Message 1981929 - Posted: 23 Feb 2019, 23:17:04 UTC

Last modified: 23 Feb 2019, 23:18:32 UTC

In late 2017, astronomers detected a visitor from beyond our solar system for the first time. Itâ€™s likely happened before, and will again, but this time it was not a product of science fiction, as entertaining, thought- provoking, or as banal those books and movies might be. While most of us would consider it unlikely that it is an alien spaceship, this interstellar visitor presents a useful opportunity to construct physical theories that account for the data, while also prompting us to refresh our considerations of the kinds of visits and communications that might be detected by SETI projects.



Astronomers today are engaged in the systematic tracking of asteroids, comets, and other non- planetary denizens of our solar neighborhood. For example, the Pan-STARRS telescope situated atop the Haleakala crater in Hawaii is dedicated to the detection of moving and variable celestial objects. In October of 2017, it sighted an object whose subsequent track revealed that it was moving far too quickly to remain in our solar system, or to have originated within it. It was first given a â€œCâ€ classification and number representing a comet in the designation system of the International Astronomical Union. Comets are chunks of icy material originating from the outer regions of our solar system. They form a gassy envelope when nearing the sun. However, the failure to detect such a cometary coma led astronomers to switch to an â€œAâ€ designation referring to an asteroid. But when its speedy hyperbolic trajectory was firmly established, a new classification of â€œIâ€ for interstellar was created. 1I/2017 U1 is the first officially named interstellar vagabond. The observing team then named it Oumuamua from the Hawaiian language meaning â€œfirst messengerâ€ or â€œscoutâ€.



How surprising is such a visitor? Our solar system is littered with objects smaller than its planets and moons which are the natural products of the accretion and collision processes that formed the sun and its orbiting bodies from a primordial rotating disc of gas and dust about 5 billion years ago. We have learned in recent years from ground-based surveys and orbiting observatories such as the Kepler Satellite that planetary systems and their dusty precursors are not at all unusual, but rather accompany roughly half of the stars. Thus, the census of the real estate of our Milky Way galaxy contains not only a few hundred billion stars (plus interstellar clouds of gas and dust), but also a comparable number of planets. Smaller rocks and ice chunks like asteroids and comets are naturally expected to be part of these planetary systems. Some objects will be kicked out of their stellar systems by gravitational encounters with planets and become interstellar wanderers. Estimates of the number of these interstellar vagabonds that will visit our neighborhood vary from very few to hundreds or more per year. With n=1 observed so far, itâ€™s hard to know.



What have we learned about our first visitor? Unfortunately, Oumuamua was already speeding away from the sun when first observed, so its entry path is a product of calculation. Its trajectory is highly inclined to the plane of planets in our solar system, and its speed well exceeds the escape velocity of our sunsâ€™ gravity. It is following a hyperbolic path which must have whipped it around the sun on September 9, 2017, closer than the orbit of Mercury, at about 200,000 miles per hour. It is now farther from the sun than Jupiter and heading out of our solar system at about 70,000 miles per hour. As it is rapidly outdistancing effective observation from even our best telescopes, we may not be able to collect more data to add to what we now have to ground our theories. The extremely large variations in its light curve indicate that it is anything but a spherical rock or ball of ice. Rather it must be a highly elongated object that has been described as â€œcigar shapedâ€ or â€œplate shapedâ€ (avoiding the media-favorite words â€œtorpedo-â€ or â€œsaucer-shapedâ€). It is tumbling at a rotation period of ~7-8 hours. Estimates of its dimensions range from 300-3000 ft x 100-500 ft x 100-500 ft. But these dimensions have been questioned, as they are based only on light measurements and assumptions of how much light its surface reflects and how smooth it might be. Its size and shape were never directly observed, since it only appears as a point source of light in the most powerful telescopes.



The plot has thickened since reports in mid 2018 that Oumuamua is not passively coasting away from the sunsâ€™ pull, but is accelerating by a slight but significant amount. Initial speculation was that Oumuamua was receiving a rocket-like push from outgassing, a jet-like evaporation that occurs in comets. But this idea ran into problems. To account for the observed acceleration it would have produced a detectable comet-like tail. But none was found. Furthermore, such outgassing is incompatible with the objectsâ€™ spin. Perhaps light provides the motive force with a push of solar radiation pressure. This â€œlight pressureâ€ is strong enough to force dust particles away from the heads of comets to produce their characteristic tails, but could it be strong enough to push Oumuamua? That depends on what it is made of â€“ whether it is a lumpy rock, a cloud of dust, a cosmic leaf or a solar sail.



Unusual astronomical observations have previously led to speculations of an intelligent alien design. When rapidly repeating high energy radio signals were first detected in the 1960s, even their discoverers whimsically tossed out a suggested code-name for them as LGM, standing for little green men. We now call them pulsars and understand them to be high energy beacons emitted from fast spinning neutron stars as their beam sweeps past our line of sight. So it may not be surprising that suggestions of alien technology soon appeared for Oumuamuaâ€™s acceleration, and not just from the media. The Chair of the Harvard Astronomy Department, Abraham Loeb and Schmuel Baily published a scholarly paper making just such a suggestion in Astrophysical Journal Letters last November.



After considering the observed trajectory and light curve, and reiterating the shortcomings of the comet-like outgasing scenario, the paper details the parameters necessary for the acceleration to be produced by radiation pressure. Only a thin, sheet-like object would allow the requisite response to the light pressure at its distance from the sun. Such a â€œlight sailâ€ has been proposed as a means to travel between our solar planets and beyond. So the authors came right out and suggested the possibility of an artificial origin for Oumuamua. Outlandish? We are reminded of the sage advice of Sherlock Holmes: â€œWhen you have excluded the impossible, whatever remains, however improbable, must be the truth.â€



Astronomers have not yet run out of natural explanations. Recently, Zdenek Sekanina proposed that Oumuamua is a â€œmonstrously fluffy dust aggregateâ€ that was recently ejected from a local object by an explosive event (Such a cloud would not be expected to rotate and survive in its current form if it had travelled interstellar distances.) No doubt other proposals will be forthcoming. Unfortunately, we are not likely to ever gain more data on Oumuamua as it races away from detection from our best instruments. And it is probably a forlorn hope that we would develop and launch a next generation space probe to catch up with it.



Still, the alien hypothesis was interesting enough that SETI investigators have listened for signals from Oumuamua, both with the SETI Instituteâ€™s Allen Telescope Array and with the Breakthrough Listen detector on the Green Bank Radio Telescope. Nothing was heard so far. More listening seems appropriate, as well as assiduous searches by astronomers to detect and analyze other interstellar vagabonds.



Is there value beyond stirring the press and the public imagination in placing in the scientific literature the provocative possibility of an artificial origin of Oumuamua? Perhaps. Adding this to the list of possibilities for Oumuamua and future unusual phenomena doesnâ€™t mean one abandons the search for more-likely natural explanations. But it does serve to remind us to keep reconsidering the nature of possible encounters with, or signals from, extraterrestrial intelligence so they can best be detected and understood. And it should help to fuel the continued efforts of SETI. Even if Uamuamua is not an artificial scout, and the odds of an extraterrestrial encounter remain low, recall that the probability of winning the LOTTO is so miniscule that anyone would reasonably round off that number to zero. But not buying a ticket truly makes the odds of winning zero. --

Richard M. Lawn In late 2017, astronomers detected a visitor from beyond our solar system for the first time. Itâ€™s likely happened before, and will again, but this time it was not a product of science fiction, as entertaining, thought- provoking, or as banal those books and movies might be. While most of us would consider it unlikely that it is an alien spaceship, this interstellar visitor presents a useful opportunity to construct physical theories that account for the data, while also prompting us to refresh our considerations of the kinds of visits and communications that might be detected by SETI projects.Astronomers today are engaged in the systematic tracking of asteroids, comets, and other non- planetary denizens of our solar neighborhood. For example, the Pan-STARRS telescope situated atop the Haleakala crater in Hawaii is dedicated to the detection of moving and variable celestial objects. In October of 2017, it sighted an object whose subsequent track revealed that it was moving far too quickly to remain in our solar system, or to have originated within it. It was first given a â€œCâ€ classification and number representing a comet in the designation system of the International Astronomical Union. Comets are chunks of icy material originating from the outer regions of our solar system. They form a gassy envelope when nearing the sun. However, the failure to detect such a cometary coma led astronomers to switch to an â€œAâ€ designation referring to an asteroid. But when its speedy hyperbolic trajectory was firmly established, a new classification of â€œIâ€ for interstellar was created. 1I/2017 U1 is the first officially named interstellar vagabond. The observing team then named it Oumuamua from the Hawaiian language meaning â€œfirst messengerâ€ or â€œscoutâ€.How surprising is such a visitor? Our solar system is littered with objects smaller than its planets and moons which are the natural products of the accretion and collision processes that formed the sun and its orbiting bodies from a primordial rotating disc of gas and dust about 5 billion years ago. We have learned in recent years from ground-based surveys and orbiting observatories such as the Kepler Satellite that planetary systems and their dusty precursors are not at all unusual, but rather accompany roughly half of the stars. Thus, the census of the real estate of our Milky Way galaxy contains not only a few hundred billion stars (plus interstellar clouds of gas and dust), but also a comparable number of planets. Smaller rocks and ice chunks like asteroids and comets are naturally expected to be part of these planetary systems. Some objects will be kicked out of their stellar systems by gravitational encounters with planets and become interstellar wanderers. Estimates of the number of these interstellar vagabonds that will visit our neighborhood vary from very few to hundreds or more per year. With n=1 observed so far, itâ€™s hard to know.What have we learned about our first visitor? Unfortunately, Oumuamua was already speeding away from the sun when first observed, so its entry path is a product of calculation. Its trajectory is highly inclined to the plane of planets in our solar system, and its speed well exceeds the escape velocity of our sunsâ€™ gravity. It is following a hyperbolic path which must have whipped it around the sun on September 9, 2017, closer than the orbit of Mercury, at about 200,000 miles per hour. It is now farther from the sun than Jupiter and heading out of our solar system at about 70,000 miles per hour. As it is rapidly outdistancing effective observation from even our best telescopes, we may not be able to collect more data to add to what we now have to ground our theories. The extremely large variations in its light curve indicate that it is anything but a spherical rock or ball of ice. Rather it must be a highly elongated object that has been described as â€œcigar shapedâ€ or â€œplate shapedâ€ (avoiding the media-favorite words â€œtorpedo-â€ or â€œsaucer-shapedâ€). It is tumbling at a rotation period of ~7-8 hours. Estimates of its dimensions range from 300-3000 ft x 100-500 ft x 100-500 ft. But these dimensions have been questioned, as they are based only on light measurements and assumptions of how much light its surface reflects and how smooth it might be. Its size and shape were never directly observed, since it only appears as a point source of light in the most powerful telescopes.The plot has thickened since reports in mid 2018 that Oumuamua is not passively coasting away from the sunsâ€™ pull, but is accelerating by a slight but significant amount. Initial speculation was that Oumuamua was receiving a rocket-like push from outgassing, a jet-like evaporation that occurs in comets. But this idea ran into problems. To account for the observed acceleration it would have produced a detectable comet-like tail. But none was found. Furthermore, such outgassing is incompatible with the objectsâ€™ spin. Perhaps light provides the motive force with a push of solar radiation pressure. This â€œlight pressureâ€ is strong enough to force dust particles away from the heads of comets to produce their characteristic tails, but could it be strong enough to push Oumuamua? That depends on what it is made of â€“ whether it is a lumpy rock, a cloud of dust, a cosmic leaf or a solar sail.Unusual astronomical observations have previously led to speculations of an intelligent alien design. When rapidly repeating high energy radio signals were first detected in the 1960s, even their discoverers whimsically tossed out a suggested code-name for them as LGM, standing for little green men. We now call them pulsars and understand them to be high energy beacons emitted from fast spinning neutron stars as their beam sweeps past our line of sight. So it may not be surprising that suggestions of alien technology soon appeared for Oumuamuaâ€™s acceleration, and not just from the media. The Chair of the Harvard Astronomy Department, Abraham Loeb and Schmuel Baily published a scholarly paper making just such a suggestion in Astrophysical Journal Letters last November.After considering the observed trajectory and light curve, and reiterating the shortcomings of the comet-like outgasing scenario, the paper details the parameters necessary for the acceleration to be produced by radiation pressure. Only a thin, sheet-like object would allow the requisite response to the light pressure at its distance from the sun. Such a â€œlight sailâ€ has been proposed as a means to travel between our solar planets and beyond. So the authors came right out and suggested the possibility of an artificial origin for Oumuamua. Outlandish? We are reminded of the sage advice of Sherlock Holmes: â€œWhen you have excluded the impossible, whatever remains, however improbable, must be the truth.â€Astronomers have not yet run out of natural explanations. Recently, Zdenek Sekanina proposed that Oumuamua is a â€œmonstrously fluffy dust aggregateâ€ that was recently ejected from a local object by an explosive event (Such a cloud would not be expected to rotate and survive in its current form if it had travelled interstellar distances.) No doubt other proposals will be forthcoming. Unfortunately, we are not likely to ever gain more data on Oumuamua as it races away from detection from our best instruments. And it is probably a forlorn hope that we would develop and launch a next generation space probe to catch up with it.Still, the alien hypothesis was interesting enough that SETI investigators have listened for signals from Oumuamua, both with the SETI Instituteâ€™s Allen Telescope Array and with the Breakthrough Listen detector on the Green Bank Radio Telescope. Nothing was heard so far. More listening seems appropriate, as well as assiduous searches by astronomers to detect and analyze other interstellar vagabonds.Is there value beyond stirring the press and the public imagination in placing in the scientific literature the provocative possibility of an artificial origin of Oumuamua? Perhaps. Adding this to the list of possibilities for Oumuamua and future unusual phenomena doesnâ€™t mean one abandons the search for more-likely natural explanations. But it does serve to remind us to keep reconsidering the nature of possible encounters with, or signals from, extraterrestrial intelligence so they can best be detected and understood. And it should help to fuel the continued efforts of SETI. Even if Uamuamua is not an artificial scout, and the odds of an extraterrestrial encounter remain low, recall that the probability of winning the LOTTO is so miniscule that anyone would reasonably round off that number to zero. But not buying a ticket truly makes the odds of winning zero.--Richard M. Lawn ID: 1981929 ·