Deep Space Transmission Strategies

One reason we need to re-think our communications strategies is that our resources are so limited. The Interplanetary Internet Project, for example, points out as a major justification for its work that if we can network spacecraft in distant planetary environments, we can sharply cut back on the amount of antenna time needed. After all, having a trio of spacecraft (including an orbiter, perhaps, and two rovers) linking their data to a single relay would mean a unified data download to Earth.

Goldstone antenna

The IPN idea would create new networking protocols that could make these things happen. But research continues on other fronts, including the technology we use to transmit signals. An upcoming paper discusses phased arrays, wherein a large number of mini-transmitters could send a combined beam into the sky. Systems like these are familiar to those working with military radar but have been hitherto unavailable for cost reasons for civilian uses. The paper, by Louis Scheffer at Cadence Design Systems (San Jose, CA) discusses the advantages of such phased arrays with comparisons to digital cell phone technology and a consideration of the steep savings offered by saving transmission time over the Deep Space Network’s huge dishes.

Image: The Goldstone antenna, a 70-meter-diameter (230-foot) dish capable of tracking a spacecraft travelling more than 16 billion kilometers (10 billion miles) from Earth. Can we find ways to make the Deep Space Network’s antennae more efficient and cost-effective? Credit: NASA/JPL.

From an American Geophysical Union press release:

Currently, planetary radars and distant spacecraft communications need transmitters with extremely high power, which has been accomplished by combining a strong microwave source with a large reflective antenna. This is now done with giant satellite dishes mechanically steered to a point in the sky. NASA’s Goldstone radar, for example, the agency’s sensitive, deep-space analysis radar, uses a 500 kilowatt transmitter and a 70-meter [230-foot] reflector for tracking asteroids that may collide with Earth. The large antenna is focused on only a small point in space at a time, and must be adjusted–and occasionally shut down–due to changing weather conditions. In addition, Scheffer points out that while almost all of the world’s largest antennas are used to both send and receive, the powerful transmissions severely hinder their ability to detect faint signals from space.

What Scheffer has in mind as an alternative is a large, flat array of transmitters controlled by computers and capable of sending a powerful beam in one or several directions simultaneously. Scheffer’s system is transmit-only, and thereby useful for spacecraft control while freeing up existing antennae to operate in receive-only mode. It leverages the mass-production technologies now common to all kinds of electronics to arrive at the conclusion that an antenna similar to Goldstone’s could be built for a quarter of the cost. Existing work in phased radar arrays makes this conclusion plausible.

Centauri Dreams‘ note: Scheffer talks about a phased communications array being used for asteroid research as we scan the skies for near-Earth objects. But he’s also willing to speculate on the potential for such arrays when it comes to SETI signals sent by us to nearby stars. The technologies for producing such signals are rarely discussed in SETI literature, but have an obvious bearing on the likelihood of other civilizations deciding to create them in the first place (are we, as some have speculated, a galaxy full of listeners with no one in the transmission business?)

The paper is Scheffer, L. “A Scheme for a High-Power, Low-Cost Transmitter for Deep-Space Applications,” slated to appear in Radio Science in its October issue (Volume 40).

The Best Way to View Terrestrial Worlds

Centauri Dreams has been a champion of Webster Cash’s New Worlds Imager for several years now. The proposal, whose initial study was funded by NASA’s Institute for Advanced Concepts, offered a way to find terrestrial planets around other stars and, in its most fully developed configuration, to create startlingly sharp images of such worlds down to the level of continents and weather patterns moving across their surfaces. Now two new developments — related in a phone call from Cash last week — bring New Worlds Imager to the fore as NASA weighs strategies for its Terrestrial Planet Finder mission.

First, Cash has changed the basic design of New Worlds Imager to move away from the enormous ‘pinhole camera’ concept discussed earlier in these pages to an occulter — a design that blocks the starlight from the central star to allow its planetary companions to be visible. The problem with occulters has always been that no matter how scientists worked with their design, they could not get rid of diffracted light. “If you’re trying to make the smallest possible shadow thrower, in fact, you end up doing exactly the opposite,” Cash told me. “You concentrate the light. This has been the problem with occulters all along.”

But Cash, an astronomer at the University of Colorado at Boulder, was in the process of finishing up his Phase I study for NIAC when he solved the diffraction problem. Centauri Dreams will have more to say about this in coming weeks as we take a more detailed look at New Worlds Imager, but the gist of the matter is this: using a star-shade that is much smaller than originally projected, perhaps on the order of 30 to 50 meters tip to tip, Cash’s design can suppress central starlight to an efficiency that not only reveals planets down to terrestrial size, but makes possible spectroscopy on their atmospheres.

Secondly, the new design drops the cost in startling fashion. Working with engineers from Northrop Grumman, Cash now has a New Worlds Imager that can fit inside a single launch vehicle and cost below $1 billion. This at a time when NASA has just announced a sharp cutback in Terrestrial Planet Finder funding for this year as it searches for a cost-effective technology to make it happen. New Worlds Imager offers a clean, practicable solution at lower cost than competing designs.

“Everything is going the right direction,” Cash added. “There are no showstoppers here. I’ve had some really good people at aerospace companies looking at this and we’d be surprised if there are any problems at this point.”

The Jet Propulsion Laboratory’s competing Terrestrial Planet Finder concept, shown on a JPL Web site, is for a two-part mission, both to launch within ten to fifteen years. The first of these, TPF-C, would work in visible light, while the second, TPF-I, would use interferometry with multiple spacecraft to make infrared studies for more precise detections. Cash is proposing a system that seems superior: it does the work of TPF-C and is also capable of doing follow-up spectroscopy to study distant planetary atmospheres. His Phase II work for NIAC will examine a basic ‘observer’ mission and a much more complicated imaging mission, which would use interferometry to share data between five spacecraft examining solar systems for planetary close-ups.

New Worlds Imager offers a low-cost way to achieve NASA’s stated goal of imaging terrestrial worlds and examining them for the presence of life. It does so using breakthroughs in the study of diffracted light that may one day give us new geographies to examine on blue and green worlds that resemble Earth and are probably found in great profusion throughout the galaxy. Centauri Dreams intends to focus on New Worlds Imager closely as this work continues. It is a design that cannot be allowed to vanish beneath bureaucratic red tape as we make crucial decisions for the next phase of exoplanetary exploration.

Brown Dwarfs May Have Planets

If brown dwarfs, those ‘failed stars’ that never make it to the stage of full nuclear burning, can have planets around them, then the speculations of Karl Schroeder’s novel Permanence (New York: Tor Books, 2002) may be closer to reality than Centauri Dreams once thought. Schroeder imagines human colonies, artificially sustained through extraordinary technologies, on planets surrounding a variety of brown dwarf stars, an entire civilization of humans living in the spaces between the ‘lit’ stars we see in the night sky.

Now the Spitzer Space Telescope has found the signs of early planet formation around six young brown dwarfs located some 520 light years away in the Chameleon constellation. Ranging in size from between 40 to 70 times the mass of Jupiter, the brown dwarfs are between 1 and 3 million years old. And five of them have disks made up of dust particles that are clearly sticking together, in what looks suspiciously like the early stages of planet formation. The astronomers doing this work found relatively large dust grains in their data and small crystals of the mineral olivine. From a Jet Propulsion Laboratory news release:

“We are seeing processed particles that are linking up and growing in size,” said Dr. Ilaria Pascucci, a co-author also of the University of Arizona. “This is exciting because we weren’t sure if the disks of such cool objects would behave the same way that stellar disks do.”

Centauri Dreams‘ take: another key sign of planetary formation is a pronounced flattening of the observed disks. Planet formation seems to occur everywhere we look, and if the observational evidence continues to mount, we may have to face the fact that the number of planets in the galaxy is far higher than previously thought.

The paper on this work is Apai, Pascucci, Bouwman, et al., “The Onset of Planet Formation in Brown Dwarf Disks,” published online in Science Express (an abstract is here, but you’ll need AAAS membership for full access). If you’re not familiar with the service, Science Express offers electronic publication of selected papers that are to appear in Science in the next few weeks. It’s clearly an attempt to leverage the online preprint phenomenon made so conspicuous by arXiv, and is a welcome addition to our toolkit.

SETI and Drake: Part II

Yesterday we looked at Milan ?irkovi?’s paper “The Temporal Aspect of the Drake Equation and SETI” (Astrobiology Vol. 4 No. 2, pp. 225-231), and pondered whether there might not be a ‘communications window’ — an interval for any society between when it reaches the technological capacity for interstellar communication and the point when it becomes a ‘supercivilization’ unlikely to use conventional SETI methods to contact us or anyone else. If so, that ‘window’ would have a profound effect on how many civilizations we might be able to contact via SETI, and would thus change our answers to the Drake Equation.

But there are other kinds of assumptions built into the equation that may be problematic. ?irkovi? notes that the equation assumes a more or less uniform physical and chemical history of our galaxy, but uniformitarianism doesn’t work well in astrophysics or cosmology (think of the Steady State theory — uniformitarian — vs. the Big Bang, which introduced the concept of epochs never experienced by observers). As ?irkovi? notes, the development of the galaxy places constraints on when and how life could develop. From the paper:

Obviously, the history of the Galaxy divides into at least two periods (or phases): before and after sufficient metallicity for the formation of Earth-like planets has been built up by global chemical evolution. But this reflects only the most fundamental division. It is entirely plausible that the history of the Galaxy is divided still finer into several distinct periods with radically different conditions for life. In that case, only weighted relative durations are relevant, not the overall age.

Indeed, there may be a regulation mechanism preventing the growth of technological civilizations early in the galaxy’s history. An outstanding candidate for this would be gamma-ray bursts, which could have caused biological extinctions over much of the galactic habitable zone. Now apply this to our thinking about the Drake Equation — if the idea of continuous habitability is invalid, then the results we derive from the equation will be flawed. In fact, the equation may cause us to seriously underestimate the number of technological civilizations now existant, if we assume a galactic history with sharp boundaries on when intelligent life is possible, and then factor in our own existence at this particular epoch of that history.

M31 galaxy

Image: M31, the Andromeda Galaxy. Is the development of life in such galaxies regulated by catastrophic events that limit how and when intelligence can arise? Credit: Adam Block/NOAO/AURA/NSF.

The galaxy may, in fact, be populated by technological civilizations in a state of development far more constrained than previous models suggest. And here is where the arguments of the SETI optimists and the ‘contact pessimists’ may be united. ?irkovi? again:

Intuitively, it is clear that in such phase-transition models it is a very sensible policy for humanity to engage in serious SETI efforts: We expect practically all extraterrestrial intelligent societies to be roughly of the same age as ours, and to be our competitors for Hart-Tiplerian colonization of the Milky Way. This class of models underlines the essential weakness in the “contact pessimist” position; as Tipler (1980) wrote: “[pessimist] argument assumes that the . . . probabilities of the Drake equation do not vary rapidly with galactic age.” Phase transition is exactly such a “rapid variation.” The price to be paid for bringing the arguments of “optimists” and “pessimists” into accord is, obviously, the assumption that we are living in a rather special epoch in galactic history, i.e., the epoch of phase transition.

Note too that this assumption is in accord with the ‘rare Earth’ hypothesis advocated by Peter Ward and Donald Brownlee in their book Rare Earth: Why Complex Life is Uncommon in the Universe (New York: Springer, 2000). Here, then, are two factors we must take account of: 1) evolutionary effects in galactic history, such as the periods before and after sufficient metallicity to create the formation of Earth-like planets and other more subtle phase shifts; and 2) catastrophic regulation (as through gamma-rays or other mechanisms) of habitability within the galaxy. A non-uniform galactic history offers surprising and positive news for SETI hunters by explaining why life may be widespread but not necessarily advanced to the level of ‘supercivilizations.’

A Hard Look at SETI and the Drake Equation

The famous Drake Equation was developed as a way to estimate how many technological civilizations might exist and thereby be targets for SETI research. Conceived in 1961 as astronomer Frank Drake worked at the National Radio Astronomy Observatory (Green Bank, WV), the equation exists in a variety of forms depending on which authors you consult (see, for example, this SETI Institute discussion of the equation). But all variants draw on the same idea: to study extraterrestrial civilizations, you must consider such factors as:

  • the mean rate of star formation in the Galaxy;
  • the fraction of stars that can support life;
  • the fraction of stars that have planetary systems;
  • the number of planets per system with conditions suitable for life;
  • the number of planets where life does originate and evolve;
  • the fraction of planets where intelligent life forms develop;
  • the fraction of planets where intelligent life develops technology;
  • and a final, crucial measure:

  • the mean lifetime of a technological civilization
  • The Drake Equation has had such currency as a way of estimating extraterrestrial civilizations that it deserves further scrutiny in light of current work, which is just what it receives in Milan ?irkovi?’s 2004 paper “The Temporal Aspect of the Drake Equation and SETI (Astrobiology Vol. 4 No. 2, pp. 225-231. ?irkovi? (Astronomical Observatory of Belgrade) is rapidly becoming a preeminent analyst of issues involving SETI and the Fermi Paradox — we looked at his recent JBIS study “Permanence — An Adaptationist Solution to Fermi’s Paradox?” in an earlier post. In the paper in question, he examines the Drake Equation from the standpoint of its unwarranted assumptions and lack of temporal structure; the equation, ?irkovi? argues, fails to take into account evolutionary processes that profoundly alter any conclusions we might draw from it.

    These issues are important because they affect the kind of time scales we might expect a sustained SETI effort to need before it actually detects either extraterrestrial signals or artifacts of an alien civilization. We’ve come a long way from the easy optimism of the 1970s, when some believed there might be as many as a billion technological civilizations in the Milky Way. In fact, a school of what ?irkovi? calls ‘contact pessimists’ (think Frank Tipler, for example, or Michael Hart) has arisen, one pointing out that self-reproducing von Neumann probes could visit all solar systems in the galaxy within a period that is only a minute fraction of the age of the galaxy itself. Can we counter such pessimism with reasons for continuing the search?

    We know from recent work by Charles Lineweaver and others that Earth-like planets around other stars in the galactic habitable zone should be, on average, 1.8 billion years older than our own planet. And this is only an average; ?irkovi? points out that there are likely to be inhabitable worlds in the galaxy as much as 3 billion years older than the Earth. The small number of oldest and most advanced societies is likely to dominate any galactic civilization, meaning we might expect to encounter civilizations significantly older than the 1.8 billion year average, cultures with whom communication seems unlikely. From the paper:

    “Remember that 1 Gyr ago the appearance of even the simplest animals on Earth lay in the distant future [Ediacaran fauna — a kind of fuse on the famous Cambrian Explosion — is now being dated at ‘only’ 565-543 Myr before the present…] Thus, the set of the civilizations interesting from the point of view of SETI is not open in the temporal sense, but instead forms a ‘communications window,’ which begins at the moment the required technology is developed…and is terminated either through extinction of the civilization or through its passing into the realm of ‘supercivilizations’ unreachable by our primitive SETI means…”

    Thus one shortcoming of the Drake Equation is that we would need to add a term to it corresponding to the duration of this ‘communications window,’ and establishing its ratio to the larger value of the lifetime of a technological civilization. The net effect of this would surely be to reduce the number of civilizations we would be likely to encounter. But there is a corresponding sense of optimism, as the author notes: “Fortunately (from the SETI point of view) this is not the only evolutionary bias hidden in the Drake equation.” And some of these biases may actually work in our favor.

    ?irkovi?’s paper is rich enough that I want to return to it tomorrow to discuss what these biases are and how they affect the result. The Lineweaver reference above appears in an earlier post here, but for those who need it, “An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect” ran in Icarus Vol. 151, No. 2 (2001), pp. 307-313 (available here in PDF form), and was followed by a paper in collaboration with Yeshe Fenner and Brad Gibson titled “The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way.” The latter ran in Science Vol. 303 (2004), pp. 59-62, and is also available as a PDF online.