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A Statite ‘Slingshot’ for Catching Interstellar Objects

I see that a white paper on Richard Linares’ interesting ‘statite’ concept became available just before Christmas, and I want to call your attention to it (and thanks to Antonio Tavani for the pointer). Back in April, the idea received funding as a Phase 1 study in the NASA Innovative Advanced Concepts (NIAC) Program, renewing attention on the matter of interstellar objects (ISO) like ‘Oumuamua. The notion is to deliver a payload to an object discovered entering our Solar System so that, unlike the two we’ve found thus far (‘Oumuamua and 2I/Borisov), we can examine them up close before they depart. Statites are to be the enabling technology.

So let’s circle back to that concept, for statites are getting more interest these days given the plans for Solar Cruiser, NASA’s solar sail mission that may experiment with maneuvers that allow it to act in non-Keplerian ways. The idea is that a solar sail can achieve ‘station-keeping’ — hovering in place — by using light pressure from the Sun to offset gravitational forces. That would allow what Robert Forward termed ‘displaced orbits.’ Forward envisioned what he called ‘polesitters,’ satellites that could, if positioned at high Earthly latitudes, offer telecommunications services through geosynchronous coverage of the polar regions.

Solar Cruiser could use these methods in its experiments to observe the Sun’s polar regions, though how much the statite idea would be explored during the mission is not yet determined. What Linares has in mind is the fact that it’s extraordinarily difficult to imagine a mission to catch an incoming interstellar asteroid or comet. Such objects show up suddenly and with high heliocentric velocities. A constellation of statites, however, located stationary in terms of the Sun, would make it possible to move quickly into the appropriate flyby or rendezvous trajectory. The payload can be quickly placed on a trajectory toward the Sun, using the sail and, perhaps, electric propulsion along the way for course adjustment.

Here’s how Linares and team describe the idea:

The proposed mission concept is to have a constellation of statites where: A) each statite enters into a stationary, “low-energy” state, which it can hold indefinitely, awaiting a potential ISO; B) once an ISO is detected, a flyby or rendezvous trajectory is calculated with an expected 4-16 months of lead time; C) a single statite releases a CubeSat which enters into a freefall trajectory with respect to the Sun or uses attitude control to orient the solar sail and adjust the solar radiation pressure force to accomplish a rendezvous; D) the CubeSat, which may use propulsion or the solar sail, then adjusts its trajectory for a flyby or rendezvous with the ISO; and E) onboard sensors are used to make critical scientific measurements.

Image: To ensure the best coverage of the Solar System, MIT’s Richard Linares envisions a constellation of “statites” that communicate and work together, activating the statite in the optimum position to fly by or rendezvous with an interstellar object. Other statites in the constellation can continue to wait for the next ISO to appear. Credit: Linares et al.

So we use solar photons and the momentum they impart to cancel the Sun’s gravitational force, enabling a constellation of statites that can hover in place indefinitely. The effort takes shape under the umbrella of what Linares describes as a “dynamic orbital slingshot for rendezvous with interstellar objects.” We learn from the white paper that the NIAC work will take in the design of a statite constellation and its configuration, as well as its capability for achieving rapid rendezvous. The potential of added solar electric propulsion will be considered as a way of improving performance, while scientific instruments and operational factors will be analyzed.

As statites must cancel out the gravitational acceleration caused by the Sun, the area-to-mass ratio of the sail is critical, for both the gravitational force and the propulsive force of the sail vary as 1/R2. Any statite can thus cancel the gravitational force as long as it has a suitable area-to-mass ratio. Linares and team assume the aluminized, temperature-resistant CP-1 — an ultra-thin reflective polymer — as the sail material.

And when an incoming interstellar object is detected? The levitating sail has an inertial velocity of zero, and when released from ‘hover,’ it enters a Keplerian orbit. In the example given in the paper, a statite at 1 AU has a free-fall trajectory of 64 days. The choice of the term ‘slingshot effect’ is dictated by the fact that the sail is essentially being used to store energy that is released when needed, with the free-fall path to solar flyby adjusted along the way. Linares’ simulations show effective rendezvous with our two previous interstellar visitors:

…a DeltaV of 104 km/s and 145 km/s are achieved for the ’Oumuamua and Borisov rendezvous trajectories, respectively. These high Delta-V values are possible due to the fact that the statite can cancel the solar gravity completely, independent of the distance to the Sun. Moreover, from these initial proof-of-concept results, we can see that the rendezvous times are relatively short for both of these notional missions. For ’Oumuamua, the rendezvous time is approximately 5 months and the rendezvous occurred at 0.76 AU. For Borisov, the rendezvous time was approximately 8 years and the rendezvous occurred at 30.6 AU. Although the Borisov trajectory had a long time of flight, it was still within typical mission timescales, while requiring neither large propulsion systems nor long lead times.

The paper argues that a statite free-falling toward the Sun from an initial position at 1 AU and then deploying its sail away from the Sun at perihelion can achieve speeds of up to 25 AU/year, making it possible to deliver payloads to the outer Solar System. Voyager 1 has reached 3.6 AU per year by comparison, making the statite concept attractive beyond its value as a station-keeper for quick response missions to interstellar comets/asteroids.

The paper is Linares et al., “Rendezvous Mission for Interstellar Objects Using a Solar Sail-based Statite Concept,” now available as a preprint. For more on Solar Cruiser, see Heliophysics with Interstellar Implications. See also Les Johnson’s analysis “The Solar Cruiser Mission Concept — Enabling New Vistas for Heliophysics,” Bulletin of the American Astronomical Society, Vol. 52, No. 3 (June, 2020). Abstract.

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{ 17 comments… add one }
  • Christopher L. Bennett January 5, 2021, 14:09

    A bunch of statites keeping station around the Sun… isn’t that basically a really, really diffuse Dyson sphere?

  • Alex Tolley January 5, 2021, 14:54

    It wasn’t until I understood the value of the “freefall” value of the statite that I realized the advantages compared to orbiting probes. It reminded me of a hawk hovering in the wind or thermals, then folding its wings to dive onto its prey below.

    For a statite, there are various options to “fold its wings” – deploy the sail, change sail orientation to edge on to the sun, or change the reflectivity of the sail (like IKAROS). My sense is that to get the best outward bound acceleration and velocity, the areal density should change from the 1.6 g/m^2 to a lower value. The best way might be to deploy a more effective sail area. I would favor making the sail design segmented so that segments could be independently orientated to allow this. This approach, perhaps looking like the COSMOS-1 design might also be used to spin the sail to aid initial sail deployment, then design it, and fine-tune the effective areal density for any orientation of the sail. Maximal effective sail area would be deployed after perihelion.

    The “Other Applications” section at the end is also instructive as the flight times once initiated from the statite’s final location seems very attractive, rather than planning specific missions many years ahead which is not suitable for objects of a more transient nature, or for erratic and capricious funding decisions.

    • Paul Gilster January 5, 2021, 16:29

      A hawk folding its wings… Great analogy!

  • Michael January 5, 2021, 15:04

    I suppose if we call a solar driven stationary satellite a statite we should call a statite held aloft by other stars light an exostatite.

  • Jens Fridthjof Knudsen January 5, 2021, 16:52

    Maybe this technique could be used to accelerate other deep space missions too?

  • Eniac January 5, 2021, 17:26

    It seems to me that this could be achieved almost as well with a highly elliptical orbit. Such objects spend most of their time moving slowly in the far part of the ellipse, with a very low delta-v difference from a statite. The advantage is you wouldn’t need a sail. The extra delta-v could go both ways, depending on the relative inclination of the two trajectories. With multiple craft, you can pick the best to make the “dive”, which would allow you to pick ones with a favorable delta-v in most cases, compared to a statite.

    • Alex Tolley January 5, 2021, 22:16

      A conventional rocket (chemical, nuclear, or electric) needs lots of delta-V to get into elliptical orbits, especially those out of the ecliptic. This is where solar sails excel. Even an elliptical orbit then needs a burn to kill the orbit or adjust it so that it makes a sun diver approach and a further burn to gain an Oberth effect. Whether this will give you the needed velocities is still somewhat doubtful.

      The solar sail approach with statite operation buys you:

      1. a low-cost method to reach almost any position in the celestial sphere and maintain it without fuel.
      2. A method to gain a high-velocity dive towards the sun without propellant and minimal power.
      3. A method to gain a larger acceleration after a close perihelion offering 100 km/s plus velocities – in any chosen trajectory.
      4. A good way to disperse the sailcraft around the celestial sphere making them ready at any time to create a matching trajectory to the transient object, especially comets and ISOs. They are almost “launch and forget” craft, ready to be used as soon as an object is detected.

      I can see that this approach might well be complemented by a desorption paint that is emitted at perihelion increasing the acceleration and velocity of the sail. (painted on the reverse side of the sail, so that reorientating the sail 180 degrees exposes the paint which will provide thrust when in the presence of a high heat source – ie the sun near perihelion. Once gone, the sail reorientates again to show the highly reflective side to the sun.)

      • Eniac January 7, 2021, 17:35

        I certainly think the statite idea is good, but the delta-v needed to reach a statite position is every bit as much as that needed for the corresponding highly elliptical orbit, precisely because the two are so similar (one is stationary at apogee, the other close to stationary). So, the only question here is whether that amount of delta-v is best achieved by sail or by rocket. Rocket is faster and simpler, sail could be cheaper. Note that the bulk of delta-v is used merely to reach the waiting orbit, and only exactly once. That’s a rocket situation, really. Sails are best when ongoing delta-v is needed. I’m not saying which method is superior, just that the highly elliptical orbit without sail should be considered as a baseline.

  • Henry Cordova January 5, 2021, 21:18

    It seems like a viable concept, although it occurs to me that to deploy and maintain a constellation of statites with the capability of intercepting a reasonable number of external visitors (remember, they can approach us from any direction in space) would be a complex and expensive proposition. Again, the question arises, its not so much as whether this is possible, but is it worthwhile? I don’t feel qualified to conduct a cost/benefit analysis, but my instincts tell me our resources could be better spent doing something else.

    I feel there is a future for the statite concept in solar exploration, with increasingly protected probes dropped deeper and deeper into the sun, each returning valuable additional data as improved future technologies
    gave each succeeding generation of ‘heliosonde’ the ability to penetrate closer and closer to the photosphere. This would not only be of value to the field of solar physics, it would have immense practical utility in detecting, understanding or even forecasting destructive solar flares or other solar eruptions. As more and more of our industrial and scientific facilities are placed in deep space, anything which sheds light on our sun’s potentially destructive activities would be great value.

    A highly reflective sail would not only be useful for maneuvering, it could also be folded up during the dive into a multilayered sun-shield to protect the sonde on its way into the corona.

    • Mike Serfas January 6, 2021, 12:06

      I tend to have the same reaction. I imagine NASA has a long list of projects that are 90+% likely to generate good data if they can be launched into interplanetary orbit, so a proposal to leave a satellite sitting and waiting for an unknown event seems unlikely to be funded.

      But … is there a way to combine this with some loftier and more fundable goal? I was wondering if optical VLBI may become feasible, as I see was asked and not that well answered here: https://astronomy.stackexchange.com/questions/29082/is-optical-vlbi-theoretically-feasible-if-not-why-not

      If you _could_ build optical VLBI telescopes recording data at many petabits per second, there are surely some long term observations for which it might help to remain at a statite position. An individual telescope could then move and begin independent observations if favorable circumstances arose.

  • George King January 6, 2021, 22:37

    NASA’s Innovative Advanced Concepts (NIAC) program granted a Phase II award back in 2019 to Grover Swartzlander with the Rochester Institute of Technology to further study a diffractive (as opposed to reflective) sail concept. See, e.g., https://www.jpl.nasa.gov/news/news.php?feature=7374

    Can’t recall on the fly seeing a discussion of that here or in the small bit of underlying papers referenced here that I’ve had time to review.

    Is the TRL of diffractive sail technology just that far behind reflective sail technology to not warrant consideration or discussion? In my review on the fly as a non-scientist, it appears that substantial materials related research and development remains for both reflective and diffractive sail materials to achieve the optimum performance level desired for the sundry photon sail missions being discussed. And diffractive sail materials seemed at least in theory to be potentially more tolerant of close perihelion approaches than reflective sails, which–if that proved to be the case–would seem to make diffractive sails highly desirable for missions employing sundiver type or other close perihelion trajectories.

    This is an interest of mine and not my bailiwick. But I am curious why diffractive sails are not also being discussed as a possibility, given that I at least understand that sail material does not appear to be a fully matured technology either way that you go.

    Anyway, would appreciate any thoughts/perspective on the diffractive sail alternative by those here for whom this is their bailiwick.

    • Paul Gilster January 7, 2021, 6:07

      George, we’ve looked at diffractive sails here and there over the years, going back to this larger article 10 years ago, which contains some background:

      https://www.centauri-dreams.org/2010/12/07/an-optical-lift-for-solar-sails/

      This is a reminder to me to get the more recent NIAC work into a new article; it’s certainly time. Dr. Swartzlander’s ideas are well circulated in the sail community and I find them promising.

      • George King January 7, 2021, 18:02

        Ah, thanks Paul. I remember that optical lift article now, and hadn’t put the two together in my memory as yet. I maybe should have just searched for Dr. Swartzlander’s name rather than using diffract as a root search term.

        This 4-30-2020 thesis by Amber Dubill at RIT, “Attitude Control for Circumnavigating the Sun with Diffractive Solar Sails,” touches on the overall fairly current status of the technology before focusing then on attitude control strategies. On my .pdf, what I believe are the math formulas don’t come through, but the specific math tends to be largely wasted on me anyway personally with my instead wordsmith background.

        https://scholarworks.rit.edu/cgi/viewcontent.cgi?article=11538&context=theses

        All of these photon sail discussions, including this one and the recent JPL discussion of using reflective sails to get to the gravitational focus line, tend to come back eventually to that need for more materials research and development.

        That appears to be a major sticking point vis-à-vis getting sail technology through that TRL valley that I recently heard of and into space specifically for some of these more advanced missions involving a close perihelion.

        Given the wide variety of otherwise practically infeasible potential missions that can be readily opened up by matured sail technology, of whatever stripe, hopefully one of the space agencies will be able to see their way through to pursuing/sponsoring the needed materials research fairly soon.

  • charlie January 6, 2021, 22:54

    While all this sounds very promising, I do wonder whether or not a significant payload is capable of being delivered to what ever object that you are attempting to reach that makes this all worthwhile. Am I right, or am I wrong that the benefit behind this concept lies in the fact that you must have the lightest payload possible to permit station keeping and being situated in a readiness state at any moment to make a sun dive?

    Finally, I wonder about the question as to how long it will take to fall from your orbit to sun dive our star and then how long will it take to reach the interstellar intruder such that a meaningful rendezvous can be accomplished?

    • Paul Gilster January 7, 2021, 6:01

      “For ’Oumuamua, the rendezvous time is approximately 5 months and the rendezvous occurred at 0.76 AU. For Borisov, the rendezvous time was approximately 8 years and the rendezvous occurred at 30.6 AU. Although the Borisov trajectory had a long time of flight, it was still within typical mission timescales, while requiring neither large propulsion systems nor long lead times.”

  • AlexTru January 9, 2021, 5:59

    Statite’s idea seams to be very attractive from engineering point of view, in same time – the idea to build and use statites fleet for ISO catching seams to be useless and expesive, i.e. non-sense…
    Suppose for ISO studying it will be much better to send to ISO something that is used for asteroid visiting present time (Haybusa asteriod etc.)
    And for remote imaging of the asteroid like objects that moving in Solar System, probably some type of MW Radars, LIDARs or asteroid illumination by powerful laser and taking direct image can be more useful , accounting the fact that we do not know nothing faster than light…

  • Michael Fidler January 13, 2021, 16:56

    Another Interstellar Comet?

    Newly found Comet Leonard might become 2021’s brightest.

    Posted by Eddie Irizarry in ASTRONOMY ESSENTIALS | SPACE | January 13, 2021
    The first comet to be found in 2021 – labeled C/2021 A1 (Leonard) – might become the brightest comet of this year! Charts and more info here.

    “Nature provides us with sky events seen once in a lifetime. Comet Leonard might be one of these, as it seems to have a hyperbolic orbit, that is, an orbit that’ll carry it only once through the inner solar system, then out again into the depths of space.

    In other words, after this current sweep close past our sun, Comet Leonard won’t be seen again from Earth.”

    https://earthsky.org/astronomy-essentials/bright-comet-c-2021-a1-leonard-2021-2022

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