by Paul Gilster | Jul 24, 2009 | Outer Solar System |
If you’ll examine the cover of Claudio Maccone’s new book carefully, you’ll see an interesting object at the lower right. It’s a spacecraft with two deployed antennae connected by a tether. The book is Maccone’s Deep Space Flight and Communications, whose subtitle — ‘Exploiting the Sun as a Gravitational Lens’ — tells us much about the author’s view of how early interstellar missions should proceed. And Maccone devoted a session at the recent conference in Aosta to these matters, making the case for taking advantage of this natural phenomenon.
Uses of Gravitational Lensing
We’ve looked at the Sun’s gravitational lens, and the FOCAL mission Maccone champions to exploit it, many times here on Centauri Dreams. But for newcomers, gravitational focusing has been an active astronomical tool since 1978, when a ‘twin’ image of a quasar was found by the British astronomer Dennis Walsh. The gravitational field of a galaxy between the Earth and the quasar had bent the light from the more distant object, yielding the double image. And it was back in 1979 that Von Eshleman studied the Sun’s own gravitational focus at 550 AU, pondering how we might send a spacecraft there to study its effects.
The Sun at that distance becomes a huge celestial magnifier, and Dr. Maccone has been arguing that before we send a mission to any star, we should send a probe to this much closer target in the exactly opposite direction. Diffracting effects from the Sun’s corona may distort the image at the minimum 550 AU distance, so we may have to go farther, but we’re still talking about getting a spacecraft to well less than 1000 AU to begin making its observations. In fact, the further the probe travels, the less distortion from the solar corona, and the focal line extends to infinity.
A Tethered Solution to the Antenna Problem
Below is an enlarged image of the FOCAL spacecraft with tether clearly visible. The key point is that sending such a mission would avail us little if we couldn’t take advantage of its trajectory to get an adequate image of the focused light from the other side of the Sun. Maccone told the assembled scientists in Aosta that working with a gravitationally-lensed image would involve more than a single antenna, but noted that tether technology could be used to solve the problem. Hence the twin antennae of the image.
An email from Dr. Maccone this afternoon clarifies the idea of using what radio astronomers call ‘aperture synthesis’:
While the spacecraft is moving away from the Sun, the tether is gradually released, and so two Archimedean SPIRALS are covered by the two antennae, yielding a full radio picture of the radio source, with a FIELD of view MUCH LARGER than the one provided by a single antenna.
Tethers have numerous space applications but they’re tricky to test on the ground because we can’t simulate zero gravity conditions sufficiently to study their characteristics. That has made for numerous space missions using tethers, dating all the way back to the latter days of the Gemini program. But note this: Maccone is talking about tethers of no more than several kilometers in length. The largest tether ever deployed successfully was on the YES-2 experiment conducted by the European Space Agency in 2007. YES-2 deployed a 31.7 kilometer tether in space, and robust work continues on the tether concept.
Maccone is convinced that a tethered system of antennae can resolve the daunting imaging issues posed by a probe at 550 AU, with stunning capabilities at magnifying the object being imaged. Thus the possibility of getting high quality images of any planets of interest in a target solar system, down to small details on the planetary surface. It’s worth remembering, too, that the distance a FOCAL probe would need to reach to study the Centauri stars is 278 times smaller than the actual distance of those stars. And the view that is potentially achievable from that distance far surpasses anything available through other kinds of observatories.
An Idea with a History
Deep Space Flight and Communications (Springer, 2009) represents Maccone’s current thinking on FOCAL as it has evolved over the years. Be aware that he was discussing the idea as far back as a 1992 conference in Turin, and in 1993 submitted a formal proposal to the European Space Agency to fund the mission design. Many things have changed in the time since, all summarized in the new book. As he did in the book, Maccone developed his ideas on the Karhunen Loeve Transform (KLT) and its uses for signal processing and analysis at the Aosta conference, showing how the KLT becomes a tool for improving the signal-to-noise ratio in SETI work as well as for communications with a fast-moving FOCAL probe.
Image: Claudio Maccone (right) explains the uses of the Karhunen Loeve Transform (KLT) in signal processing to me at a fine lunch we enjoyed in the Italian Alps (note the cheese plate that sits between us — fabulous!). This photo was snapped by Roman Kezerashvili (City University of New York).
It’s interesting to see the tether concept being applied here. And I also want to note that Michel van Pelt’s new book Space Tethers and Space Elevators (Copernicus, 2009) goes into considerable detail on the history of tether missions in space — the number is far higher than I had realized. I’ll have more to say about tethers and their uses once I’ve finished the book, but it’s clear that refining the tether concept is going to take time, and that its potential uses are so beneficial as to make it a high priority.
by Paul Gilster | Jul 23, 2009 | Exoplanetary Science |
The hunt for exomoons — satellites of planets around other stars — gets more interesting all the time. This morning I received a note from David Kipping (University College London), who has been studying methods for finding such objects. Kipping and colleagues have a paper soon to be published by Monthly Notices of the Royal Astronomical Society that discusses how to detect habitable exomoons using Kepler-class instrumentation. And it turns out that finding such worlds is well within our present capabilities.
A bit of background: Kipping’s method is to analyze two useful sets of signals. Transit timing variations (TTV) are variations in the time it takes a planet to transit its star. Kipping and team acquire these data and then weave the TTV information together with what is called transit duration variation (TDV). The latter is detectable because as the planet and its moon orbit their common center of mass, velocity changes can be observed over time. Put TTV and TDV together and exomoon detections become possible.
Image: A tropical moon around a gas giant in an imagined solar system. We should soon begin to learn whether such worlds are common. Credit: Dan Durda (SwRI).
The new paper extends exomoon studies to habitable zones, and its findings are exciting. Assuming the kind of photometry available to the Kepler mission (and, in short order, ground-based telescopes), exomoons in the habitable zone down to one-fifth of an Earth mass should be within range of our instruments. Indeed, habitable exomoons around M, K and lighter G-class stars up to 100-200 parsecs away should be detectable. Kipping notes that up to 25,000 stars within Kepler’s field of view could be surveyed for exomoons of up to one Earth mass, while an extended survey of the galactic plane could extend the number to two million.
The most likely planet for an exomoon detection? A world something like Saturn, as Dr. Kipping explained in his email message:
…we find that low-density planets, like Saturn, provide a much better chance of detecting an exomoon, as opposed to say a Jupiter-like planet. This is because the transit depth increases with planetary radius, allowing for a more precise transit time measurement. In contrast, a low-mass planet allows for large wobbles in the orbital motion and therefore a large exomoon signature.
An additional finding is that we should be able to find lower-mass exomoons around M-dwarfs because their habitable zones are much closer to the star, allowing a larger number of transits for the observing time available. Given that these studies show a sensitivity down to 0.2 Earth masses, it’s also interesting to see in the paper that 0.3 Earth masses is the minimum habitable mass for a planet as calculated by earlier theorists.
How common are Earth-like exomoons? We don’t know, but note this (from the paper):
Our results suggest it is easier to detect an Earth-like exoplanet than an Earth-like exomoon around a gas giant. However, we have no statistics to draw upon to estimate which of these scenarios is more common. If a roughly equal number of both are discovered, it would indicate that the latter is more common due to the detection bias.
And that’s where we are now, waiting for the tsunami of data that Kepler and later space-based missions should provide us. The prospect of finding habitable moons around distant planets boggles the mind, but Kipping makes a strong case for our ability to make such a detection within a few short years. In any case, equipment like Kepler should be able to find such worlds if they exist. All-sky surveys focusing on M-dwarf stars would be ideally suited for continuing the hunt.
The paper is Kipping et al., “On the detectability of habitable exomoons with Kepler-class photometry,” accepted by Monthly Notices of the Royal Astronomical Society and available online.
by Paul Gilster | Jul 22, 2009 | Asteroid and Comet Deflection |
It’s a lively Solar System indeed. In yet another confirmation of the value of amateur astronomy, Australia’s Anthony Wesley tipped off scientists on July 19 that a new object had struck Jupiter and observatories around the world zeroed in on the event. It comes exactly fifteen years after the ‘string of pearls’ comet Shoemaker-Levy 9 struck the giant planet. Infrared images show a likely impact point near the south polar region, visible in the image below.
Image: A large impact shown on the bottom left on Jupiter’s south polar region captured on July 20, 2009, by NASA’s Infrared Telescope Facility in Mauna Kea, Hawaii. Image credit: NASA/JPL/Infrared Telescope Facility.
Unlike Shoemaker-Levy 9, this event may have been caused by a single object. UC Berkeley and SETI Institute astronomer Franck Marchis explains:
“The analysis of the shape and brightness of the feature will help in determining the energy and the origin of the impactor. We don’t see other bright features along the same latitude, so this was most likely the result of a single asteroid, not a chain of fragments like for SL9.”
It’s entertaining to note that UC Berkeley astronomer Paul Kalas, in Greece at the time, decided to use scheduled observing time on the Keck II telescope in Hawaii to investigate the Jupiter strike because he read about it on Marchis’ blog. So we have the astronomy world lit up by an Australian amateur making a find that’s quickly disseminated through various channels including weblogs, triggering quick and intense observing sessions with instruments worldwide. Hubble’s Wide Field Camera 3 will soon be on the case as well with visible and ultraviolet observations.
A planetary scar the size of the Pacific Ocean gets the attention, and should remind us, as did Shoemaker-Levy 9, that although the days of the Late Heavy Bombardment are long gone, there are objects wandering around the system that could end our civilization. The need for a mission to a near-Earth object to learn more about these impactors should be apparent, and it’s a technology driver for later deep space missions as well.
by Paul Gilster | Jul 21, 2009 | Exotic Physics |
Tibor Pacher has been kind enough to publish the text of my public lecture in Aosta, Italy on his PI Club site. The lecture took place at the Aosta town hall and wasn’t part of the ongoing conference just down the street, although some conference participants attended. It’s a broad overview of earlier work on interstellar flight. My intention was to acquaint non-scientists with the fact that the subject has been under study for decades in ways that do not violate the laws of known physics. A major challenge is how to scale some of the colossal engineering involved down to realistic levels.
Although I only touched upon it in the lecture, I often talk about the twin tracks of interstellar studies. The first track comprises work that tries to scale current technology up for an interstellar mission. The second track is oriented toward examining physical laws in hopes of finding potential breakthroughs that current theory doesn’t allow. No one knows if such breakthroughs are possible, but we want to keep banging on prevailing ideas to see if there are areas that need revision.
Into the Eclipse
On that score, be aware of tomorrow’s total solar eclipse, which gets particular attention because six Chinese teams will be monitoring the event to look for the possibility of anomalous gravitational effects. Does gravity undergo a slight change during a total eclipse? The French physicist Maurice Allais noted unusual behavior in a swinging pendulum back in 1954 during the eclipse that passed over Paris that year. Since then, measurements to pin down what is going on have been inconclusive. A New Scientist story provides more background, as does this NASA page, which details subsequent, often contradictory follow-ups.
The first step in this kind of investigation is to find out whether the anomaly actually exists. The six Chinese monitoring sites will include gravimeters and pendulums, and will encompass a large enough area (3000 kilometers between those that are farthest apart) that local changes in weather or problems with instrumentation should be ruled out. If General Relativity is in need of a tweak, these results could provide a clue.
Explaining an Anomaly
Back in 2004, Chris Duif (Delft University of Technology, The Netherlands) looked at anomalous eclipse observations to see whether they could be explained by seismic disturbances, meteorological conditions, changes in the geomagnetic field or other possibilities. He found none of these solutions satisfactory, and went on to write:
Although, despite all proposed conventional explanations fail to explain the observations either qualitatively or quantitatively, it is still possible that the reported anomalies will turn out to be due to a combination of some of these effects and instrumental errors. And, of course, there may be yet unidentified conventional causes which play a role. The judgment of some of the experimental results is hampered by the lack of a statistical analysis and/or data of sufficient length. Nevertheless, there exist some strong data which cannot be easily explained away.
So we’ll see what turns up in China. New Scientist quotes Tang Keyun (Chinese Academy of Sciences), as saying: “If our equipment operates correctly, I believe we have a chance to say the anomaly is true beyond all doubt.” That would be a fascinating result, and one that would lead to a great deal of new theorizing about how to incorporate the eclipse anomaly into our current views on how gravity works.
The Duif paper is “A review of conventional explanations of anomalous observations during solar eclipses,” available online.
by Paul Gilster | Jul 20, 2009 | Culture and Society |
I sometimes wonder whether Neil Armstrong wrestled all the way to the Moon with what he would say when he stepped out onto the surface. The answer is probably tucked away somewhere in the abundant literature on the Moon landings. I know that if it were me, I’d be turning over the options in my mind for months in advance. What do you say upon achieving what is obviously one of the most significant accomplishments in history? Did Armstrong ponder alternatives even as he descended from the lander?
In any case, the words carried a great truth. Giant leaps are made up of small steps, and not just the first step of a single astronaut leaving a footprint. It wasn’t just a Saturn V that got Apollo 11 to the Moon — it was also Einstein, and Newton, and Leibniz, and thousands of mathematicians, physicists, engineers and yes, philosophers throughout history whose work pushed the possibility forward. This is, not coincidentally, the philosophy of the Tau Zero Foundation: ad astra incrementis. To the stars one step at a time. The operating principle is that each step is a little bigger than the last.
My uncle had come up from Florida for his yearly visit when Apollo 11 landed. I remember that he and I both found one moment in the descent utterly magical. It was when Buzz Aldrin called out ‘picking up some dust.’ Eagle was descending over that fractal landscape — very hard to tell just how high you were using vision alone — but suddenly there was the confirmation. The little craft was low enough that its engine was pushing around dust that had lain undisturbed for millions of years, and here were human beings seeing that with their own eyes.
I used to think that Armstrong’s ‘one giant leap for all mankind’ statement was too canned, a bit of celestial boilerplate. But over the years I’ve come to appreciate it more and more. The philosopher Lao Tzu said “You accomplish the great task by a series of small acts.” Most of us lose sight of the larger picture in the minutiae of daily life, but the small acts are the things we do every day that accumulate and, when chosen well, push the envelope a little bit further. Armstrong and Lao Tzu remind us that it’s time not only to celebrate Apollo 11’s achievements, but also to get back to work.
