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.
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 Skimmer 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.
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.
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 ? 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 ? 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).
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 Haleakal?, 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.