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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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{ 47 comments… add one }
  • Alex Tolley October 8, 2021, 11:35

    For all the missions proposed, it does seem that a craft already prepared to fly, or loiter in a suitable orbit, is required. Spending time to develop a mission is just not going to work.

    The high delta v needed for these missions seems to require NTR for both the needed delta v and the high thrust if an Oberth maneuver is required. Despite the suggestion that a zero-loss LH2 tank be available, this might in practice prove difficult. I wonder if a more storable propellant can be used, and take the lower Isp hit?

    Forr missions that do not require high thrust, then the usee of sails or electric engines seems more appropriate. Both technologies can allow for long loiter times to await a suitable target, and ideally these can be made in volume and placed in a variety of orbits to ensure at least one can do the mission whatever the target’s trajectory.

    Despite the optimism of the 1960s for advanced nuclear and electric propulsion, it is a shame that propulsion technology has largely stagnated. Chemical rocket technology is akin to optimizing carriages and horse teams, while the internal combustion and electric engines are still being tinkered with in garages. We really need a good business case for advanced propulsion to drive its development with far more resources so that we achieve a number of competing approaches and designs with mature technology that is being used. Then we could live in an age when papers like this need not speculate on propulsion needs, but rather select the best “off the shelf” options to satisfy time and cost constraints.

    • Andreas Hein October 9, 2021, 5:47

      Dear Alex,
      Appreciate your detailed and insightful comments as always.

      I certainly agree that the DeltaV requirements are relatively high (10-20 km/s) for e.g. ‘Oumuamua.

      Using storable propellants is a good point, which we did not consider in detail for NTP.

      Our point of view is certainly that the Oberth maneuver is less difficult to perform than the recent results from the Interstellar Probe team suggest, which we feel are based on overly conservative assumptions. Nevertheless, we found that there are even options without the Oberth maneuver, which would allow for missions to ISOs years after their passage, using a combination of flybys and a deep space maneuver. More in “Project Lyra: Catching 1I/’Oumuamua – Mission Opportunities After 2024”
      https://arxiv.org/abs/1902.04935

      Thanks for mentioning the “ready-to-launch” and loitering options. Laia Lopez Llobet from ISU has looked into this trade-off in more detail in her ISU Individual Project, which we co-supervised. There is no straightforward answer to whether to prefer build-and-wait missions (spacecraft stored on Earth and launched when a suitable ISO comes across) and loitering. This is a more complex trade-off, which depends on a variety of factors such as propellant storability, spacecraft reliability, likelihood of ISO approaching at the “right” place.

      • Antonio October 11, 2021, 8:13

        Hello Andreas,

        Since you are here… do you know what happened to the Icarus Interstellar project? The website has been down for a month or so. Is the project dead? Will there be a final report?

    • Jeff Wright October 10, 2021, 0:44

      That means SLS…which itself costs about the same as a whole Titan IV mission and so is reasonable. Musk doesn’t do hydrogen. Everyone wants to put Boeing down…but a Block 2 with nuclear thermal and a nuclear electric payload will get you a probe that is more than foil or wires.

      Even all chemical Block 1B will get you interstellar in 15 years.

      • Alex Tolley October 10, 2021, 12:49

        If by “Musk doesn’t do hydrogen” you mean that the NTR uses LH2 as propellant to get the Isp, then unless you have a really good cryogenic storage system that effectively eliminates losses due to boil off, then any NTR will need to launch and be on its way quickly – no long term loiter is possible to have the vehicle in place. But let me remind you that the US ICBM program abandoned liquid fuel launches because of the difficulty in maintaining readiness and opted for solid boosters instead (e.g. Minuteman series).

        When considering approaches to ISO interception as the Hein paper does well, one should recognize that technology is always improving, especially in miniaturization. It has enabled us to launch Cubesats that can do quite a lot that once required a much larger chassis. Miniaturization reduces teh need to throw mass, and hence launcher size. It was miniaturization of warheads that allowed teh US to reduced the size of its launches during the early years of the cold war, and made those launchers unsuitable for human spaceflight. The Russians, unable to do the same, had their much heavier launchers that could be adapted for their first Vostok flights and have continued with improvements to the basic launcher to the present. The SLS may be the best launcher to throw a large payload into interplanetary space today, but is it going to be needed in future? (I won’t further argue about the system cost as we had that discussion elsewhere.)

        We cannot miniatize people, so there will always be a need for man-rated launchers for human spaceflight. But newer technologies might be a lot better, smaller, and cheaper for robotic probes. It is those technologies that I think will win out to do the sort of needed surveillance and interception of bodies moving through our system.

        • Jeff Wright October 11, 2021, 3:23

          Miniaturization also allowed underperforming Delta II sounding rockets as I call them to be JPL’s crutch addiction.

          JIMO type craft can dispense cubesats and release hem…but for radars—you need power beyond just RTGs.

          • Alex Tolley October 11, 2021, 14:42

            Businesses are comptitive. Lower costs offer an advantage. Technology changes drive lower costs, which in turn allows for new uses (markets) . Market expansion feeds on itself to create new markets and technologies.
            Whether to use mature technology or a new, low-cost, but “good enough” technology was the meme of Clayton Christensen’s 1997 book: The Innovator’s Dilemma: When New Technologies Cause Great Firms to Fail. Cheaper, good enough, undercuts the more expensive, full featured product, eventually displacing it as the product improves, pushing the existing product into the high-end niche.This was seen most clearly in IT, where once giants like IBM limp along trying to convince companies that there is still a role for mainframes and expensive custom software, while hardware increasingly is dominated by mobile devices and edge computing, and ubiquitoous, cheap software eats the world (according to Marc Andreesen).

            Space technology has been slower to achieve cost advantages. Does anyone seriously question the importance of the cost of space access as THE MOST IMPORTANT driver of expanding space exploration and industrialization? The SLS may represent the equivalent of the mainframe computer. It may be the last word in reliable, heavy lift vehicles, but its cost versus the reusable launchers will fate it to become a niche option should it ever fly.

            One day the SLS will become an exhibit at the Smithsonian, with a placard explaining it was the last of the giant ELVs. People will marvel that we ever built such wasteful machines, and none will depart for their homes in space on such a monster.

            • Antonio October 11, 2021, 18:17

              I don’t think the problem with SLS is the technology (or at least not the main problem). I’m sure the NASA of the 70’s could do it much much cheaply (not at SpaceX’s level, but much much cheaply than current NASA nevertheless).

              • Alex Tolley October 11, 2021, 21:09

                It is almost as if the Golgafrincham B Ark crew all ended up employed at NASA. ;)

                • ljk October 12, 2021, 13:42

                  NASA is becoming IBM: They were the unrivaled kings of their realm back in the day, but now smaller, faster, and cheaper upstart companies are taking over the market. I hope NASA knows how to adapt and change like IBM so it doesn’t disappear like so many other big computer companies did. It is already starting to look like a subsidiary of SpaceX.

                  I don’t want NASA to disappear, but they really have to push to start looking like a space agency of the 21st Century and stop resting on their laurels, especially the Apollo era now that there are real plans for a manned base or two there by the end of this decade.

                  The SLS is doing nothing to help NASA’s image. They haven’t even launched a single test mission yet and it probably won’t happen until 2022, barring any other problems. Partnering with Boeing and its perpetually faltering Starliner isn’t helping their image, either.

            • Jeff Wright October 13, 2021, 3:07

              It ISN’T wasteful. It is worth it. I can see Block 2 with NTRs and an NEP payload catching up with Ouamuamua. Smaller craft perhaps drilling pieces out for return via tether sling back…the spine as gauss gun…etc…beyond Musk’s bodging. SLS is the only reason Clipper exists.

  • FrankH October 8, 2021, 14:26

    In terms of an Oberth maneuver – a semi-facetious suggestion would be to use the same “technology” used successfully by ʻOumuamua – if artificial, a large solar sail or if natural, a large block of liquid nitrogen (or water) to protect the spacecraft.

    Why re-invent the wheel when Nature or aliens have given us a working example?

    • Andreas Hein October 9, 2021, 5:52

      Dear Frank,

      Thank you for your comment. This is an interesting thought if one could use an abundant source of liquid nitrogen or water in space. My concern is that the mass of such a shield would be much higher than the one used for the Parker Solar Probe.

      But high-temperature solar sails are, in principle, a possibility. One issue with solar sails is that they are difficult to control and it will be difficult to steer them precisely enough during an Oberth maneuver to fly to the desired target.

    • Alex Tolley October 10, 2021, 13:03

      Perhaps not so facetious. While we know that fluffy comets often break up as they approach the sun, perhaps ice layers and highly reflective foils might provide the needed protection, albeit at the cost of a lot of extra mass. It might be interesting to compute the tradeoffs between a close solar Oberth maneuver with such a vehicle vs one with more conventional shielding further out vs a solar sail with no shielding even further out. Clearly, there will be a mass cost to decelerate the craft to do the sun diver orbit, as well as the mass cost to accelerate the shielding at perihelion which will still be subliming pre-and post- approach. Is ice even optimal, or would a carbon-based ablative material be better?

      [I will say that I found the prior post’s paper convincing that a Jupiter assist was likely a better option than a close solar Oberth maneuver, but it did assume near term technology rather than considering other possible options.]

      • Patient Observer October 13, 2021, 20:16

        My heart nearly skipped a beat when you mentioned “fluffy comets” as such bodies were supposedly detected and allegedly bombard the earth in sufficient quantity to account for the world’s oceans over billions of years

        https://www.pas.rochester.edu/~blackman/ast104/smallcomets.html

        http://smallcomets.physics.uiowa.edu/

        If true, the moon’s surface should be a little damp. In any event, the fluffy snowball comet theory seems to have faded into the past to join poly water. For me, a sad loss.

  • ljk October 8, 2021, 16:37

    Can we build the equivalent of Breakthrough Starshot Lite? Since it won’t have to be pushed all the way to Alpha Centauri, could we use a less powerful – and therefore less complicated and less expensive – laser to move the sails?

    The probe could be equipped with all those little Sprites rather than some singular big probe, which I assume could be constructed faster and cheaper. Plus this could be good practice for the Alpha Centauri mission.

    I agree with Alex Tolley we are at the stage where we should have probes ready to go since we now know that interstellar objects are coming through the Sol system. These would also be good for anything else celestial that might surprise us. Do we need to go to Elon Musk first again to make this happen?

    https://lweb.cfa.harvard.edu/~loeb/Loeb_Starshot.pdf

    • Andreas Hein October 9, 2021, 5:57

      Thank you for this great point, ljk!

      We actually looked into this and proposed a Starshot-light for catching ISOs: Hibberd, A., & Hein, A. M. (2020). Project Lyra: Catching 1I/’Oumuamua–Using Laser Sailcraft in 2030. arXiv preprint arXiv:2006.03891. https://arxiv.org/ftp/arxiv/papers/2006/2006.03891.pdf
      The results are fantastic! We could fly to ‘Oumuamua in 2030 within little over a year and much quicker, within days or months to other potential ISOs. Such a system would definitely revolutionize the exploration of such bodies.

      • Michael October 11, 2021, 13:44

        A second ejectable sail would allows us to enter into orbit around them as well.

        • Alex Tolley October 11, 2021, 14:23

          If you are referencing Robert Forward’s sail design, then a third sail could even return a sample.

  • Henry Cordova October 8, 2021, 16:57

    A landing, rendezvous or orbital mission seems highly unlikely. Even if we are capable of reaching the required velocities using advanced propulsion tech or clever, intricate orbital ballets and slingshot maneuvers, it is unlikely we’ll have enough time to put a mission together to encounter this object successfully unless we’ve seen it coming years in advance. My suspicion is it will come in out of the dark and fly through the inner solar system in a highly inclined hyperbolic ballistic at velocities on the order of solar escape velocity–at the very least. And it may be moving much, much faster than that!

    But if everything comes together just right, it may be possible to hastily improvise a flyby, or even a high-speed collision, with the object by commandeering some other spacecraft already being prepared for another, different mission, or even already en route to another mission elsewhere in the solar system. Unfortunately, it would mean abandoning that mission and hastily assigning another and sending a sensor package totally unsuited for its new objective to greet the interloper.

    It might be worth the sacrifice if it is truly an interstellar visitor. Still, it is highly unlikely everything will line up just right, and there will certainly not be enough fuel left over to bring our probe home. This will be no Rendezvous with Rama, it will be strictly a one-way trip. If a human crew is involved, it will be a suicide mission.

    • Andreas Hein October 9, 2021, 6:04

      Dear Henry, thank you for your comment! We would be more optimistic regarding the time available for putting together a mission. We have shown for the example of ‘Oumuamua that even for such a fast object, we would have years, if not decades to prepare a mission. It depends on the acceptable trade-off between the time necessary for the preparation and the mission duration / detectability. We were ourselves surprised that the potential launch window is so large.

      • Henry Cordova October 9, 2021, 13:07

        I can see that the potential launch window might be quite large, but would this not also depend on knowing way in advance when the extrasolar object was coming, as well as the precise elements of its orbit? In addition, using gravity assist encounters with other solar system objects to help speed up our probe might also involve waiting long periods of time for all the planets to line up–I doubt our visitor would be cooperating with our schedule.

        We can always get lucky, I suppose, and we may have an existing mission in preparation which can be commandeered and hastily modified for the intercept. But we would need a sophisticated set of sensors to patrol the skies for years ahead of time and detect these objects a long way off, and we would require an active presence in space with multiple probes in various stages of preparation so that there would be a probability one of them might prove suitable. Having dedicated high velocity probes on standby would be ideal, but would it be justifiable financially? Perhaps a system-wide space technology and science/exploration/settlement program could provide the infrastructure, but we’re nowhere near that now.

        The intruder could be approaching from any direction, at any speed, and its path could be any of an infinite number of possible orbits. Once detected, we could certainly compute its orbital elements and possible gravitational perturbations while in our system, We could probably quickly determine if a rendezvous was within our technical capabilities, but whether we’ll have enough time and money to get ready for it is something else again.

        This is especially the case for the REALLY interesting objects, the ones that our moving VERY fast.

        I would suggest that we instead spend our resources on detection systems for such objects, especially if that detection capability can be piggy-backed onto some other research problem (perhaps a planetary defense sensor net) with a higher probability of success. If it turns out these interstellar interlopers are common, and their visits frequent, then perhaps investing in an intercept program
        might be worthwhile.

  • Jens Fridthjof Knudsen October 8, 2021, 18:38

    Is it possible to use C14/diamond batteries in place of solar panels and RTGs? https://ndb.technology/

    • Alex Tru October 9, 2021, 4:40

      I suppose this (ndb) technology will be limited to very low power applications, as potential replacement of Lithium oxide cells, no more.

    • Andrei October 9, 2021, 11:03

      Diamond Nuclear Voltaic batteries could add a bit of useful electricity while those also function as heaters to keep instruments on a spacecraft in a nominal temperature range.
      So I am a bit more optimistic than Alex here that they with clever engineering might find a place in some space missions.

      • Alex Tru October 12, 2021, 3:48

        Andrei, as well as I know NDB technology is not about heating – it is about electricity, it is not RTG…

  • gbaikie October 8, 2021, 22:35

    “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.”

    Falcon heavy with it’s current second stage does not do as much payload as the Atlas V-541 to Jupiter
    To LEO or GTO it does better any other rocket currently flying.
    SLS is planned to have a better second stage which planned to fly in 2026:
    https://en.wikipedia.org/wiki/Exploration_Upper_Stage
    And that should a lot more than any other rocket- but just that stage adds 800 million to launch cost. wiki {above}:
    “Due to the possible cost of EUS (about US$800 million each), NASA invited proposals for alternatives, but in May 2019 rejected Blue Origin’s proposal. NASA ordered eight EUSs from Boeing.”

    The only significant about SpaceX in regards such mission is the planned refueling of starship in LEO and how cheap Starship could be.
    Or if Falcon Heavy gets better second stage.

    Also, when we explore the Moon and determine whether there mineable lunar water, then one could get refueling depots in Lunar orbit.
    [And/or Gateway might serve as rocket fuel depot}
    If you can refuel in lunar orbit, you can return to a low perigee with high elliptical earth orbit, and with that higher velocity near earth, get a Oberth effect, which could get going faster and with more payload than compared to any rocket launched from Earth surface.

    • Andreas Hein October 10, 2021, 14:39

      Thank you for pointing at the cost of the EUS upper stage. We did not go that much into the details of cost, as the focus was more on the near-term technical feasibility. But cost is obviously an important aspect to cover in future studies.

    • gbaikie October 10, 2021, 16:42

      Oh, I am relying on other people [and NASA’s} calculations, and it seems falcon heavy could lift a Centaur stage. And put the Centaur stage into highly elliptical orbit and when centaur stage and payload return to perigee, then lit it and go into some solar escape trajectory. Thereby allowing Centaur stage to get an Oberth effect [which should amount to a couple km/sec to it’s total delta-v {as compared burning at lower velocity far from Earth}].

  • Patient Observer October 9, 2021, 10:42

    Still uncertain as to whether it will actually fly but Russia is moving forward on a nuclear electric rocket energized by a 1.0 megawatt (electrical) nuclear power plant. Per the link below, it would be placed in orbit by the Angara V which implies a weight of around 20,000 kg. It is purported to have 18 N thrust which, if I did the math correctly, will impart an acceleration of abou4 450 k/h initially with increasing acceleration as the reaction mass is used up. Could not find the empty weight of the rocket so a terminal velocity could not be estimated. But, just as speculation, assuming a 50% load of reaction mass and the stated ISP of 7,000, the terminal velocity would be around 48 k/sec. The payload would probably be relatively small but there would be huge amounts of power for instrumentation and communications.

    Cooling schemes mentioned include tube-fin and liquid droplet radiator (the later would be awesome). The droplet concept has been tested on the ISS with purportedly good results.

    https://en.wikipedia.org/wiki/TEM_(nuclear_propulsion)

    https://russianspaceweb.com/tem.html

    • Alex Tolley October 9, 2021, 12:23

      Whether or not this craft ever flies, the demonstration of a 1MWe space nuclear reactor would be proof of concept for a high thrust VASIMR electric propulsion system. The claim that such reactors are unfeasible has been used to shoot down VASIMR as a viable high thrust, high-Isp engine. A working version even just demonstrated on earth, would be a boost for developing a craft with such an engine. I await more information on specs and performance if it materializes.

    • Andreas Hein October 10, 2021, 14:36

      Very interesting and using a VASIMR type engine plus nuclear reactor could greatly facilitate missions to ISOs. We did not consider it in the article but it would be an interesting analysis to conduct.

  • Patrick Underwood October 9, 2021, 13:43

    Would the probe’s trajectory be called the Hein Line?

    • Paul Gilster October 9, 2021, 20:39

      Love it!

    • Andreas Hein October 10, 2021, 13:23

      Dear Patrick, I had a good laugh! :D Adam Hibberd did the trajectory analysis and Hibberd line would be more appropriate but then, of course, the joke no longer works.

  • Robin Datta October 9, 2021, 23:54

    It is perhaps general consensus that at least a few meteorites on Earth are of interstellar origin. Developing methods to find and identify them may be rewarding. While some solar system swarms are well-recognzed, the distances covered by interstellar wayfarers is likely to separate them from their companions. Even so, they may be paths through the galaxy where they are more frequent; speculative trajectories to landfall on earth and a quest guided by such “educated guesses” could be fruitful.

    • Alex Tolley October 10, 2021, 12:51

      Even better on the Moon, as they will be far more pristine buried in teh regolith after millions/billions of years.

    • Andreas Hein October 10, 2021, 13:27

      Dear Robin,
      Excellent point and Avi Loeb has published a paper along these lines: https://arxiv.org/abs/1904.07224
      Regarding frequent paths, Marshall Eubanks has recently authored a paper on galactic streams, which deal with streams of interstellar objects in the galaxy: “High-Drag Interstellar Objects And Galactic Dynamical Streams” https://arxiv.org/abs/1903.09496
      Eubanks, T. M., Schneider, J., Hein, A. M., Hibberd, A., & Kennedy, R. (2020). Exobodies in Our Back Yard: Science from Missions to Nearby Interstellar Objects. arXiv preprint arXiv:2007.12480.

  • Mike Serfas October 11, 2021, 7:08

    Is it feasible to *listen* for fast-moving objects in space? I know that sounds ridiculous, but we’ve heard some remarkable recordings of plasma vibrations from Saturn and other locales. If we had a probe at the heliopause, where there is a build-up of nearly stationary charged particles from the solar wind, using the most sensitive instruments we can conceive, how far away could it detect incoming objects?

  • wdk October 12, 2021, 11:19

    In the discussions of trades that have run from beamed nano-spacecraft such as Starshot, to large platforms supported by electric thruster
    propulsion, there would appear to be considerable variance in payloads.
    The obvious issue here would be one of sensors to determine characteristics of an ISO. For example, how about a flashlight?
    Or for that matter, the equivalent in any bandwidth.
    In the Oort Cloud, there is not much illumination. And illumination
    requires a power budget. Perhaps something like a photo-flash from a capacitor. But that still poses a problem of how to re-charge.
    The bigger the platform, the more such features are built in.
    As for the case of Starshot, at the very least, we would expect the stellar target to provide the background lighting. That would be lacking on
    an ISO intercept mission.

    To date, our knowledge of exoplanets is increasing faster with exo-telescopes rather than space probes. The imbalance there is so great that we wouldn’t know where to point interstellar space probes were it not for improvements in observatories locally. I don’t have any idea of how we can build a better ISO observatory on the ground or space, save that we look for deep space comets with eccentricities greater than 1. But we should put space observatories for such on the table for trade too. That might make the inner solar system reception party planning easier.

    • Alex Tolley October 12, 2021, 17:52

      Ways to extract the light needed is to use long exposures, greater aperture for light collection, even potentially, photomultipliers (this last requires a little power). I am sure one can “see” even the darkest objects using these techniques.

      IDK about terrestrial telescopes, but I assume good all-sky survey ‘scopes coupled with bigger small-field telescopes is the required approach. An expert would need to detail the current gaps in our approach. If anything, we need better ways to analyze ISOs from the ground so that more can be examined as soon as they are detected, with sufficient instruments to characterize them well. The ‘Oumuamua case shows how poorly we were able to observe it allowing all sorts of different interpretations of the object.

  • ljk October 12, 2021, 14:13

    Speaking of space rock interlopers, should one of them be headed at Earth with no obvious application of any brakes, we may have the option of nuking them successfully:

    https://room.eu.com/news/disrupting-earth-bound-asteroids-with-nukes-is-very-effective-new-study-says

  • Patient Observer October 13, 2021, 21:04

    IIRC, Arthur C. Clarke in”Rendezvous with Rama” described the detonation of a nuclear device (somewhere beyond Pluto’s orbit) encased in a material that would be excited to emit copious amounts of microwave radiation. Detectors would catch microwaves reflected off of innumerable objects thus mapping of the Kuipper belt and Ort cloud. I may have extrapolated a little but the basic idea is clever and quite compelling.

    • Mike Serfas October 14, 2021, 16:51

      Wouldn’t it be simpler to scan slowly and continuously with a bright narrow beam of microwaves from Earth, tracking with one good telescope to see what bounces back?

    • ljk October 14, 2021, 17:33

      Patient Observer, that was actually in Clarke’s science fiction novel from 1993, The Hammer of God. However I can understand the error since both novels involved SPACEGUARD.

      The nuclear detonation was originally designed to detect every sizable object in the Sol system with its pulse. As a result, the explosion was eventually detected by some distant ETI, who sent back a “ping” of sorts containing no discernable information. Basically it was just an initial “Hello” as the detonation contained no actual information itself.

      The father of the Soviet hydrogen bomb, Andrei Sakharov, had an idea to attract others in the Milky Way galaxy by detonating nuclear bombs at the edge of our Sol system. I have little doubt Clarke was influenced by Sakharov’s concept, which he described in 1971:

      http://lnfm1.sai.msu.ru/SETI/eng/articles/sakharov.html

      The audio version of the novel here:

      https://www.youtube.com/watch?v=QBshwX2CIPY

      The original short story online here:

      http://content.time.com/time/subscriber/article/0,33009,976752,00.html

      • Patient Observer October 15, 2021, 15:19

        Thanks for the correction and the new information!

  • ljk October 14, 2021, 17:37

    http://spaceref.com/comets/to-watch-a-comet-form-a-spacecraft-could-tag-along-for-a-journey-toward-the-sun.html

    To Watch A Comet Form, A Spacecraft Could Tag Along For A Journey Toward The Sun

    Press Release – Source: University of Chicago

    Posted October 13, 2021 11:35 PM

    Deep in the solar system, between Jupiter and Neptune, lurk thousands of small chunks of ice and rock.

    Occasionally, one of them will bump into Jupiter’s orbit, get caught and flung into the inner solar system–towards the sun, and us.

    This is thought to be the source of many of the comets that eventually pass Earth. A new study lays out the dynamics of this little-understood system. Among the findings: it would be doable for a spacecraft to fly to Jupiter, wait in Jupiter’s orbit until one of these objects gets caught in the planet’s gravity well, and hitch a ride with the object to watch it become a comet in real time.

    “This would be an amazing opportunity to see a pristine comet ‘turn on’ for the first time,” said Darryl Seligman, a postdoctoral researcher with the University of Chicago and corresponding author of the paper, which is accepted to The Planetary Science Journal. “It would yield a treasure trove of information about how comets move and why, how the solar system formed, and even how Earth-like planets form.”

    Thanks in part to discoveries of several major asteroid belts, scientists over the last 50 years have revamped their theories of how our solar system came to be. Rather than big planets quietly evolving in place, they now envision a system that was much more dynamic and unstable–chunks of ice and rock scattered and smashing into each other, re-forming and moving around within the solar system.

    Many of these objects eventually coalesced into the eight major planets, but others remain loose and scattered in several regions of space. “These minor bodies show you the solar system is actually this very dynamic and almost living place that’s constantly in a state of flux,” said Seligman.

    Scientists are very familiar with the asteroid belt near Mars, as well as the larger one out past Neptune called the Kuiper belt. But between Jupiter and Neptune, there lurks another, lesser-known population of objects called the centaurs (named after the mythical hybrid creatures due to their classification halfway between asteroids and comets).

    Occasionally, these centaurs will get sucked into the inner solar system and become comets. “These objects are very old, containing ice from the early days of the solar system that has never been melted,” said Seligman. “When an object gets closer to the sun, the ice sublimates and produces these beautiful long tails.

    “Therefore comets are interesting not only because they’re beautiful; they give you a way to probe the chemical composition of things from the distant solar system.”

    In this study, scientists examined the centaur population and the mechanisms by which these objects occasionally become comets bound for the sun. They estimate that about half of the centaurs-turned-comets are nudged into the inner solar system by interacting with both Jupiter and Saturn’s orbits. The other half come too close to Jupiter, then get caught in its orbit and flung toward the center of the solar system.

    The latter mechanism suggested a perfect way to get a better look at these soon-to-be comets: Space agencies, the scientists said, could send a spacecraft to Jupiter and have it sit in orbit until a centaur bumps into Jupiter’s orbit. Then the spacecraft could hitch a ride alongside the centaur as it heads toward the sun, taking measurements all the way as it transforms into a comet.

    This is a beautiful but destructive process: A comet’s beautiful tail is produced as its ice burns off as the temperature rises. The ice in comets is made up of different kinds of molecules and gases, which each start to burn up at different points along the way to the sun. By taking measurements of that tail, a spacecraft could learn what the comet was made up of. “You could figure out where typical comet ices turn on, and also what the detailed internal structure of what a comet is, which you have very little hope of figuring out from ground-based telescopes,” Seligman said.

    Meanwhile, the surface of the comet erupts as it heats up, creating pockmarks and craters. “Charting all of this would help you understand the dynamics of the solar system, which is important for things like understanding how to form Earth-like planets in solar systems,” he said.

    While the idea sounds complicated, NASA and other space agencies already have the technology to pull it off, the scientists said. Spacecraft routinely go to the outer solar system; NASA’s Juno mission, currently taking wild photos of Jupiter, only took about five years to get there. Other recent missions also show that it’s possible to visit objects even as they’re moving: OSIRIS-REx visited an asteroid 200 million miles away, and Japan’s Hayabusa 2 spacecraft brought back a handful of rocks from another asteroid.

    There’s even a possible target: A year and a half ago, scientists discovered that one of the centaurs, called LD2, will likely be sucked into Jupiter’s orbit in about the year 2063. And as telescopes become more powerful, scientists may soon discover many more of these objects, Seligman said: “It’s very possible there would be 10 additional targets in the next 40 years, any of which would be attainable by a spacecraft parked at Jupiter.”

    Moreover, Seligman said, “We have records of comets dating back thousands of years; how cool would it be to see how that happens up close?”

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