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 | Nov 14, 2017 | Asteroid and Comet Deflection |
I used to think the Kuiper Belt object Quaoar was hard to pronounce (“Kwawar”), and even muffed it despite having plenty of time to practice before the recent Tennessee Valley Interstellar Workshop. Pontus Brandt (JHU/APL) had mentioned Quaoar in his talk in Huntsville as a target that lined up in useful ways with a proposed interstellar precursor mission he was presenting, one designed to examine dust distribution from within the system by looking back at our heliosphere at distances up to 1000 AU, seeing it as we see other stars’ dust environments.
So I summarized Brandt’s ideas in my wrap-up talk and couldn’t get Quaoar pronounced properly without multiple tries. But even Quaoar pales into the realm of everyday lingo when compared to 1I/’Oumuamua. Please tell me how to do this. The word is a Hawaiian term for ‘scout,’ and the Ulukau: Hawaiian Electronic Library’s online dictionary tells me it’s pronounced this way: ?’u-mu’-a-mu’-?. I could work with that and maybe get it right in a talk, with extra practice. At least until I look at it — all those vowels defeat me.
The object originally tagged A/2017 U1, then, is now tagged as interstellar in the combined 1I/’Oumuamua, the 1 indicating it is the first such object to be observed, the I indicating interstellar. It is fitting that 1I/’Oumuamua was the name chosen by the Pan-STARRS team in Hawaii that first brought this object to our attention.
Image: The trajectory of 1I/ ‘Oumuamua, which made its closest approach to Earth on October 14, coming within 24,000,000 km, or about 60 times the distance to the moon. Credit: NASA/JPL-Caltech.
As far as the sheer number of objects making long interstellar journeys, consider what New Horizons PI Alan Stern recently told Astronomy Magazine (see The First Known Interstellar Interloper):
According to Stern, Jupiter, Saturn, Uranus, and Neptune combined probably ejected 1013 to 1014 objects larger than 1 km early in our solar system’s history, when it was still cluttered with debris left over from the planet-formation process. Multiply that by the 1011 stars in the Milky Way, and one comes up with numbers like 1024 to 1025 objects larger than a kilometer. Smaller objects like 1I/‘Oumuamua must be orders of magnitude more plentiful.
We’ve just seen, in the work of members of the Initiative for Interstellar Studies, how reaching such an object might be attempted. Now the question becomes, where did it come from? On that score, we have quick work indeed from Eric Gaidos (University of Hawaii at Manoa), working with Jonathan Williams at the same institution and Adam Kraus (University of Texas at Austin). The trio believes it has traced 1I/‘Oumuamua’s origins and presents its case in a paper submitted to Research Notes of the American Astronomical Society.
Gaidos and team traced the path of 1I/’Oumuamua backwards along its route and took into account the numerous variables along the way in so extended a journey. The likelihood, the researchers believe, is that the object originated in a nearby young stellar cluster. They point to the Carina and Columba Associations, the word ‘association’ referring to stellar associations, which are loosely bound star clusters whose stars share a common origin. They still move together through space but are at this point gravitationally unbound. The estimated distances to the Carina and Columba Associations range from 50 to 85 parsecs (163 to 277 light years); the age of stars within these groups is on the order of 45 million years. An object ejected at 1-2 kilometers per second soon after star formation would thus have had time to reach the Sun.
From the paper:
We suggest that A/2017 U1 formed in a protoplanetary disk in the Carina/Columba associations and was ejected by a planet ?40 Myr ago. The absence of ice indicates an origin inside the “ice line” of the disk plus an ejection velocity of 1-2 km sec?1 (assuming the cluster was already unbound), constrain the mass mP and semi-major axis aP of the planet.
What kind of planet could have ejected this object? The paper examines the issue both for solar mass stars as well as M-dwarfs:
Permitted values (grey zone) center around a 20-30M? planet forming by a few Myr within a few AU, reminiscent of the core accretion scenario for giant planet formation. In contrast, a “super-Earth” at ?1 AU could eject ice-free planetesimals from a lower-mass M dwarf.
Image: This is half of the paper’s Figure 1, showing only the projection from a protoplanetary disk around a solar mass star. The paper’s second chart, not shown here, gives equivalent information for an M-dwarf. The caption continues: “Below the red line planets accrete rather than scatter planetesimals. Above the green line planets eject planetesimals at > 1 km sec?1. Below the purple line planetesimals are captured into clouds by the cluster tide. Below the black line planetesimals require > 10 Myr to escape. To the right of the blue lines planetesimals contain ices.” Credit: Gaidos, Williams & Kraus.
As the paper notes, future interstellar interlopers may well have radiants similar to 1I/’Oumuamua. Practicing our skills on this celestial wanderer may thus tune us up for another.
The paper is Gaidos, Williams and Kraus, “Origin of Interstellar Object A/2017 U1 in a Nearby Young Stellar Association?” submitted to Research Notes of the American Astronomical Society (abstract). Our interstellar wanderer seems to be spawning a growth industry in these early days following its detection. See also Zwart et al., “The origin of interstellar asteroidal objects like 1I/2017 U1,” submitted to Monthly Notices of the Royal Astronomical Society (preprint). I haven’t read this one yet and thus won’t comment.
by Paul Gilster | Sep 20, 2017 | Outer Solar System |
One of the great values of the Kepler mission has been its ability to produce a statistical sample that we can use to analyze the distribution of planets. The population of asteroids in our own Solar System doubtless deserves the same treatment, given its importance in future asteroid mining as well as planetary protection. But when it comes to main belt asteroids, we’re able to look up close, even though the number of actual missions thus far has been small.
Thus it’s heartening to see Pekka Janhunen (Finnish Meteorological Institute), long a champion of intriguing ‘electric sail’ concepts, looking into how we might produce just such an asteroid sampling through a fleet of small spacecraft.
“Asteroids are very diverse and, to date, we’ve only seen a small number at close range. To understand them better, we need to study a large number in situ. The only way to do this affordably is by using small spacecraft,” says Janhunen.
The concept weds electric sails riding the solar wind with a fleet of 50 small spacecraft, the intent being that each should visit six or seven asteroids, collecting spectroscopic data on their composition and taking images. Dr. Janhunen presented the idea at the European Planetary Science Congress (EPSC) 2017 in Riga on Tuesday September 19.
Image: The single-tether E-sail spacecraft. Credit: Janhunen et al.
Electric sails ride the solar wind, that stream of charged particles that flows constantly out of the Sun. While solar sails take advantage of the momentum imparted by photons on the sail, and beamed energy sails are driven by microwave or laser emissions, electric sails use the solar wind’s charged particles to generate all the propulsion they need without propellant. What Janhunen envisions is a tether attached to one end of a spacecraft, to which is attached an electron emitter and a high-voltage source, all connected to a remote unit at the other end.
The tether makes a complete rotation every 50 minutes, creating a shallow cone around a center of mass close to the primary spacecraft. Each small craft can change its orientation to the solar wind, and thereby alter its thrust and direction. Janhunen’s presentation at the EPSC made the case that a 5 kg spacecraft with a 20 kilometer tether could accelerate at 1 millimeter per second squared at the Earth’s distance from the Sun. Coupled with the boost provided by the launch itself, this is enough to complete a tour through the asteroid belt and return within 3.2 years.
Image: Artist’s concept of the spacecraft. Credit: FMI.
These spacecraft are small enough (Janhunen refers to them as ‘nanosats’) that they cannot carry a large antenna. Instead, the mission concept calls for each spacecraft to make a final flyby of the Earth to download mission data. The financial numbers are compelling. Billions of Euros would be involved in attempting a flyby of 300 asteroids with conventional methods, while Janhunen’s fleet of nanosats could, he believes, fly this mission for 60 million Euros.
The payoff could be substantial:
“The nanosats could gather a great deal of information about the asteroids they encounter during their tour, including the overall size and shape, whether there are craters on the surface or dust, whether there are any moons, and whether the asteroids are primitive bodies or a rubble pile. They would also gather data on the chemical composition of surface features, such as whether the spectral signature of water is present.”
Working with ever smaller payloads is a recurring theme in deep space exploration, the ultimate example being Breakthrough Starshot’s study of a laser-beamed sail mission to Proxima Centauri’s planet, one that would deploy a fleet of sails just meters to the side, each carrying a payload as small as a microchip. In Janhunen’s concept, the spacecraft are capable of carrying a 4-centimeter telescope capable of resolving asteroid surface features, along with an infrared spectrometer that can analyze reflected light to determine the object’s mineralogy.
With flybys at a range of about 1000 kilometers, the spacecraft would be able to image features to a resolution of 100 meters or better, and with multiple targets for each, the catalog of main belt asteroids that we have seen up close would suddenly mushroom. The mission, being referred to as the Asteroid Touring Nanosat Fleet, would help us catalog the various types of asteroids and provide valuable analysis of their composition and structure, all of which would add to our expertise if and when we ever have to nudge an asteroid into a new trajectory.
200,000 Euros per asteroid is a strikingly efficient use of resources, and the engineering involved in deploying Janhunen’s fleet of electric sails would give us priceless experience as we work with other mission concepts that involve the solar wind. But can we ride a wind as mutable and unpredictable as this one while ensuring the kind of pinpoint navigation we need? Questions like these will need answering in space through the necessary precursors.
by Paul Gilster | Aug 3, 2017 | Outer Solar System |
The Voyagers’ continuing interstellar mission reminds us of how little we know about space just outside our own Solar System. We need to learn a great deal more about the interstellar medium before we venture to send fast spacecraft to other stars. And indeed, part of Breakthrough Starshot’s feasibility check re small payloads and sails will be to assess the medium and determine what losses are acceptable for a fleet of such vehicles.
The definitive work on the matter is Bruce Draine’s Physics of the Interstellar and Intergalactic Medium, and thus it’s no surprise that Draine has been involved as a consultant with Starshot. As we saw yesterday, we have only one spacecraft returning data from outside the heliosphere (soon to be joined by Voyager 2), making further precursor missions explicitly designed to study ‘local’ gas and dust conditions a necessity.
Another reminder of the gaps in our knowledge comes from an analysis of WISE data. The Wide-field Infrared Survey Explorer satellite has given us a look at objects perturbed in some fashion within the Oort Cloud and now making occasional forays into nearby space. A distribution of comets and other icy bodies beginning some 300 billion kilometers from the Sun and extending outwards perhaps as far as 200,000 AU, the Oort Cloud’s extent makes it possible that it may extend into similar clouds of icy material around the Alpha Centauri stars.
Image: The fact that this image is logarithmic gives a startlingly clear idea of the extent of the Oort Cloud. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. One AU is the distance from the sun to the Earth, which is about 150 million kilometers. At the outer edge of the Oort Cloud, the gravity of other stars begins to dominate that of the sun. The inner edge of the main part of the Oort Cloud could be as close as 1,000 AU. Voyager 1, our most distant spacecraft, is around 125 AU. It will take about 300 years for Voyager 1 to reach the inner edge of the Oort Cloud and possibly about 30,000 years to fly beyond it. Credit: NASA / JPL-Caltech.
There turns out to be more of such material than we had thought. The outer Oort Cloud is only loosely bound, meaning that gravitational interactions with passing stars or ‘rogue’ planets, not to mention effects from the Milky Way itself, can dislodge comets from their orbits and bring them into the inner system. Such comets may have periods not just in the hundreds but millions of years. The WISE data were gathered during the spacecraft’s primary mission, before its recommissioning as NEOWISE, with the charter of studying near-Earth objects.
Measuring the size of long-period comets is difficult because the cloud of gas and dust around the comet — its coma — makes it difficult to measure the actual cometary nucleus. WISE was able to get around this problem by probing comets in the infrared, subtracting the glow of the coma from the signature of the nucleus. 2010 WISE observations of 95 Jupiter family comets — with periods of 20 years or less — and 56 long-period comets were used in the study.
The result: Comets that move regularly into the inner system are found to be, on average, as much as four times smaller than long-period comets, those moving only rarely near the Sun. Moreover, there are seven times more long-period comets in the size range of one kilometer in diameter and above than had previously been thought. In the eight months of the study period, three to five times more long-period comets were observed moving in the vicinity of the Sun than had been predicted.
“The number of comets speaks to the amount of material left over from the solar system’s formation,” said James Bauer, lead author of the study and now a research professor at the University of Maryland, College Park. “We now know that there are more relatively large chunks of ancient material coming from the Oort Cloud than we thought.”
Image: This illustration shows how scientists used data from NASA’s WISE spacecraft to determine the nucleus sizes of comets. They subtracted a model of how dust and gas behave in comets in order to obtain the core size. Credit: NASA/JPL-Caltech.
The results presumably reflect the fact that, coming closer to the Sun on a much more frequent basis, Jupiter-class short-period comets lose volatiles through sublimation, along with surface materials. An observed clustering in the orbits of long-period comets also suggests that many of these could have been part of larger bodies at some point in the past. The findings may have a bearing on our estimates of water delivery to the early Earth.
Co-author Amy Mainzer (JPL), principal investigator of the NEOWISE mission, points out that, traveling much faster than asteroids, long-period comets like these, many of them quite large, have to be factored into our analyses of impact risk. We’re developing an extensive catalog of near-Earth objects, but a long-period comet dislodged from the Oort Cloud, moving faster than any near-Earth asteroid, poses a risk that is badly in need of assessment.
The paper is Bauer et al., “Debiasing the NEOWISE Cryogenic Mission Comet Populations,” Astronomical Journal Volume 154, Number 2 (14 July 2017) (abstract). This NASA news release is also helpful.
by Paul Gilster | Dec 16, 2016 | Outer Solar System |
Ceres, that interesting dwarf planet in the asteroid belt, is confirmed to be just as icy as we had assumed. In fact, a new study of the world, led by Thomas Prettyman (Planetary Science Institute), was the subject of a press conference yesterday at the American Geophysical Union fall meeting in San Francisco. Prettyman and team used data from the Dawn spacecraft’s Gamma Ray and Neutron Detector (GRaND) instrument to measure the concentrations of iron, hydrogen and potassium in the uppermost meter of Ceres’ surface.
Prettyman, who is principal investigator on GRaND, oversees an instrument that works by measuring the number and energy of gamma rays and neutrons coming from Ceres. The neutrons are the result of galactic cosmic rays interacting with the surface, some of them being absorbed while others escape. The number and kind of these interactions allows researchers to investigate surface composition. Hydrogen on Ceres is thought to be in the form of frozen water, allowing the researchers to study the global distribution of ice.
The result of the GRaND study: The elemental data show that the materials were processed by liquid water within the interior. The top layer of Ceres’ surface is hydrogen rich, with the higher concentrations found at mid- to high latitudes, a finding consistent with near surface water ice, with the ice table closest to the surface at the higher latitudes. Says Prettyman:
“On Ceres, ice is not just localized to a few craters. It’s everywhere, and nearer to the surface with higher latitudes. These results confirm predictions made nearly three decades ago that ice can survive for billions of years within a meter of the surface of Ceres. The evidence strengthens the case for the presence of near-surface water ice on other main belt asteroids.”
Image: This image shows dwarf planet Ceres overlaid with the concentration of hydrogen determined from data acquired by the gamma ray and neutron detector (GRaND) instrument aboard NASA’s Dawn spacecraft. The hydrogen is in the upper yard (or meter) of regolith, the loose surface material on Ceres. The color scale gives hydrogen content in water-equivalent units, which assumes all of the hydrogen is in the form of H2O. Blue indicates where hydrogen content is higher, near the poles, while red indicates lower content at lower latitudes. In reality, some of the hydrogen is in the form of water ice, while a portion of the hydrogen is in the form of hydrated minerals (such as OH, in serpentine group minerals). The color information is superimposed on shaded relief map for context. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI.
But we have no solid ice layer here. Instead, Ceres’ surface appears to be a porous mixture of rocky materials, with ice filling the pores, as this Institute for Astronomy (University of Hawaii) news release makes clear. The GRaND findings show about 10 percent ice by weight.
Also interesting is that the elemental composition of Ceres differs from CI and CM carbonaceous chondrite meteorites, which represent some of the most primitive, undifferentiated meteorites we know (Cl and CM are two of several different subgroupings within the carbonaceous chondrite family). These meteorites were also altered by water, but the GRaND data tell us that their parent body would have differed markedly from Ceres.
The researchers offer two explanations, the first being that large scale convection occurring within Ceres may have separated ice and rock components, leaving the surface with a different composition than the bulk of the object. The other possibility is that Ceres formed in a different location in the Solar System than the parent object of this class of meteorite.
A second paper on Ceres has also appeared, this one in Nature. It is the work of Thomas Platz (Max Planck Institute for Solar System Research, Göttingen) and colleagues, who focus on craters that are found in persistently shadowed regions. These ‘cold traps’ are cold enough (about 110 K) that little of their ice turns into vapor. Bright material found in some of these craters is thought to be ice, and Dawn’s infrared mapping spectrometer has indeed confirmed ice in at least one.
As is the case with the Moon and Mercury, ice in such cold traps is thought to be the result of impacting bodies, although solar wind interactions are also a possibility. Each of these bodies has a small tilt compared to its axis of rotation, producing numerous permanently shadowed craters. “We are interested in how this ice got there and how it managed to last so long,” said co-author Norbert Schörghofer (University of Hawaii at Manoa). “It could have come from Ceres’ ice-rich crust, or it could have been delivered from space.”
But the comparison between what we find on Ceres and elsewhere in the Solar System reminds us how much we still have to learn about the process. From the paper::
The direct identification of water-ice deposits in PSRs [permanently shadowed regions] on Ceres builds on mounting evidence from Mercury and the Moon that PSRs are able to trap and preserve water ice. For the Moon, the abundance and distribution of cold-trapped ice is little understood. On Mercury, the cold traps are filled with ice, and the planet traps about the same fraction of exospheric water as Ceres, so either the PSRs on Ceres are not able to retain as much water ice as those on Mercury or the amount of available water is much lower.
The Prettyman paper is “Extensive water ice within Ceres’ aqueously altered regolith: Evidence from nuclear spectroscopy,” published online by Science 15 December 2016 (abstract). The Platz paper is “Surface water-ice deposits in the northern shadowed regions of Ceres,” published online by Nature Astronomy 15 December 2016 (abstract). Video of the press briefing at the AGU meeting can be accessed here.
