VISTA (Visible and Infrared Survey Telescope for Astronomy) is a near-infrared instrument located at the European Southern Observatory’s Paranal site, and is by all accounts the world’s largest survey telescope, with extremely wide field of view and sensitive detectors. On the peak next to ESO’s Very Large Telescope (VLT), VISTA shares its exceptional viewing conditions using a 4.1-meter primary mirror and a three-tonne camera with 16 infrared detectors.
With its time devoted to six surveys ranging from complete southern sky coverage to small patches of sky looking for extremely faint objects, VISTA was bound to come up with interesting data, especially in the survey known as VVV, which stands for VISTA Variables in Via Láctea. Here, astronomers are homing in on regions that are obscured by dust in the bulge and southern Galactic disk, using pulsating RR Lyrae and Cepheid variables as distance indicators, with a focus on microlensing events, eclipsing binaries and pre-main sequence variable stars.
Image: VVV-WIT-07 in the centre of a star field. Credit: Saito et al.
This is fertile ground for discovery, but even so, the object known as VVV-WIT-07 is taking everyone a bit by surprise. We’re dealing with a star that reminds everyone of Boyajian’s Star (KIC 8462852), famed for its unusual dips in lightcurve. Were they signs of a Dyson Sphere under construction, or a natural phenomenon the likes of which we had never seen? Uneven rings of dust, dusty planetesimals or comets are still in contention. Now we have a star that is apparently even more extreme, one whose range in lightcurve variation is extraordinary. Viewed by the survey over a period of eight years, VVV-WIT-07 has been seem to dim first by a factor of 2, then by almost 80 percent. A week later, it was back to normal.
Here’s what the ESO blog had to say about the matter:
“The first observations showed nothing strange — simply a mild scatter in the brightness measurements, consistent with the observational uncertainties. However, in August–September 2011, just before the end of the observing season, the star dimmed by a factor of almost two! By June 2012, when we began re-observing it, the star’s brightness was nearly back to normal. But by mid-July, it had dimmed by almost 80%! Then it was back to its usual self in about a week. The data taken since then contain hints of additional drops in brightness, but nothing so dramatic.”
The WIT designation is fun — it stands for What Is This, and the VVV team is using it to describe any objects that do not apparently fit known classes of stellar variability. WIT may remind you of another appellation for Boyajian’s Star, which was the ‘WTF Star,’ its ambiguous acronym standing surely for ‘Where’s the Flux?’ Have a look at the lightcurve of VVV-WIT-07:
Image: Lightcurve of VVV-WIT-07 showing how it varied in brightness between 2010 and 2018. The insert shows an expanded view of the particularly dramatic dimming event that occurred in July 2012. Credit: Saito et al.
Is it possible we are looking at some kind of circumstellar disk with huge variations in it, a clumpy disk that blocks the star’s light in this highly irregular way? The odds on that seem long. Here is what the paper says:
Alternative scenarios for VVV-WIT-07 include a “dipper” T Tauri star with clumpy dust structures orbiting in the inner disk that transit our line of sight (e.g. Rodriguez et al. 2017), or even a long period, high-inclination X-ray binary. The deep, narrow eclipse delayed with respect to a broad and shallower dip is reminiscent of the morphology seen in high-inclination low-mass X-ray binaries (LMXB, e.g. Parmar et al. 1986; Baptista et al. 2002). However, LMXBs are restricted to orbital periods of less than a few days while high-mass x-ray binaries (HMXB) can be found at Porb up to hundreds of days (e.g. X1145-619 has Porb = 187.5 d, Watson et al. 1981). Moreover, in this scenario optical and IR spectra would be dominated by the mass-donor companion star, and should show rotationally-broadened hydrogen absorption lines at epochs of no mass ejection episodes, which is not the seen in the spectra of VVV-WIT-07.
So we have something that gives us echoes of Boyajian’s Star, and may also remind us of Mamajek’s Object (J1407), an interesting pre-main sequence K5 star with a ring system eclipsing it. Or something we haven’t yet identified, perhaps in the form of multiple objects, may be moving between us and the host star, even a dense family of comet-like objects. VIV-WIT-07 would be easy enough to explain if it were a binary, but the observations clearly rule that out.
VISTA has observed VIV-WIT-07 85 times already. Needless to say, it will be the subject of even more intense scrutiny. The paper also notes recent discoveries like OGLE LMC-ECL-11893, an eclipsing star consistent with a dense circumstellar dust disk structure, and PDS 110, an eclipsing system with likely transits by a companion with a circumstellar disc.
The paper is Saito et al., “VVV-WIT-07: another Boyajian’s star or a Mamajek’s object?” in process at Monthly Notices of the Royal Astronomical Society (preprint).
I’ll wrap up this three-part series on ‘lurker’ probes and ways of finding them with Keith Cooper’s provocative take on the matter. A contributor to Centauri Dreams whose far-ranging ideas have fueled a number of dialogues here (see the archives), Keith is editor of Astronomy Now and the author of the upcoming book The Contact Paradox: Challenging Assumptions in the Search for Extraterrestrial Intelligence. I’ve read the manuscript and can tell you that you’re going to want this one on your shelves. Today, Keith takes us into the practical realm. If we were to find a Bracewell probe in our Solar System, what would we do with it? Who might discover it, who would claim its technologies, and what, under international law, would be its legal status? Plenty of material for science fiction plots here as we embark on the search to see what’s out there among Earth’s co-orbitals.
by Keith Cooper
It’s enough to keep me awake at night.
Suppose that an extraterrestrial probe is discovered in our Solar System, anchored to a co-orbiting near-Earth asteroid. It could potentially instigate one of the biggest crises in all of human history.
Consider this nightmare scenario: the alien probe would quite possibly be discovered by one of our probes. If we’re lucky, it will be one of our science missions, like Hayabusa2 or OSIRIS-REx. If we’re unlucky, it could be an autonomous asteroid-mining spacecraft, sent into space to chew up asteroids and refine their rocks for their precious minerals.
In that case, first contact could well end up with us destroying the alien probe along with the asteroid. Maybe the probe will defend itself and destroy the mining craft. Or, if the probe is passive, then maybe its last moments relayed back to its home civilisation will be of a human-built robot looming threateningly. Either way, it’s not the kind of first impression we’d really want to make.
Maybe that scenario is too far-fetched for you. Okay, let’s just go with the option of an alien probe being discovered during a science mission, or by an astronomical survey. We might try and contact it, but could we really resist the lure of advanced extraterrestrial technology just a few million kilometres from Earth? Of course not. Nations and private space companies will rush to try and capture the probe and to learn its secrets. Who, if anyone, would have the rights to claim it? It could lead to conflict between competitors, or maybe the probe would again try and defend itself, an act that we might deem to be hostile.
What we can learn from these scenarios, besides the importance of making a good first impression on behalf of humanity, is that there is a startling unpreparedness for any such discovery of a watching probe.
In his new paper (see A SETI Search of Earth’s Co-orbitals), Jim Benford has proposed searching asteroids in co-orbiting orbits – objects in a 1:1 gravitational resonance with Earth such that they orbit the Sun in exactly the same time as our planet, shadowing our world but on orbits that are a little more elliptical. Relative to Earth, these orbits trace out ‘horseshoe’- or ‘tadpole’-shaped paths. The co-orbiting objects include the asteroid Cruithne, which starred in Stephen Baxter’s novel Manifold: Time and gets as close as 12 million kilometres, and Earth’s first known Trojan asteroid, 2010 TK7, which comes as close as 19 million kilometres.
Benford recommends that Breakthrough Listen target these co-orbiting neighbours, since they would give an alien probe an ideal vantage point from which to watch us. It’s a great idea, and although I think there’s a very low probability of success, as with anything to do with SETI, we won’t know until we look. However, as per the scenarios above, the discovery of a probe could be fraught with danger.
If a probe is discovered, who has the right to claim its technology? In an ideal world I would say nobody, and that aside from trying to communicate with it, we should leave it be. However, we’re talking about human beings here, and the temptation to capture the probe and learn its secrets will be too hard to resist. But it could lead to one almighty free-for-all.
So I asked Frans von der Dunk, who is Professor of Space Law at the University of Nebraska-Lincoln, what the Outer Space Treaty says about extraterrestrial probes. Unsurprisingly, it doesn’t say anything. Instead, von der Dunk suggests that if we substitute ‘natural resources’ for ‘technology’ , then the best interpretation of the current law would be to treat the probe as any other celestial object, such as an asteroid, which can be mined or otherwise appropriated.
“In my view, [this] would indeed not make it illegal for states to unilaterally condone someone going for the technology and trying to appropriate it, if under an appropriate license and otherwise complying with whatever international space law dictates with respect to the use of outer space, as per Art. VI of the Outer Space Treaty,” he says.
However, von der Dunk accepts that rivals for the probe’s technology would be unlikely to accept this if it looks like someone else is going to get there first, leading to all kinds of diplomatic shenanigans, and potentially conflict.
There are other issues with the law. If the probe contains a sentient artificial intelligence, should it not be treated as a sentient being according to the law? The problem is, other species on Earth, such as dolphins, are also sentient but the law does not recognise them with the same rights as human beings.
Then there is the concern that the probe would defend itself against any action it deemed hostile. Given that we would not know its capabilities, nor the motivations of the civilisation that programmed it, this could be exceptionally risky on an existential scale. Should we destroy or capture the probe then we could incur the wrath of whoever sent it; if it defends itself, we cannot know what lengths it would go to before ceasing fire. There’s even a case for leaving the probe alone after discovery, and not attempting to communicate with it; the probe could be hibernating and unaware of our modern technological developments until we start poking and prodding it. As the saying goes, we might not want to wake the dragon. Visions of Fred Saberhagen’s berserker probes abound.
To be fair, I don’t necessarily agree with that assessment. If an alien probe is going to venture into our backyard, it would seem unreasonable to then not expect us to be curious. However, we do need to act accordingly, and sensibly. So far, little to no thought has been put into this. If we are going to embark on a search, as Benford encourages, then we need to start thinking about what the consequences will be and how we’ll manage them, and we need to start thinking about this with some haste. Should SETI discover a radio signal from another civilisation, it will have originated many light years away; sheer distance mitigates the danger. A probe, however, will be a cosmic stone’s throw from us, and any potential danger would be immediate.
So, this is what I recommend: any search must be in conjunction with educating space agencies and private space companies about the possibility, however small, of a probe being discovered. The asteroid mining companies of the future would especially have to bear this in mind, and survey their target asteroids thoroughly before mining, to make sure there are no probes there.
In the meantime, we should develop first contact protocols to be implemented if a probe is discovered, and we also need to update the law, such that if a probe is found, then it doesn’t result in a potentially violent mêlée as we race to learn its secrets before our rivals, and that whatever benefits can be gleaned from the probe are made available to the whole of humankind.
If we can meld the societal issues with the practical issues of the search, it would stand SETI in good stead. It would act as a precedent for that other area of controversy, the METI versus SETI debate, which revolves around the societal complications of instigating contact. And, if we could find and make contact with a probe, what a wonderful opportunity that would be, bypassing traditional SETI and providing instant contact. Since Jean-Luc Picard is only a fictional character, the question then becomes, who else should we nominate to speak to the probe on behalf of Earth?
But that’s a topic for another time.
Because I’ve been re-reading Gregory Benford’s Galactic Center sequence (now into Furious Gulf), I want to quickly mention the galactic center simulation available here, which offers a 360-degree, ultra-high-definition view based on Chandra X-ray observations as massaged by NASA supercomputers. It’s lively stuff, showing “the effects of dozens of massive stellar giants with fierce winds blowing off their surfaces in the region a few light years away from the supermassive black hole known as Sagittarius A* (Sgr A* for short).” Just remember Greg got there first. But back to the probe question we’ve been examining. Jim Benford’s take on a SETI search for ‘lurkers,’ probes that fit into the Bracewell category, examines targets known as Earth co-orbitals, as we saw on Friday. UCI physicist Greg Benford’s comments about his brother’s article examine the question of what the presence of such a probe in our system might imply. The possible scenarios take us into the realm of what Greg has called ‘deep time.’
by Gregory Benford
Consider time scales. Some tech society within a few hundred light years may have sent a Lurker which remains operating now. But our routine radio signals are only a bit more than a century old. If a Lurker reports back to its origin, it may well have not gotten an answer yet about how to proceed.
This means we should consider searches over decades-long time scales. In any case, we can carry out interrogations by radar and consider missions to orbit nearby targets and survey for Lurker sites, which may have gone dead in the distant past, or be intermittently active now.
We humans measure our eras in millennia at most. Within a century or so we may plausibly have made computer minds that could manage a Lurker that voyages for centuries and watches a biologically active world for more millennia still. If societies can persist for very long times, they will have alien artificial intelligences.
If ETI dispatches Lurkers here, these may report back any attempt at contact but do no more. Or Lurkers may be free to respond, if they were instructed so by their home society (or societies) earlier. Or further still, Lurkers from the far past may have done their duty and slowly failed, finally, into their own demise.
On scales of millennia, the ruins of Lurker installations, including mining for resources on nearby orbiting sites, may be visible, even though their animating intelligences are long gone. Surveying, exploring satellites could see these ruins and learn much from them. Thus a lack of response to our pings and messages may simply imply a needed further study through exploring probes.
This implies a staging of research. At each level, we disprove a hypothesis. Moving on in this way will disallow models of alien societies ranging through ever-larger distances, and ever longer past eras. This program can go forward for decades, as our capabilities grow. Our understanding of the near-orbital objects will grow as well, giving us a gamut of possibilities we can imagine and then test.
These minds must be made for social species; otherwise it seems unlikely that loner species like our carnivores will care much for speaking to distant others, a point made by E.O. Wilson. As Wittgenstein said, “If a lion could speak, we could not understand him.”
Translation would be unlikely if an ET mind were a single integrated intelligence, without social species drives; yet a Lurker may well be that. As well, a Lurker may be thrifty in energy use. If it came here using a nuclear drive or energy source, it would have to mine for radioactives to keep itself running—or erect large solar panels. (This is another reason to lurk near Earth: higher solar flux.) It may well be an occasional watchman, not continuous. Or it may have a small alert system to summon the larger intelligence when certain events or symptoms occur—such as a hail from Earth that repeats and contains self-explaining images that convey interest, such as a picture of the Lurker itself seen from afar. All these possibilities speak for a repeating observation and hailing pattern. This should be a long game, over years at least.
One objection to SETI is that it is not falsifiable — there is no point at which a lack of signals can prove that extraterrestrial civilizations do not exist. But there are some aspects of SETI that can be falsifiable. Consider a class of objects near enough for us to investigate not only with listening efforts but with probes, one small enough to be thoroughly covered, and one most people know almost nothing about. Could these offer a listening post for ‘Bracewell probes,’ a way of watching the development of our culture and reporting home to ETI? And if so, could we combine SETI with METI to advance both disciplines without compromising our own security?
If the idea of nearby probes seems far-fetched today, it was even more so when Ronald Bracewell advanced his ‘sentinel hypothesis.’ Bracewell took the question of SETI and stood it on its ear. That was no mean feat in 1960, for SETI was just being born in that year through the efforts of Frank Drake at the Green Bank instrument in West Virginia. While Drake was, reasonably enough, asking whether we might pick up signs of an extraterrestrial civilization around another star, Bracewell had begun to wonder whether there might be a different way to study an alien culture. A long-lived probe could be planted in any system under investigation.
Image: Stanford’s Ronald Bracewell, who in 1960 advanced the idea of long-lived probes investigating other planetary systems. Credit: Stanford University.
Add Von Neumann-style self repair and such an object might stay on sentry duty for millennia, for aeons, all the while returning useful data about the changes occurring on an interesting habitable planet. And if a civilization arose on that planet and reached the level of electromagnetic communications, then the probe could be programmed to make contact, at whatever threshold its builders chose.
Jim Benford has been thinking about Bracewell probes and their possibilities of late because they offer advantages over traditional forms of SETI. For one thing — and this is a huge advantage — a contact once made with a local probe could initiate dialog in more or less real time, without interstellar lightspeed delays, although of course we would be querying an intelligence that was itself subject to those delays if it communicated with its home world.
Image: Plasma physicist Jim Benford (Microwave Sciences).
The question is interesting enough that is has inspired some top-notch science fiction, in particular David Brin’s novel Existence, where the idea is extended to not one but a series of different probes at work in Earth orbit and in the asteroids. But it would be Michael Papagiannis who in 1978 wrote seriously about the asteroid belt as a possible venue for such ‘lurkers,’ (to use Benford’s term). Benford is not sold on the asteroid belt as a target.
For we also have an all but undiscussed body of targets that can be called co-orbital objects with Earth. These small objects approach the Earth closely and on an annual basis, for they have the same orbital period as Earth. We might study them for signs of artificiality through spectroscopy in the visible or near-infrared as well as pinging them with radar or other signals.
What strange orbits these objects occupy, with some in so-called ‘horseshoe’ orbits — these can actually become quasi-satellites for a time before returning to earlier orbital parameters. Have a look at a horseshoe orbit.
Image: A horseshoe orbit. No wonder these objects took so long to find. Credit: James Benford.
And from another view:
Image: Plan showing possible orbits along gravitational contours. In this image, the Earth (and the whole image with it) is rotating counterclockwise around the Sun. Credit: Wikimedia Commons.
As Benford explains, think of a quasi-satellite as an object in a 1:1 orbital resonance with a planet, so that the object stays close to the planet over many orbital periods. Outside the Hill sphere (that region where an astronomical body dominates the attraction of satellites), quasi-satellites cannot be considered true satellites. Instead, while their period around the Sun is the same as the planet, they seem to travel in an oblong retrograde loop around it.
Beyond horseshoe orbits we also find ‘tadpole’ and ‘quasi-satellite’ orbits as shown in the figure below. Here we find stable orbits for centuries and possibly longer, much longer. Co-orbitals include Cruithne (3753), a 5-kilometer object with closest approach to Earth of 0.080 AU — interestingly, this one experienced a close encounter with Mars in historical timescales, around the time of Periclean Athens. Another is Earth Trojan 2010 TK7, which oscillates around the Sun-Earth Lagrangian point L4, and 2016 HO3, which Benford describes as “currently the smallest, closest, and most stable (known) quasi-satellite of Earth,” with a minimum distance of 0.0348 AU. A number of other quasi-satellites are known.
Image: Three types of co-orbital orbits. Credit: James Benford.
How might we investigate these objects with the tools of SETI, and why? Benford calls for a multi-year program of observations in radio and optical wavelengths as well as planetary radars, with the main burden of the work falling upon the Lick Observatory and other platforms involved in the Breakthrough Listen project. Here we’re looking for size, shape, rotation periods and spectra. At the same time, he urges SETI observations of this range of objects.
Planetary radar also comes into play, and with an interesting consequence. From the paper:
These objects have not been pinged or imaged by any planetary radar as yet. Recent developments in planetary radars have shown they can detect the presence and trajectories of spacecraft in lunar orbit, even though their size is a few meters. Whether these radars are sensitive or powerful enough to get a return signal from any of the presently known co-orbital objects requires analysis. In any case, they can ‘ping’ the objects, meaning that a signal reaches there but the return signal may be too weak to detect at Earth.
Here the Bracewell idea comes into full view:
If there is an ET probe there, it might sense that it had been noticed by us.
What an interesting campaign Benford has in mind. It includes simultaneous use of planetary radar on the target and SETI observations. Readers at this point may be recalling that Benford is on the record with strenuous objections to METI, the idea of Messaging to Extraterrestrial Civilizations, given the limits on what we know about what is around us in the cosmos, and the need for international agreement on how to proceed, as opposed to sending signals to the stars in random bursts of activity and with wildly varying content.
Yet here we are talking about an activity that, in the unlikely event there is a probe in our own Solar System, could conceivably activate it and cause it to respond. The Bracewell probe is front and center here, recalling Duncan Lunan’s 1974 proposition that a Bracewell probe could be the cause of long-delayed echoes of radio transmissions heard in the 1920s. Benford notes that the phenomena Lunan identified have subsequently been explained as unusual propagation patterns in Earth’s magnetosphere. But it’s interesting to see Benford’s response to the idea of METI in this new context:
This would be ‘Active SETI’, which could solicit a response from a hypothetical probe. This does not incur the objections to sending interstellar messages, messaging to ETI (METI), because any such alien lurkers would already know we are here. Of course, this is at very short range compared to the interstellar ambitions of METI enthusiasts. We presume that Lurkers already know that we have radar, but might not respond to a single radar painting such as we have done to many asteroids. If we want to send a message, as Paul Davies suggested for the LaGrange points in 2010, how would a signal be designed to elicit such a response?
An interesting question indeed, and as the author points out, actually working on a near-term use of METI at a nearby target could benefit research into message creation and drive the field forward. The problem is an easy one to state: What kind of message would one send to a lurking probe that would ‘awaken’ it to the possibility of communicating with us?
As to falsification and SETI:
In my view SETI has suffered from being seen as somewhat nonscientific. That’s because it doesn’t offer itself as a study with falsifiable propositions, which is the very definition of science, as Popper said.
I advance a falsifiable proposition: “There exist in near-Earth space extraterrestrial probes which are observing Earth and it may be possible for us to find and contact them.
This proposition can be disproved. We can observe them, ping them with radar, transmit messages to them, send robotic probes to them and visit them with human spacecraft missions.
What a lively concept. We blend SETI’s listening to the stars with astronomical imaging and spectroscopy, while simultaneously turning METI into what Benford calls a ‘local experiment.’ And as we do this, our efforts at studying co-orbital objects advance the cause of astronomical science, which is engaged in the great process of mapping the entire Solar System.
The paper is Benford, “Looking for Lurkers: Objects Co-orbital with Earth as SETI Observables,” submitted to the Astrophysical Journal (preprint).
We know by now to expect surprises when we do something for the first time with a spacecraft. The latest case in point is OSIRIS-REx, which has revealed multiple unexpected facets of the asteroid Bennu, near which it has been operating since December. Consider the surface of the asteroid, a key factor in how the mission goes forward since this is a sample return mission, and that involves finding a place relatively free of surface debris from which to take the sample.
The problem: This smallest body ever to be orbited by a spacecraft turns out to be strewn with boulders. The original sample collection plan — christened Touch-and-Go (TAG) — will have to be altered, for it was dependent on a sample site with a 25-meter radius free of hazards. The OSIRIS-REx team has been unable to identify any site that meets those requirements. A new type of candidate site will have to be found, demanding higher performance using an updated sampling approach called Bullseye TAG that will be tailored for smaller sample zones.
Nonetheless, the OSIRIS-REx team remains optimistic:
“Throughout OSIRIS-REx’s operations near Bennu, our spacecraft and operations team have demonstrated that we can achieve system performance that beats design requirements,” said Rich Burns, the project manager of OSIRIS-REx at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Bennu has issued us a challenge to deal with its rugged terrain, and we are confident that OSIRIS-REx is up to the task.”
Image: Bennu’s surface is rockier than expected, creating challenges for the team whose mission is to scoop up a sample of pristine material and return it to Earth in 2023. Credit: NASA/Goddard/UA.
The larger issue facing asteroid investigations is the question of computer modeling. The reason scientists have assumed that Bennu’s surface would be generally smooth is that observations from Earth of the object’s thermal inertia and radar measurements of its surface roughness have been integrated into computer models that made this prediction. We now learn that the interpretation of these models was not correct. Indeed, data from Bennu should help us refine such models to better predict what we’ll find on the rocky surfaces of small asteroids.
A suite of papers covering the Bennu findings has appeared in Nature following presentations at the recent 50th Lunar and Planetary Conference in Houston (citations below). On the matter of Bennu’s boulder strewn surface, a team from SwRI presents results showing that the surface geology of the asteroid is between 100 million and 1 billion years old. That too can be considered a surprise, to judge from the remarks of SwRI’s Kevin Walsh:
“We expected small, kilometer-sized NEAs to have young, frequently refreshed surfaces,” said SwRI’s Dr. Kevin Walsh, a mission co-investigator and lead author of a paper outlining the discovery published March 19 in Nature Geoscience. “However, numerous large impact craters as well as very large, fractured boulders scattered across Bennu’s surface look ancient. We also see signs of some resurfacing taking place, indicating that the NEA retains very old features on its surface while still having some dynamic processes at play.”
Some of Bennu’s boulders are larger than 45 meters (150 feet) in size, much larger than earlier observations had predicted, and according to Walsh, they are simply too big to be the result of cratering. Instead, the scientist believes they date back to the formation of the asteroid.
But OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer — I have to untangle the acronym once in each post on the mission) continues to seize our attention long before the sample return with the news that particle plumes are erupting from its surface. This one caught everyone by surprise as well when, on January 6, the science team discovered the plumes while the spacecraft was about 1.6 kilometers away, with further detections in the ensuing months. Some of these particles were ejected from Bennu entirely, while others returned to the asteroid. None are thought to pose a danger to the spacecraft.
“The discovery of plumes is one of the biggest surprises of my scientific career,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. “And the rugged terrain went against all of our predictions. Bennu is already surprising us, and our exciting journey there is just getting started.”
Image: This view of asteroid Bennu ejecting particles from its surface on January 19 was created by combining two images taken on board NASA’s OSIRIS-REx spacecraft. Other image processing techniques were also applied, such as cropping and adjusting the brightness and contrast of each image. Credit: NASA/Goddard/University of Arizona/Lockheed Martin.
We’ve already talked about the change in Bennu’s spin rate as an apparent result of the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect (see Asteroid Bennu: Changes in Rotation Rate), a fascinating find in itself, but it’s also encouraging in terms of understanding asteroid composition to learn that the MapCam color imager and the OSIRIS-REx Thermal Emission Spectrometer (OTES) have detected magnetite on Bennu’s surface, which points to rock and liquid water interactions on the asteroid’s much larger parent body.
Learning about the sources of organic molecules and water on Earth may be enhanced by our analysis of such asteroids, and we’re also beginning to learn what resources will be available in near-Earth space. Bennu truly offers us a window into the early days of the Solar System.
The papers are Lauretta et al., “The unexpected surface of asteroid (101955) Bennu,” Nature 19 March 2019 (abstract); Barnouin et al., “Shape of (101955) Bennu indicative of a rubble pile with internal stiffness,” Nature Geoscience 19 March 2019 (abstract); Walsh et al., “Craters, boulders and regolith of (101955) Bennu indicative of an old and dynamic surface.” Nature Geoscience 19 March 2019 (abstract); Hamilton et al. “Evidence for widespread hydrated minerals on asteroid (101955) Bennu.” Nature Astronomy 19 March 2019 (abstract); Hergenrother et al., “Operational environment and rotational acceleration of asteroid (101955) Bennu from OSIRIS-REx observations,” Nature Communications 19 March 2019 (abstract); Scheeres et al., “The dynamic geophysical environment of (101955) Bennu based on OSIRIS-REx measurements,” Nature Astronomy 19 March 2019 (abstract); and DellaGiustina et al., “Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis,” Nature Astronomy 19 March 2019 (abstract).