by Paul Gilster | Aug 24, 2012 | Deep Sky Astronomy & Telescopes |
The Magellanic Clouds, visible in the southern hemisphere, are two dwarf galaxies that orbit the Milky Way, a fact that has always captivated me. We see the galaxy from the inside, but I have always wondered what it would be like to see it from the perspective of the Magellanics. The Large Magellanic Cloud (LMC) is, after all, only 160,000 light years out, while the Small Magellanic (SMC), its companion cloud, is about 200,000 light years away. Add in the recently discovered Sagittarius Dwarf Elliptical at 50,000 light years from the galactic core and you have three exotic venues from which to gain a visual perspective on the Milky Way, at least in the imagination.
We’re so used to thinking that our solar and galactic neighborhoods are utterly commonplace that it may come as a surprise to learn that the configuration of spiral galaxy and satellite galaxies that we see in the Milky Way is actually quite unusual. New work on this comes from Aaron Robotham (International Center for Radio Astronomy Research and University of St. Andrews), whose team looked for groups of galaxies in something like our own configuration. The data for this effort is drawn from the Galaxy and Mass Assembly project (GAMA), a spectroscopic survey of ~340,000 galaxies using the AAOmega spectrograph on the Anglo-Australian Telescope that builds on and is augmented by earlier spectroscopic efforts.
Image: This image shows one of the two ‘exact matches’ to the Milky Way system found in the survey. The larger galaxy, denoted GAMA202627, which is similar to the Milky Way clearly has two large companions off to the bottom left of the image. In this image bluer colours indicate hotter, younger, stars like many of those that are found in our galaxy. Image Credit: Dr Aaron Robotham, ICRAR/St Andrews using GAMA data.
The effort is interesting because we have never had a good understanding of how unusual the configuration of the Milky Way and its halo really is. The halo should contain the faintest known satellite galaxies because it is so close, and we should be able to study its characteristics to learn more about the galaxy’s formation history, knowledge which can then be extended to other galaxies. According to the paper on this work, simulations have trouble predicting the full distribution of satellite galaxies around the Milky Way halo, especially for the brightest of these, the Magellanics. Satellites in this configuration have been thought to be a rare occurrence.
The new work, presented at the International Astronomical Union General Assembly in Beijing, bears out this conclusion. Think of the halo as the spherical component surrounding the galaxy, which contrasts with the flat disk of the Milky Way. We’re on the cusp of major strides in our studies of galactic halos, because missions like the soon to be launched GAIA will let us measure the properties of 2 billion stars in the Local Group, including samples from all the member galaxies. For now, the Galaxy and Mass Assembly project has produced redshift data that allow Robotham and his colleagues to search for Milky Way Magellanic Cloud Analogs in the halos of galaxies with close companions much like our own.
The results bear out our simulations. The researchers found that only 3 percent of spiral galaxies like the Milky Way have satellite companions like the Magellanic Clouds. In the GAMA data, 14 galaxy systems were roughly similar to ours, but only two turned out to be a truly close match. Many galaxies have smaller galaxies in orbit around them, but few have satellites as large as the two Magellanic Clouds. About the closest matches, the paper has this to say:
Only two full analogues to the MW [Milky Way]-LMC-SMC system were found in GAMA, suggesting such a combination of late-type, close star-forming galaxies is quite rare: in GAMA only 0.4% (0.3%–1.1%) of MW mass galaxies have such a system (a 2.7? event). In terms of space density, we ?nd 1.1 × 10?5 Mpc?3 full analogues in GAMA (in a volume of 1.8 × 105 Mpc3). The best example found shares many qualitative characteristics with the MW system. The brightest pair galaxy has spiral features, as does the bigger minor companion. The minor companions are ?40 kpc in projected separation, so not in a close binary formation like the SMC and LMC.
Adds Robotham: “The galaxy we live in is perfectly typical, but the nearby Magellanic Clouds are a rare, and possibly short-lived, occurrence. We should enjoy them whilst we can, they’ll only be around for a few billion more years.”
The paper is Robotham et al., “Galaxy And Mass Assembly (GAMA): In Search of Milky-Way Magellanic Cloud Analogues,” Monthly Notices of the Royal Astronomical Society, Volume 424, Issue 2 (2012), pp. 1448-1453 (preprint).
by Paul Gilster | Aug 23, 2012 | Exoplanetary Science |
Polish astronomer Aleksander Wolszczan (Penn State) is best known as the discoverer of the first confirmed planet outside our Solar System. That was back in the early 1990s, when Wolszczan was working with Dale Frail (NRAO), using observations from the Arecibo dish to demonstrate that the pulsar PSR B1257+12 was orbited by two planets. These are relatively small worlds (3.9 and 4.3 Earth masses respectively), and in an era where new planet candidates number in the thousands, it’s easy to forget how striking Wolszczan’s work appeared at the time, and how it gave impetus to the developing exoplanet hunt.
A pulsar planet looks to be an extremely inhospitable place, but learning how planets are distributed among the stars involves studying every conceivable kind of world. Wolszczan’s latest work targets an equally hostile environment, the former habitable zone of a star that has begun expanding into a red giant. The star, BD+48 740, has 11 times the Sun’s radius and is significantly older. Wolszczan and an international team of astronomers have been studying its chemical composition by way of figuring out events in its recent past. Using data gathered at the Hobby-Eberly Telescope (UT-Austin), they discovered that BD+48 740 contains an unusual amount of lithium, an element created primarily in the early days of the universe.
Finding lithium in such high amounts is a flag to the astronomer:
“Theorists have identified only a few, very specific circumstances, other than the Big Bang, under which lithium can be created in stars,” Wolszczan added. “In the case of BD+48 740, it is probable that the lithium production was triggered by a mass the size of a planet that spiraled into the star and heated it up while the star was digesting it.”
Image: The first evidence of a planet’s destruction by its aging star indicates that the missing planet was devoured as the star began expanding into a “red giant” — the stellar equivalent of advanced age. “A similar fate may await the inner planets in our solar system, when the Sun becomes a red giant and expands all the way out to Earth’s orbit some five-billion years from now,” said Alexander Wolszczan, Evan Pugh Professor of Astronomy and Astrophysics at Penn State and the discoverer of the first planet ever found outside our solar system. Credit: Marty Harris/McDonald Obs./UT-Austin.
The work on BD+48 740 is part of the Penn State-Toru? Planet Search, which specializes in detecting planetary systems around stars more evolved than the Sun. Some 50 red giants are currently known to host planets or brown dwarf companions, some of them (like HD 102272) in multi-planet systems. Although earlier phase sub-giants are found with ‘hot Jupiters’ orbiting them, the more evolved red giants show few planets in close orbits and few planets whose orbit is eccentric.
That makes the discovery of a long-period planet in a highly eccentric orbit around a red giant an interesting find. BD+48 740 is orbited by a planet of at least 1.6 Jupiter’s mass in such an orbit, one the researchers call the most elliptical planetary orbit yet detected around an evolved star. The culprit should be gravitational interactions between planets, and the suspicion is that the missing planet was the cause, its dive into the star giving the outer planet a gravitational boost.
The elliptical orbit of the newly discovered planet is not in itself enough to imply the engulfment of an inner planet. The paper on this work points out that we have studied planetary systems like HAT P-13 b, c and HD 217107 b, c, both systems around main sequence stars in which there is a close-in planet in a nearly circular orbit and a more distant companion in an eccentric orbit. But the lithium abundance in BD+48 740 suggests a missing planet and a recent planet-planet scattering event, one that would have occurred after the star had left the main sequence.
Says team member Eva Villaver (Universidad Autonoma de Madrid):
“Catching a planet in the act of being devoured by a star is an almost improbable feat to accomplish because of the comparative swiftness of the process, but the occurrence of such a collision can be deduced from the way it affects the stellar chemistry. The highly elongated orbit of the massive planet we discovered around this lithium-polluted red-giant star is exactly the kind of evidence that would point to the star’s recent destruction of its now-missing planet.”
The paper is M. Adamów at al., “BD+48 740 – Li overabundant giant star with a planet. A case of recent engulfment?” Accepted at Astrophysical Journal Letters (preprint). A Penn State news release on this work is also available.
by Paul Gilster | Aug 22, 2012 | Exotic Physics |
SETI always makes us ask what human-centered assumptions we are making about extraterrestrial civilizations. When it comes to detecting an actual technology, like the starships we’ve been talking about in the last two posts, we’ve largely been forced to study concepts that fit our understanding of physics. Thus Robert Zubrin talks about how we might detect a magsail, or an antimatter engine, or a fusion-powered spacecraft, but he’s careful to note that the kind of concepts once studied by the Breakthrough Propulsion Physics Project at NASA may be undetectable, since we really don’t know what’s possible and what its signature might be.
I mentioned zero-point energy in a previous post because Zubrin likewise mentions it, an idea that would draw from the energy of the vacuum at the quantum level. Would a craft using such energies — if it’s even possible — leave a detectable signal? I’ve never seen a paper on this, but it’s true that one classic paper has looked at another truly exotic mechanism for interstellar travel, the wormhole. These shortcuts through spacetime make space travel a snap. Because they connect one part of the universe to another, you go in one end and come out the other, emerging into another place and, for all we know, another time.
The fact that we don’t know whether wormholes exist doesn’t mean we can’t think about how to detect one, although the authors of the classic paper on wormhole detection make no assumptions about whether or not any intelligent species would actually be using a wormhole. The paper is “Natural Wormholes as Gravitational Lenses,” and it’s no surprise to find that its authors are not only wormhole specialists like Matt Visser and Michael Morris, but physicists with a science fiction connection like John Cramer, Geoffrey Landis, Gregory Benford and the formidable Robert Forward.
Image: A wormhole presents a shortcut through spacetime. Can one be detected? Credit: Wikimedia Commons.
The analysis assumes that the mouth of a wormhole would accrete mass, which would give the other mouth a net negative mass that would behave in gravitationally unusual ways. Thus the GNACHO (gravitationally negative anomalous compact halo object), which playfully echoes the acronym for massive compact halo objects (MACHOs). Observationally, we can look for a gravitational lensing signature that will enhance background stars by bending light in a fundamentally different way than what a MACHO would do. And because we have MACHO search data available, the authors propose checking them for a GNACHO signature.
In conventional gravitational lensing, when a massive object moves between you and a much more distant object, a greatly magnified and distorted image of the distant object can be seen. Gravitational lensing like this has proven a useful tool for astrophysicists and has also been a means of exoplanet detection. But when a wormhole moves in front of another star, it should de-focus the light and dim it. And as the wormhole continues to move in relation to the background star, it should create a sudden spike of light. The signature, then, is two spikes with a steep lowering of light between them.
The authors think we might find the first solid evidence for the existence of a wormhole in our data by looking for such an event, saying “…the negative gravitational lensing presented here, if observed, would provide distinctive and unambiguous evidence for the existence of a foreground object of negative mass.” And it goes without saying that today’s astronomy, which collects information at a rate far faster than it can be analyzed, might have such evidence tucked away in computer data waiting to be discovered by the right search algorithms.
Would a wormhole be a transportation device? Nobody knows. Assuming we discover a wormhole one day, it would likely be so far away that we wouldn’t be able to get to it to examine its possibilities. But it’s not inconceivable that a sufficiently advanced civilization might be able to create an artificial wormhole, creating a network of spacetime shortcuts for instantaneous travel. Matt Visser has discussed a wormhole whose mouth would be held open by negative energy, ‘…a flat-space wormhole mouth framed by a single continuous loop of exotic cosmic string.’ A primordial wormhole might survive from the early universe. Could one also be created by technology?
Civilizations on the Brink
More conventional means of transport like solar or laser-powered sails present serious problems for detection. In Jerry Pournelle and Larry Niven’s The Mote in God’s Eye, an alien lightsail is detected moving at seven percent of the speed of light, its spectrum the same as the star that it is approaching but blueshifted, which is how analysts have determined it is a sail. The novel’s detection occurs with far more sophisticated observatories than we have in our day, when finding a solar or lightsail in transit would be a tricky thing indeed. A fusion rocket, for example, would emit largely in the X-ray range and could be detectable for several light years, but a lightsail is a highly mutable catch.
I remembered reading something about this in Gregory Matloff’s Deep Space Probes (Springer, 2005) and checked the book to extract this:
If ET prefers non-nuclear travel, he might utilise a laser or maser light sail. If the starship is near enough and the laser/maser is powerful enough, reflections from the sail might be observable as a fast-moving and accelerating monochromatic ‘star.’ However, detection will depend on sail shape and orientation as well as other physical factors.
Therefore, it is not as easy to model the spectral signature of these craft as it is energetic nuclear craft. A starship accelerated using lasers or masers may be easier to detect during deceleration if a magsail is used.
Writing in the comments to yesterday’s post, Centauri Dreams reader James Jason Wentworth recalls Larry Niven’s short story “The Fourth Profession,” which has a lightsail detection something like the one in The Mote in God’s Eye:
“All right. The astronomers were studying a nearby nova, so they caught the intruder a little sooner. It showed a strange spectrum, radically different from a nova and much more constant. It got even stranger. The light was growing brighter at the same time the spectral lines were shifting toward the red.
“It was months before anyone identified the spectrum.
“Then one Jerome Finney finally caught wise. He showed that the spectrum was the light of our own sun, drastically blue-shifted. Some kind of mirror was coming at us, moving at a hell of a clip, but slowing as it came.”
Some sails could be truly gigantic, and we can imagine worldships large enough to require sails the size of a planetary radius, which could be detected when near their home or destination stars, but would be hard to find when in cruise. Matloff goes on to suggest that any search for this kind of ship should look near stars from which an entire civilization might be emigrating. A star like Beta Hydri is a possibility, a nearby (21 light years) solar-type star now expanding from the main sequence. This is the longest shot of all, but finding unusual signatures in visible light near a star leaving the main sequence would at least compel a second look.
The wormhole paper is John Cramer, Robert L. Forward, Gregory Benford et al., “Natural Wormholes as Gravitational Lenses,” Physical Review D (March 15, 1995): pp. 3124-27 (available online). See also Matloff and Pazmino, “Detecting Interstellar Migrations,” in Astronomical and Biochemical Origins and the Search for Life in the Universe, ed. C. B. Cosmovici, S. Bowyer and D. Werthimer, Editrici Compositori, Bologna, Italy (1997), pp. 757-759.
by Paul Gilster | Aug 21, 2012 | Uncategorized |
Yesterday we pondered the possibility of detecting an interstellar craft as a new kind of SETI. If the energies needed to drive such a vessel are as titanic as we think, there could be a detectable signature, as Robert Zubrin pointed out in a 1995 paper. Zubrin’s best case in visible light involved an antimatter engine whose exhaust could be detected from as far as 300 light years from Earth. That would cover a huge number of stars, as 100,000 exist within 200 light years of our planet.
I suppose the classic starship detection occurs in Larry Niven and Jerry Pournelle’s 1975 novel The Mote in God’s Eye, where human starfarers using the ‘Alderson Drive’ — which allows instantaneous jumps between stars — detect an alien, laser-pushed lightsail. The starship is a throwback, an older technology that human interstellar methods have long superseded, one that contains a strange, asymmetric alien being, the first extraterrestrial humans have encountered. It’s no surprise to learn that Niven and Pournelle fine-tuned the laser lightsail idea through conversations with Robert Forward, who studied such concepts intensively in the scientific literature.
And then there’s Gregory Benford’s 2006 novelette about an astronomer who begins to suspect the anomalous object he’s looking at isn’t natural. Whereas Zubrin looked at antimatter, fusion, fission and magsails, Benford’s astronomer thinks he’s found something like a Bussard ramjet:
“What you wrote,” she said wonderingly. “It’s a…star ship?”
“Was. It got into trouble of some kind these last few days. That’s why the wake behind it – ” he tapped the Fantis’ image – “got longer. Then, hours later, it got turbulent, and—it exploded.”
She sipped her coffee. “This is…was…light years away?”
“Yes, and headed somewhere else. It was sending out a regular beamed transmission, one that swept around as the ship rotated, every 47 seconds.”
Her eyes widened. “You’re sure?”
“Let’s say it’s a working hypothesis.”
The story is “Bow Shock,” which ran in Jim Baen’s Universe and is reprinted in Benford’s new collection Anomalies. Benford speculates that synchrotron radiation in the bow shock of a magnetically screened starship would be detectable at microwave and radio frequencies, and perhaps an easier catch than the torch of Zubrin’s antimatter spacecraft from a great distance. Ralph, Benford’s astronomer, had been examining what he thought was a runaway neutron star, ‘a faint finger in maps centered on the plane of the galaxy, just a dim scratch,’ and if we ever do make a starship detection, this could be more or less what we find, a changeable, ambiguous object.
Image: Note the bow shock in the binary star system BZ Cam, created as the system moves through surrounding interstellar gas. In most cataclysmic variables, matter from a normal star accumulates on the surface of the companion white dwarf star, eventually causing a nova-like flare. In BZ Cam, however, light appears to flicker unpredictably, and an unusually large wind of particles is being expelled. BZ Cam lies about 2500 light-years away toward the constellation of Camelopardalis. Credit: R. Casalegno, C. Conselice et al., WIYN, NOAO, MURST, NSF.
Magsail Braking and Detection
A magnetic sail (‘magsail’) that deflects charged particles is also an interesting possibility. In his 1995 paper, Zubrin points out that a sail like this would be of value because it could decelerate a starship without the use of propellant, allowing it to brake against the stellar wind of the destination star. Here again we get a bow shock which will heat the interstellar medium to a degree dependent on the ship’s velocity, creating a plasma that will encounter the magnetic field of the magsail and produce radiation. This is an ingenious analysis — Zubrin, having gone through the equations governing the rate of deceleration of a magsail in the interstellar medium, notes the changes that occur as the velocity of the ship decreases and the bow shock heating begins to fall, causing the cyclotron radiation emitted by the bow shock to fall at the same time.
A large enough starship decelerating as it neared its destination should, according to Zubrin, put out emissions in the X-ray and radio ranges. Thus we get our starship signature: “The decline in bremsstrahlung energies and cyclotron frequencies in time in accord with the form of equation (2) [governing the rate of deceleration of the magsail] would be a dead giveaway that the emitting object was a decelerating magsail.” Governing the radiation emitted is the ship’s velocity and the density of ions in the interstellar medium. Zubrin calculates that at a density of 1 ion per cubic centimeter, a ship decelerating from a cruise velocity of 0.1 c will produce radiation at about 12 kHz.
Would we have a real chance at such a detection? From the paper:
It can be seen that the magsail radiation of a characteristic fusion starship being decelerated from a cruise velocity of 0.1c could be detected by a 6 km orbiting antenna from a distance of 400 light years, while that emitted by a characteristic antimatter photon rocket in its deceleration phase could be seen as far away as 2,000 light-years. There are about 100,000,000 stellar systems to be found within the latter distance. This extended range detection capability combined with magsail radiation’s unique time-dependent frequency spectrum appears to make a search for magsail radiation the most promising option for extraterrestrial starship detection.
You’ll recall that Zubrin estimated the characteristic mass of his ‘standard’ starship as 1,000,000 tons, something he describes as ‘a speculative guess.’ But even if the guess is off the mark, the idea of starship detection remains sound, in his view, because a decrease of ship mass by two orders of magnitude only decreases the detectability distance by one order of magnitude. We’re still looking at detecting a 10,000 ton starship (using a magsail) at 40 light years via a 6 km antenna, while a 30 km antenna could make the detection at a range of 200 light years.
Further thoughts on starship detection tomorrow. But let me add one note before closing: Al Jackson pointed out in the comments to yesterday’s post that Zubrin does not reference an important paper. It’s D. R. J. Viewing, C. Horswell, E. W. Palmer, “Detection of Starships,” JBIS, 30, 99-104 (1977), and it’s not one I’ve seen yet either. Because of that, let me quote Al:
The Viewing , et.al., paper looks at two kinds of starships, Innocuous and Energetic. Almost the same propulsion systems are considered as in Zubrin’s paper. That paper got me interested in what truly relativistic ships would look like in the simple case when a propulsion system does is not 100% efficient, and loses energy by waste heat. Suppose that in its rest frame it radiates isotropically. A simple relativistic kinematic effect will be that the radiation will be beamed relative to another rest frame. Called the ‘head light’ effect in special relativity. See: Correspondence: A. A. Jackson, IV, “Ultra-Relativistic Starships,” JBIS, 32, 240 (1979).
The Zubrin reference is “Detection of Extraterrestrial Civilizations via the Spectral Signature of Advanced Interstellar Spacecraft,” Progress in the Search for Extraterrestrial Life, ASP Conference Series Vol. 74 (1995). Available online.
by Paul Gilster | Aug 20, 2012 | Astrobiology and SETI |
Several Centauri Dreams readers passed along Seth Shostak’s latest article on SETI in IEEE Spectrum, a piece that invokes the ‘Wow!’ signal at Ohio State and goes on to make the case for continuing the hunt. Shostak thinks both the ongoing search for exoplanets and refinements in our signal detection technology, including optical SETI, should keep us active. “No, we haven’t found any signals so far, but there’s a growing incentive provided by new findings in astronomy and biology, and the instruments are getting better,” he writes. “Thirty-five years from now, we may really find a signal that will make us say ‘Wow!'”
The IEEE Spectrum piece doesn’t break any new ground, but it’s another example of the ‘Wow!’ signal getting broader coverage, and I now find that people routinely bring the Ohio State event up when I talk to audiences about SETI. Meanwhile, let’s think about some truly exotic possibilities when it comes to detecting extraterrestrial life. Would it be possible, for example, to detect signs of an alien civilization not just by a beacon, but by a spacecraft in transit between the stars? I was intrigued to learn that Robert Zubrin, of Mars Society fame, has investigated the question in a 1995 paper.
Modeling an Interstellar Craft
Zubrin has interstellar credentials, of course, most notably in his work with Dana Andrews on the Bussard ramjet and the question of drag, which has fed into studies of magsails and their uses. I also highly recommend his book Entering Space: Creating a Spacefaring Civilization (Tarcher, 1999) for its overview of interstellar concepts, a helpful source for those new to the interstellar field. In the 1995 paper on spacecraft detection, he makes an interesting point. One of the things that muddies the SETI investigation is our lack of knowledge of how aliens think, so that we are left to guess at what a species might do if trying to communicate. By searching for the spectral signal of an interstellar transportation system, we can abandon that concern:
The advantage of our approach is that the characteristic power levels associated with interstellar transportation systems are many orders of magnitude greater than those required for communication, and so the signal strength may be much greater. Furthermore, unlike communication which is governed by a fairly arbitrary selection of technology and mutually agreed upon conventions, transportation systems are governed much more stringently by the laws of physics. No understanding of alien psychology is necessary to detect a starship.
But what kind of starships are we looking for? For the purposes of analysis, Zubrin has to rule out breakthroughs in physics that an extraterrestrial culture may have made but we have not — we have no way to characterize a ship propelled by something exotic like zero-point energy. But antimatter, fusion and fission rockets, as well as magnetic sails, come within his province. If the alien starship works within the limits of physics as we know them, then a speed of about 10 percent of lightspeed seems minimal — Zubrin selects this figure under the assumption of human-like life-spans so that the mission can be completed within a working lifetime. He also assumes that technological creatures are social and need a large crew for a long voyage.
Image: Could we detect an interstellar spacecraft more readily than an accidental signal or even a beacon from an extraterrestrial civilization? Credit: Adrian Mann.
We’re left with an optimum starship of considerable mass, especially when we throw in shielding against cosmic rays and near relativistic interstellar particles. Let’s assume 1,000,000 tons, with an exhaust velocity of 0.1c, with average accelerations on the order of 0.1 m/s2. Zubrin works out a power requirement for his ‘standard’ starship of 1500 TW, which equals 0.9 percent of all the sunlight falling on the Earth, a figure only 11 orders of magnitude less than the total output of the Sun. By comparison, a Saturn V’s S1 stage had a power output of 0.1 TW. 1500 TW sounds extreme (Zubrin cites a collective human use of about 12 TW today) but if you keep power production growing at a rate of 2.6% per year, the rate current when the paper was written, then we hit 1500 TW around the year 2180 and get to 30,000 TW in 2300:
…the maximum size of individual power plants has been growing at a rate of 2 orders of magnitude per century for the past two hundred years. Thus if present trends continue, the apparently astronomical power required of our standard starship should be common in 3 or 4 centuries, a blink of an eye on the cosmic time scale.
Signature of a Starship
Zubrin is interested in the spectral signature of the standard starship, which should be different from an astrophysical object because its position and speed should change over time. The hope would be that future space telescopes or even terrestrial instruments could detect an object like this in many wavebands. The problem becomes one of collecting enough photons on a detector. High-power antimatter drives should emit huge amounts of gamma rays, but at interstellar distances we get a rate of impacts per square meter of collection area that is quite small. The standard starship may evade detection.
Consider: Zubrin calculates that a starship putting out 10,000 TW of 200 MeV gamma radiation at a distance of one light year from Earth would cause just 7.5 photons per year to strike a 1 square meter collection device. Such a spacecraft would be undetectable. The same problem occurs with X-ray emissions produced as bremsstrahlung radiation from a fusion engine (assuming D/He3), and while a detection within one light year is remotely possible, at 10 light years the impact rate falls to two per hour and the signal fades into the background noise. It’s in the realm of visible light that things begin to get more interesting. Visible light radiation from the ship’s exhaust becomes detectable through a telescope for many light years.
Thus our best case: An antimatter photon rocket with a jet power of 120,000 TW using a reflective nozzle to focus emitted light to a half angle of 30 degrees should produce an exhaust stream we can detect:
Such an object at a distance of 1 light year would be seen from Earth as a 17th magnitude light source, and could be detected on film by a first class amateur telescope. The 200 inch telescope on Mount Palomar could image it at 20 light-years, and the Hubble Space Telescope at a distance of about 300 light-years… Since at least for the upper-end telescopes considered, the number of stellar systems within range is significant (100,000 stars are within 200 light-years of Earth) this approach offers some hope for a successful search. The light from the photon rocket could be distinguished from that of a dim star by the lack of hydrogen lines in the rocket’s emissions.
More on this tomorrow, when we’ll look at magsail detection and the possibilities for other kinds of sails as well as worldships. The paper is Zubrin, “Detection of Extraterrestrial Civilizations via the Spectral Signature of Advanced Interstellar Spacecraft,” Progress in the Search for Extraterrestrial Life, ASP Conference Series Vol. 74 (1995). Available online.
