Centauri Dreams
Imagining and Planning Interstellar Exploration
Planetary Composition: Enter the ‘Super-Mercuries’
The idea that the composition of a star and its rocky planets are connected is a natural one. Both classes of object accrete material within a surrounding gas and dust environment, and thus we would expect a link between the two. Testing the hypothesis, researchers from three institutions — the Instituto de Astrofísica e Ciências do Espaço (Portugal), the NCCR PlanetS project at the University of Bern, and the University of Zürich — have confirmed the concept while fine-tuning the details. After all, we still have to explain iron-rich Mercury as an outlier in our own Solar System.
Image: Mercury has an average density of 5430 kilograms per cubic meter, which is second only to Earth among all the planets. It is estimated that the planet Mercury, like Earth, has a ferrous core with a size equivalent to two-thirds to three-fourths that of the planet’s overall radius. The core is believed to be composed of an iron-nickel alloy covered by a mantle and surface crust. Credit: NASA.
Starlight contains the spectroscopic signature of the star’s composition, but because we have directly imaged few planets, the composition of rocky planets has to be inferred by examining their mass and radius. A significant factor in this study is what is known as the Bern Model of Planet Formation and Evolution, which covers quite a bit of ground, from processes in the protoplanetary disk, accretion models of a planet’s growing core, and the eventual gravitational interactions of young planets. The authors apply the model in estimating the iron mass fraction of rocky exoplanets.
Christoph Mordasini (University of Bern), a co-author of the paper on this work, comments on the method:
“…since stars and rocky planets are quite different in nature, the comparison of their composition is not straightforward. Instead, we compared the composition of the planets with a theoretical, cooled-down version of their star. While most of the star’s material – mainly hydrogen and helium – remains as a gas when it cools, a tiny fraction condenses, consisting of rock-forming material such as iron and silicate.”
The researchers, led by Vardan Adibekyan (Instituto de Astrofísica e Ciências do Espaço), chose the planets for their study from an initial cut of 364 worlds orbiting F, G and K-class stars. They then narrowed the list to 56 planets with the highest precision in mass and radius, excluding planets whose masses had been determined by transit-timing variations because these results can differ from mass determined by radial velocity methods. They then whittled their list down to 22 potentially rocky planets with radii less than twice that of Earth in 21 stellar systems.
While the analysis confirms that the composition of terrestrial-class worlds is linked to the composition of the host star, the abundance of planetary iron can be higher than what is found in the star. The correlation exists but not precisely in a 1:1 ratio. The implication: Planets in formation may shed lighter materials while leaving dense iron behind. The paper identifies five planets (K2-38 b, K2-106 b, K2-229 b, Kepler-107 c, and Kepler406 b) with a higher iron content than the rest, all seemingly higher-mass analogs of Mercury as planets with Earth-like composition but higher mass.
The likely formation and evolution of these ‘super-Mercuries’ demands investigation, and early system collisions alone may not suffice: From the paper:
The five super-Mercuries we identify have a wide range of masses, unlike the concentration around ~5 M? predicted by simulations of giant impacts. We suggest that a giant impact alone is not responsible for the high density of super-Mercuries. Planet formation simulations that incorporate collisions are unable to produce the highest-density super-Mercuries.
If not collisions, then what? All five of the super-Mercuries found in the study orbit stars with high iron abundance, which the authors consider a proxy for the overall content of heavy elements in stars:
The first trend may suggest that the mechanism responsible for the overabundance of iron in these planets is related to the composition of the protoplanetary disk. The second trend could imply a more efficient planet formation, leading to a formation of multiple planets and resulting in frequent collisions. We suggest that both iron enrichment and collisional mantle stripping may need to be invoked to produce an iron enrichment in the general planet population and explain the presence of super-Mercuries.
The findings of the paper regarding the correlation between planet and star in terms of iron abundance remain significant even if the five super-Mercuries are removed from the sample. Thus the iron mass fraction, computed for planets through their mass and radius, suggests that distinct populations of super-Earths and planets like Mercury can occur, their composition reflecting factors involved in their formation. But the broader picture is that given that density is but one clue to composition, if the host star’s composition is a reliable marker we are justified in making inferences about the makeup of its planets.
The paper is Adibekyan et al., “A compositional link between rocky exoplanets and their host stars,” Science Vol 374, Issue 6565 (15 October 2021), pp. 330-332. Abstract.
A Jupiter-class Planet Orbiting a White Dwarf
A gas giant similar to Jupiter, and with a somewhat similar orbit, revolves around a white dwarf located about 6500 light years out toward galactic center. As reported in a paper in Nature, this is an interesting finding because stars like the Sun eventually wind up as white dwarfs, so we have to wonder what kind of planets could survive a star’s red giant phase and continue to orbit the primary. If Earth one day is engulfed, will the gas giants survive? The new discovery implies that result, and marks the first confirmed planetary system that looks like what ours could become.
Image: An artist’s rendition of a newly discovered Jupiter-like exoplanet orbiting a white dwarf. This system is evidence that planets can survive their host star’s explosive red giant phase, and is the first confirmed planetary system that serves as an analogue to the face of the Sun and Jupiter in our own Solar System. Credit: W. M. Keck Observatory/Adam Makarenko.
Underlining just how faint white dwarfs are is the method of discovery and the follow-up observations that made the paper on this work possible. A gravitational microlensing event called MOA-2010-BLG-477 was detected at Mount John Observatory (New Zealand) in 2010, later observed by more than 20 telescopes. A team led by Joshua Blackman (University of Tasmania) made infrared observations using the Keck Observatory’s adaptive optics system and its Near-Infrared Camera (NIRC2).
The microlensing analysis had revealed the star and its planet, while the Keck observations confirmed the faintness of the star. The paper’s analysis of the data is lengthy as the authors worked to rule out a variety of stellar possibilities in the main sequence given the faintness of the event. This is what emerged:
As all of the possible main-sequence lenses for the event are brighter than the Keck detection limit and no such star is observed, the lens cannot be a main-sequence star. The same analysis also excludes brown dwarf lenses owing to an upper limit on the microlensing parallax parameter, ?E?<?1.03, which leads to an implied limit on the lens system mass of ML?>?0.15 M?. Similarly, the lower microlensing parallax limit of ?E?>?0.26 implies an upper mass limit of ML?<?0.78 M?, which rules out neutron stars and black holes as the host stars. As main-sequence stars, brown dwarfs, neutron stars and black holes are ruled out, we conclude that the lens must be a white dwarf.
Image: This is Figure 1 from the paper. Caption: a, An image obtained with the narrow-camera on the NIRC2 imager in 2015 centred on MOA-2010-BLG-477 with an FOV of 8?arcsec. b, A 0.36-arcsec zoomed-in view of the same image as in a. The bright object in the centre is the source. To the northeast (top left) is an unrelated H?=?18.52?±?0.05?star 123?mas from the source, which we refer to as star 123NE. c, The field in 2018. The contours indicate the probable positions of a possible main-sequence host (probability of 0.393, 0.865, 0.989 from light to dark blue) using constraints from microlensing parallax and lens–source relative proper motion. No such host is detected. Credit: Blackman et al.
The authors used a sample of 130 white dwarfs within 20 parsecs of the Sun, excluding binary systems, and ran their calculations under the assumption that all white dwarfs are equally likely to host planets. We wind up with a white dwarf that is, typical of the type, about the size of the Earth, and about 55 percent the mass of the Sun. The gas giant is found to be approximately 40 percent more massive than Jupiter, orbiting at least 3 AU from the host. Thus we find our first analogue to the final stages of our own system some 2 kiloparsecs away toward the center of the galaxy.
It’s likely, according to this work, that the planet is indeed a survivor of the red giant phase of its host star, which in itself is an interesting aspect of the story. The authors discuss orbital change only sparingly, but point out that mass loss in the star pushes a planet toward a wider orbit, while tidal forces have the opposite effect when the star expands beyond about 1 AU. What little work I can find in the literature on this suggests a consensus that Jupiter-class planets orbiting white dwarfs are likely to be found at separations greater than 5 AU, higher than the ~3 AU we find here.
The paper is Blackman et al., “A Jovian analogue orbiting a white dwarf star,” Nature 598 (13 October 2021), 272-275 (abstract).
Interesting Transient: A New Class of Object toward Galactic Center?
The 36 dish antennae at ASKAP — the Australian Square Kilometre Array Pathfinder in outback Western Australia — comprise an interferometer with a total collecting area of about 4,000 square meters. ASKAP has commanded attention as a technology demonstrator for the planned Square Kilometer Array, but today we’re looking at the discovery of a highly polarized, highly variable radio source labeled ASKAP J173608.2?321635, about 4 degrees from galactic center in the galactic plane.
According to Ziteng Wang, who is lead author of the study on this signal and a University of Sydney PhD student, the observations are strikingly different from other variable radio sources:
“The strangest property of this new signal is that it has a very high polarisation. This means its light oscillates in only one direction, but that direction rotates with time. The brightness of the object also varies dramatically, by a factor of 100, and the signal switches on and off apparently at random. We’ve never seen anything like it.”
Variable celestial objects are common enough, from supernovae to pulsars, not to mention interesting sources like Fast Radio Bursts and, of course, the Cepheid variable stars that have played such a large role in astronomical history in helping us determine the scale of the universe. Any new variable source might be looked upon in light of such objects, perhaps as a type of flare star intermittently spewing out bursts of radiation. But none of these match the odd behavior of the new source. While J173608.2?321635 was found at ASKAP, Wang and team performed follow-up observations with the MeerKET telescope in South Africa.
So we have a source toward galactic center that is at first unseen, then brightens, fades, and reappears. Having detected six such signals from the source over nine months in 2020, the astronomers searched in vain for it in visible light, even as a search with the Parkes radio telescope turned up nothing. That’s when the team turned to MeerKAT, where it was once again detected. Tara Murphy, who is Wang’s PhD supervisor at Sydney, notes what happened next:
“Because the signal was intermittent, we observed it for 15 minutes every few weeks, hoping that we would see it again. Luckily, the signal returned, but we found that the behaviour of the source was dramatically different — the source disappeared in a single day, even though it had lasted for weeks in our previous ASKAP observations.”
Image: The ASKAP telescope array. Credit: CSIRO.
Other low frequency transients from galactic center have been detected in recent years, including GCRT J1745-3009, which was quickly labeled a ‘burper’ by its discoverers due to its intermittent bursts after detection in 1998. Five bursts of equal brightness were noted, each about ten minutes in duration, and occurring every 77 minutes. No explanation has been agreed upon for that one either, although a pulsar, a neutron star pair, or a radio-emitting white dwarf have all been discussed in the literature.
For the ASKAP transient, the authors have considered pulsar scenarios, a transient magnetar, and “a low-mass star/substellar object with extremely low infrared luminosity,” with none of these providing a satisfactory answer. The suspicion grows that this is a new class of objects that future radio imaging surveys will observe as our capabilities improve. With the Square Kilometer Array coming online in the next decade, we are probably looking at a phenomenon that will generate a great deal of study and, doubtless, many more examples.
The paper is Wang et al., “Discovery of ASKAP J173608.2–321635 as a Highly Polarized Transient Point Source with the Australian SKA Pathfinder,” The Astrophysical Journal Vol. 920, No. 1 (12 October 2021), 45. Abstract.
Enlarging Perspectives on Space (and Time)
What do we mean by an ‘interstellar mission’? The question came up in relation to Interstellar Probe, that ‘Voyager Plus’ concept being investigated by the Johns Hopkins Applied Physics Laboratory. I do indeed see it as an interstellar mission, as Interstellar Probe takes us outside the heliosphere and into the local interstellar medium. We need to understand conditions there because it would be folly to mount a mission to another star without knowing the dynamics of the heliosphere’s movement through the interstellar cloud we are currently in, or the ramifications of moving between it and the adjacent cloud as we make our crossing.
How could it be otherwise? Journeys need maps and knowledge of conditions along the way. Thus we push into the fringes of interstellar space, and gradually extend our reach. As we do this, we inevitably produce changes in the way we perceive our place in the cosmos.
Cultural expectations about space have been shaped by what I might call a ‘planar’ approach to astronomy. First there is the Moon, then Mars, then the main asteroid belt, and so on, all of these things at increasing distances but roughly along the great disk of the ecliptic. In the 1950s science fiction film Rocketship X-M, a Moon mission misses its target through a series of odd misadventures and winds up landing on Mars. It was entertaining in its way as Lloyd Bridges and team explored the Red Planet, but it depicts a view of the Solar System in which if you go one distance, you’re at one target, and if you go another, you’re at the next. Never mind that the rocket’s mishap was entirely random and it could have gone anywhere.
Long-period comets and odd objects like Sedna teach us much about what goes on outside the ecliptic, but most deep space missions that have commanded the public’s attention have had destinations somewhere within it. The two Voyagers have a more complicated story given their gravitational encounters, Voyager 1 having taken a jog at Saturn to fly by Titan and thus propel itself out of the ecliptic on an interstellar trajectory, while its twin, Voyager 2, left the system and ecliptic in another direction after its encounter with Neptune. Neither was designed for interstellar operations but both now comprise our only live craft beyond the heliosphere.
As our missions become still more ambitious, we push into this wider, spherical realm of reference, which inevitably shapes public attitudes about our relationship with the galaxy. New Horizons’ mission to Pluto reminds us that the dwarf planet is at a 17° tilt to the ecliptic. Going to other stars would shed this culturally embedded planar concept, for the most part, though it’s interesting that one nearby destination, Epsilon Eridani, lines up well enough with the ecliptic to offer a boost from the angular momentum available to a departing craft. Alpha Centauri, well south of the ecliptic, demands a trajectory bend that loses this bit of assistance. This is a point APL’s Ralph McNutt made to me almost 20 years ago, as I was reminded recently in going through my notes from that period.
Image: Voyager 1 and 2 trajectories. Voyager 1 visited Jupiter and Saturn, and then veered northward off of the plane of our solar system. Voyager 2 visited all four giant planets of the outer solar system before departing southward toward interstellar space. Credit: NASA.
When we start contemplating interstellar missions, we have the chance to do what Voyager did just once, to look back at the Solar System, but this time in a much broader context. The focus will not be on the planets and the pale blue dot of Earth, but rather on the heliosphere, from a vantage well beyond its outer regions. Interstellar Probe is a heliophysics mission in its attempt to understand the Sun and planets as a system moving through the interstellar medium. It pushes perspectives as we visualize the entire Solar System as a moving, interacting environment where life can emerge.
The burgeoning catalog of exoplanets clearly plays into the concept, for we see thousands of stellar systems, each with their own context in what we can call an ‘astrosphere.’ The host stars we study, a tiny fraction of the several hundred billion in the galaxy, all move through plasma and dust within the interstellar medium. We have little enough information about how the Sun’s solar wind carves out the magnetic bubble surrounding our Solar System, but about astrospheres around other stars, we know next to nothing. Our view is flattened; we see their planets, or their circumstellar disks, our instrumentation not up to the challenge of seeing an astrosphere.
Image: This is Figure 3-1 from the JHU/APL report on Interstellar Probe from 2019; the latest report will be out in December. Caption: As our type-G2V star plows through the galactic interstellar medium, it forms the habitable astrosphere harboring the entire solar system we live in. Of all other astrospheres, one of our habitable type has never been observed, and yet we are only at the very beginning of uncovering our own. An interstellar probe through the heliospheric boundary into the LISM would enable us to capture its global nature and would represent humanity’s first step into the galaxy, where unpredictable discoveries await. Credit: NASA/Rosine Lallement, 2020.
Make no mistake, the crossing of the heliopause by both Voyagers has supplied us with data on the plasma physics at work in this region, while from inside the heliosphere, missions like IBEX have revealed unusual features that demand clarification. Interactions at heliosphere’s edge involve solar plasma, and magnetic fields both solar and interstellar, as well as neutral particles in the medium and galactic cosmic rays. Charge-exchange processes between interstellar hydrogen atoms and solar plasma protons shape the heliosphere as does the solar magnetic field pervading it.
A mission that gets to a vantage as distant as 1000 AU will be able to see these interactions from the outside, to determine the heliosphere’s overall shape and the distribution of plasma within it, even as missions like the upcoming IMAP (Interstellar Mapping and Acceleration Probe) study the heliosphere’s boundary from well within it. A probe into the interstellar medium would allow us to examine how the Sun’s activity cycle affects the heliosphere’s recorded shock and pressure waves, as found in Voyager data. Voyager has also shown that the heliosphere shields the Solar System from approximately 75 percent of incoming galactic cosmic rays, a factor in habitability.
But back to movement through the medium. Many interstellar clouds are found in what is called the Local Bubble,a region of hot gas that extends several hundred light years from the Sun. The conception of the Solar System as moving through interstellar clouds of varying dust, plasma and gas content backs out the field of view yet again. The Sun moves at 26 kilometers per second toward the edge of the Local Interstellar Cloud and will exit it in about 1900 years, and the question of what cloud we move through next is open. Fifteen interstellar clouds have been identified within 15 parsecs of our system.
Our Voyagers will run out of power somewhere in the range of 160 AU from the Sun, a long way from what astronomers consider the undisturbed local interstellar medium. Putting a probe well beyond this range would provide the first sampling of the interstellar medium that is unaffected by the heliosphere, and thus teach us a great deal about what our solar bubble moves through. As interstellar dust grains are the foundation of both stellar and planetary systems, they hold clues to the formation of matter in the galaxy and the evolution of stars. All this is applicable, of course, not just to our own heliosphere but the astrospheres around exoplanetary systems.
Image: This is Figure 3-10 from the JHU/APL report. Caption: The Sun is on the way to exiting the Local Interstellar Cloud and entering another unexplored interstellar region. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan.
A mission designed to be returning data 50 years after launch, expressly interstellar in its conception, also elevates our thinking about time as we confront operations long after our own demise. Such a mission puts the blip of our present existence into the context of galactic rotation, the chronological equivalent of the pale blue dot image.
Deeper awareness of ourselves as part of a great astrophysical complex that renders life possible helps to place us in a galactic setting. Going interstellar demands looking a long way out, but it also demands looking back, in our data and imagery, to understand the bubble within which we emerged. That shift in perspective in turn feeds the interstellar ambition, as we expand the frame of reference to other stars.
Interstellar Reach: Exploration as Choice
Two missions with interstellar implications have occupied us in recent days. The first, Interstellar Probe, has significance in being the first dedicated mission into the local interstellar medium. Here the science return would be immense, as we would have the opportunity to view the heliosphere from the outside. Culturally, Interstellar Probe is the kind of mission that can force resets in how we view exploration, a thought I want to expand on in the next post.
The other mission — multiple mission options, actually — involves interstellar objects like the odd 1I/’Oumuamua and 2I/Borisov, the latter clearly a comet, the former still hard to categorize. In fact, between the two, what I think we can just call Comet Borisov seems almost pedestrian, with a composition so like comets in our own system as to suggest such objects are commonplace among the stars. Whereas to explain ‘Oumuamua as a comet, we have to stretch our definitions into bizarre objects of pure hydrogen (a theory that seems to have lost traction) or consider it a shard of a Pluto-like world made of nitrogen ice. We may never know exactly what it was.
The point of Andreas Hein and team was to show not just what might be capable with an all-out effort to catch ‘Oumuamua, but more important, to offer mission options for the next interstellar wanderer that makes its way through our system. Thus the implication for future interstellar activities is that we have the opportunity to study materials from another star long before we have the capability of putting human technologies near one. These objects become nearby, fast-moving destinations that form part of the morphology of our interstellar effort.
I use the term ‘morphology’ deliberately because of its dexterity. In linguistics, the study of a language’s morphology takes us deep into its internal structure and the process of word formation. In biology, the word refers to biological form and the arrangement of size, structure and constituent parts. Here I’m using it in a philosophical sense, to argue that we continually shape cultural expectations of exploration that govern what we are willing to attempt, and that doing this is an ongoing process that will decide whether or not we choose to move beyond Sol.
Going interstellar is a decision. It comes with no guarantees of success, but we know beyond doubt that only by learning what is possible and attempting it can we ever succeed.
It seems a good time to revisit an image of 2I/Borisov from the Hubble Space Telescope as we ponder strategies for future missions amidst these reflections. The instrument had been observing the comet since October of 2019, following its discovery by Crimean amateur astronomer Gennady Borisov in August of that year. The Hubble work revealed among other things the surprising fact that the comet turned out to be no more than about 975 meters across. This was unexpected, as David Jewitt (UCLA) explained at the time:
“Hubble gives us the best upper limit of the size of comet Borisov’s nucleus, which is the really important part of the comet. Surprisingly, our Hubble images show that its nucleus is more than 15 times smaller than earlier investigations suggested it might be. Our Hubble images show that the radius is smaller than half a kilometer. Knowing the size is potentially useful for beginning to estimate how common such objects may be in the solar system and our galaxy. Borisov is the first known interstellar comet, and we would like to learn how many others there are.”
All fodder for crafting mission concepts. The image below was taken in November of 2019. Here we have an interstellar interloper in our own system, framed along with the distant background spiral galaxy 2MASX J10500165-0152029. Notice the smearing of the galaxy image, a result of Hubble tracking the comet, which was at the time of image acquisition about 327 million kilometers from Earth. The blue color is artificial, used to draw out detail in the comet’s coma surrounding the nucleus (Credit: NASA, ESA and D. Jewitt (UCLA).
The immensity of the cosmos taunts us with our limitations, but in considering them, we choose directions for our thinking, our aspirations and our science. This image is emblematic. Out of the darkness comes something interstellar that we now believe is just one of many such objects open to investigation, and reachable by near-term technologies. A galaxy lies behind it. How far into our own galaxy can we push as our technologies morph into new capabilities?
Exploration is a decision. How far will we choose to go?
Reaching an Interstellar Interloper
The ongoing Interstellar Probe study at the Johns Hopkins University Applied Physics Laboratory reminds us of the great contribution of the Voyager spacecraft, but also of the need to develop their successors. Interstellar flight is a dazzling goal considered in the long term, but present technologies develop incrementally and missions to other stars are a multi-generational goal. But as we continue that essential effort with projects like Interstellar Probe, we can also make plans to explore objects from other stellar systems (ISOs) closer to home.
I refer of course to the appearance in the last three years of two such objects, 1I/’Oumuamua and 2I/Borisov, the ‘I’ in their names referencing the exciting fact that these are interstellar in nature, passing briefly through our system before moving on. Papers have begun to appear to examine missions to one or the other of these objects, or to plan how, with sufficiently early discovery, we could get a spacecraft to the next one. And keep in mind the ESA’s Comet Interceptor mission, which sets its sights on a long-period comet but could be used for an ISO.
Are missions to interstellar objects possible with near-term technology? A new paper from lead author Andreas Hein (Initiative for Interstellar Studies) and an international team of researchers answers the question in the affirmative. The paper characterizes such missions by the resources required to perform them, which in turn relates to the ISO’s trajectory. Unbound ISOs — those that pass through our system only once — can be contrasted with bound objects that have remained in the Solar System after their entry. If the ISO is unbound, a mission launched before perihelion would have the best chance of producing data and perhaps sample return.
Image: An artist’s impression of 2I/Borisov, an interstellar comet. Credit: NRAO/AUI/NSF, S. Dagnello.
In previous papers, Hein and team have considered chemical propulsion complemented by a reverse gravity assist at Jupiter and a Solar Oberth Maneuver to reach 1I/’Oumuamua, although they have also looked at thermal nuclear propulsion with gravity assist at Jupiter. Uncertainties in the object’s orbit are challenging but, the authors believe, surmountable through the use of a telescope like that of New Horizons (LORRI) or, a highly speculative idea, a swarm of chipsats that could be launched ahead of the probe to refine navigational data. This approach goes well beyond existing technology, though, as the authors acknowledge by citing the work on Breakthrough Starshot’s laser architecture, which is a long way from realization.
I’m also concerned about that notion of a Solar Oberth Maneuver, given what we’ve learned recently in connection with the research on Interstellar Probe, for the kind of spacecraft described here to intercept 1I/’Oumuamua would carry the needed upper stage kick engine, along with the heat-shield technology Interstellar Probe has been investigating. All this adds to mass. The authors believe Falcon Heavy (or, unlikely, a future SLS) would be up to the challenge, but I think the proposed Solar Oberth Maneuver at 6 solar radii is a problematic goal in the near-term.
The authors echo these sentiments in terms of the perihelion burn itself as well as the navigation issues to reach the ISO which will ensue. A propulsive burn at perihelion for a probe trying to intercept an interstellar object is a long way from proven technology, particularly when we’re hoping to deliver a substantial instrument package to the ISO for science return. The authors call for developing nuclear thermal propulsion in order to make a wider range of ISOs reachable without relying on the Oberth maneuver.
The paper usefully offers a taxonomy of interstellar objects, matched to their associated science and conceivable mission types. Objects with low inclinations, low hyperbolic escape velocity (v?), and those discovered well before perihelion are the most reachable targets. Of course, this survey of options for reaching an ISO isn’t intended to be specific to a given object but applicable to many, suggesting what is possible with present and near-term technologies. In the discussion of a mission to 1I/’Oumuamua, the authors also note the wide range of details that need to be considered:
Our brief analysis (and its attendant caveats) should not be regarded as exhaustive. Other issues that we have not delineated include the difficulties posed by long CCD exposure times (11 hours in our scenario) such as the cumulative impact of cosmic rays and the necessity of accounting for parallax motion of the object during this period. Obstacles with respect to measuring the position of the object, calculating offsets, and relaying it to the spacecraft may also arise. Hence, we acknowledge that there are significant (but not necessarily insurmountable) and outstanding challenges that are not tackled herein, as they fall outside the scope of this particular paper.
In any event, 1I/’Oumuamua may be quite a tricky object to catch at this juncture even for this kind of fast flyby. Objects detected earlier in their entry into our system should present a much more workable challenge, and with the Vera Rubin Observatory coming into play, we are probably going to be finding many more of them, some well before perihelion. Hence the need to know what is possible for future operations at ISOs, ensuring we have a plan and resources available to fly when we next have the opportunity.
A rendezvous mission may one day be in the cards, with the authors relying on electric or magnetic sail propulsion schemes to allow the spacecraft to slow down and study the target at close hand. But it may be more reasonable to consider rendezvous with captured interstellar objects in bound elliptical orbits. These are missions which are examined here in relation to two potential ISOs (not yet confirmed as such), (514107) Ka’epaoka’awela, a Jupiter co-orbital in retrograde orbit, and the Centaur 2008 KV42. The paper examines rendezvous strategies and provides trajectories for multiple years. 2008 KV42, for example, should be reachable for rendezvous with launch in 2029 and a flight duration of 15 years.
Finally, nuclear thermal technologies should allow sample return from some interstellar objects using a pre-positioned interceptor at the Sun/Earth L2 point. The paper considers an interceptor mission to comet C/2020 N1, serving as a surrogate for particular types of ISOs. The spacecraft, using nuclear thermal or solar electric propulsion, would deploy an impactor on approach to the object and travel through the plume, perhaps using swarm subprobes to return samples to the main craft depending on whether or not the plume is thought likely to be hazardous.
Even without nuclear thermal capability, though, missions can be flown to some types of interstellar objects with technologies that are currently in use. From the paper:
Our results indicate that most mission types elucidated herein, except for sample return, could be realized with existing technologies or modified versions of existing technologies, such as chemical propulsion and a Parker Solar Probe-type heat shield (Hein et al., 2019; Hibberd et al., 2020). Collisions with dust, gas, and cosmic rays and spacecraft charging in the interplanetary or interstellar medium will engender deflection of the spacecraft trajectory and cause material damage to it, but both effects are likely minimal even at high speeds (Hoang et al., 2017; Hoang & Loeb, 2017; Lingam & Loeb, 2020, 2021), and the former can be corrected by onboard thrusters.
So we learn that missions to interstellar objects are feasible, with some fast flyby scenarios capable of being accomplished with today’s technologies. Rendezvous and sample return missions await the maturation of solar electric and nuclear thermal propulsion. Here the concept ‘near-term’ is speculative. When will we have nuclear thermal engines available for this kind of mission? I am speaking in a practical sense — we know a great deal about nuclear thermal methods, but when will we deploy workable engines at a high enough Technology Readiness Level to use?
There is much we could learn from an ISO intercept, whether a flyby, a rendezvous or a sample return. Given that we are a long way from being able to sample interstellar objects in other stellar systems (I doubt seriously we’ll have this capability in a century’s time), ISOs represent our best bet to discover the structure and composition of extrasolar objects. This and the capability of doing interplanetary dust and plasma science along the way should be enough to keep such missions under active study as our new generation telescopes come online.
The paper is Hein et al., “Interstellar Now! Missions to Explore Nearby Interstellar Objects,” in press at Advances in Space Research (abstract / preprint).
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