Reaching ‘Oumuamua through some kind of statite technology, an idea we’ve been kicking around recently, brings up the interesting work of Richard Linares at MIT, who has been working on a “dynamic orbital slingshot” for rendezvous with future objects from the interstellar depths (ISOs). Linares received a Phase I grant from the NASA Innovative Advanced Concepts (NIAC) Program to pursue the idea of a network of statites on sentry duty, any one of which could release the stored energy of the sail to enter a trajectory that would take it to a flyby of an object entering our system on a hyperbolic orbit.
The concept is simplicity itself, once you realize that a statite balances the pressure of solar photons against the Sun’s gravitational pull, and essentially hovers in place. As I mentioned when covering Greg Matloff and Les Johnson’s paper on using statites to achieve fast rectilinear trajectories to reach interstellar interlopers, Robert Forward was the one who came up with the idea and practical uses for it. He could envision, for example, communications satellites in polar position to cover high latitudes on Earth.
Here’s what Forward said about the statite concept in his wonderful essay collection Indistinguishable from Magic (1995):
…I have the patent on it — U.S. Patent 5,183,225 “Statite: Spacecraft That Utilizes Light Pressure and Method of Use”… The unique concept described in the patent is to attach a television broadcast or weather surveillance spacecraft to a large highly reflective lightsail, and place the spacecraft over the polar regions of the Earth with the sail tilted so the light pressure from the sunlight reflecting off the lightsail is exactly equal and opposite to the gravity pull of the Earth.
You can see why we need a new term here. If you deploy a sail in the configuration Forward describes, it essentially sits over the polar region while the Earth rotates below it. In other words, technically it is not a satellite. ‘Statite’ is a Forward coinage to describe such a hovering object in space. He wrote of a statite he dubbed the ‘Hovering Hawke’ in one of his short stories. It would be placed too far from the surface to be effective as a communications satellite, but could offer direct broadcasting to places on Earth that are without that capability. Weather surveillance is another use.
Polesitters become interesting when we consider the nature of a geostationary orbit. Put a satellite directly over the equator at 35,786 kilometers altitude and it will appear stationary over the Earth, a useful trait for communications. But the satellite must be positioned directly above the equator, matching Earth’s rotation, to maintain its position relative to the surface.
If we put our satellite at an angle relative to the equator, its apparent motion on Earth will be a figure eight, in what is called an inclined geosynchronous (not geostationary) orbit. That’s useful for areas not covered by geostationary satellites but not good enough for continuous coverage of a specific area, especially the more latitudinally challenged regions like the poles, and that’s why the polesitter is attractive. It can give us continuous coverage even when the region it sits above is far from the equator.
Image: Analog‘s December, 1990 issue contained an article by Robert Forward describing the ‘polesitter’ concept, one of many innovative ideas the scientist introduced to a broad audience. Credit: Condé Nast.
There’s always a catch, and here’s the catch with polesitters, as Forward explained it in his article. When the summer months arrive and the polar regions are in sunlight, keeping the statite precisely balanced (to maintain the hover) becomes quite tricky. He saw that such seasonal instability demanded that a statite be relatively far from Earth, and calculated that it cannot get any closer than 250 Earth radii to the surface.
But Linares and team are not thinking about statites supplying services to Earth. The NIAC work explores using statites to set up an early warning system for interstellar objects, one that will allow fast intercepts before these interlopers blow through our system and return to interstellar space. Consider what happens when we ‘turn off’ the statite capability on our satellite (as from rotating the sail to an edge-on position, for example, or simply releasing a CubeSat). At this point, the released object has no forces impinging upon it but gravity. Let me quote Linares from a white paper on the subject:
…a statite at 1 AU has a free-fall trajectory of about 64 days. This fast response time to a potential ISO can be thought of as a slingshot effect, since the solar sail is used to “store energy” that is released when desired. Additionally, to achieve a flyby some Delta-V is required to adjust from the free-fall path to a flyby trajectory. The proposed mission for the statite concept is to utilize a constellation of such devices to achieve wider coverage over a spherical region of 1 AU for potential ISO missions. Additionally, the orbital plane can be adjusted with relatively low Delta-V.
Image; A constellation of statites as envisioned by Richard Linares for intercepting a future interstellar interloper. Credit: MIT/Richard Linares.
The levitating sail has an inertial velocity of zero, and when released from ‘hover,’ it enters a Keplerian orbit. So as Linares points out, we can turn any one of the statites in our constellation of statites into a ‘sundiver,’ hurtling toward the Sun before its trajectory is adjusted by use of the sail (or perhaps other propulsion). Which statite is deployed simply depends upon the optimum trajectory to the incoming ISO.
We are now on a fast track toward reaching the interstellar object with at least a flyby. Linares calls this a “dynamic orbital slingshot for rendezvous with interstellar objects.” And the idea is to have a constellation of these statites always at the ready for the next ‘Oumuamua. Or, considering how odd ‘Oumuamua seems to be, perhaps I should say “the next Borisov.” Even so, with this net, who knows what we might catch?
The paper makes the case 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. Now we’re in Matloff/Johnson ‘sundiver’ territory. 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.
For more on Richard Linares’ work, see “Rendezvous Mission for Interstellar Objects Using a Solar Sail-based Statite Concept,” a white paper available on arXiv.
I suspect the electric drive could work as well or maybe better as the electron gun can be adjusted very quickly.
https://electric-sailing.fi/
Just came off a several week exercise looking at application of Lambert’s theorem to orbital maneuvers. While this does involve impulsive maneuvers at launch and rendezvous, it still has some application to this problem. If a continuous burn ( such as solar or nuclear electric) method is actually applied someday, it is not necessarily a waste to use the Lambert method for analysis. In fact, over the distances and times of flight involved, finite burns might look impulsive relatively speaking. Additionally, had we examined a deep space mission decades ago, our mission spacecraft might have resembled Pioneer or Voyager, vehicles of hundreds of kilograms mass. Quite possibly a micro-sat or multi-unit cubesat could be built in the near term, which would allow more focus on impulsive maneuvers whether chemical, nuclear or electric “approximation”.
Basically, if you have two radius vectors in space, there are families of conic sections that connect them. The 180 degree transfer is a divide by zero case, however. Other angles have families of solutions addressed in various textbooks (E.g., Battin’s Astronautical Guidance and Kaplan’s Modern Spacecraft Dynamics and Control). The family of conic paths have one thing in common: they have a difference in true anomaly angles, but the two values are not explicitly defined. But beside the radii r1 and r2, the chord length c between their tips and the general ( e.g., not-necessarily right) angle solution, the analyst has an infinite number of solutions, even though they are limited to certain spaces.
For the case of a hyperbolic passage, to apply this method with a change of velocity, we might assume that the start is at some arbitrary 1 AU origin and the hyperbolic object is either entering the solar system or leaving.
For example, to get on an arc from the Earth’s circular path, you could anticipate a
a radial as well as an out plane component of velocity. Substantially less of an angle however in a hyperbolic catchup.
But although this system of search will work, it should be noted that the object from interstellar space is NOT necessarily on the Earth’s ecliptic plane. Likely there is an intermediate transfer plane that needs to be adopted for the transfer and then shift to the “comet” plane at a specific intercept time.
So, likely the time of flight and the delta velocity trades will drive the solution to a rendezvous condition. Ellipses might work in the inner solar system, but distant rendezvous going out will likely need a hyperbolic solution for catch up.
Since appearance of a hyperbolic comet could be quite sudden, I can well imagine that rendezvous or inspection spacecraft would have to be on orbit in standby for quite some time. But I suspect that relatively impulsive transfer systems should not be thrown out of the trades. They might be even easier to keep on alert and then deploy and respond.
IUIC, the statite will fall towards the sun until it reaches its perihelion velocity, and then the redeployment of the statite configuration will allow it to maintain this velocity to rendezvous. This seems very similar to the Matloff & Jonson idea except that initial deployment is at 1 AU, and then a sundiver maneuver is made to reach an even higher velocity than the velocity if deployed as a statite at the same orbital distance as the perihelion.
As the sail cannot slow down (other than by reducing its thrust so that gravity slows it down,) it needs to match the velocity of the ISO at the rendezvous point, which I assume determines the perihelion and the statite selected.
As before, we need sail material that, in concert with a payload, can achieve the needed areal density. I hope there is a path to this, and it is not like “When can we have a fusion or anti-matter drive?” The Johnson presentation in 2017 Solar Sail Propulsion: A Roadmap from Today’s Technology to Interstellar Sailships indicates that the needed statite areal density of 1.6 g/m^2 is 2 orders of magnitude lower than sails at that time e.g. Lightsail and NEA Scout.
If we cannot develop the needed sail material, perhaps we can use a beamed sail instead without a sundiver maneuver. A sail orbing the Earth or Moon could be given the needed velocity by a laser that would place the sail into the required trajectory and rendezvous velocity. If the laser array is on Earth, then the greatest delay is 24 hours. If the sails deeply only when needed, it avoids the concern of astronomers over polluting the night sky with light or occultations. Whether a ground-based phased laser array is a sensible investment remains to be seen.
Sails definitely evoke in me the same feelings as sailing ships, and weather balloons, using other means than combustion to achieve their performance. However, their manufacture and deployment has proven spotty so far. It took decades to even get a working solar sail to orbit, but at least there are people working on developing the technology. I wonder if sails need to be constructed in space as Eric Drexler once proposed, to achieve the low areal densities we want.
From the paper:
Can someone explain what this means?
I really don’t know, but here’s a possibility…
The sun’s gravitational acceleration at 1 AU is 5.92 x 10^-3 m/s^2
At that linear acceleration for 1 year (3.16 x 10^7 s) you would be traveling at
187 x 10^3 m/s. I don’t know just what the physical significance of that result is, but the number seems about right. Here’s where I got my data..
https://www.grc.nasa.gov/www/k-12/Numbers/Math/Mathematical_Thinking/sunlight_exerts_pressure.htm
https://space.stackexchange.com/questions/36834/what-gravity-does-the-sun-affect-items-with-at-the-distance-of-1-au
Just for comparison, the tangential velocity for any object orbiting the Sun at a 1 AU distance is about 30 km/s. If our statite were to suddenly luff its sail into the solar photons it would take off tangentially to its orbit at that speed, in a direction at right angles to the Sun-statite axis. At 1 AU, the solar escape velocity is 42 km/sec. Again, I have no idea what that 187 km/s number is all about.
Please ignore my Dec 24 post. I obviously didn’t think it through! Clearly, without the solar photon pressure, a statite would just fall straight into the Sun. By definition, statites have no orbital or tangential velocity. I have no idea what I was thinking when I wrote that.
Merry Christmas
hc
I’m still confused. If the statite is hovering above one of Earth’s poles, then isn’t it orbiting the sun as the Earth is? If it folds the sail, why doesn’t it fall toward the Earth rather than the sun?
The way I understand it,
the statite hovering over the earth’s pole is a special case, one relevant to polar communications orbits, say bringing cable tv to to the tundra. It would behave as you suggest. The use of sailing statites to take advantage of the speed boost for solar missions can be placed in any solar orbit, as required. They are placed in solar orbits far from any planet. The 1 AU station is just employed to give you a reasonable example. A typical 1 AU place to park one of these would be far from the earth, but near earth’s orbit (say at the L4 or L5 Lagrangian points).
Exactly so, Henry. At least that’s my understanding of Linares’ concept as compared to Forward’s.
Henry
It’s a potential energy equation converted to kinetic energy, the escape velocity to infinity from the sun is around 600km/s so there plenty of energy available if you have time and distance from the sun. That’s why a laser system on earth could be so powerful by dropping a spacecraft from earth orbit towards the sun.
The solar escape velocity is 618 km/s from its SURFACE. At 1 AU its 42.1 km/sec.
https://en.wikipedia.org/wiki/List_of_artificial_objects_leaving_the_Solar_System#:~:text=The%20initial%20speed%20required%20to,a%20distance%20of%20100%20AU.
Agreed it is 42km/s but as you drop towards the sun your velocity increases substantially. The parker probe reaches around 200km/s on closest approach and experience 475 times the light force at earths orbit. With that much sunlight the boost on the way out is quite large, that’s the way i understand it. I doubt a light weight probe could survive those harsh conditions though and would be better further out.
Perhaps a solar Parker type probe with a ribbon like sail trailing behind it, at closest approach the sun shield blows apart sideways exposing the ribbon sail to the full force of the sun’s rays. I am thinking of a folded ribbon design like a parachute in which the sunlight opens up the ribbon sail to its full extent.
When taught swimming in the US Navy, the instructor advised: “Don’t try to fight the water; the water will ultimatery win 100% of the time. Work with the water and let the water help you.”
Using statites to stay and to go seems to be a basic step in learning to work with nature. Maybe that is how advanced civilizations might ride the currents in spacetime and matter-energy, with minimal disruptions of all these rendering them inconspicuous.
I am not sure about your analogy of moving in water to statutes. If anything, the more natural “go with the flow” is an orbit. Statites require a knife-edge balance to maintain position, which in practice probably requires “periodic” changes in balancing solar pressure and gravity rather like a thermostat controls internal temperatures. This would be entirely unnecessary for a sail or other probe in orbit that then makes a similar maneuver. For example, a sail that was undeployed in orbit could be deployed on demand, and execute the maneuver to cancel its orbital velocity. How much time would that take to cancel the orbital velocity at 1 AU. Does the sail need to be capable of being a statite? Would it be better to have sails with areal densities even lower than needed?
Could a statite be part of a big radiometer?
Cut cables when you see something of interest…ends of a radiometer the either side of a libration point?
Tethersats
https://centauri-dreams.org/2023/12/22/a-novel-strategy-for-catching-up-to-an-interstellar-object/
In the past, I have suggested lightsails on cables attached to a body such that the cables intersect in an X where the intersection can be raised of lowered by tacking.
That could spin up a flying windlass to pull up a third cable/payload.
Gaudi used strings not drawings for his cathedral after all.
Sorry here is the proper link
https://www.eurekalert.org/news-releases/1006752
Certainly not water and statites. Rather maximizing use of buoyancy vs. weight; and inertia & momentum vs. gravity.
A statite at 1 AU is not in orbit, hence the Earth (from which it is launched) is moving at 30 Km/sec relative to the desired position. How do they propose to get it there? The solar sail? How long would it take to get there?
I think that having something that can deliver a fast response time to any unexpected thing we wish to check-out, or smash-into, is a very good tool for mankind to have.
It does seem as though this would require an awful lot of statites in a rough sphere around the Sun in order to provide full coverage — since interstellar objects can come in from any direction. At some point the cost of launching them all would exceed the cost of keeping an earth-launched spacecraft ready to just chase it directly. WIth reusable heavy boosters available, such a mission wouldn’t have to be scheduled years in advance.
Although we would need a few probes get a big coverage it may not be as many as we think as we can still use the sunlight to tac on the way in and out and change direction somewhat.
That’s good to know
Once again, it appears someone hasn’t thought this through.
On first look, this statite/sundiver concept seems technically feasible. The concept could be employed if we are presented with a mission that requires the dynamics it can generate. It also makes sense to have one of these set up and primed, ready to let loose if we need to unleash a high speed probe out of the solar system at some time in the future, on a moment’s notice.
Having a “constellation” of these on standby so we have the capability to chase down extrasolar interlopers regardless of where on the celestial sphere they come from may very well be technically within our grasp. But what about the expense of preparing this fleet, and maintaining it once it is in place? The resources required to carry out this program could probably be better spent on missions with specific objectives and which can exploit future known planetary alignments.
Space exploration, regardless of our technical capabilities, is going to be very expensive, perhaps even dangerous. A rigorous cost/benefit analysis will have to be carried out for every mission. Another factor is long-range planning; we should favor multiple-destination missions over single use flights, and they should all be coordinated as parts of a larger program so that subsequent explorations can build upon the findings of previous ones. Piggy-backing multiple experiments on each launch should also be taken advantage of wherever possible.
These considerations are further complicated by the time it takes to conceive of and then prepare each voyage, not to mention the time it will take the spacecraft to get there. All the time, technology can be expected to improve, and favorable future windows for orbital configurations will arise which can be taken advantage of.
In other words, a meaningful space program must be coordinated, and it must be managed. Otherwise, we can expect a waste of resources and a duplication of effort which can only be detrimental to our future in space. These considerations are, I believe, more important than simply taking advantage of a particular mission profile that suddenly becomes technically available.
I am intrigued by this idea of statites, and I look forward to applications of it where the benefits can be seen to far outweigh the costs. But I also feel we should be spending more time and effort discussing the strategies of our space exploration, and less on the tactics of individual missions and technologies. Perhaps this philosophy may be a bit premature at our present level of technical capability and societal commitment, but we should still keep it in mind.
I fear that space exploration may be abandoned as a societal activity because some short-sighted government, or public opinion, or even genuine need will dictate our resources should be spent closer to home. The best way to avoid this is to take care now that our resources are used efficiently. Somewhere out there is the technical breakthrough or great discovery that will change EVERYTHING. We need to keep the program going until that happens.
That sentiment is quite close to the complaint that the NSF spends far too much of its resources on minor knowledge-seeking and far too little on “big ideas” that could truly change our knowledge for the fraction that pan out.
My complaint about scientific space exploration is that it is unnecessarily expensive as each new probe is custom=built, and as a one-off, built with a lot of redundancy. I would like to see low-cost, off-the-shelf hardware and software that works out-of-the-box and need not be put through expensive design, construction, and testing before launch. It is the only way to do missions like this statite idea without incurring truly massive costs.
In this particular example, there is no mention of how many statutes would be desirable to be sure that a new ISO could be intercepted. Neither is the method of eliminating the orbital velocity, although that is self-evident. What is ignored is that a 1 AU position will result in the Earth sweeping up all the sails in a coplanar orbit within a year, so they must either be in non-coplanar positions and/or at a creature or lesser distance from the sun. (However, I think these are minor considerations).
I do think we need more papers comparing the pros and cons of each method and between different methods to achieve the same missions. If a beamed sail was ready to be launched quickly from Earth or LEO/MEO/GEO would that work better? What about other more conventional propulsion techniques?
Paul,
Almost all ANALOG number are here free : https://readitfree.org/SF/AN_2.htm
…except the one you mention :) Too bad, but Merry Christmas anyway !
Fred
And to you as well, my friend.
About 20 comments ago, opened up with some feedback that was a little off-topic and intuitive. But I suspect that design dialogs have to start off in such a manner anyway while participants formulate their thoughts about a new concept.
Still mulling but still a devil’s advocate. So here goes:
Right now our database for high eccentricity comets comes to about two. The first was quite extraordinary and the second appears to be more conventional. Extrapolating the nature of future comets from a sample of two…? Well, intuition would suggest that most would be similar to other comets, perhaps jarred in the Oort Cloud into a higher energy orbit.
But the Oort Cloud is something we don’t see; it is more like an explanation for a supply of falling matter beyond the range of the Kuiper Belt. We presume it has a mass density and a large population of individual objects. Correspondingly we would expect that neighboring stars have similar regions. And maybe by now, what with zodiacal dust observations of some, there might more evidence out there for Oort Clouds around other stars, Literature is vast and developments are rapid too.
But since we cannot see the Oort Cloud very well – or at all – that suggests that a new telescope might help.
Currently, we have two remarkable instruments that come close, but need another to get some overlap of their capabilities. The JWST has high resolution and near infrared. When it comes to moons of Neptune and rings, it seems to do all right. But it has not turned up Oort Cloud objects to my knowledge.
The Gaia telescope doing stellar astrometry of billions has excellent mapping capabilities in the visual band and tracked over 100,000 asteroids. – but they are not as cold as Oort Cloud objects or interstellar comets…
So we really need a deep infrared or microwave scope mapping the sky for cold dark objects headed this way. That would give us more warning or preparation time for sending out flybys or rendezvous craft. Rendezvous if it looks intrinsically interesting. Flyby for the sake of routine science.
Going back to the Statite vs. more conventional means to address these missions, in summary I would argue on the basis of cost and technical readiness. For example, a multi-stage solid rocket motor system could stand by at a Lagrangian point awaiting a go ahead for a comet flyby. The terminal stage would need more flexibility, but the first phase of flight could be addressed with storable propellants such as solids. If the payload is small and the accounting starts on orbit, there could be a couple dozen kilometers per second change of velocity without going to an extremely minimized payload. Six unit cube sats are already employed in interplanetary missions. The observatory described would give the low tech response considerably more warning.
But the observatory portion of this approach should go ahead for reasons beyond the hyperbolic comet issue. At this point, as explained above, the Oort Cloud is not so much explored as offered as an explanation. If it exists, then it should have a signature as a host of small cold objects with temperatures around 20 Kelvin. Some of them moving around erratically based on previous passages… Which might throw light on other phenomena.
Merry Christmas and Happy New Year to everyone. Another wonderful year of stories and commentaries has nearly passed. Here’s to a better 2024 full of new discoveries and great steps forward in space.