Centauri Dreams
Imagining and Planning Interstellar Exploration
White Dwarfs: Planetary System Rebirth?
Let’s catch up with white dwarfs, a kind of star that may spawn planetary systems of its own. For I’ve just found another case of archival data being put to good use in the form of a study of a white dwarf system called G238-44. Here, the data come from the Hubble instrument (specifically, its Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph), the Far Ultraviolet Spectroscopic Explorer (FUSE), and the Keck Observatory’s High Resolution Echelle Spectrometer (HIRES) in Hawaii.
What astronomers presented at a recent AAS conference is a picture of a system severely disrupted by its star’s transition to white dwarf status. Moreover, this is a star in the process of accretion with a distinct twist from earlier such discoveries. For the white dwarf – the remnant left behind after the system’s star went through its red giant phase – is actively drawing rocky and metallic material as well as ices from the debris of the disrupted system. These are the stuff of planet formation. We’re learning how extreme are conditions in what astronomers call an ‘evolved’ planetary system as it undergoes destruction and what may be a kind of rebirth.
Image: This illustrated diagram of the planetary system G238-44 traces its destruction. The tiny white dwarf star is at the center of the action. A very faint accretion disk is made up of the pieces of shattered bodies falling onto the white dwarf. The remaining asteroids and planetary bodies make up a reservoir of material surrounding the star. Larger gas giant planets may still exist in the system. Much farther out is a belt of icy bodies such as comets, which also ultimately feed the dead star. Credit: NASA, ESA, Joseph Olmsted (STScI).
In relation to main sequence stars, white dwarfs give us new assumptions about planet formation. Such a star contains half the mass of the Sun, for example, but while it’s only a bit bigger than the Earth, it sports a density of 1 x 109 kg/m3. The average white dwarf is 200,000 times as dense as the Sun, a remnant stellar core with a temperature in the range of 100,000 Kelvin. And the system it finds itself in, surviving the red giant phase of the star, is hardly a static place.
To analyze what is happening at G238-44, we have to take into account that the original red giant, perhaps much like the Sun in its earlier days, would have cast off its outer layers as nuclear burning ceased. This shedding of mass can cause asteroids and small moons to be scattered gravitationally by remaining large planets, their own orbits disrupted. Materials like these experience tidal forces that can tear them apart as they move inward toward the star. The result: A disk of gas and dust that, over time, settles onto the surface of the white dwarf and throws a distinct observational signal.
At G238-44, the white dwarf left behind is seen in the process of accreting two such objects, a process observed before in a number of white dwarf systems but never with both icy and rocky-metallic components in the mix. Now we have a case of a white dwarf evidently drawing on a planetary system that was once abundant in ices. As UCLA’s Benjamin Zuckerman, a co-author of the paper on this work, notes:
“Life as we know it requires a rocky planet covered with a variety of elements like carbon, nitrogen, and oxygen. The abundances of the elements we see on this white dwarf appear to require both a rocky and a volatile-rich parent body – the first example we’ve found among studies of hundreds of white dwarfs.”
Within about 100 million years of the white dwarf’s formation, the star will be capturing material from regions analogous to our asteroid and Kuiper Belt. The total mass involved in this study is relatively small, about that of a large asteroid. Nitrogen, oxygen, magnesium, silicon and iron have been measured in the debris disk here, and in interesting proportions. Lead researcher Ted Johnson, a colleague of Zuckerman’s at UCLA, sees a 2-1 mix of Mercury-like material – high in iron and suggestive of a metallic planetary core – mixing with the comet-like debris.
Image: This illustration shows a white dwarf star siphoning off debris from shattered objects in a planetary system. The Hubble Space Telescope detects the spectral signature of the vaporized debris that reveals a combination of rocky-metallic and icy material, the ingredients of planets. The findings help describe the violent nature of evolved planetary systems and the composition of its disintegrating bodies. Credit: NASA, ESA, Joseph Olmsted (STScI).
Terrestrial Planet in the Habitable Zone?
With this destruction derby in mind, let’s catch up with the white dwarf WD1054-226, found not so long ago to have objects – apparently of small moon or asteroid size – orbiting close to the star. Their presence is an indication, according to astronomers at University College London, that there may be a nearby planet in the star’s small habitable zone. This finding is based on data from the ESO’s 3.5m New Technology Telescope (NTT) at the La Silla Observatory in Chile. Fully 65 dips in the star’s light show the extent of the orbital material, whipping around the star in clouds every 25 hours.
A planet farther out seems the best explanation for how this arrangement stays in place, and if it is there, it would be in an orbit about 1.7 percent of the distance between the Earth and the Sun (roughly 2.5 million kilometers). That’s in the liquid water habitable zone, and the planet would be about the size of the Earth, based on these calculations.
What interesting scenarios stars like these represent. 95 percent of the stars in the galaxy will eventually become white dwarfs, with our Sun joining their ranks in four or five billion years. At WD1054-226, we’re hypothesizing the existence of a kind of planet that has yet to be confirmed around such a star. UCL’s Jay Farihi is lead author of the paper on this work:
“This is the first time astronomers have detected any kind of planetary body in the habitable zone of a white dwarf. The moon-sized structures we have observed are irregular and dusty (e.g. comet-like) rather than solid, spherical bodies. Their absolute regularity is a mystery we cannot currently explain. An exciting possibility is that these bodies are kept in such an evenly-spaced orbital pattern because of the gravitational influence of a nearby major planet. Without this influence, friction and collisions would cause the structures to disperse, losing the precise regularity that is observed. A precedent for this ‘shepherding’ is the way the gravitational pull of moons around Neptune and Saturn help to create stable ring structures orbiting these planets.”
Image: An artist’s impression of the white dwarf star WD1054-226 orbited by clouds of planetary debris and a major planet in the habitable zone. Credit Mark A. Garlick / markgarlick.com. License type Attribution (CC BY 4.0).
Tantalizing, but remember that the ‘planet’ here is unconfirmed. JWST data on the debris disk would be helpful as we learn more. White dwarf planets of terrestrial size should eventually turn up if they’re out there in any numbers. If we could find a transit, we’d be looking at a world as large as the star it orbits. The transit depth that would afford if we can find a system so aligned would make for an unforgettable light curve.
The paper is Farihi et al., “Relentless and Complex Transits from a Planetesimal Debris,” Monthly Notices of the Royal Astronomical Society Vol. 511, No. 2 (April 2022), 1647-1666 (full text).
‘Lurker’ Probes & Disappearing Stars
We’ve looked before at the growing interest in exploring near-Earth space for evidence of probes from other civilizations that may have been sent in the distant past to monitor and report home on the progression of life in our Solar System. If extraterrestrial civilizations exist, the idea that one of them might have explored our system and left behind what Jim Benford calls a ‘lurker’ probe is sensible enough. We send probes to places we want to learn more about, and we would certainly have probes around the nearest stars if we had the means. Breakthrough Starshot is an example of such interest. A century from now, human probes to other stars may be commonplace.
Various places to search for lurker probes have been suggested, from Lagrange points – where objects placed there tend to stay put, with minimal need for fuel consumption – to barely studied Earth co-orbitals to the surface of the Moon. But what about Earth orbit? Surveillance of the Earth could involve probes in long-term high altitude orbits, the geosynchronous realm of our present-day communication satellites, which can always remain above the same location on the planet. As opposed to low-Earth orbits, GEO offers stable conditions over millions, perhaps billions of years.
The immediate objection is that looking into Earth’s sky is confounded by multiple factors. We have close to 5,000 satellites already in one kind of Earth orbit or another. We must also cope with centimeter-scale debris in lower orbits that seems to be increasing over time, another reason why higher orbits would be preferable for searching for something anomalous. Even so, human contamination near our planet means that using modern survey tools like Pan-STARRS is complicated and time-consuming.
If we had a time machine, we could see the sky as it was before Sputnik. But as Beatriz Villarroel and colleagues note in a new paper in Acta Astronautica, we have much easier ways of doing this. Photographic plate projects like the First Palomar Sky Survey (POSS-1) are available from earlier periods, and Villarroel (Stockholm University) is behind a new citizen science project called VASCO (Vanishing & Appearing Sources during a Century of Observations) to exploit these resources.
VASCO builds upon an earlier project of the same name in which Villarroel and colleagues analyzed old sky catalogs looking for stars that appear in the older plates but are not found in later imaging. In the earlier work, about 100 red transients turned up, interesting objects that likely represent flare stars worth follow-up investigation. The image below is drawn from this work (citation at the end of the article).
Image: A source visible in an old plate (left, seen as the bright source at the centre of the square) has disappeared in a later plate (right). Credit: Villarroel et al. (2019).
With the online VASCO project, the focus shifts to human volunteers, who work in the cause of finding anomalous features that may point to a technology in Earth orbit before the first Earth satellite flew. While VASCO uses the POSS-1 dataset, other plate material is available from the Lick and Sonneberg observatories and the Carte du Ciel, a decades-long mapping project from the early 20th Century. Interest in photographic plates is quickening because this is a resource ripe for analysis with our new digital tools. Thus projects like DASCH (Digital Access to a Sky Century @ Harvard), which has spent two decades thus far scanning photographic plates and archiving data.
Long-term Centauri Dreams readers will know that I champion the idea of using older datasets, which are priceless windows into the pre-digital sky. How we can exploit this material and expand our understanding with our new digital tools is an exciting area of research, and here we have the great benefit that the data have already been collected. We need build no new observatories to acquire information, but can concentrate on mining older plates for what may turn out to be new discoveries.
In the case of VASCO, the trick is to come up with the necessary filters to isolate anything that may be artificial, i.e., a technosignature. Low Earth orbits remain relatively uninteresting because they do not fit the long-term survival we’re presuming in a lurker, although fast-moving point sources in a long exposure can readily show natural objects like asteroids or meteors. An object in geosynchronous orbit may throw a glint when observer and reflective surface happen to be precisely aligned. But single glints are not enough. The authors are after indicators that cannot be confused with natural phenomena and are not the result of instrumental or photographic defects.
Image: This is Figure 2 from the paper. Caption: Fig. 2. A typical streak. The POSS-I streak found in a red image identified through the citizen science project shows the effect of tumbling and is a possible near-Earth asteroid but is also a possible candidate. The streak is roughly 40 arcminutes in length and unlikely to be a meteor with its angular velocity and pattern of first being dim, then brightens, and then dims once again. The typical exposure time for POSS-I images is about 50 minutes. Credit: Villarroel et al.
While a reflective satellite can produce a short, powerful glint, the glint shows a Point Source Function (PSF) shape that does not help us much. A point source is one that is smaller than maximum resolution of the equipment. An image of it seems to spread, a factor we must consider because of the diffraction of the telescope aperture. From the paper:
Satellites that are uniformly illuminated at low- or medium altitude orbits leave clear streaks in the long time exposures from old photographic plates as they move at speeds projected as hundreds of arcseconds per second. At higher or GEO altitudes the presence of satellites or space debris can be detected by fast, transient glints caused by surface reflection of the Sun. When the reflective surface of the satellite coincides perfectly with the position of the observer and the Sun, a short but powerful glint can be observed. Despite the fast movement of the satellite, the very brief reflective alignment means that the resulting short duration glint has a Point Source Function (PSF)-like shape…
And it turns out that a single glint on older photographic plates is indistinguishable from an astrophysical transient. In fact, ground-based searches for such transients today often pull up solar reflections from artificial objects in geosynchronous orbits. Moreover, 75 percent of glints from GEO, while not associated with any known object, are almost certainly centimeter-sized human space debris.
Going beyond the single glint, then, the paper analyzes multiple glints, and notes in particular glints with point source functions that occur along a straight line – which can occur when a spinning object reflects sunlight – and triple glints, another sign of possible rotation. And indeed, multiple glints have been found in at least one image exposed in 1950, though it is impossible to rule out contamination or emulsion defects on old photographic plates. What the authors are after is something more reliable:
The smoking-gun observation that settles the question unequivocally, is the one of repeating glints with clear PSFs along a straight line in a long-exposure image. When an object spins fast around itself and when its reflective surface faces the Earth, some of its parts could reflect sunlight. That results in multiple glints following a trail in an image. The number of glints might depend on the geometry and the speed of the rotation of the object. An object with only one single reflective surface that spins slowly will produce fewer glints than an object with several reflective surfaces that, moreover, spins fast. From the period one can also determine the shape of the glinting object.
This, then, is the kind of signature Villarroel and colleagues hope to find during the course of the VASCO investigations:
An exciting aspect of these suggestions is that precisely these type of objects could be found during the course of the VASCO project [8], [29]. Among the many objects classified as “Vanished”, we could discover both single and multiple glint objects. Also through automatised methods, we seek to identify all cases of multiple glints within a small area of 10 × 10 arcmin2, and to see if any of these represent cases where the glints follow a straight line.
Image: This is Figure 5 from the paper. Caption: Fig. 5. Triple glints. An example of a triplet glints in a red POSS-I image from 1950s. The left column shows the POSS-I image, and the right column the Pan-STARRS image ( year 2015). The example is from Villarroel et al. (2021) [54] and uses the VASCO citizen science web interface. Credit: Villarroel et al.
A series of multiple glints along a line in photographic plate images, if found in the VASCO plates, would be of great interest, but there is a ticking clock, because trying to locate any such object today comes up against the growing volume of human-made space debris. The authors argue that searches for technosignatures in photographic plates should thus be done as soon as possible, and preferably performed on datasets beyond the POSS-1 material now used by VASCO.
A sky without human contamination in orbit is available through these plates, and if an object in geosynchronous orbit has been left behind – perhaps millions of years ago – as an observing platform or other kind of probe, this method is one way citizen science can be employed to spot it. Just how long a reflective surface can endure in an environment of dust grain and micrometeorite collisions is debatable, but of course we know nothing about what measures probe builders might take to protect their equipment. The authors think the imponderables keep VASCO a viable project.
The paper is Villarroel et al., “A glint in the eye: Photographic plate archive searches for non-terrestrial artefacts,” Acta Astronautica Vol. 194 (May 2022), 106-113 (full text). For earlier work, see Villarroel et al., “The Vanishing and Appearing Sources during a Century of Observations Project. I. USNO Objects Missing in Modern Sky Surveys and Follow-up Observations of a “Missing Star,” Astronomical Journal Vol. 159, No. 1 (2020) 8 (full text). Thanks to my friend Antonio Tavani for the pointer to the 2022 paper.
Comet Interceptor Could Snag an Interstellar Object
It pleases me to learn that Dutch astronomer Jan Oort was among the select group of people who have seen Halley’s Comet twice. At the age of 10, he saw it with his father on the shore at Noordwijk, Netherlands. In 1986, he saw it again from an aircraft. What a fine experience that would have been for a man who brought so much to the study of comets, including the idea that the Solar System is surrounded by a massive cloud of such objects in orbits far beyond those of the outer planets.
Image: Dutch astronomer Jan Oort, a pioneer in the study of radio astronomy and a major figure in mid-20th Century science. Credit: Wikimedia Commons CC BY-SA 3.0.
Halley’s Comet is a short-period object, roughly defined as a comet with an orbit of 200 years or less, and thus not a member of the Oort Cloud. But let’s linger on it for just a moment. The most famous person associated with two appearances of Halley’s Comet is Mark Twain, who was born in 1835 with the comet in the sky, and who sensed that its approach in 1910 would also mark his demise. As Twain put it:
I came in with Halley’s Comet… It is coming again … and I expect to go out with it… The Almighty has said, no doubt: ‘Now here are these two unaccountable freaks; they came in together, they must go out together.’
And so they did.
Edging in from the Oort
The Oort Cloud is an intriguing concept because by some accounts, it may extend halfway to the nearest star, meaning that it’s conceivable that the cometary cloud around the Sun nudges into a similar cloud around Centauri A/B, assuming there is one there. We use the Oort to explain the appearance of long-period comets, assuming that among these trillions of objects, a few are occasionally nudged out of their orbits and fall toward the Sun. The concept makes sense but observational data is sparse, as these dark objects are not directly observable until one of them moves inward.
Image: The presumed distance of the Oort cloud compared to the rest of the Solar System. Credit: NASA / JPL-Caltech / R. Hurt.
We’ve recently learned about a long-period comet with interesting properties indeed. C/2014 UN271 (Bernardinelli-Bernstein) is the object in question, named after the two astronomers who discovered it in Dark Energy Survey (DES) data at a heliocentric distance of 29 au. Recent work with the Hubble Space Telescope has determined that the object may be as much as 130 kilometers across, making it the largest nucleus ever seen in a comet. Moreover, we can assume that it’s not an aberration.
David Jewitt (UCLA) is a co-author of the paper on this work:
This comet is the tip of the iceberg for many thousands of comets that are too faint to see in the more distant parts of the solar system. We’ve always suspected this comet had to be big because it is so bright at such a large distance. Now we confirm it is.”
Getting an accurate read on an object like this was no easy matter. At this distance from the Sun, the nucleus is too faint to be resolved even by the Hubble instrument, so Jewitt and team had to rely on data showing the spike of light where the nucleus was thought to be. Lead author Man-To Hui (Macau University of Science and Technology) led the development of a computer model of the surrounding coma, adjusting it to the Hubble data and then subtracting its glow, leaving behind the nucleus. Observations from the Atacama Large Millimeter/submillimeter Array (ALMA) confirmed its size and also made it clear that the nucleus is, as Jewitt puts it, “blacker than coal.”
Image: Sequence showing how the nucleus of Comet C/2014 UN271 (Bernardinelli-Bernstein) was isolated from a vast shell of dust and gas surrounding the solid icy nucleus. Credit: NASA, ESA, Man-To Hui (Macau University of Science and Technology), David Jewitt (UCLA). Image processing: Alyssa Pagan (STScI).
Intercepting a Comet
If long period comets are difficult objects to study from Earth orbit, we may need to get up close with a spacecraft. It’s good to hear that the European Space Agency has approved the mission known as Comet Interceptor for construction, slotting it to fly in 2029 in the same launch that will carry the Ariel exoplanet finder into space. We’ve studied comets before, of course, including Halley’s, with notable success. But it’s obvious that short-period comets like the former and Rosetta target 67/P Churyumov-Gerasimenko would have been changed by their long proximity to the inner Solar System. What will we find when we study a newly arriving Oort object?
Michael Küppers is an ESA scientist working on the Comet Interceptor mission:
“A comet on its first orbit around the Sun would contain unprocessed material from the dawn of the Solar System. Studying such an object and sampling this material will help us understand not only more about comets, but also how the Solar System formed and evolved over time.”
Both Ariel and Comet Interceptor will proceed to the L2 Lagrangian point 1.5 million kilometers from the Earth, where the latter will wait for a target, presumably an Oort object jostled inward by gravitational interactions. Here we rely on the fact that comets are often detected more than a year before they reach perihelion, a time too short to allow for the construction of a dedicated space mission. The plan is to make Comet Interceptor ready to move when the time comes, performing a flyby of the incoming object and releasing twin probes to build up a 3D profile of the comet.
Image: An illustration of the L2 point showing the distance between the L2 and the Sun, compared to the distance between Earth and the Sun. Credit: ESA.
ESA will build the spacecraft and one of the two probes, the other being developed by the Japanese space agency JAXA. Given that over 100 comets are known to come close to Earth in their orbit around the Sun, along with the 29,000 asteroids cataloged so far, it will likewise be useful to have a better understanding of the composition of a pristine comet in case it ever becomes necessary to take action to avert an impact on Earth.
And if the target turns out to be an interstellar new arrival like ‘Oumuamua? So much the better. We should be finding more such newcomers shortly, given the success of the Pan-STARRS observatory and the development of the Large Synoptic Survey Telescope, now known as the Vera C. Rubin Observatory, under construction in Chile. Waiting in space for an Oort object or an interstellar comet means we won’t need to know the target in advance, but can adjust the mission as data become available. In any case, ESA is optimistic, saying Comet Interceptor “is expected to complete its mission within six years of launch.”
An ESA factsheet on Comet Interceptor can be found here. The paper on C/2014 UN271 (Bernardinelli-Bernstein) is Man-To Hui et al, “Hubble Space Telescope Detection of the Nucleus of Comet C/2014 UN271 (Bernardinelli-Bernstein),” Astrophysical Journal Letters Vol. 929, No. 1 (12 April 2022) L12 (abstract).
Europa: Catching Up with the Clipper
I get an eerie feeling when I look at spacecraft before they launch (not that I get many opportunities to do that, at least in person). But seeing the Spirit and Opportunity rovers on the ground at JPL just before their shipment to Florida was an experience that has stayed with me, as I pondered how something built by human hands would soon be exploring another world. I suppose the people who do these things at the Johns Hopkins Applied Physics Laboratory and the Jet Propulsion Laboratory itself get used to the feeling. For me, though, the old-fashioned ‘sense of wonder’ kicks in long and hard, as it did when Europa Clipper arrived recently at JPL.
Not that the spacecraft is by any means complete, but its main body has been delivered to the Pasadena site, where it will see final assembly and testing over a two-year period. Here I fall back on the specs to note that this is the largest NASA spacecraft ever designed for exploration of another planet. It’s about the size of an SUV when stowed for launch, but we know from the James Webb Space Telescope how large these things can become when fully deployed. In Europa Clipper’s case, the recently delivered main body is 3 meters tall and 1.5 meters wide. Extending the solar arrays and other deployable equipment takes it up to basketball court size.
Image: The main body of NASA’s Europa Clipper spacecraft has been delivered to the agency’s Jet Propulsion Laboratory in Southern California, where, over the next two years, engineers and technicians will finish assembling the craft by hand before testing it to make sure it can withstand the journey to Jupiter’s icy moon Europa. Here it is being unwrapped in a main clean room at JPL, as engineers and technicians inspect it just after delivery in early June 2022. Credit: NASA.
Eight antennas are involved, powered by a radio frequency subsystem that will service a high-gain antenna measuring three meters wide, and as JPL notes in a recent update, the electrical wires and connectors collectively called the ‘harness’ themselves weigh 68 kilograms. Stretch all that wiring out and you get 640 meters, taking us twice around a football field. The main body will include a fuel tank and an oxidizer tank connecting to an array of 24 engines. Tim Larson is JPL deputy project manager for Europa Clipper:
“Our engines are dual purpose. We use them for big maneuvers, including when we approach Jupiter and need a large burn to be captured in Jupiter’s orbit. But they’re also designed for smaller maneuvers to manage the attitude of the spacecraft and to fine tune the precision flybys of Europa and other solar system bodies along the way.”
So what is arriving, or has arrived at JPL, is a spacecraft in pieces, its main body now joining key instruments like E-THEMIS, a thermal emission imaging system developed at Arizona State, and Europa-UVS, the mission’s ultraviolet spectrograph. E-THEMIS is an infrared camera that should give us insights into temperatures on the Jovian moon, and hence offer information about its geological activity. Given that we’re interested in finding places where liquid water is close to the surface, the data from this instrument should be extremely valuable during the spacecraft’s nearly fifty close passes.
The theory here is that as Europa’s surface cools after local sunset, the areas of the most solid ice will retain heat longer than areas with a looser, more granular texture. E-THEMIS will be able to map cooling rates across the surface. The infrared camera works in three heat-sensitive bands, and the warmer regions it should see may be the result of liquid water close to the surface, or possible impacts or convection activity. Not surprisingly, E-THEMIS lead project engineer Greg Mehall points to the radiation environment in Jupiter space as one of the team’s biggest issues:
“The extreme radiation environment at Europa gave far more design challenges for the ASU engineering team than on any previous instrument we’ve developed. We had to use dense shielding materials, such as copper-tungsten alloys, to provide the necessary protection from the expected radiation. And to ensure that E-THEMIS will survive during the mission, we also carried out radiation tests on the instrument’s electronic components and materials.”
Image: The thermal imager will use infrared light to distinguish warmer regions on Europa’s surface, where liquid water may be near the surface or might have erupted onto the surface. The thermal imager will also characterize surface texture to help scientists understand the small-scale properties of Europa’s surface. In the image above, we’re seeing a diurnal temperature color image from the first light test of Europa Clipper’s thermal imager (called E-THEMIS), taken from the rooftop of the Interdisciplinary Science and Technology Building 4 on the Tempe Campus of Arizona State University (ASU). The top image was acquired at 12:40 PM, the middle at 4:40 PM, and the bottom image at 6:20 PM (after sunset). Temperatures are approximations during this testing phase. Credit: ASU.
As to the Europa-UVS instrument, this ultraviolet spectrograph will search for water vapor plumes and study the composition of both the surface and the tenuous atmosphere as it uses an optical grating to spread and analyze light, identifying basic molecules like hydrogen, oxygen, hydroxide and carbon dioxide.
The spacecraft’s visible light imaging system (EIS) is going to upgrade those well-studied images from the Galileo mission enormously. The plan is to map 90 percent of the moon’s surface at 100 meters per pixel, which is six times more of Europa’s surface than Galileo, and at five times better resolution. And when Europa Clipper swings close to Europa during a flyby, it will produce images with a resolution fully 100 times better than Galileo. The Europa Imaging System includes both wide- and narrow-angle cameras, each with an eight-megapixel sensor. Both of these cameras will produce stereoscopic images and include the needed filters to acquire color images.
All told, the spacecraft’s nine science instruments should be able to extract information about the depth and salinity of the ocean under the ice and, crucially, the thickness of the ice crust (I can imagine wagers on that issue going around in certain quarters). Gathering information about the moon’s surface and interior should further illuminate the issue of plumes from the ocean below that may break through the ice.
Assembly, test and launch is a two year phase that, by the end of this year, should see assembly of most of the flight hardware and the remaining science instruments. Kudos to JHU/APL, which has just delivered a flight system that is the largest ever built by engineers and technicians there. Now we look toward bolting on the radio frequency module, radiation monitors, power converters, the propulsion electronics and those hundreds of meters of wiring. Not to mention the electronics vault that must stand up to hard radiation.
The full instrument package will include an imaging spectrometer, ice-penetrating radar, a magnetometer, a plasma instrument, a mass spectrometer and a dust analyzer. Only two years and four months before launch onto a six-year journey of 2.9 billion kilometers. Europa Clipper isn’t a life-finder, but it does have the capability of detecting whether the moon’s ocean really does allow for the possibility of life to develop. It’s our first reconnaissance of Europa since the 1990s. What surprises will it reveal?
Bear in mind, too, that we still have ESA’s JUICE (JUpiter ICy moons Explorer) in the offing, with launch planned for 2023. I note with interest that on June 19, Europa will occult a distant star, which should be useful in tweaking our knowledge of the moon’s orbit before the arrival of both missions. Destined to end its life as a Ganymede orbiter, JUICE will make only two close passes of Europa, but its period of operations will coincide with part of Europa Clipper’s numerous flybys of the moon.
Solar Sailing: The Beauties of Diffraction
Knowing of Grover Swartzlander’s pioneering work on diffractive solar sails, I was not surprised to learn that Amber Dubill, who now takes the idea into a Phase III study for NIAC, worked under Swartzlander at the Rochester Institute of Technology. The Diffractive Solar Sailing project involves an infusion of $2 million over the next two years, with Dubill (JHU/APL) heading up a team that includes experts in traditional solar sailing as well as optics and metamaterials. A potential mission to place sails into a polar orbit around the Sun is one possible outcome.
[Addendum: The original article stated that the Phase III award was for $3 million. The correct amount is $2 million, as changed above].
But let’s fall back to that phrase ‘traditional solar sailing,’ which made me wince even as I wrote it. Solar sailing relies on the fact that while solar photons have no mass, they do impart momentum, enough to nudge a sail with a force that over time results in useful acceleration. Among those of us who follow interstellar concepts, such sails are well established in the catalog of propulsion possibilities, but to the general public, the idea retains its novelty. Sails fire the imagination: I’ve found that audiences love the idea of space missions with analogies to the magnificent clipper ships of old.
We know the method works, as missions like Japan’s IKAROS and NASA’s NanoSail-D2 as well as the Planetary Society’s LightSail 2 have all demonstrated. Various sail missions – NEA Scout and Solar Cruiser stand out here – are in planning to push the technology forward. These designs are all reflective and depend upon the direction of sunlight, with sail designs that are large and as thin as possible. What the new NIAC work will examine is not reflection but diffraction, which involves how light bends or spreads as it encounters obstacles. Thus a sail can be built with small gratings embedded within the thin film of its structure, and the case Swartzlander has been making for some time now is that such sails would be more efficient.
A diffractive sail can work with incoming light at a variety of angles using new metamaterials, in this case ‘metafilms,’ that are man-made structures with properties unlike those of naturally occurring materials. Sails made of them can be essentially transparent, meaning they will not absorb large amounts of heat from the Sun, which could compromise sail substrates.
Moreover, these optical films allow for lower-mass sails that are steered by electro-optic methods as opposed to bulky mechanical systems. They can maintain more efficient positions while facing the Sun, which also makes them ideal for the use of embedded photovoltaic cells and the collection of solar power. Reflective sails need to be tilted to achieve best performance, but the inability to fly face-on in relation to the Sun reduces the solar flux upon the sail.
The Phase III work for NIAC will take Dubill and team all the way from further analyzing the properties of diffractive sails into development of an actual mission concept involving multiple spacecraft that can collectively monitor solar activity, while also demonstrating and fine-tuning the sail strategy. The description of this work on the NIAC site explains the idea:
The innovative use of diffracted rather than reflected sunlight affords a higher efficiency sun-facing sail with multiplier effects: smaller sail, less complex guidance, navigation, and attitude control schemes, reduced power, and non-spinning bus. Further, propulsion enhancements are possible by the reduction of sailcraft mass via the combined use of passive and active (e.g., switchable) diffractive elements. We propose circumnavigating the sun with a constellation of diffractive solar sails to provide full 4? (e.g., high inclination) measurements of the solar corona and surface magnetic fields. Mission data will significantly advance heliophysics science, and moreover, lengthen space weather forecast times, safeguarding world and space economies from solar anomalies.
Delightfully, a sail like this would not present the shiny silver surface of the popular imagination but would instead create a holographic effect that Dubill’s team likens to the rainbow appearance of a CD held up to the Sun. And they need not be limited to solar power. Metamaterials are under active study by Breakthrough Starshot because they can be adapted for laser-based propulsion, which Starshot wants to use to reach nearby stars through a fleet of small sails and tiny payloads. The choice of sail materials that can survive the intense beam of a ground-based laser installation and the huge acceleration involved is crucial.
The diffractive sail concept has already been through several iterations at NIAC, with the testing of different types of sail materials. Grover Swartzlander received a Phase I grant in 2018, followed by a Phase II in 2019 to pursue the work, a needed infusion of funding given that before 2017, few papers on diffractive space sails existed in the literature. In a 2021 paper, Dubill and Swartzlander went into detail on the idea of a constellation of sails monitoring solar activity. From the paper:
We have proposed launching a constellation of satellites throughout the year to build up a full-coverage solar observatory system. For example a constellation of 12 satellites could be positioned at 0.32 AU and at various inclinations about the sun within 6 years: Eight at various orbits inclined by 60 and four distributed about the solar ecliptic. We know of no conventionally powered spacecraft that can readily achieve this type of orbit in such a short time frame. Based on our analysis, we find that diffractive solar sails provide a rapid and cost-effective multi-view option for investigating heliophysics.
Image: The new Diffractive Solar Sailing concept uses light diffraction to more efficiently take advantage of sunlight for propulsion without sacrificing maneuverability. Incidentally, this approach also produces an iridescent visual effect. Credit: RIT/?MacKenzi Martin.
Dubill thinks an early mission involving diffractive sails can quickly prove their value:
“While this technology can improve a multitude of mission architectures, it is poised to significantly impact the heliophysics community’s need for unique solar observation capabilities. Through expanding the diffractive sail design and developing the overall sailcraft concept, the goal is to lay the groundwork for a future demonstration mission using diffractive lightsail technology.”
A useful backgrounder on diffractive sails and their potential use in missions to the Sun is Amber Dubill’s thesis at RIT, “Attitude Control for Circumnavigating the Sun with Diffractive Solar Sails” (2020), available through RIT Scholar Works. See also Dubill & Swartzlander, “Circumnavigating the sun with diffractive solar sails,” Acta Astronautica
Volume 187 (October 2021), pp. 190-195 (full text). Grover Swartzlander’s presentation “Diffractive Light Sails and Beam Riders,” is available on YouTube.
Venus Life Finder: Scooping Big Science
I’ve maintained for years that the first discovery of life beyond Earth, assuming we make one, will be in an extrasolar planetary system, through close and eventually unambiguous analysis of an exoplanet’s atmosphere. But Alex Tolley has other thoughts. In the essay below, he looks at a privately funded plan to send multiple probes into the clouds of Venus in search of organisms that can survive the dire conditions there. And while missions this close to home don’t usually occupy us because of Centauri Dreams’ deep space focus, Venus is emerging as a prominent exception, given recent findings about anomalous chemistry in its atmosphere. Are the clouds of Venus concealing an ecosystem this close to home?
by Alex Tolley
Introduction
The discovery of phosphine (PH3), an almost unambiguous biosignature on Earth, in the clouds of Venus in 2021 increased interest in reinvestigating the planet’s clouds for life, a scientific goal that had been on hiatus since the last atmospheric entry and lander vehicle mission, Vega-2 in 1984. While the recent primary target for life discovery has been Mars, whether extinct, or extant in the subsurface, it has taken nearly half a century since the Viking landers to once again look directly for Martian life with the Perseverance rover.
However, if the PH3 discovery is real (and it is supported by a reanalysis of the Pioneer Venus probe data), then maybe we have been looking at the wrong planet. The temperate zone in the Venusian clouds is the nearest habitable zone to Earth. If life does exist there [see Figure 1] despite the presence of concentrated sulfuric acid (H2SO4), then it is likely to be in this temperate zone layer, having evolved to live in such conditions.
Figure 1. Schematic of Venus’ atmosphere. The cloud cover on Venus is permanent and continuous, with the middle and lower cloud layers at temperatures that are suitable for life. The clouds extend from altitudes of approximately 48 km to 70 km. Credit: J Petkowska.
But why launch a private mission, rather than leave it to a well-funded, national one?
National space agencies haven’t been totally idle. There are four planned missions, two by NASA (DAVINCI+, VERITAS), one by ESA (EnVision) to investigate Venus, all due to be launched around 2030, as well a Russian one (Venera-D) to be launched at the same time:
VERITAS and DAVINCI+ are both Discovery-class missions. They are budgeted up to $500 million each. EnVision is ESA’s mission launching in the same timeframe. All three missions have target launch dates ranging from 2028 (DAVINCI+, VERITAS) to 2031 (EnVision). As with any large budget mission, these missions have taken a long time to develop. DAVINCI was proposed in 2015, the revised DAVINCI+ proposed again in 2019, and selected in 2021 for a 2028 launch. VERITAS was proposed in 2015, and selected only in 2021. Then there is the seven years of development, testing, and finally launch in 2028. EnVision was selected in 2021, and faces a decade before launch.[5,6,7].
DAVINCI+’s goals include:
1. Understanding the evolution of the atmosphere
2. Investigating the possibility of an early ocean
3. Returning high resolution images of the geology to determine if plate tectonics ever existed.
[PG note: NASA GSFC just posted a helpful overview of this mission.]
Image: The Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI) mission, which will descend through the layered Venus atmosphere to the surface of the planet in mid-2031. DAVINCI is the first mission to study Venus using both spacecraft flybys and a descent probe. Credit: NASA.
VERITAS’s rather similar goals involve answering these questions:
1. How has the geology of Venus evolved over time?
2. What geologic processes are currently operating on it?
3. Has water been present on or near its surface?
EnVision’s goals include:
1. Determining the level and nature of current activity
2. Determining the sequence of geological events that generated its range of surface features
3. Assessing whether Venus once had oceans or was hospitable for life
4. Understanding the organizing geodynamic framework that controls the release of internal heat over the history of the planet
In addition, Russia has the Venera-D mission planned for a 2029 launch that has a lander. One of its goals is to analyze the chemical composition of the cloud aerosols. [8]
There is considerable overlap in the science goals of the four missions, and notably none have the search for life as a science goal, although the 3rd EnVision science goal could be the preparatory “follow the water” approach before a follow-up mission to search for life if there is evidence that Venus did once have oceans.
As with the Mars missions post Viking up to Perseverance, none of these missions is intended to look directly for life itself. Given the 2021 selection date for all three missions and the end of decade launch dates, it will be somewhat frustrating for scientists interested in searching for life on Venus.
Cutting through the slow progress of the national missions, the privately funded Venus Life Finder mission aims to start the search directly. The mission to look for life is focused on small instruments and a low-cost launcher. Not just one but a series of missions is planned, each increasing in capability. The first is intended to launch in 2023, and if the three anticipated missions are successful, Venus Life Finder would scoop the big science missions in being the first to detect life in Venus should it exist.
Some history of our views about Venus
Before the space age, both Venus and Mars were thought to have life. Mars stood out because of the seasonal dark areas and Schiaparelli’s observation of channels, followed by Lowell’s interpretation of these channels as canals, which carried the implication of intelligence. Von Braun’s “Mars Projekt” (1952) inferred that the atmosphere was thin, but the astronauts would just need O2 masks, and his technical tale had the astronauts discover an advanced Martian civilization. The popular science book “The Exploration of Mars” (1956) written by Willy Ley and Wernher Von Braun and illustrated by Chesley Bonestell, supported the idea of Martian vegetation, speculating that it was likely to be something along the lines of hardy terrestrial lichens.
Unlike Mars, the surface of Venus was not observable, just the dense permanent cloud cover. It was believed that Venus was younger than Earth and that the clouds covered a primeval swamp full of animals like those in our planet’s past. With the many probes starting in 1962 with the successful flyby of Mariner 2, it was determined that the surface of Venus was a hellish 438-482 C (820-900 degrees F), by far the hottest place in the Solar System. Worse, the clouds were not water as on Earth, but H2SO4, in a concentration that would rapidly destroy terrestrial life. Seemingly Venus was lifeless.
Some scientists thought Venus was much more Earth Like in the past, and that a runaway greenhouse state accounted for its current condition. If Venus was more Earth Like, there could have been oceans, and with them, life. On Earth, bacteria are carried up from the surface by air currents and have been found living in clouds and are part of the cloud formation process. Bacteria have been found in Earth’s stratosphere too. Bacteria living in the Venusian oceans would likely have been carried up into the atmosphere and occupy a similar habitat. If so, it has been hypothesized that bacterial life may have evolved to live in the increasingly acidic Venusian clouds just as terrestrial extreme acidophiles have evolved, and that this life is the source of the detected PH3.
The First Science Instrument
Is there any other evidence for life on Venus? Using two instruments, a particle size spectrometer and a nephelometer, the Pioneer Venus probe (1978) suggested that some tiny droplets in the clouds were not spherical, as physics would predict, and therefore might be living [unicellular] organisms.
But these probes could not resolve some anomalies of the Venusian atmosphere that might as a whole, indicate life.
1. Anomalous UV Absorber – spatial and temporal variability reminiscent of algal blooms.
2. Non-spherical large droplets – possible cells
3. Non-volatile elements such as phosphorus that could reduce the H2SO4 concentration and a required element of terrestrial life
4. Gases in disequilibrium, including PH3, NH3
Enter the Venus Life Finder (VLF) team, led by Principal Investigator Sara Seager, whose team includes the noted Venus expert David Grinspoon. The project isfunded by Breakthrough Initiatives. The initial idea was to do some laboratory experiments to determine if the assumptions about possible life in the clouds were valid.
As the VLF document states up front:
The concept of life in the Venus clouds is not new, having been around for over half a century. What is new is the opportunity to search for life or signs of life directly in the Venus atmosphere with scientific instrumentation that is both significantly more technologically advanced and greatly miniaturized since the last direct in situ probes to Venus’ atmosphere in the 1980s.
The big objection to life in the Venusian clouds is their composition: extremely concentrated sulfuric acid. Any terrestrial organism subjected to the acid is dissolved. [There is a reason serial killers use this method to remove evidence of their victims!]
To check on the constraints of cloud conditions on potential life and the ability to detect organic molecules, the VLF team conducted some experiments that showed that:
1. Organic molecules will autofluoresce in up to 70% H2SO4. Therefore organic molecules are detectable in the Venusian cloud droplets.
2. Lipids will form micelles in up to 70% H2SO4 and are detectable. Cell membranes are therefore possible containers for biological processes.
3. Terrestrial macromolecules – proteins, sugars, and nucleic acids – all rapidly become denatured in H2SO4, ruling out false positives from terrestrial contamination
4. The Miller-Urey experiment will form organic molecules in H2SO4. Therefore abiogenesis of precursor molecules is also possible on Venus.
With these results, the team focused on building a single instrument to investigate both the shapes of particles and the presence of organic compounds. Non-spherical droplet shapes containing organic compounds would be a possible indication of life. This instrument, an Autofluorescing Nephelometer, is being developed from an existing instrument, as shown in figure 2.
Figure 2. Evolution of the Autofluorescing Nephelometer (AFN) from the Backscatter Cloud Probe (BCP) (left of arrow) to the Backscatter Cloud Probe with Polarization Detection (BCPD) (right of arrow). The BCPD is further evolved to the AFN by replacement of the BCPD laser with a UV source and addition of fluorescence-detection compatible optics.
All this in a package of just 1 kg to be carried in the atmosphere entry vehicle.
Reaching Venus
The VLF team has partnered with the New Space company Rocket Lab which is developing its Venus mission. The company has small launchers that are marketed to orbit tiny satellites for organizations that don’t want to use piggy-backed rides with other satellites as part of a large payload. Its Electron rocket launcher has so far racked up successes. The Electron can place up to 300kg in LEO.
For the Venus mission, the payload includes the Photon rocket to make the interplanetary flight and deliver a 20 kg Venus atmosphere entry probe that includes the 1 kg AFN science package. To reach Venus, the Photon rocket using bi-propellant generates the needed 4 km/s delta V. It employs multiple Oberth maneuvers in LEO to most efficiently raise the orbit’s apogee until it is on an escape trajectory to Venus. Travel time is several months.
The Electron rocket, the Photon rocket, and the entry probe are shown in the next three figures.
The photon rocket powers the cruise phase from LEO to Venus intercept. This rocket uses an unspecified hypergolic fuel and will carry the entry probe across the 60 million km trajectory of its 3-month Venus mission.
Figure 3a. Electron small launch vehicle. The Electron ELV has successfully launched 146 satellite missions to date for a low per launch cost. A recent test of a helicopter retrieval of the 1st stage indicates that reusability is possible using this in-flight capture approach, therefore potentially saving costs. The kick stage in the image is replaced by the Photon rocket for interplanetary flight.
Figure 3b. High-energy Photon rocket and Venus entry probe.
Figure 3c. The small Venus probe is a 45-degree half-angle cone approximately 0.2m in diameter. Credit: NASA ARC.
Fast and Cheap
The cost of the mission to Venus is not publicized, but we know the cost of a launch of the Electron rocket to LEO is $7.5m [12]. Add the photon cruise stage, the entry probe, the science instruments, the operations and science teams. All in, a fraction of the Discovery mission costs, but with a faster payback and more focussed science. Rocket Lab has not launched an interplanetary mission before, so there is risk of failure. The company does have other interplanetary plans, including a Mars mission using two Photon cruise stage rockets for a Mars orbiter mission in 2024.
Is the Past the Future?
The small probe and dedicated instrument package, while contrasting with the big science missions of the national programs, harkens back to the early scientific exploration of space at the beginning of the space age. The smaller experimental rockets had limited launch capacity and the scientific payloads had to be small. Some examples include the Pioneer 4 lunar probe [11] and the Explorer series [10].
These relatively simple early experiments resulted in some very important discoveries. The lunar flyby Pioneer 4, launched in 1958, massed just 6.67 kg, with a diameter of just 0.23 m, a size comparable to the VLF’s first mission [11]. These early missions could be launched with some frequency, each probe or satellite containing specific instruments for the scientific goal. Today with miniaturization, instruments can be made smaller and controlled with computers, allowing more sophisticated measurements and onboard data analysis. Miniaturization continues, especially in electronics.
Breakthrough StarShot’s interstellar concept aims at have a 1 gm sail with onboard computer, sensors, and communications, increasing capabilities, reducing costs, and multiplying the numbers of such probes. With private funding now equaling that of the early space age experiments, and the lower costs of access to space, there has been a flowering of the technology and range of such private space experiments. The VLF mission is an exemplar of the possibilities of dedicated scientific interplanetary missions bypassing the need to be part of “big science” missions.
Just possibly, this VLF series of missions will return results from Venus’ atmosphere that show the first evidence of extraterrestrial life in our system. Such a success would be a scoop with significant ramifications.
References
1. Seager S, et al “Venus Life Finder Study” (2021) Web accessed 02/18/2022
https://venuscloudlife.com/venus-life-finder-mission-study/
2. Clarke A The Exploration of Space (1951), Temple Press Ltd
3. Ley, W, Von Braun W, Bonestell C The Exploration of Mars (1956), Sidgwick & Jackson
4. RocketLab “Electron Rocket: web accessed 02/18/2022 https://www.rocketlabusa.com/launch/electron/
5. Wikipedia “List of missions to Venus” en.wikipedia.org/wiki/List_of_missions_to_Venus
6. Wikipedia “DAVINCI” en.wikipedia.org/wiki/List_of_missions_to_Venus
7. Wikipedia “VERITAS” en.wikipedia.org/wiki/VERITAS_(spacecraft)
8. Wikipedia “EnVision” en.wikipedia.org/wiki/EnVision
9. Wikipedia “Venera-D” en.wikipedia.org/wiki/Venera-D
10. LePage, A, “Vintage Micro: The Second-Generation Explorer Satellites” (2015) www.drewexmachina.com/2015/09/03/vintage-micro-the-second-generation-explorer-satellites/
11. LePage, A, “Vintage Micro: The Pioneer 4 Lunar Probe” (2014)
www.drewexmachina.com/2014/08/02/vintage-micro-the-pioneer-4-lunar-probe/
12. Wikipedia “Rocket Lab Electron”, en.wikipedia.org/wiki/Rocket_Lab_Electron
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