by Paul Gilster | Feb 20, 2018 | Outer Solar System |
A Hubble project called Outer Planet Atmospheres Legacy (OPAL) has been producing long-term information about the four outer planets at ultraviolet wavelengths, a unique capability that has paid off in deepening our knowledge of Neptune. If you kept pace with Voyager 2 at Neptune, you’ll recall that the spacecraft found huge dark storms in the planet’s atmosphere. Neptune proved to be more atmospherically active than its distance from the Sun would have suggested, and Hubble found another two storms in the mid-1990’s that later vanished.
Image: Neptune’s Great Dark Spot, a large anticyclonic storm similar to Jupiter’s Great Red Spot, observed by NASA’s Voyager 2 spacecraft in 1989. The image was shuttered 45 hours before closest approach at a distance of 2.8 million kilometers. The smallest structures that can be seen are of an order of 50 kilometers. The image shows feathery white clouds that overlie the boundary of the dark and light blue regions. Credit: NASA/JPL.
Now we have evidence of another storm, discovered by Hubble in 2015 and evidently vanishing before our eyes. This is a storm that was once large enough to have spanned the Atlantic, evidently visible because primarily composed of hydrogen sulfide drawn up from the deeper atmosphere. UC-Berkeley’s Joshua Tollefson, a co-author on the new paper on this work, notes that the storm’s darkness is relative: “The particles themselves are still highly reflective; they are just slightly darker than the particles in the surrounding atmosphere.”
Image: This series of Hubble Space Telescope images taken over 2 years tracks the demise of a giant dark vortex on the planet Neptune. The oval-shaped spot has shrunk from 5,000 kilometers across its long axis to 3,700 kilometers across, over the Hubble observation period. Immense dark storms on Neptune were first discovered in the late 1980s by the Voyager 2 spacecraft. Since then only Hubble has tracked these elusive features. Hubble found two dark storms that appeared in the mid-1990s and then vanished. This latest storm was first seen in 2015. The first images of the dark vortex are from the Outer Planet Atmospheres Legacy (OPAL) program, a long-term Hubble project that annually captures global maps of our solar system’s four outer planets. Credit: NASA, ESA, and M.H. Wong and A.I. Hsu (UC Berkeley).
The differences between Neptune’s storms and famous Jovian features like the Great Red Spot are interesting though not yet fully understood. The Great Red Spot has been a well described feature on Jupiter for more than two centuries, still robust though varying in size and color. A storm that once encompassed four Earth diameters had shrunk to twice Earth’s diameter in the Voyager 2 flyby of 1979, and has now dropped to perhaps 1.3. As to its heat sources, they are still under investigation, as we saw in 2016 (check Jupiter’s Great Red Spot as Heat Source).
Neptune is another story, with storms that seem to last but a few years. Thus the fading of the recent dark spot, which had been observed at mid-southern latitudes. Michael H. Wong (UC-Berkeley) is lead author of the paper:
“It looks like we’re capturing the demise of this dark vortex, and it’s different from what well-known studies led us to expect. Their dynamical simulations said that anticyclones under Neptune’s wind shear would probably drift toward the equator. We thought that once the vortex got too close to the equator, it would break up and perhaps create a spectacular outburst of cloud activity.”
But the storm drifted not toward the equator but the south pole, not constrained by the powerful alternating wind jets found on Jupiter. Moreover, we have no information on how these storms form or how fast they rotate. And as the new paper notes, the five Neptune dark spots we’ve thus far found have differed broadly in terms of size, shape, oscillatory behavior and companion cloud distribution. We have much to learn about their formation, behavior and dissipation.
When you think about flyby missions like Voyager 2 at Neptune, the value of getting that first look at a hitherto unknown object is obvious. But we are moving into an era when longer-term observations become paramount. The OPAL program with Hubble is an example of this, studying in the case of Neptune a phenomenon that seems to exist on a timescale that suits an annual series of observations. Hubble has been complemented by observations from other observatories, including not just Spitzer but, interestingly, Kepler K2. A robotic adaptive optics system tuned for planetary atmospheric science is being prepared for deployment in Hawaii, offering a way to scrutinize these worlds over even smaller periods.
From the paper:
Clearly, there is much room in the discovery space of solar system time domain science. There is room in this discovery space for exploration by a dedicated solar system space telescope, a network of ground facilities, and cadence programs at astrophysical observatories with advanced capabilities.
The paper is Wong et al., “A New Dark Vortex on Neptune,” Astronomical Journal Vol. 155, No. 3 (15 February 2018). Abstract.
by Paul Gilster | Feb 16, 2018 | Astrobiology and SETI |
Juggling all the factors impacting the emergence of extraterrestrial civilizations is no easy task, which is why the Drake equation has become such a handy tool. But are there assumptions locked inside it that need examination? Robert Zubrin thinks so, and in the essay that follows, he explains why, with a particular nod to the possibility that life can move among the stars. Although he is well known for his work at The Mars Society and authorship of The Case for Mars, Zubrin became a factor in my work when I discovered his book Entering Space: Creating a Spacefaring Civilization back in 2000, which led me to his scientific papers, including key work on the Bussard ramjet concept and magsail braking. Today’s look at Frank Drake’s equation reaches wide-ranging conclusions, particularly when we begin to tweak the parameters affecting both the lifetime of civilizations and the length of time it takes them to emerge and spread into the cosmos.
by Robert Zubrin
There are 400 billion other solar systems in our galaxy, and it’s been around for 10 billion years. Clearly it stands to reason that there must be extraterrestrial civilizations. We know this, because the laws of nature that led to the development of life and intelligence on Earth must be the same as those prevailing elsewhere in the universe.
Hence, they are out there. The question is: how many?
In 1961, radio astronomer Frank Drake developed a pedagogy for analyzing the question of the frequency of extraterrestrial civilizations. According to Drake, in steady state the rate at which new civilizations form should equal the rate at which they pass away, and therefore we can write:
Equation (1) is therefore known as the “Drake Equation.” Herein, N is the number of technological civilizations is our galaxy, and L is the average lifetime of a technological civilization. The left-hand side term, N/L, is the rate at which such civilizations are disappearing from the galaxy. On the right-hand side, we have R?, the rate of star formation in our galaxy; fp, the fraction of these stars that have planetary systems; ne, is the mean number of planets in each system that have environments favorable to life; fl the fraction of these that actually developed life; fi the fraction of these that evolved intelligent species; and fc the fraction of intelligent species that developed sufficient technology for interstellar communication. (In other words, the Drake equation defines a “civilization” as a species possessing radiotelescopes. By this definition, civilization did not appear on Earth until the 1930s.)
By plugging in numbers, we can use the Drake equation to compute N. For example, if we estimate L=50,000 years (ten times recorded history), R? = 10 stars per year, fp = 0.5, and each of the other four factors ne, fl, fi, and fc equal to 0.2, we calculate the total number of technological civilizations in our galaxy, N, equals 400.
Four-hundred civilizations in our galaxy may seem like a lot, but scattered among the Milky Way’s 400 billion stars, they would represent a very tiny fraction: just one in a billion to be precise. In our own region of the galaxy, (known) stars occur with a density of about one in every 320 cubic light years. If the calculation in the previous paragraph were correct, it would therefore indicate that the nearest extraterrestrial civilization is likely to be about 4,300 light years away.
But, classic as it may be, the Drake equation is patently incorrect. For example, the equation assumes that life, intelligence, and civilization can only evolve in a given solar system once. This is manifestly untrue. Stars evolve on time scales of billions of years, species over millions of years, and civilizations take mere thousands of years.
Current human civilization could knock itself out with a thermonuclear war, but unless humanity drove itself into complete extinction, there is little doubt that 1,000 years later global civilization would be fully reestablished. An asteroidal impact on the scale of the K-T event that eliminated the dinosaurs might well wipe out humanity completely. But 5 million years after the K-T impact the biosphere had fully recovered and was sporting the early Cenozoic’s promising array of novel mammals, birds, and reptiles. Similarly, 5 million years after a K-T class event drove humanity and most of the other land species to extinction, the world would be repopulated with new species, including probably many types of advanced mammals descended from current nocturnal or aquatic varieties.
Human ancestors 30 million years ago were no more intelligent than otters. It is unlikely that the biosphere would require significantly longer than that to recreate our capabilities in a new species. This is much faster than the 4 billion years required by nature to produce a brand-new biosphere in a new solar system. Furthermore, the Drake equation also ignores the possibility that both life and civilization can propagate across interstellar space.
So, let’s reconsider the question.
Estimating the Galactic Population
There are 400 billion stars in our galaxy, and about 10 percent of them are good G and K type stars which are not part of multiple stellar systems. Almost all of these probably have planets, and it’s a fair guess that 10 percent of these planetary systems feature a world with an active biosphere, probably half of which have been living and evolving for as long as the Earth. That leaves us with two billion active, well-developed biospheres filled with complex plants and animals, capable of generating technological species on time scales of somewhere between 10 and 40 million years. As a middle value, let’s choose 20 million years as the “regeneration time” tr. Then we have:
where N and L are defined as in the Drake equation, and ns is the number of stars in the galaxy (400 billion), fg is the fraction of them that are “good” (single G and K) type stars (about 0.1), fb is the fraction of those with planets with active biospheres (we estimate 0.1), fm is the fraction of those biospheres that are “mature” (estimate 0.5), and nb, the product of these last four factors, is the number of active mature biospheres in the galaxy.
If we stick with our previous estimate that the lifetime, L, of an average technological civilization is 50,000 years, and plug in the rest of the above numbers, equation (2) says that there are probably 5 million technological civilizations active in the galaxy right now. That’s a lot more than suggested by the Drake equation. Indeed, it indicates that one out of every 80,000 stars warms the home world of a technological society. Given the local density of stars in our own region of the galaxy, this implies that the nearest center of extraterrestrial civilization could be expected at a distance of about 185 light years.
Technological civilizations, if they last any time at all, will become starfaring. In our own case (and our own case is the only basis we have for most of these estimations), the gap between development of radiotelescopes and the achievement of interstellar flight is unlikely to span more than a couple of centuries, which is insignificant when measured against L=50,000 years. This suggests that once a civilization gets started, it’s likely to spread. Propulsion systems capable of generating spacecraft velocities on the order of 5 percent the speed of light appear possible. However, interstellar colonists will probably target nearby stars, with further colonization efforts originating in the frontier stellar systems once civilization becomes sufficiently well-established there to launch such expeditions.
In our own region of the galaxy, the typical distance between stars is five or six light years. So, if we guess that it might take 1,000 years to consolidate and develop a new solar system to the point where it is ready to launch missions of its own, this would suggest the speed at which a settlement wave spreads through the galaxy might be on the order of 0.5 percent the speed of light. However, the period of expansion of a civilization is not necessarily the same as the lifetime of the civilization; it can’t be more, and it could be considerably less. If we assume that the expansion period might be half the lifetime, then the average rate of expansion, V, would be half the speed of the settlement wave, or 0.25 percent the speed of light.
As a civilization expands, its zone of settlement encompasses more and more stars. The density, d, of stars in our region of the galaxy is about 0.003 stars per cubic light year, of which a fraction, fg, of about 10 percent are likely to be viable potential homes for life and technological civilizations. Combining these considerations with equation 2, we can create a new equation to estimate C, the number of civilized solar systems in our galaxy, by multiplying the number of civilizations N, by, nu, the average number of useful stars available to each.
For example, we have assumed that the average lifespan, L, of a technological species is 50,000 years, and if that is true, then the average age of one is half of this, or 25,000 years. If a typical civilization has been spreading out at the above estimated rate for this amount of time, the radius, R, of its settlement zone would be 62.5 light years (R = VL/2 = 62.5 ly), and its domain would include about 3,000 stars. If we multiply this domain size by the number of expected civilizations calculated above, we find that about 15 billion stars, or 3.75 percent of the galactic population, would be expected to lie within somebody’s sphere of influence. If 10 percent of these stars are actually settled, this implies there are about 1.5 billion civilized stellar systems within our galaxy. Furthermore, we find that the nearest outpost of extraterrestrial civilization could be expected to be found at a distance of 185-62.5 = 122.5 light years.
The above calculation represents my best guess as to the shape of things, but there’s obviously a lot of uncertainty in the calculation. The biggest uncertainty revolves around the value of L; we have very little data to estimate this number and the value we pick for it strongly influences the results of the calculation. The value of V is also rather uncertain, although less so than L, as engineering knowledge can provide some guide. In Table 1 we show how the answers might change if we take alternative values for L and V, while keeping the other assumptions we have adopted constant.
Table 1 The Number and Distribution of Galactic Civilizations
| V=0.005 c | V=0.0025 c | V=0.001 c |
| L=10,000 years | | | |
| N (# civilizations) | 1 million | 1 million | 1 million |
| C (# civilized stars) | 19.5 million | 2.4 million | 1 million |
| R (radius of domain) | 25 ly | 12.5 ly | 5 ly |
| S (Separation between civilizations) | 316 ly | 316 ly | 316 ly |
| D (distance to nearest outpost) | 291 ly | 304 ly | 311 ly |
| F (fraction of stars within domains) | 0.048% | 0.006% | 0.0025% |
| | | |
| L=50,000 years | | | |
| N (# civilizations) | 5 million | 5 million | 5 million |
| C (# civilized stars) | 12 billion | 1.5 billion | 98 million |
| R (radius of domain) | 125 ly | 62.5 ly | 25 ly |
| S (Separation between civilizations) | 185 ly | 185 ly | 185 ly |
| D (distance to nearest outpost) | 60 ly | 122.5 ly | 160 ly |
| F (fraction of stars within domains) | 30% | 3.75% | 0.245% |
| | | |
| L=200,000 years | | | |
| N (# civilizations) | 20 million | 20 million | 20 million |
| C (# civilized stars) | 40 billion | 40 billion | 18 billion |
| R (radius of domain) | 500 ly | 250 ly | 100 ly |
| S (Separation between civilizations) | 131 ly | 131 ly | 131 ly |
| D (distance to nearest outpost) | 0 ly | 0 ly | 31 ly |
| F (fraction of stars within domains) | 100% | 100% | 44% |
In Table 1, N is the number of technological civilizations in the galaxy (5 million in the previous calculation) , C is the number of stellar systems that some civilization has settled (1.5 billion, above), R is the radius of a typical domain (62.5 ly above), S is the separation distance between the centers of civilization (185 ly above), D is the probable distance to the nearest extraterrestrial outpost (122.5 ly, above), and F is the fraction of the stars in the galaxy that are within someone’s sphere of influence (3.75% above).
Examining the numbers in Table 1, we can see how the value of L completely dominates our picture of the galaxy. If L is “short” (10,000 years or less), then interstellar civilizations are few and far between, and direct contact would almost never occur. If L is “medium” (~50,000 years), then the radius of domains is likely to be smaller than the distance between civilizations, but not much smaller, and so contact could be expected to happen occasionally (remember, L, V, and S are averages; particular civilizations in various localities could vary in their values for these quantities). If L is a long time (> 200,000 years), then civilizations are closely packed, and contact should occur frequently. (These relationships between L and the density of civilizations apply in our region of the galaxy. In the core, stars are packed tighter, so smaller values of L are needed to produce the same “packing fraction,” but the same general trends apply.)
Any way you slice it, one thing seems rather certain: There’s plenty of them out there.
What are these civilizations like? What have they achieved?
It would be good to know.
by Paul Gilster | Feb 15, 2018 | Deep Sky Astronomy & Telescopes |
“Wherever you go, there you are.” So goes an old saw that makes a valid point: You can’t escape yourself by changing locations. Translating the great Greek poet C. P. Cavafy, Lawrence Durrell tweaked the language of “The God Abandons Antony” to come up with these closing lines:
Ah! don’t you see
Just as you’ve ruined your life in this
One plot of ground you’ve ruined its worth
Everywhere now — over the whole earth?
All this in the service of Durrell’s Alexandria Quartet, noting the fact that not even a Roman autocrat could escape his fate. Bear with me — I think about stuff like this when I’m out walking late at night and the stars are particularly stunning. Before my walk, I had been looking at images of M31, the Andromeda galaxy, and doing my usual “What would it be like to be there” routine. Minus Durrell/Cavafy’s dark vision, I might still ask myself what had changed. From a vantage in the Andromeda galaxy, there would be a Milky Way in my sky. And what else?
Then David Herne dropped me a note, providing a link to new research from Australia’s International Centre for Radio Astronomy Research dealing with this very galaxy. The work described therein raised the question anew: Just how alike are Andromeda and the Milky Way? For some previous estimates have held that M31 was actually two to three times the size of our galaxy, while others have found a rough parity, with Harvard’s Mark Reid and colleagues arguing in 2009 that our galaxy is about as massive as Andromeda, with a mass of up to 3 trillion Suns.
Image: The Andromeda Galaxy, perhaps a twin of our own. We can see Andromeda from without, but determining the structure of the galaxy we move through ourselves is a continuing challenge. Credit: NASA.
I described Reid’s work in 2009 (see How Many Stars in the Galaxy), and I mention it here because when we’re discussing these matters, it’s necessary to add a caveat. We have to ask ourselves, what exactly does a mass of 1 to 3 trillion stars actually mean? Much has happened since 2009, but I wrote this back then:
Does that mean that the Milky Way contains three trillion stars? Absolutely not. I’m seeing the three trillion star number popping up all over the Internet, and almost reported it that way here when I first encountered the work. The misunderstanding comes from making mistaken assumptions about galactic mass. Reid used the Very Long Baseline Array to examine regions of intense star formation across the galaxy, a study the scientist reported at the American Astronomical Society’s winter meeting this past January [2009]. The Milky Way does indeed turn out to have much more mass than earlier studies had indicated.
But a heavier than expected Milky Way means — according to much current thinking — a larger amount of dark matter. Reid and team had found that the Milky Way was rotating 15% faster than previously assumed, matching the rotation rate of M31 and implying similar overall mass and size. But only a fraction of this would be normal matter, so that a mass of three trillion Suns would still translate to, say, five hundred billion actual stars, and they would be spread over all stellar classes. In any case, the 3 trillion figure is now in doubt.
Back to the ICRAR work. In 2014, Prajwal Kafle (University of Western Australia) revised back downward the mass of the Milky Way, studying the kinematics of halo stars to determine the underlying distribution of mass, revealing about half as much dark matter as had been previously thought. Now Kafle and colleagues have gone to work on Andromeda, reaching the conclusion that the galaxy is about 800 billion times heavier than the Sun. Our nearest galactic neighbor thus turns out to be roughly the same mass as the Milky Way.
As in their earlier study, Kafle’s group looked at the orbits of high-velocity stars as a way of gauging galactic escape velocity, a technique developed by British astronomer James Jeans in 1915. For the Milky Way, this value is thought to be in the neighborhood of 550 kilometers per second, a figure Kafle and team confirmed in 2014. The new paper’s data mean that the value for Andromeda is not dissimilar. Like the Milky Way, M31 turns out to have much less dark matter than previously thought, perhaps only a third of earlier high-end estimates.
If this is the case, then we can start to re-think the remote future, when the two giant spiral galaxies (over two million light years apart) begin to approach each other. Indeed, galactic interactions within the local group — 54 galaxies, most of them dwarfs — are affected by these changes, given that the gravitational center is located between the Milky Way and Andromeda. The two galaxies are now shown to be evenly matched in terms of size. Adds Kafle:
“It completely transforms our understanding of the local group. We had thought there was one biggest galaxy and our own Milky Way was slightly smaller but that scenario has now completely changed.”
We have five billion years to wait before the merger of our two galaxies, which happens to be the same timescale that takes in the growth of our Sun to red giant stage. What will the Solar System become as the Sun swells and the galaxies begin their close encounter? Kafle’s simulations of the event are spectacular, as you can see below.
The paper is Kafle et al., “The Need for Speed: Escape velocity and dynamical mass measurements of the Andromeda galaxy,” Monthly Notices of the Royal Astronomical Society February 15th, 2018 (abstract). Kafle’s 2014 paper on the Milky Way’s mass is “On the Shoulders of Giants: Properties of the Stellar Halo and the Milky Way Mass Distribution,” Astrophysical Journal 794, No. 1 (24 September 2014). Abstract.
by Paul Gilster | Feb 14, 2018 | Astrobiology and SETI |
Anomalous objects are a problem — we need more than one to figure them out. One ‘hot Jupiter’ could have been an extreme anomaly, but we went on to find enough of them to realize this was a kind of planet that had a place in our catalog. Or think of those two Kuiper Belt objects that New Horizons imaged, as discussed in yesterday’s post. Soon we’ll have much closer imagery of MU69, but it will take more encounters — and more spacecraft — to begin to fathom the full range of objects that make up the Kuiper Belt. Ultimately, we’d like to see enough KBOs up close to start drawing statistically valid conclusions about the entire population.
So where does the intriguing ‘Oumuamua fit into all this? It was the first interstellar asteroid we’ve been able to look at, even if the encounter was fleeting. A friend asked me, having learned of the Breakthrough Listen SETI monitoring of the object, whether it wasn’t absurd to imagine it could be a craft from another civilization. I could only say that the idea was highly unlikely, but given how little time we had and how rare the object was, how could we not have listened? I favor throwing whatever resources we have at an opportunity this unusual.
And time was short, as Joshua Sokol recently noted in Scientific American. We found ‘Oumuamua in late October of last year, but getting a probe to it on the best possible trajectory would have demanded a launch the previous July. I see that Greg Laughlin (UC-Santa Cruz), working with Yale doctoral student Darryl Seligman, has been exploring how we might drive an impactor into a future interstellar visitor, allowing the kind of analysis we did with the Deep Impact mission. I’ll have more on the idea as the paper wends its way through peer review.
Image: This animation shows the path of ‘Oumuamua, which passed through our inner solar system in September and October 2017. From analysis of its motion, scientists calculate that it probably originated from outside of our Solar System. Credit: NASA/JPL-Caltech.
We appear to be getting into the era of comparative interstellar object studies. One, two, many ‘Oumuamuas, not to mention their cousins, who may not just pass through but stick around. Harvard’s Avi Loeb, working with Manasvi Lingam (Harvard-Smithsonian Center for Astrophysics), offers a paper on ‘Oumuamua that’s now available on the arXiv server. Here we get a sense of the broader population of interstellar objects, not all of which may have departed.
The authors have approached the question by asking how likely it is for interstellar objects to be captured in our Solar System, performing the same kind of analysis for the Alpha Centauri system. The scientists believe several thousand captured interstellar objects may be within the Solar System at any given time, with the largest of these reaching tens of kilometers in size.
‘Oumuamua came and went quickly, but a long-lingering population offers us ample grounds for investigation. Likening the effects of the Sun and Jupiter to a fishing net, the authors peg the number of interstellar objects currently within the system at ~ 6 x 103, pointing out that they offer us the potential to study exoplanetary debris without leaving our own system.
But how to determine whether an object now bound to our Solar System really is interstellar in origin? The answer may lie in the chemical constitution of water vapor found associated with the object. The oxygen isotope ratios may hold the key, as the paper explains:
…if the oxygen isotope ratios are markedly different from the values commonly observed in the Solar system, it may suggest that the object is interstellar in nature; more specifically, the ratio of 17O/18O is distinctly lower for the Solar system compared to the Galactic value (Nittler & Gaidos 2012), and hence a higher value of this ratio may be suggestive of interstellar origin.
To make this work, we could analyze these isotopes through high-resolution spectroscopy, working in the optical, infrared and submillimeter ranges of water vapor in cometary tails, just as the Herschel observatory was able to measure the isotope ratio of comet C/2009 P1 in the Oort cloud. A flyby and perhaps even a sample return mission could not be ruled out either, with the interesting implication that a technology like Breakthrough Starshot‘s could be used to explore much closer targets than Proxima Centauri with short mission times.
But if thousands of interstellar objects are within our Solar System now, what implications does this offer for the emergence of life? The paper notes that some 400 interstellar objects with a radius in the 0.1 kilometer range could have struck the Earth prior to abiogenesis, and about 10 could have been kilometer-sized. The possibility of interstellar panspermia is evident. The paper continues:
If a km-sized interstellar object were to strike the Earth, we suggested that it would result in pronounced local changes, although the global effects may be transient. Habitable planets could have been seeded by means of panspermia through two different channels: (i) direct impact of interstellar objects, and (ii) temporary capture of the interstellar object followed by interplanetary panspermia. There are multiple uncertainties involved in all panspermia models, as the probability of alien microbes surviving ejection, transit and reentry remains poorly constrained despite recent advancements.
It’s interesting to note on this score that while the Solar System might have snared objects up to tens of kilometers in size, the Alpha Centauri system could capture objects up to Earth size, making for the possibility of a life-bearing world being acquired in its entirety.
‘Oumuamua work continues in a letter from Carlos de la Fuente Marcos (Complutense University of Madrid) that analyzes the orbits of 339 known hyperbolic objects and models their histories, finding eight possible interstellar objects within past astronomical observations. Unlike Loeb and Lingam’s population of captured objects, these visitors followed the ‘Oumuamua model, making a single brief appearance, but they offer the possibility that our archives contain further examples of such wanderers. The onset of observations with the Large Synoptic Survey Telescope in the early 2020s may help us further constrain the population of unbound objects.
The paper is Lingam & Loeb, “Implications of Captured Interstellar Objects for Panspermia and Extraterrestrial Life” (preprint). The de la Fuente Marcos paper is “Where the Solar system meets the solar neighbourhood: patterns in the distribution of radiants of observed hyperbolic minor bodies,” Monthly Notices of the Royal Astronomical Society 20 February 2018 (abstract).
by Paul Gilster | Feb 13, 2018 | Outer Solar System |
New Horizons continues to push our limits, revealing new sights as it makes its way through the Kuiper Belt enroute to a January 1, 2019 encounter with the KBO 2014 MU69. No object this far from the Sun has ever been visited by a spacecraft. Adding further interest is the unusual nature of the target, for MU69 is thought to be a contact binary, two independent bodies that have touched (comet Churyumov-Gerasimenko is likely a contact binary as well). The beauty of this kind of exploration, of course, is that we so often get surprised when we reach our destination.
Below is an image of NGC 3532, also known as the Wishing Well Cluster, an open cluster in the constellation Carina that has its own place in our observational history, becoming the first target ever observed by the Hubble Space Telescope. That was in May of 1990; this is New Horizons’ view in December.
The Wishing Well Cluster is a naked eye object for southern hemisphere observers, one of the most spectacular clusters of its type. It’s worth noting that astronomer John Herschel (1792-1871) considered NGC 3532 one of the most beautiful clusters in the sky, describing “several elegant double stars” during a residence in southern Africa in the 1830s. The New Horizons image below doesn’t bring out its aesthetic appeal (see the following image for that), but it’s stirring nonetheless when we consider how far from home the image was made.
Image: For a short time, this New Horizons Long Range Reconnaissance Imager (LORRI) frame of the “Wishing Well” star cluster, taken Dec. 5, 2017, was the farthest image ever made by a spacecraft, breaking a 27-year record set by Voyager 1. About two hours later, New Horizons later broke the record again. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
Here we’re seeing the work of New Horizons’ Long Range Reconnaissance Imager (LORRI), taken when the spacecraft was 6.12 billion kilometers (40.9 AU) from Earth. And yes, that puts it further away than Voyager 1 was when it took the ‘Pale Blue Dot’ photo back in 1990 — Voyager at that time was 6.06 billion kilometers away. Because the Voyager cameras were turned off not long after that image was made, its distance record for images has stood until now.
By way of comparison, and in the spirit of the great John Herschel, here’s the Wishing Well Cluster in all its glory in an image from the European Southern Observatory’s La Silla site, using the Wide Field Imager instrument.
Image: The MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile captured this richly colourful view of the bright star cluster NGC 3532. Some of the stars still shine with a hot bluish colour, but many of the more massive ones have become red giants and glow with a rich orange hue. Credit: ESO/G. Beccari.
Two hours after the Wishing Well image from New Horizons, LORRI set still another distance record, imaging Kuiper Belt objects 2012 HZ84 and 2012 HE85. The spacecraft’s travels in the Kuiper Belt will be replete with observations of KBOs other than MU69, although none will be approached nearly as closely as the latter. This update from JHU/APL tells us that the plan is to observe at least two dozen KBOs, dwarf planets and Centaurs, hoping to determine their shapes and examine their surface properties, while likewise looking for moons and rings. Meanwhile, measurements of plasma, dust and the neutral-gas environment in this region proceed, useful data for future missions to the heliosphere and beyond.
Image: With its Long Range Reconnaissance Imager (LORRI), New Horizons has observed several Kuiper Belt objects (KBOs) and dwarf planets at unique phase angles, as well as Centaurs at extremely high phase angles to search for forward-scattering rings or dust. This December 2017 false-color image of KBO 2012 HZ84 is, for now, one of the farthest from Earth ever captured by a spacecraft. At the time it was among the closest observations yet made of the mysterious, distant objects known as KBOs. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
Image: A second KBO image. Here, New Horizons’ range to the KBO 2012 HE85 was only 51 million kilometers, or 0.34 AU – closer than the planet Mars ever comes to Earth. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
