by Paul Gilster | Apr 25, 2008 | Astrobiology and SETI |
Alexander Zaitsev’s latest contribution to the debate over sending messages to the stars is a short paper that looks at how visible our planet might be thanks to transmissions from planetary radars like Arecibo, Goldstone or the Evpatoria site from which directed transmissions have already been sent. METI (Messaging to Extra-Terrestrial Intelligence) is broadly dedicated to transmitting messages to stars likely to have habitable planets, but so far the number of transmissions is relatively sparse. The debate over METI discusses the wisdom of continuing them without broader discussion.
But tucked within that debate is the specific question of our civilization’s visibility. For in addition to the messages that have already been sent, beginning with the Arecibo message in 1974 and continuing in the far more targeted transmissions from Evpatoria between 1999 and 2003, we are using our planetary radars to perform crucial astronomical studies. The work these dishes do in refining our knowledge of potentially dangerous asteroids is too significant to stop, but Zaitsev (Institute of Radio Engineering and Electronics, Russia) argues that their radar signature is far more obvious than the METI messages.
The argument goes like this: Roughly 1400 sets of radar transmissions have been produced from the three sites, with a distribution covering a broad swath of sky compared to the small number (16) of METI transmissions, which covered an area 2000 times smaller. Zaitsev then factors in the total duration of the radar transmissions, which exceeds the METI broadcasts by a factor of 500. He concludes that the radar work on asteroids and other objects is one million times more likely to be detectable than the signals sent as communications to other stars.
If this is the case, then making our civilization less visible involves shutting down activities necessary for planetary protection, an obviously dangerous move. But we still have a problem of degrees. A directed signal sent to a star with potentially habitable planets brightens our planetary signature significantly to receivers near that star, a place pre-selected for its astrobiological interest. The fleeting pulses of planetary radar transmissions covering broad areas of sky should likewise be detectable, but I find it hard to agree that this kind of transmission is at the same order of visibility, precisely because it is far more widely and randomly dispersed. [Addendum: See Dr. Zaitsev’s comment below re my misuse of the term ‘pulses’ in this context. These are not pulsed systems.]
The more we learn about how our activities might be detected elsewhere, the better, and I think that raising the issue of ongoing radar transmissions is completely valid. Similarly, we have much to learn about how the radio and television transmissions of the past century might or might not be receivable at various distances. The METI debate takes place in a context of apparent degrees of visibility, and questions whether making a specific attempt to raise that visibility to carefully chosen targets is wise. Settling that debate should involve not just astronomers and physicists but a broad spectrum of informed opinion. It is a debate that Dr. Zaitsev is fully engaged in, but one that most media outlets (with striking exceptions like SEED Magazine) have chosen to ignore.
The paper is Zaitsev, “Detection Probability of Terrestrial Radio Signals by a Hostile Super-civilization,” available online.
by Paul Gilster | Apr 24, 2008 | Sail Concepts |
A Finnish design making the news recently is hardly the only concept for near-term space sailing, but the possibility of testing it in space for a relatively small sum of money is attractive. This is especially true at a time when strapped budgets like NASA’s are focused on ratcheting up conventional propulsion techniques to get us back to the Moon and on to Mars. Yes, let’s keep pushing outward into nearby space with what we’ve got, but we need next-generation thinking, too, and the Finnish sail, the work of Pekka Janhunen and Arto Sandroos, points in that direction.
Unlike magnetic sails that create an artificial magnetosphere around the spacecraft, the Finnish concept is to use long, thin conductive wires that are kept at a positive potential through the use of an onboard electron gun. The two researchers considered how the charged particles of the solar wind would interact with a single charged wire in a 2007 paper that we looked at in this Centauri Dreams article just over a year ago. A full-scale mission would use fifty to one hundred 20-kilometer long charged tethers. Supercomputer simulations come up with potential speeds of 100 kilometers per second, which is about five times what New Horizons is doing on its way to Pluto/Charon.
That’s also a speed that gets you into the nearby interstellar medium in about fifteen years, a time frame that should quicken the heart of many a deep space scientist. When he looked at some of the potential mission concepts in Next Big Future, Brian Wang mentioned the possibility of transporting raw materials from the asteroids for use in making fuel at high Earth orbit. I see that Janhunen noted the asteroid idea in a recent interview, tying it to a broader human future: “Starting the long-awaited asteroid resource utilization could be significant for the longer-term well-being and survival of our civilization on this planet.”
That article, published in Space.com (and thanks to John Hunt for the link), notes the nature of the sail’s first prototype, seen as a smaller sail using 8-kilometer long tethers in an elliptical Earth orbit, a scenario that would allow tests on the force of the solar wind on the spacecraft. The team would also investigate using radio waves to excite solar wind particles in an attempt to boost the possible thrust.
So many good concepts, so many budgetary constraints. Long an admirer of Robert Winglee’s Mini-Magnetospheric Plasma Propulsion concept, I watched with growing enthusiasm as it sailed through Phase I and Phase II rounds at NASA’s Institute for Advanced Concepts and went on to further scrutiny, but getting some kind of solar, magsail or electric sail concept into actual space testing now seems a remote possibility. The Finnish team’s sail awaits the resolution of its own funding issues, a quick fix being the infusion of somewhere around 5 million Euros.
One thing is for sure: Propulsion concepts that let us leave the fuel on Earth have a huge future in opening up the outer planets and the interesting places beyond. Solar sails can do this by using the momentum provided by photons from the Sun, but these effects drop dramatically as we move beyond Jupiter. The solar wind, streaming outward from the Sun at speeds approaching 1.5 million kilometers per hour, may offer a way to boost sail performance through magsail and electric concepts, but we have much to learn about how sails might interact with it. In both cases, we need sail deployment in space to take the necessary next steps.
A good way to keep up with the Finnish sail studies is to track the latest papers and press releases here. You’ll also find the latest paper I know about, which is Mengali et al., “Electric sail performance analysis,” Journal of Spacecraft and Rockets Volume 45, Issue 1 (Jan-Feb, 2008), pp. 122-129, available as an abstract with included figures on the site. It’s interesting as well to see that a workshop on electric sailing will be held at the European Space Research and Technology Centre in the Netherlands on Monday, May 19.
by Paul Gilster | Apr 23, 2008 | Culture and Society |
Sometimes it’s hard to believe that Stephen Hawking is only sixty-six. Not just because of his indomitable fight against Lou Gehrig’s disease, which is a story in its own right, but because his position at the summit of modern physics has kept him in the public eye for an exceedingly long time. Now, in a speech commemorating the fiftieth anniversary of NASA, Hawking has taken aim at the question of why space matters. And it’s not surprising that this Star Trek fan quoted his favorite show.
“If the human race is to continue for another million years, we will have to boldly go where no one has gone before … It will not solve any of our immediate problems on planet Earth, but it will give us a new perspective on them and… Hopefully, it will unite us to face a common challenge.”
But of course, that question of solving our immediate problems on Earth is what is often subject to debate. Although the space budget is usually overestimated (a recent conversation illustrated this, a friend citing the ‘trillions we’re spending on space’) Hawking notes that you could increase the international space budget twenty times and still be talking about no more than 0.25 percent of the collective gross domestic product (GDP). That’s a quarter of a percent of the world’s financial resources to potentially save the species.
Some of us point to the threat of impacts from near Earth objects (NEOs) in this debate, but Hawking has in the past mentioned the dangers of climate change and nuclear war as reasons for expanding first to other planets and eventually to other solar systems. He views the current NASA goal of a return to the Moon and a trip to Mars as steps within this broader vision, one that includes work on interstellar propulsion that may pay off on a time frame of from two to five hundred years.
Image: Stephen Hawking’s vision takes us out a good five hundred years. Do we have the cultural commitment to apply consistent effort over such time frames? Our future in interstellar space may depend upon the answer. Credit: British Council.
Hawking spoke at George Washington University on Monday, where the bulk of the audience may have found a two to five-hundred year time frame uncomfortable. After all, we’re not used to thinking in such terms, and in an era that demands fast turn-arounds, wouldn’t it be grand to simply come up with a star drive tomorrow? Indeed, do we have the patience to embark on a project that might last five centuries, whose outcome will always remain in doubt, and whose funding will have to be continually extracted from reluctant governments or, more likely, drawn from the philanthropic donations of a small number of visionaries?
A quick solution to star drives is unlikely. But Stephen Hawking is himself an example of the power of the long-term even as it applies to daily life. Speaking last week at Caltech (again via a pre-recorded talk), Hawking answered questions that had been submitted to him in advance. There were only five questions, but according to Kip Thorne, a close friend and upcoming physics legend in his own right, it took Hawking several days to program his answers. Thorne describes him as “about the most patient, stubborn man I know.” Patience and stubborness. Those are the traits that may get us to the stars, markers of a core belief in the long haul.
by Paul Gilster | Apr 22, 2008 | Exoplanetary Science |
Serendipity is a wondrous thing. Start writing about the early history of the Solar System, as I intended to do yesterday, and you wind up discussing ‘hot Jupiters’ around other stars. But there actually is a bridge between the two concepts, and it comes in the form of a question. If we find gas giants in scorchingly hot orbits around other stars, why was there no hot Jupiter in our own Solar System? Or was there? That question was what originally led me to the paper by Avi Mandell, Sean Raymond and Stein Sigurðsson that occupied yesterday’s post.
For in their analysis of how giant planets migrate through an early planetary system, wreaking havoc on newly forming worlds but also scattering them interestingly throughout nearby space, these researchers paused to examine the implications of these studies for our own system. Having demonstrated through their simulations that the migration of a gas giant through an inner system may be common, and that systems that experience it often form terrestrial worlds, why rule out earlier generations of giant planets right here in Sol space?
Image: A ‘hot Jupiter’ moves breathtakingly close to its star. Found around a number of other stars, could such a world have once moved through our own Solar System? Credit: NASA/JPL-Caltech.
If such were the case, then the natural follow-up is to ask whether it may be possible to find the signature of early planetary scattering in the system today. One problem with that concept is that although the mixing of large bodies that would occur during migration should leave traces in the makeup of elements found in the final system, this signature would be obscured by other kinds of mixing. Thus the authors give up on the idea of tracing it in the inner Solar System, saying in their paper:
…radial mixing of dust and smaller bodies is also thought to occur through a variety of other processes in the planet formation region besides planetary scattering…, and it is clear from the final abundances of planets in our simulations and others that any signature of the scattering of massive bodies early in the formation history will be quickly erased through accretion in the inner system due to continued mixing.
But the further we move from the Sun, the more interesting things get. Here we’ve dealing with lower densities of solids and gases and longer time-frames, with accretion and other factors less likely to obscure what may be evidence of planetary movements in the early system. Finding comets whose orbits make them appear to be Oort Cloud objects, but whose physical characteristics suggest asteroids, could provide evidence for the scattering of inner disk objects by a migrating gas giant.
Even better would be an object in the right place, one made of materials showing signs of high temperature origins. From the paper:
A more conclusive sign of giant planet migration would be a classical KBO with a composition primarily composed of refractory materials; this would imply the re-circularization of a scattered inner-disk object, which would most likely only be possible in the presence of damping by significant amounts of gas or dust for long timescales.
So there’s a lively vector for another research project, examining objects from the Edgeworth/Kuiper belt and the Oort Cloud for evidence of early planetary migration. It would take a lot of work and much more information about the outer system than we currently have before we could remotely conclude that a hot Jupiter once perturbed our own neighborhood. But the idea that extrasolar systems with these planets are necessarily of a different order than our own Solar System may one day be shown to be false. We may find that hot Jupiters are not uncommon as a system with terrestrial worlds continues its development.
by Paul Gilster | Apr 21, 2008 | Exoplanetary Science |
What happens to potentially habitable planets when a gas giant swings through the neighborhood? It’s a pertinent question when you consider the surprises that ‘hot Jupiters’ have given us. 22 percent of known extrasolar planets show an orbital radius of less than 0.1 AU, and 16 percent are located within 0.05 AU of their host star. That’s a surprise given the assumption that these gas giant planets must form much further out in their systems, but it can be explained by inward migration of the giant planet, a process under much study that is generally thought to be caused by interactions with the protoplanetary disk.
Such a migration would seem to spell trouble for planets already orbiting closer to the star, leading some to believe that systems with hot Jupiters are unlikely homes for living worlds. But recent simulations of the growth of such systems make it clear that a hot Jupiter isn’t necessarily a deal-breaker for habitable worlds. Are we going to have to add such systems into our target lists for future terrestrial planet finder missions?
Image: A hot Jupiter seared by its star. Could terrestrial planets survive the migration of such a planet inward from its place of origin in the outer system? Credit: Harvard-Smithsonian Center for Astrophysics.
Sean Raymond (University of Colorado), Avi Mandell and Steinn Sigurðsson (the latter two from the University of Pennsylvania) have been modeling circumstellar disks, beginning with a disk containing seventeen Earth masses of rocky/icy material divided between 80 ‘planetary embryos’ and 1200 planetesimals. The inner disk was modeled as iron-rich and water-poor, the outer as water-rich and iron-poor. These are values that are much like the Solar System, where it is believed that comets are as much as half water ice, and asteroids outside 2.5 AU contain large amounts of water.
Given these initial disk conditions, the team ran a Jupiter-mass planet from 5.2 AU in to 0.25 AU over a period of 100,000 years, studying the effects on planetary formation in the following 200 million years under a variety of scenarios. The first result is eye catching: A lot of things start to happen inside the orbit of the migrating giant planet, where planetesimals and embryos are drawn (‘shepherded’) by motion resonances and gas drag. ‘Hot Earths’ with masses up to five times that of Earth form relatively quickly.
Then something equally interesting occurs. Materials that have been scattered outward by the giant planet’s migration begin to have their orbits re-circularized by gas drag from the disk. Planetesimals outside the orbit of the ‘hot Jupiter’ begin to deliver materials including large amounts of water to growing terrestrial planets. Note this (internal references omitted for brevity):
Planets formed in the Habitable Zone in a significant fraction of our simulations. These planets have masses and orbits similar to those seen in previous simulations including only outer giant planets… However, Habitable Zone planets in systems with close-in giant planets tend to accrete a much larger amount of water than those in systems with only outer giant planets… The reason for these high water contents is twofold: 1) strong radial mixing is induced by the giant planet’s migration, and 2) in-spiraling icy planetesimals are easily accreted by planets in the terrestrial zone. Although we have not taken water depletion into account, these planets contain about twenty times as much water as those formed in similar outer giant planet simulations… These planets are likely to be “water worlds”, covered in kilometers-deep global oceans…
Interesting, no? We see the possibility of Earth-like planets (along with ‘hot Earths’ inside the gas giant’s orbit) that are water-rich, in some cases containing more than 100 times the water content of Earth, and low in iron (the iron content being diluted by outer, water-rich material). The influx of ices via outer planetesimals is unimpeded by the kind of scattering that Jupiter provides in our Solar System.
Out of all this comes a rough limit on the orbital distance of the inner giant planet that would allow the formation of terrestrial planets in the habitable zone: A terrestrial planet just inside the outer edge of the zone at 1.5 AU would demand the hot Jupiter’s orbit to be inside about 0.5 AU, although up to 0.7 AU may still be feasible. And when the researchers applied this limit to the known extrasolar giant planets, they found that 65 out of 178 of the systems in their sample permitted an Earth-like planet of at least 0.3 Earth masses or more to form in the habitable zone.
In other words, some of the known ‘hot Jupiter’ systems may turn out to yield potentially habitable planets after all, a key thought when drawing up target lists for next-generation planet-finder missions. “Our results suggest that terrestrial planets can coexist with both close-in giant planets and giant planets in outer orbits,” write the authors, “expanding the range of planetary systems that should be searched with these upcoming missions.” The paper is Mandell, Raymond and Sigurðsson, “Formation of Earth-like Planets During and After Giant Planet Migration,” Astrophysical Journal Vol. 660, Issue 1 (May, 2007), pp. 823-844 (abstract). Citation amended from the original as per the comment below.
