferometry (EDI), which boosts the power of spectrographs by inserting a small interferometer in the beam. EDI methods can be used on existing spectrographs, creating features in the processed light that extract useful data and boost the resolution of the spectrograph by factors of 2 to 6 times. The Exoplanet Tracker team at the University of Florida used EDI methods to detect a planet around HD 102195, the first confirmed EDI find.
The TEDI project at the 200-inch Mt. Palomar instrument uses EDI coupled with an infrared spectrograph called TripleSpec, built at Cornell University. Developed with the Cornell team and researchers from the University of California, Berkeley and Lawrence Livermore National Lab, TEDI’s expected result is described on the EDI site:
The interferometer will add fringes to the spectrum which will greatly increase the sensitivity to Doppler shifts caused by a planet tugging on the star. The infrared (0.8 – 2.5 micron) sensitivity of Triplespec will allow detections of planets around small cool stars that emit too little visible light to be included in current Doppler searches.
This is exciting stuff because of the ubiquity of smaller stars in our galaxy. M dwarfs alone are thought to account for 75 percent or more of all stars in the Milky Way. That makes understanding the kind of planets that form around them crucial in working our way through various planet formation theories. Perhaps just as significant is the fact that low-mass stars should more readily yield up information about smaller planets once we’re able to refine the methods for measuring their Doppler shifts. Extending radial velocity methods to these stars could allow us to begin taking the measure of stars like Proxima Centauri and its ilk.
Unrelated to TEDI but perhaps of interest is the fact that one reason the Project Daedalus team chose Barnard’s Star as its destination for an interstellar probe was the belief that the star had planets. Astronomer Peter Van de Kamp (Sproul Observatory, Swarthmore College) took 25 years of data on Barnard’s Star before announcing he had detected a wobble that indicated a planet of 1.6 Jupiter masses orbiting at 4.4 AU. As many as three planets were ultimately posited, but John Hershey, also at Swarthmore, was able to show that problems with the Swarthmore instrument were the actual reason for the mistaken detection.
With the radial velocity game much further refined, we can assume no such errors with TEDI. The plan is to observe 100 M dwarfs, 25 L dwarfs and 10 T dwarfs. More on the program and the needed tests for getting it up to speed is available in Edelstein et al., “TEDI: the TripleSpec Exoplanet Discovery Instrument,” slated to appear in SPIE Volume 6693, Techniques and Instrumentation for Detection of Exoplanets III (available online). Thanks to Mike Wirth for valuable pointers on this story.
by Paul Gilster | Mar 8, 2008 | Outer Solar System |
As if we didn’t have enough trouble getting to the outer Solar System, now comes word that the US inventory of plutonium-238 is diminishing. That’s what NASA administrator Mike Griffin told a House appropriations subcommittee this past week, pointing out that after the Mars Science Laboratory launches in 2009, the agency will find itself running out of the plutonium needed to fuel radioisotope power systems. Even New Horizons, on the way to Pluto/Charon, is using Russian plutonium, the periodic purchase of which has been forced by NASA’s dwindling supplies.
New Horizons’ principal investigator Alan Stern told the committee that beyond the Mars Science Laboratory, NASA probably has enough plutonium on hand or on order to fuel the outer planets flagship mission targeted at 2017 and an interim Discovery class mission scheduled to fly a few years earlier, the latter intended, ironically enough, to test more efficient radioisotope power systems now under development.
Meanwhile, the outer planets continue to surprise us with findings like the apparent ring system around Saturn’s moon Rhea. Or is it just a halo of dust and debris? Geraint Jones (University College, London) is lead author on a paper looking at the Rhea possibilities. In the current issue of Science, Jones and colleagues note that the small moon (some 1500 kilometers in diameter) is circled by a debris disk perhaps 5000 kilometers from end to end, with particles ranging from small pebbles to boulders. Simulations show that such rings could be stable for lengthy periods.
Image: An artist concept of the ring of debris that may orbit Saturn’s second-largest moon, Rhea. The suggested disk of solid material is exaggerated in density here for clarity. Due to a decrease in the number of electrons detected by NASA’s Cassini spacecraft on either side of the moon, scientists suggest that rings are the likeliest cause of these electrons being blocked before they reach Cassini. Credit: Jet Propulsion Laboratory.
Are we looking at a ring system composed of the remnants of an ancient asteroid or comet collision with the moon? Saturn’s lively system shows evidence of other catstrophic impacts of the sort that could have created the needed debris field. And just as we’re now looking at asteroids with moons (the Eugenia occultation is imminent — see yesterday’s story here), we also deal with the intriguing thought that, as Jones puts it, “…Rhea seems to have some family ties to its ringed parent Saturn.”
The paper is Jones et al., “The Dust Halo of Saturn’s Largest Icy Moon, Rhea,” Science Vol. 319, No. 5868 (7 March 2008), pp. 1380-1384 (abstract). See also this Los Alamos National Laboratory news release.
