Paul Gilster's Blog, page 30

Great Winds from the Sky

Do we need to justify pushing our limits? Doing so is at the very heart of the urge to explore, which is embedded in our species. Recently, while doing some research on Amelia Earhart, I ran across a post on Maria Popova’s extraordinary site The Marginalian, one that examines the realm of action within the context of the human spirit. Back in 2016, Popova was looking at Walter Lippmann (1889-1974), the famed journalist and commentator, who not long after Earhart’s fatal flight into the Pacific discussed the extent of her achievement and the reasons she had flown.

Here’s a passage from Lippmann’s New York Herald Tribune column, written on July 8, 1937, just six days after the aviator and her navigator, Fred Noonan, disappeared somewhere near Howland Island between Hawaii and Australia. Lippmann asks whether such ventures must be justified by a utilitarian purpose and concludes that what is at stake here transcends simple utility and speaks to the deepest motivations of our explorations. It is a belief in a goal and the willingness to risk all. Practicality carries little weight among those who actually do the deed:

“The best things of mankind are as useless as Amelia Earhart’s adventure. They are the things that are undertaken not for some definite, measurable result, but because someone, not counting the costs or calculating the consequences, is moved by curiosity, the love of excellence, a point of honor, the compulsion to invent or to make or to understand. In such persons mankind overcomes the inertia which would keep it earthbound forever in its habitual ways. They have in them the free and useless energy with which alone men surpass themselves.

Such energy cannot be planned and managed and made purposeful, or weighted by the standards of utility or judged by its social consequences. It is wild and it is free. But all the heroes, the saints, the seers, the explorers and the creators partake of it. They do not know what they discover. They do not know where their impulse is taking them. They can give no account in advance of where they are going or explain completely where they have been. They have been possessed for a time with an extraordinary passion which is unintelligible in ordinary terms.

No preconceived theory fits them. No material purpose actuates them. They do the useless, brave, noble, the divinely foolish and the very wisest things that are done by man. And what they prove to themselves and to others is that man is no mere creature of his habits, no mere automaton in his routine, no mere cog in the collective machine, but that in the dust of which he is made there is also fire, lighted now and then by great winds from the sky.”

Image: Amelia Earhart’s Lockheed Electra 10E. During its modification, the aircraft had most of the cabin windows blanked out and had specially fitted fuselage fuel tanks. The round RDF loop antenna can be seen above the cockpit. This image was taken at Luke Field in Hawaii on March 20, 1937. Earhart’s final flight in this aircraft took place on July 2, 1937, taking off from Lae, New Guinea. Credit: Wikimedia Commons. Scanned from Lockheed Aircraft since 1913, by René Francillon. Photo credit USAF.

Lippmann’s tribute is a gorgeous piece of writing, available in The Essential Lippmann (Random House, 1963). Naturally, it makes me think of other flyers who rode those same winds, people like Antoine de Saint-Exupéry and Beryl Markham, who in 1936 was the first to dare a solo non-stop flight across the Atlantic from east to west. As I’ve recently re-read Markham’s elegant West With the Night (1942), she as well as Earhart has been on my mind. What a shame that Earhart didn’t live to pen a memoir as powerful, but perhaps Lippmann in some small way did it for her.

Juno: First Image from Europa

Juno’s close pass of Europa on September 29 (1036 UTC) took it within 352 kilometers of the icy moon, marking the third close pass in history below 500 kilometers. The encounter saw the spacecraft come within a single kilometer of Galileo’s 351 kilometers from the surface back in January of 2000, and it provided the opportunity for Juno to use its JunoCam to home in on a region north of Europa’s equator. Note the high relief of terrain along the terminator, with its ridges and troughs starkly evident.

Image: The complex, ice-covered surface of Jupiter’s moon Europa was captured by NASA’s Juno spacecraft during a flyby on Sept. 29, 2022. At closest approach, the spacecraft came within a distance of about 352 kilometers. Credit: NASA/JPL-Caltech/SWRI/MSSS.

This first image from JunoCam captures features at the region called Annwn Regio, and was collected in the two-hour window available to Juno as it moved past Europa at 23.6 kilometers per second. What we hope to gain from analysis of the data should be high resolution images at approximately 1 kilometer per pixel, along with data on the ice shell covering the moon’s ocean, along with a good deal more about its surface composition, its internal structure and tenuous ionosphere. Says Candy Hansen, a Juno co-investigator (Planetary Science Institute, Tucson):

“The science team will be comparing the full set of images obtained by Juno with images from previous missions, looking to see if Europa’s surface features have changed over the past two decades. The JunoCam images will fill in the current geologic map, replacing existing low-resolution coverage of the area.”

In other words, more JunoCam imagery to come, all useful to the upcoming Europa Clipper and JUICE missions. In particular, data from the spacecraft’s Microwave Radiometer should fill in our understanding of variations in Europa’s ice beneath the crust, and possibly point to regions where liquid water may be captured in subsurface pockets.

Colors of a Habitable Exoplanet

When it comes to planetary habitability, it is all too easy to let our assumptions slide past without review. It’s a danger to be avoided if we want to understand what may distinguish various types of habitable worlds. That’s the implication of a presentation at the recent Europlanet Science Congress (EPSC), which finished its work on September 23 at the Palacio de Congresos de Granada (Spain). Tilman Spohn (International Space Science Institute) and Dennis Höning (Potsdam Institute for Climate Impact Research) have been investigating the ratio of land to ocean and the evolution of biospheres.

The assumptions the duo are examining revolve around the kind of habitable world our Earth represents. Our planet draws on solar energy through continents balanced against large oceans that produce abundant rainfall. Would a given exoplanet have similar geological properties? According to the scientists, it is a balance between the emergence of continents and the volcanism and continental erosion of subduction that maintains Earth’s particular ratio of ocean to land. If we assume a similar internal state on an exoplanet, we could wind up with a similar equilibrium between the production of continents and their erosion, producing a continental land fraction like Earth’s.

But this is conjecture, not observation, and Spohn and Höning believe that several different outcomes may be produced depending on the coupling of continental crust cycle and water in the mantle. We might well wind up with a ‘land planet,’ one having 80 percent of its surface in the form of continental crust (that makes for about 70 percent land surface when continental shelves covered with water are accounted for).

At the other extreme is a planet only 20 percent or so covered in continents; here the land fraction is about 10 percent. Both worlds maintain equilibrium, and it’s rather startling that 80 percent of the team’s randomly chosen sets of initial conditions end up with the land planet outcome.

Image: Planet Earth is visible as a bright speck within the sunbeam just right of center and appears softly blue in the famous ‘pale blue dot’ image originally published in 1990. This updated version uses modern image-processing software and techniques to revisit the well-known Voyager view while attempting to respect the original data and intent of those who planned the images. Earth is the famous ‘pale blue dot,’ but would other habitable planets present the same aspect? Credit: NASA/JPL-Caltech.

Earth-like planets (with continental coverage in the range of about 40 percent, or a land fraction of 30 percent) result in only one percent of these evolutionary models, suggesting that the kind of equilibrium we see on our planet is unstable. Here’s Spohn on the matter:

“In the engine of Earth’s plate tectonics, internal heat drives geologic activity, such as earthquakes, volcanoes and mountain building, and results in the growth of continents. The land’s erosion is part of a series of cycles that exchange water between the atmosphere and the interior. Our numerical models of how these cycles interact show that present-day Earth may be an exceptional planet, and that the equilibrium of landmass may be unstable over billions of years. While all the planets modeled could be considered habitable, their fauna and flora may be quite different.”

Image: Terrestrial planets can evolve in three scenarios of land/ocean distribution: covered by lands, oceans or an equal mix of both. The land-covered planet is the most probable scenario ( around 80%), while our “equal mix” Earth (<1% chance) is even more unique than previously thought. Credit: Europlanet 2024 RI/T Roger.

When the authors included CO2 outgassing in their model as well as the long-term carbonate-silicate cycle, they found only a 5 K average surface temperature difference between the land planet outcome and the ocean planet. But the scientists point out that a land planet’s climate would be considerably different from our own, accounting for the difference in flora and fauna that Spohn alludes to above: “…we would expect that the land planet has a substantially dryer, colder and harsher climate,” they write in their presentation abstract, “possibly with extended cold deserts in comparison with the ocean planet and with the present-day Earth.”

We need look no further than Earth’s geological history to see analogues. A land planet scenario produces the kind of climate that Earth would have had in the Pleistocene, while the ocean planet conditions are similar to the climate in Earth’s Paleocene. We’ll see how these numbers stack up when this work evolves out of the conference presentation stage and into a formal paper, but if they hold, the implications for habitable planet detection seem clear. We’re far more likely to find land planets and water worlds than the expected ‘pale blue dot’ signature characteristic of Earth.

The presentation is Spohn, T. and Hoening, D., “Land/Ocean Surface Diversity on Earth-like (Exo)planets: Implications for Habitability,” Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-506, 2022. Abstract. See also this useful overview on habitability and its geological constraints: Dehant et al., “Geoscience for understanding habitability in the solar system and beyond,” Space Science Reviews 215, 42 (20 August 2019).Abstract.

DART’s Palpable Hit

Although I had Europa on my mind yesterday, I hadn’t thought to find a connection between the icy Jovian moon and the DART mission. Yet it turns out the Double Asteroid Redirection Test imaged Jupiter and Europa in July and August as the spacecraft moved toward yesterday’s encounter with the binary asteroid Didymos. Controllers used the spacecraft’s DRACO imager (Didymos Reconnaissance and Asteroid Camera for Optical navigation) to examine the visual separation between moon and planet, homing in on variations in the pixel count and intensity as the targets moved across the detector. All this in anticipation of the spacing that would soon be detected between the larger asteroid Didymos and its tiny companion Dimorphos.

Says Peter Ericksen, SMART Nav software engineer at APL:

“Every time we do one of these tests, we tweak the displays, make them a little bit better and a little bit more responsive to what we will actually be looking for during the real terminal event.”

Image: This is a cropped composite of a DART Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO) image centered on Jupiter taken during tests of DART’s SMART Nav system. DART was about 435 million miles (700 million kilometers) from Jupiter, and about 16 million miles (26 million kilometers) from Earth, when the image was taken. Two brightness and contrast stretches, made to optimize Jupiter and its moons, respectively, were combined to form this view. From left to right are Ganymede, Jupiter, Europa, Io and Callisto. Credit: NASA/Johns Hopkins APL.

Jupiter and Europa were only part of the extensive testing before last night’s event, involving thousands of pictures of stars. A successful impact was the result. Nice work by the DART team!

It will take time to determine how well the experiment worked, which means measuring the impact’s effect on the tiny asteroid, but the data will help enormously as we continue to assess strategies for adjusting the trajectory of any future objects that may pose a danger to the Earth. We’ll be getting imagery from the Italian LiciaCube spacecraft within days, and further information from ESA’s Hera mission, which will make follow-up studies at Didymos and Dimorphos in four years.

I’ve long believed that efforts like these, necessary to ensure planetary security, will be a powerful driver for space technologies going forward. The threat of a catastrophic collision with an asteroid is small, but the image below, likewise from JHU/APL, gives us a sense of the possibilities. I think of Arthur C. Clarke’s Rendezvous with Rama (1972), where an impact in 2077 causes catastrophic damage to parts of Europe, leading to the development of the protective system of technologies that eventually spots Rama, the enigmatic alien vessel entering our Solar System.

Let’s hope we’re far enough ahead in the game to have the technologies in place to avoid that kind of impact in the first place. DART is an early step in that direction.

Juno Closes on Europa

As the Europlanet Science Congress (EPSC) has just wrapped up in Spain’s Palacio de Congresos de Granada, I’m reminded how little time I’ve had recently to keep up with such gatherings. I do hope to have some entries on EPSC-announced findings in the near future. Today I simply note the news of an unexpected ‘heat wave’ (700℃) extending 130,000 kilometers just below Jupiter’s northern aurora, one traveling at high speed toward the equator, as announced by James O’Donoghue at the EPSC.

Says JAXA’s O’Donoghue:

“While the auroras continuously deliver heat to the rest of the planet, these heat wave ‘events’ represent an additional, significant energy source. These findings add to our knowledge of Jupiter’s upper-atmospheric weather and climate, and are a great help in trying to solve the ‘energy crisis’ problem that plagues research into the giant planets.”

I mention this work in particular because of my interest in the EPSC results but also because Jupiter has been on my mind thanks to the Juno mission extension. The spacecraft will now remain in operation in Jupiter space through September of 2025, becoming an explorer of the moon system here. Specifically, multiple rendezvous are planned for Ganymede, Europa and Io, even as the spacecraft continues magnetic field studies and radio occultation science. The extended mission will also take multiple passages through the planet’s thin system of rings.

Image: A far more distant encounter with Europa than the one about to happen. This image from the spacecraft’s JunoCam was taken at a distance of about 82,000 kilometers. Color and reflectance variations across Europa’s regions can be seen Although the resolution of the images is just 50 to 60 km per pixel, the data fills in a previously un-imaged area around the north pole near the center of the image. Credit: NASA/JPL-Caltech/SwRI/MSSS. Image processing: Andrea Luck CC BY.

The words ‘previously un-imaged’ have a nice resonance, reminding us how reliant we have been on the Galileo dataset for Europa surface studies. As Juno’s orbit evolves, the spacecraft continues to investigate this system from new angles, with perijove (closest approach to Jupiter) migrating northward over the course of the mission, allowing for example close views of cyclones at Jupiter’s northern poles. Note the beautiful gravitational ballet going on here, as multiple satellite flybys steer our spacecraft through the Jupiter system and reduce its orbital period.

But my attention is drawn primarily to the implications for Europa Clipper and JUICE (Jupiter Icy Moons Explorer), and not just because of the close observations of icy moon surfaces that the Juno orbits will allow. Note this: During the course of these investigations, Juno will fly through the Europa and Io ring tori. These are ring shaped clouds of ions whose characterization will assist planning as Europa Clipper and JUICE controllers anticipate the radiation environment near the large icy moons.

Obviously, the more we can learn about the radiation situation we’ll face near Europa, the better for conducting operations around that intriguing world. Juno’s icy moon encounters have already begun, with a low-altitude flyby of Ganymede in June of 2021. Gravitational interactions there have in turn set up the close flyby of Europa we can expect in just a few days, on September 29, which will be followed by close approaches of Io, one on December 30 of 2023, the next on February 3 of 2024.

The upcoming Europa flyby will reduce Juno’s orbit around Jupiter from 43 to 38 days, and will represent the closest a NASA spacecraft has approached Europa since the days of the Galileo probe. The latter came within 351 kilometers of the moon back in 2000, and we’ve been examining that precious data ever since as we investigate the moon’s surface looking for clues about the ocean underneath. Now we’ll come within 358 kilometers while collecting high-resolution images of portions of the surface.

If passage through the ring tori will be valuable for future missions, so will the additional data on Europa’s ionosphere and its interactions with Jupiter’s magnetosphere help us understand more about this world and its intriguing interior. Juno will throw every science instrument and sensor it has into the effort, from the Jovian Auroral Distributions Experiment (JADE) to its X-band medium-gain radio antenna. The Jupiter Energetic-Particle Detector Instrument (JEDI) and Magnetometer will collect information about the ionosphere and plasma environment.

Moreover, Juno’s Microwave Radiometer (MWR) should return data on the icy crust. And for the visually oriented (which probably means all of us), JunoCam will take four visible light images that should be helpful in finding any changes in surface features since Galileo. Expect a resolution better than 1 kilometer per pixel. We’ll also get a high-resolution black-and-white image from Juno’s star camera, while the Jovian Infrared Auroral Mapper (JIRAM) will take infrared images of the surface.

Data collection is to begin about an hour before closest approach, with the spacecraft still over 80,000 kilometers from Europa. That makes for a swift flyby indeed. John Bordi is Juno deputy mission manager at the Jet Propulsion Laboratory:

“The relative velocity between spacecraft and moon will be 14.7 miles per second (23.6 kilometers per second), so we are screaming by pretty fast. All steps have to go like clockwork to successfully acquire our planned data, because soon after the flyby is complete, the spacecraft needs to be reoriented for our upcoming close approach of Jupiter, which happens only 7½ hours later.”

Image: Juno’s extended mission includes flybys of the moons Ganymede, Europa, and Io. This graphic depicts the spacecraft’s orbits of Jupiter – labeled “PJ” for perijove, or point of closest approach to the planet – from its prime mission in gray to the 42 orbits of its extended mission in shades of blue and purple. Credit: NASA/JPL-Caltech/SwRI.

Nearly 50 Europa flybys are on tap from Europa Clipper, while JUICE will add several more as the spacecraft adjusts its trajectory enroute to extended orbital operations at Ganymede. Oh for faster propulsion! It takes a long time to get to Jupiter with today’s methods, but the early 2030s should be a harvest of information about the small world that seems to be the Solar System’s most likely place for life as we don’t know it.

What Happens Next

I’m going to need to take some time off, a decision prompted by responsibilities outside the interstellar community that have grown to the point where I lack the time to maintain a consistent schedule on the site. I’ll keep moderating comments as usual, and I have some first-rate essays coming up from other authors, but my own writing is going to have to be sporadic for the time being.

Long-term, I plan to keep Centauri Dreams active for a long time, so bear with me. As soon as I can do it, I will get back to a more consistent schedule. For now, though, expect a slower pace of new posts from me.

Arrokoth: The Unbearable Lightness of Being

We’re in that earliest phase of interstellar exploration that is all about nudging outward from our system into the local interstellar medium. That has already involved the Voyagers, but my plan is to keep checking in on both the Interstellar Probe concept at the Johns Hopkins Applied Physics Laboratory and the SGL probe study steadily maturing at the Jet Propulsion Laboratory. These are absorbing ventures as scientists figure out ways to do propulsion, in-flight maintenance (and in the case of SGL, in-flight assembly) and data return on timescales the Voyager team wasn’t imagining when those doughty craft were launched in 1977.

Nudging outward. Let’s check in a bit with New Horizons, because here we have a Kuiper Belt explorer that is fully operational, and with instruments specifically designed for the environment it explores, now some 54 AU from the Sun. It’s striking to think that the Juno mission is ten times closer to our star than New Horizons. The Pluto/Charon flyby seems a long time ago, as does that of the KBO Arrokoth. Indeed the spacecraft is now 1.6 billion kilometers further out than Arrokoth, which it visited in 2019.

Meanwhile the pace of analysis has been intense, with more than 65 publications from the New Horizons science team making their way into the literature last year alone. Have a look at Arrokoth as visualized from recent analysis and realize that data from the encounter continue to stream back to Earth even now. In fact, as New Horizons begins its second extended mission on October 1, completing the data transfer is among the priorities, according to principal investigator Alan Stern in his latest PI’s Perspective.

Image: Recently published discoveries from New Horizons have run the gamut across astrophysics, heliophysics and planetary science. This image is one of many geophysical data products resulting from New Horizons’ 2019 flight past Arrokoth, the first and (so far) only Kuiper Belt object explored by spacecraft, It shows surface slopes on Arrokoth derived from New Horizons stereo imagery, and illustrates one important aspect to understanding both the origin and the geological evolution of Arrokoth. Credit: From a paper led by James Tuttle Keane in the June 2022 issue of Journal of Geophysical Research (JGR) Planets (citation below).

The illustration above is from a recent paper in which lead author James Keane (JPL) and colleagues delve into the peanut-shaped Arrokoth, which the authors point out is probably the least evolved object ever explored by a spacecraft. The paper takes on the ambitious challenge of figuring out the object’s gravity field, noting that bright material seems to collect in its lowest locations. New Horizons was not able to measure Arrokoth’s density directly, but the latter can be inferred using methods that have been fine-tuned in the study of asteroids and comets. It turns out to be unusually low.

The authors describe Arrokoth as akin to fluffy snow on Earth, making it one of the lowest density objects ever explored. It’s intriguing to see that there are comparisons between Arrokoth and some of the smallest moons found within Saturn’s rings. From the paper:

The only objects in the Solar System with consistently low, Arrokoth-like, measured densities are Saturn’s ring moons. These small worlds are thought to form from the gentle accretion of icy ring particles—which may not be unlike the formation of planetesimals via streaming instability and other processes in the early Kuiper Belt, although this comparison requires more investigation.

A useful analogue indeed, if it can be shown that a ring system that Cassini has already provided huge amounts of data on can illuminate processes from the earliest days of the Solar System. The paper continues:

Expanded models of ring and planetesimal dynamics may partially support testing this hypothesis, as could continued analysis of Cassini data. New, ultra-high-resolution observations of dense rings around gas giants (like those proposed by the Saturn Ring Skimmer mission concept; Tiscareno et al., 2021) may be particularly illustrative of how these small, low-density worlds form and evolve.

Let’s pause for a moment on Saturn Ring Skimmer, which comes out of an effort led by Matthew S. Tiscareno (SETI Institute) and has been submitted to the 2023 Planetary Science Decadal Survey. The mission is described as a “ballistic tour” that makes repeated low altitude passes over Saturn’s main rings in a span of 162 days without the use of propellant, covering the main ring regions in 13 low-altitude flybys. The authors of the white paper on the idea say that Saturn Ring Skummer would get 100 times closer to the ring system than Cassini when its best ring images were taken, and would be able to measure material surrounding the rings in situ.

Image: This is Figure 1 from the paper on Ring Skimmer. Caption: Polar plot (left) illustrating 13 passes over Saturn’s rings corresponding to the 162-day long prototype ballistic tour; the altitude (middle) and relative velocity (right) curves represent the passes over the rings. The black solid lines on the left panel represent the region of the rings shadowed by the Sun and, thus, in eclipse. This ring-skimming trajectory is ballistic and exploits four Titan gravity assists. For reference, the ring passes are color coded and grouped by Titan flybys. Figure from Vaquero et al. (2019).

So we have one possibility for augmenting even our Cassini data with measurements that may shed light on the New Horizons findings at Arrokoth. Out of the exhaustive analysis in the Keane paper, we begin to build a picture of Kuiper Belt Objects assembling gently in the outer Solar System, probably within the first few million years of its formation. The New Horizons extended mission should also be able to help if suitable targets can be found. As of now, 24 KBO systems have density constraints, all of these determined by studying multiple object systems (other than Triton, which is most likely a KBO that was captured). Returning to the Keane paper:

Arrokoth is a much smaller object than these other characterized KBOs (it is the smallest KBO with an inferred density). All other characterized KBOs are binary or multiple systems with individual components at least 3× larger than Arrokoth. It is widely recognized that KBO densities decrease with decreasing size, however it is unclear how far that trend goes (and it clearly cannot go to zero density at zero size). Without a more complete sample, it is unclear if Arrokoth’s inferred low density is (a) representative of other small KBOs, (b) an outlier, or (c) simply incorrect due to some flawed assumption(s) used in inferring its density.

So we have a lot to learn. The authors point out how many questions emerge from the Arrokoth flyby. To understand KBO densities, we need further density analysis of comets (only 67P/Churyumov-Gerasimenko has precise density measurements). For that matter, we need more information about KBO binaries and their rotation and orbital dynamics, a thought that Arrokoth emphasizes because there is close alignment between its two lobes. Did two tidally locked objects slowly spiral together before merging? Finding a KBO binary ahead would be pure gold for New Horizons as we try to refine our understanding of the evolution of these objects.

The possibility of another KBO flyby is enhanced by the fact that New Horizons has about 11 kilograms of fuel onboard, though finding an object within range is a daunting task. Both Gemini South (Chile) and the Subaru telescope in Hawaii are looking for a target now, aided by new machine learning tools recently developed. Says Stern:

…by the time New Horizons emerges from hibernation at the beginning of March, we’ll be deep into planning observations of new, much more distant KBOs, as well as a look back at distant Uranus and Neptune to observe how these two “ice giant” planets reflect sunlight – which will tell us more about what drives their internal energy balance. We also plan to make the most extensive and sensitive studies of the cosmological visible light and ultraviolet light backgrounds ever made; such measurements constrain origin theories of the universe while shedding new light on the total number of galaxies in the universe.

Flybys make the news, while much of the essential data gathering proceeds quietly and relatively behind the scenes, which is why I focus on it here. Now in hibernation, New Horizons has until next spring in a relatively quiescent state, though as Stern points out, the Venetia Burney Student Dust Counter (SDC) as well as the PEPSSI and SWAP charged-particle plasma spectrometers remain active. The second priority of the extended mission is collecting and archiving data on the Kuiper Belt environment and also looking outward to the interactions between the heliosphere and the interstellar medium. The plan is to continue these observations while creating the first all-sky ultraviolet maps of the heliosphere, as well as studying clouds in the interstellar medium.

The paper is Keane et al., “The Geophysical Environment of (486958) Arrokoth—A Small Kuiper Belt Object Explored by New Horizons,” JGR Planets 15 May 2022 (full text). The paper on Saturn Ring Skimmer is Tiscareno et al, “The Saturn Ring Skimmer Mission Concept: The next step to explore Saturn’s rings, atmosphere, interior, and inner magnetosphere,” a white paper submitted to the 2023 Planetary Science Decadal Survey (2020). Full text.

Tuning Up for Europa

The new Jupiter photos from JWST’s Near Infrared Camera (NIRCam) are unusual, enough so that I decided to fold one into today’s post. It’s a pretty good fit because I had already put together most of the material I was going to use about Europa. It would have been an additional plus if Europa showed up in the image below, but even without it, note that we can see moons as small as Adrasta here. Imke de Pater (UC-Berkeley), who led the observations, noted that both tiny satellites and distant galaxies show up in the same image. And here’s Thierry Fouchet, a professor at the Paris Observatory, who likewise worked on the observing effort:

“This image illustrates the sensitivity and dynamic range of JWST’s NIRCam instrument. It reveals the bright waves, swirls and vortices in Jupiter’s atmosphere and simultaneously captures the dark ring system, 1 million times fainter than the planet, as well as the moons Amalthea and Adrastea, which are roughly 200 and 20 kilometers across, respectively. This one image sums up the science of our Jupiter system program, which studies the dynamics and chemistry of Jupiter itself, its rings and its satellite system.”

And yes, these images are significantly processed, in this case by citizen scientist Judy Schmidt in California and Ricardo Hueso (University of the Basque Country, Spain). Hurdo is a co-investigator on the Early Release Science program and also leads the NIRCam observations of Jupiter’s Atmosphere. I think Schmidt, who has been working with space observations for a decade, says it best when she describes her goal as “to get it to look natural, even if it’s not anything close to what your eye can see.”

Image: This false-color composite image of Jupiter was obtained July 27 with the NIRCam instrument on board the JWST. Jupiter’s faint rings — a million times dimmer than the planet — and two of its small satellites, Amalthea (left) and Adrastea (dot at edge of ring), are clearly visible against a background of distant galaxies. The diffraction pattern created by the bright auroras and the moon Io (to the left out of the image), form a complex background of scattered light around Jupiter. (Image credit: NASA, European Space Agency, Jupiter Early Release Science team. Image processing: Ricardo Hueso [UPV/EHU] and Judy Schmidt).

I had this image on-screen this morning as I looked into progress on Europa Clipper, which is in the midst of its most significant year so far. By the end of 2022, most flight hardware and all the science instruments are expected to be installed at the Jet Propulsion Laboratory’s Spacecraft Assembly Facility. Engineers and technicians will be assembling the spacecraft’s main body in the installation’s High Bay 1. That includes installation of the craft’s science instruments as well as the aluminum electronics vault that shields the electronics from Jupiter’s radiation. Launch is currently scheduled for October, 2024. We should get nearly 50 close passes of Europa out of all this.

Image: Engineers and technicians use a crane to lift the core of NASA’s Europa Clipper spacecraft in the High Bay 1 clean room of JPL’s Spacecraft Assembly Facility. Credit: NASA/JPL-Caltech.

Watching a spacecraft come together is a fascinating exercise, and we’ll keep an eye on NASA’s updates on the Clipper as the process continues. Just as fascinating, though, is the continual inflow of information about what Europa Clipper’s science instruments will be looking for, a process just as critical if we are to interpret its data correctly.

On that score, what an interesting paper has recently turned up in Astrobiology. In the hands of lead author Natalie Wolfenbarger, it comes out of the University of Texas at Austin, where Europa Clipper’s radar instrument has been developed. A key issue is the composition of the moon’s ice shell, which in turn will feed our conclusions about the ocean lying beneath. Europa’s ocean has been likened to the waters beneath an Antarctic ice shelf on Earth. A good comparison?

To find out, Wolfenbarger and colleagues went to work on how water freezes under ice shelves, which takes us into two unusual terms. ‘Congelation ice’ forms under the ice shelf, while ‘frazil ice’ floats upward in the form of ice flakes in supercooled seawater. These form a kind of snow that coats the bottom of the ice shelf. Interestingly, both ice production mechanisms produce ice with less salinity than seawater itself.

In other words, we may have been assuming that Europa’s ocean is saltier than it actually is, particularly given the paper’s finding that scaling up what happens under Antarctica to an ice shell the size and age of Europa’s produces ice that is less salty still. Frazil ice in particular retains only a small fraction of seawater salt, and the authors make the case that it should be common on Europa. A less saline ice shell is significant because salinity governs its strength and the movement of heat through it.

Image: Mounds of snow-like ice under an ice shelf. Credit: ©Helen Glazer 2015 from the project Walking in Antarctica.

Thus we use our own planet as a research model to understand mechanisms likely at play on a Jovian moon, in ways that help us prepare for Europa Clipper’s look via its ice penetrating instrument, which is called REASON (Radar for Europa Assessment and Sounding: Ocean to Near-surface). This is the only one of the spacecraft’s nine instruments that can look directly into the ice shell, a process that we have experience with on Earth, as REASON principal investigator Don Blankenship notes: “We’ve used ice-penetrating radar for decades. That’s how we know Earth’s ice sheets’ thickness.”

The thickness of the ice is important for everything from getting future probes through the shell into the ocean beneath to creating conditions for an ocean with more likelihood of habitable conditions. For Europa is constantly bathed in particles flung against its surface by Jupiter’s magnetic field, so that compounds emerge that would be useful to life below. A thinner ice sheet would make it more likely that these compounds enter the ocean. No wonder the thickness of the ice has been such a contentious matter among scientists, whose estimates range from a few kilometers to tens of kilometers thick.

Europa Clipper’s REASON instrument uses different wavelengths of radio waves and will be capable of penetrating the ice shell as much as 30 kilometers. This should get interesting.

The paper is Wolfenbarger et al., “Ice Shell Structure and Composition of Ocean Worlds: Insights from Accreted Ice on Earth,” Astrobiology Vol. 22, No. 8 (25 July 2022). Abstract.

Mapping Out Interstellar Clouds

Although I’ve written on a number of occasions about the project called Interstellar Probe, the effort to create what we might call a next-generation Voyager equipped to study space beyond the heliosphere, it’s always been in terms of looking back toward the Solar System. What is the shape of the heliosphere once we see it from outside, and how does it interact with the local interstellar medium? The Voyagers have given us priceless clues, but they were never designed for this environment and in any case will soon exhaust their energies.

Pontus Brandt (JHU/APL), who is project scientist for the Interstellar Probe effort, takes us beyond these heliosphere-centric ideas as he talks to Richard Stone in a fine article about the mission called The Long Shot that ran recently in Science. Because when you launch something moving faster than Solar System escape velocity, you just keep going, and while 1000 AU is often cited as a target for this mission, it’s really only a milestone marker telling us how long we’d like the spacecraft to fly with all its equipment functioning and robust. Beyond the heliosphere, though, we’re looking at interstellar clouds we know fairly little about, and in the long-term view, future interstellar missions will have to know this terrain.

When stars are born, clouds of gas and dust that were not incorporated into the final stellar system remain. Moving on an orbit around the Milky Way that takes some 230 million years to complete, the Solar System encounters these clouds, one of which is the Local Interstellar Cloud, although as Brandt told Richard Stone in the Science article, we really know so little about the cloud environment that our conception is on the order of a child’s sketch. According to the sketch, the Sun has been in the Local Interstellar Cloud for thousands of years (Brandt cites 60) but we’re on its boundaries and appear to be approaching the edge of the so-called G Cloud now. I should add that, as we’ll see in a moment, there are scientists who disagree.

Image: Our solar journey through space is carrying us through a cluster of very-low-density interstellar clouds. Right now the Sun is inside of a cloud (Local cloud) that is so tenuous that the interstellar gas detected by the IBEX (Interstellar Boundary Explorer mission) is as sparse as a handful of air stretched over a column that is hundreds of light-years long. These clouds are identified by their motions, indicated in this graphic with blue arrows. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan.

What happens when, in perhaps as little as two millennia, we make this crossing? It would be useful to know more about the heliosphere to answer the question. And we also need to know more about the temperature and density of a cloud like this, because the heliosphere itself seems malleable, capable of being deformed by the medium through which it moves. Compress the heliosphere and there are implications for life on Earth, for we are protected from dangerous cosmic rays – at least a high percentage of them – by its protective embrace. It would be good to know just how far the heliosphere can be compressed. All the way down to Earth’s orbit?

Here I want to quote from the article:

There’s evidence of such an event around the time early hominids were just beginning to pick up stone tools, and Brandt muses on a possible connection. “Let that creep up your spine for a moment,” he says. In recent years, scientists have discovered iron-60 isotopes in ocean crust samples dating from 2 million to 3 million years ago. Iron-60 is not found naturally on Earth: It’s forged in the cores of large stars. So, either a nearby supernova blasted the heliosphere with the iron dust, or the heliosphere drifted through a dense cloud laden with iron-60 from a previous supernova. Either way, Brandt says, “The heliosphere was way in, and we had a full blast of galactic cosmic rays and interstellar matter for a long, long time.” To look for relics of other such events, IP could use plasma wave antennas to essentially take the temperature of nearby electrons. Hot regions might mark the blast paths of material from past supernovae.

And here’s an interesting factoid, which I’m pulling from the Harvard & Smithsonian Center for Astrophysics: About half of the interstellar gas, almost entirely hydrogen and helium, is spread through 98% of the space between stars, hot but extremely low in density. The other half of the interstellar gas is compressed into 2% of the volume, and we observe it as interstellar clouds, the densest of which are molecular clouds, primarily formed of molecular hydrogen though including carbon monoxide and some organic compounds, with higher concentrations of dust than in the rest of the ISM.

We know about the interstellar medium both by astronomical measurements and spacecraft within the medium – our Voyagers again – that move amidst neutral gas and dust grains, some of which penetrate the heliosphere and can also be measured by spacecraft like New Horizons. Clearly, a craft designed from scratch to make these measurements outside the heliosphere would free us from the uncertainties of astronomical observation looking through the heliosphere to the medium outside.

The Local Interstellar Cloud moves toward us from the direction of Scorpius and Centaurus. All of this movement through clouds and voids is a reminder that the Sun orbits the galaxy and moves through different environments all the time. JPL notes that interstellar densities ranging from 10-5 to 105 atoms/cm3 can be observed near our system in the Milky Way.

Thus the interest in what happens next, as Brandt notes. For one thing, our future hopes for interstellar exploration focus particularly on the nearest stars, and Alpha Centauri is within the G-cloud the Sun now moves toward. We rely on hydrodynamic simulations to estimate the effects of the Solar System moving into a cloud of denser material. A spacecraft like Interstellar Probe could be launched to move ‘upstream’ of the Sun’s motion, essentially exploring the future environment through which we will pass. We’ll someday send much faster exploratory missions to sample the G Cloud in situ.

The Interstellar Probe concept study goes to the National Academies of Sciences, Engineering, and Medicine, which essentially prioritizes where we are going in space exploration over ten year periods. We won’t know how the panel enjoined with making these decisions will come down until 2024, and remember that competing ideas involving space beyond the heliosphere are out there, the most visible of which is the Solar Gravitational Lens (SGL) mission now in advanced study at the Jet Propulsion Laboratory. Be aware as well of a Chinese effort known as Interstellar Express.

One way or another, and this is true with or without the endorsement of a Decadal study, we will get spacecraft fully designed for the interstellar environment out beyond the heliosphere. I make no predictions on timing other than to say that the earliest we might expect a launch of this kind of mission is in the 2030s, and who knows what other factors may come into play if none of the current studies is funded? Nonetheless, the long-term picture I embrace makes robotic exploration beyond our Solar System inevitable whether time to launch is 10 or 100 or 1000 years from now. I think we’re wired to do it.

Space missions always bring surprises, and it’s only fair to note that the model of the Sun’s nearby cloud environment has been challenged in recent work. What alternate outcomes might an Interstellar Probe mission alert us to? Here is a snip from an interesting 2014 paper by Cécile Gry and Edward Jenkins proposing a model that is:

…fundamentally different from previous models (e.g., Lallement et al. 1986; Frisch et al. 2002, RL08) where the LISM is constituted of a collection of small clouds or cloudlets that are presented as separate entities moving as rigid bodies at different velocities in slightly different directions. In particular, in our picture, the LIC, the G cloud, and other distinct clouds of the RL08 model, are unified in a single local cloud.

And this, of course, would become apparent to any future mission pushing upstream from the heliosphere. We may have to send such a mission to make this call.

Back to the present, though. Pontus Brandt now takes his heliophysics expertise into an ongoing mission, with a new role as part of the New Horizons science team at JHU/APL. This is clearly a good fit, given that this spacecraft is already out there, operating more than 50 AU from the Sun with a suite of plasma and dust instruments that is exploring the dust and charged particles in the full flow of the solar wind. We have only one operating spacecraft in the Kuiper Belt, and this is it, as Brandt notes:

“New Horizons remains a pathfinder on a historic journey, and since we’re equipped with instrumentation not flown on Voyager, we will be able to answer some of the big questions about what upholds our vast heliosphere as it plows through the interstellar medium. Leaving the foreground ‘haze’ of the solar system’s dust and gas, New Horizons is also in a position to make some game-changing discoveries that not only give us glimpses into our changing local interstellar medium, but also discoveries on cosmological scales.”

Image: Pictured in the New Horizons mission operations center at the Johns Hopkins Applied Physics Laboratory, Pontus Brandt brings a new kind of expertise to the New Horizons science leadership team — heliophysics, where the team expects to make breakthroughs that no other mission can, with new capabilities never before available so far from the Sun. Credit: Johns Hopkins APL/Craig Weiman.

Brandt’s work at New Horizons is a reminder that the deeper we push into the Solar System, the more we also explore basic interactions between our star and an interstellar medium we are learning how to map. All of this couples with a continuing effort from Earth to locate future flyby targets for close observation. Going interstellar demands knowing where we’re coming from as much as knowing where we’re heading.

And once we do get to the point of sending a spacecraft all the way to another star? Let me quote something Ian Crawford said on this topic in a paper some years back:

If the Sun does lie within the LIC, then a mission to α Cen would sample the outer layers of the LIC, an interval of low density LB material, the edge of the G cloud, and the deep interior of the G cloud. This would sample one of the most diverse ranges of interstellar conditions of any mission to another star located with 5 pc of the Sun, as most other potential targets lie within the LIC… Even if the Sun lies just outside the LIC…, the trajectory to α Cen would still permit detailed observations of the boundary of the G cloud (and its possible interaction with the LIC), and determine how its properties change with increasing depth into the cloud from the boundary.

The paper on a new model for nearby interstellar clouds that I mentioned above is Gry & Jenkins, “The interstellar cloud surrounding the Sun: a new perspective,” Astronomy & Astrophysics 567 (2014), A58 (abstract). For further background on the interstellar medium, see Ian Crawford’s “The Astronomical, Astrobiological and Planetary Science Case for Interstellar Spaceflight,” published in the Journal of the British Interplanetary Society Vol. 62 (2009), pp. 415-421 (preprint). The definitive book on the matter is Bruce Draine’s Physics of the Interstellar and Intergalactic Medium (Princeton University Press, 2011).

Laser Communications: A Step at a Time to Deep Space

My last look at laser communications inside the NASA playbook was a year ago, and for a variety of reasons it’s time to catch up with the Laser Communications Relay Demonstration (LCRD), which launched in late 2021, and the projects that will follow. LCRD has now been certified for its mission of shaking out laser systems in terms of effectiveness and potential for relay operations. Ideally, we’d like to receive data from other missions and relay to the ground in a seamless optical network. How close are we to such a result?

Image: The Laser Communications Relay Demonstration payload. Credit: NASA Goddard Space Flight Center.

LCRD is now in geosynchronous orbit almost 36,000 kilometers above the equator, poised for its two year mission, but before we proceed, note this. The voice is that of Rick Butler, project lead for the LCRD experimenters program at NASA GSFC:

“We will start receiving some experiment results almost immediately, while others are long-term and will take time for trends to emerge during LCRD’s two-year experiment period. LCRD will answer the aerospace industry’s questions about laser communications as an operational option for high bandwidth applications.

“The program is still looking for new experiments, and anyone who is interested should reach out. We are tapping into the laser communications community and these experiments will show how optical will work for international organizations, industry, and academia.”

The Opportunities for Experiments page at GSFC offers the overview for anyone looking to join this effort with ideas for experiments to test optical communications links. Contact information for proposals is provided, and I also note that NASA intends to use LCRD to relay New Year’s resolutions submitted by the public through social media accounts as a demonstration of laser communications capabilities. Sure, it’s a bit of a stunt, but it makes optical communications visible to a general audience as we move into the era of laser networking for space missions near and far.

TeraByte InfraRed Delivery (TBIRD) is to follow, having launched on May 25 of this year. Here scientists are pushing the data downlink, going to 200 gigabits per second, which will represent the highest optical rate NASA has yet achieved. A single 7-minute pass of this CubeSat in low-Earth orbit will return terabytes of data. TBIRD, build by MIT, is integrated into the PTD-3 CubeSat as part of a technology demonstrator mission.

This is exciting stuff in its own right: The Pathfinder Technology Demonstrator program emphasizes using the same spacecraft bus and avionics platform designs across various missions, which moves toward modular spacecraft that are more efficient and easier to produce.

Image: Illustration of TBIRD downlinking data over lasers links to Optical Ground Station 1 in California (not drawn to scale). Credit: NASA/Dave Ryan.

The plan for TBIRD is to demonstrate the stability of laser pointing, with the spacecraft directed toward the ground station at Table Mountain, California. Without moving parts, the laser communications testing will rely on the pointing ability of the entire spacecraft. Beth Keer (NASA GSFC) is TBIRD project manager:

“In the past, we’ve designed our instruments and spacecraft around the constraint of how much data we can get down or back from space to Earth. With optical communications, we’re blowing that out of the water as far as the amount of data we can bring back. It is truly a game-changing capability.”

I’ll also mention a component of laser testing that will go to the International Space Station in the form of ILLUMA-T, which stands for Integrated LCRD Low-Earth Orbit User Modem and Amplifier Terminal. Sending data at 1.2 gigabits per second, the device will communicate with LCRD, which will then relay data on ISS experiments and other information to ground stations at Haleakalā, Hawaii or Table Mountain.

Image: Illustration of LCRD relaying data from ILLUMA-T on the International Space Station to a ground station on Earth. Credit: NASA’s Goddard Space Flight Center/Dave Ryan

While NASA has been using communication relay satellites since 1983, the ability of LCRD to send and receive data from both missions and ground stations from its geosynchronous orbit means we will have achieved the agency’s first two-way, end-to-end optical relay. ILLUMA-T will shake out this system, demonstrating low-Earth orbit to geosynchronous orbit to ground station links in an end-to-end system.

SpaceX uses laser links to move Internet traffic from spacecraft to spacecraft in its Starlink system, and the European Space Agency does the same for its system of environmental monitoring satellites, but both of these use conventional radio to return data to Earth, and a direct link of optical data to Earth is the logical next step.

Extending further from the Earth, the Artemis II mission will carry its own Optical Communications System aboard the Orion spacecraft, making it the first crewed lunar flight demonstrating laser communications. With a downlink rate as high as 260 megabits per second, the system will be able to send high-resolution images and video.

While we wait to see when the Psyche mission will fly, I note that the Deep Space Optical Communications package is aboard, an attempt to increase communications performance by up to 100 times over conventional deep space missions. Now we take laser technologies outside the Earth-Moon system, with the Hale Telescope at Palomar receiving high-speed data from the transceiver aboard the spacecraft. The uplink will be from a laser transmitter at the JPL Table Mountain facility. This experimental effort is scheduled to begin not long after launch and will extend for at least a year and perhaps longer depending on results.

Can we look at laser communications from an interstellar perspective? Early work on this points to the potential as well as the difficulties. According to one JPL study, it would take an installation the size of the Hubble Space Telescope, beaming a 20-watt laser signal, to reach us from Alpha Centauri, so we have a long way to go before we can contemplate such methods between stars.

We can work wonders up to a point: The Deep Space Network can pick up Voyager’s 23-watt radio signal even though it is billions of times weaker than the power it would take to operate a digital wristwatch. But going interstellar will require moving to lasers to narrow beam diffraction (the Voyager signal is now over 1000 times Earth’s diameter). We know how to communicate if we can put the equipment where we need it, but getting payloads of any size – even a microchip – to another star continues to challenge our best scientists.

Exploring that gravitational lens communications relay described by Claudio Maccone may be one way around the problem. We already have a mission under study at JPL to reach 550 AU and beyond with the express purpose of imaging a planet around a nearby star. One step at a time, then, both for exoplanet observation using the Sun’s gravitational lens and, in some latter mission, possibly exploiting its magnification for communications. And one step at a time for lasers. Let’s get Psyche launched and see what DSOC can do.