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The 1000 AU Target

One reason I wanted to run yesterday’s article about the Opher et al. paper on the heliosphere, aside from its innate scientific interest (and it is a very solid, well crafted piece of work) is to illustrate how much we still have to learn about the balloon-like bubble carved out by the solar wind. The entire Solar System fits within it easily, but we observe only from inside and have little knowledge of its structure. None of the paper’s authors would argue that we have the definitive answer on the shape of the heliosphere. That will take a good deal more data, as the paper notes:

Future remote-sensing and in situ measurements will be able to test the reality of a rounder heliosphere. In Fig. 6, we show our prediction for the interstellar magnetic field ahead of the heliosphere at V2. In addition, future missions such as the Interstellar Mapping and Acceleration Probe will return ENA [energetic neutral atom] maps at higher energies than present missions and so will be able to explore ENAs coming from deep into the heliospheric tail. Thus, further exploration of the global structure of the heliosphere will be forthcoming and will put our model to the test.

We’ll learn more from the Voyagers, in other words, as well as from IMAP (more about this one in a later article), New Horizons, and whatever probe we next send out to system’s edge. Our two Voyager spacecraft may well last another seven years, which would give them 50 years of data return since their launch in 1977.

Image: And here’s something we’ve learned from New Horizons. The SWAP instrument aboard the spacecraft has confirmed that the solar wind slows as it travels farther from the Sun. This schematic of the heliosphere shows the solar wind begins slowing at approximately 4 AU radial distance from the Sun and continues to slow as it moves toward the outer solar system and picks up interstellar material. Current extrapolations reveal the termination shock may currently be closer than found by the Voyager spacecraft. However, increasing solar activity will soon expand the heliosphere and push the termination shock farther out, possibly to the 84-94 AU range encountered by the Voyager spacecraft. Credit: Southwest Research Institute; background artist rendering by NASA and Adler Planetarium.

The interstellar probe NASA has been contemplating, under study at various centers but most visibly at the Johns Hopkins Applied Physics Laboratory (APL) would, unlike Voyager, be built from the start with a 50 year goal in mind. Voyager 1 is now about 141 AU from Earth (21.2 billion kilometers). Interstellar Probe (APL capitalizes its design) would go for 1000 AU, but at much improved speeds, reaching the distance in 50 years.

How to do this? For one thing, achieve a boost from one of the huge rockets now coming onto the market, perhaps NASA’s own Space Launch System (SLS), or a commercial entry from a private company, perhaps SpaceX or Blue Origin. We’re not talking about launching until 2030, and that’s assuming the mission gets the green light in the upcoming heliophysics decadal survey, which will put in place missions related to the Sun over a ten year period.

A gravity assist at Jupiter added on to its kick from a massive booster would put us in familiar territory, given Jupiter’s history of flinging spacecraft like Voyager and New Horizons on their way, but a solar gravity assist is also contemplated, one that would take Interstellar Probe a good deal closer to the Sun than the Parker Solar Probe. You’d think closer is better, but at this stage in our technology, the perihelion numbers will be decided by factoring the weight of the required heat shielding. A balancing act ensues to get the most bang for the buck.

Exactly which instruments would fly on this modern era Voyager Plus would depend upon how instrument packages can be combined to save mass while maximizing power and data rates on the communications side. If you have a look at the APL page devoted to Interstellar Probe, you’ll see a notional payload, meaning this is what we’d like to cover with an ideal probe. The instrumentation includes:

A particle and fields suite for exploring the interstellar medium and its interaction with the heliosphere, with detectors such as:

  • energetic neutral atom (ENA) camera
  • energetic particles/cosmic rays
  • solar /interstellar plasma and neutral wind
  • vector helium magnetometer
  • plasma wave

Beyond the particle and fields instrumentation, the probe should include:

  • Optical cameras for flyby imaging and astrometry
  • A suite to measure dust and its basic composition
  • Infrared cameras for obtaining the 3D distribution of dust beyond our planetary neighborhood

We know that Voyager 1 and 2 have both left the heliosphere, Voyager 1 in August of 2012 and Voyager 2 in November of 2018, the two craft on widely divergent trajectories (recall Voyager 1’s dogleg at Saturn to get a look at Titan, whereas Voyager 2 moved on for close passes at Uranus and Neptune). Yesterday’s paper offered a new proposal for the shape of the heliosphere which is rather interesting in this regard. If the heliosphere really is more circular, lacking that presumed cometary ‘tail,’ then getting outside it won’t necessarily be determined by what would have been considered the shortest route, avoiding a tail that was estimated to trail thousands of AU. Here astrophysics and engineering work together in the choice of optimum trajectories.

Yesterday we looked at the need to get beyond the heliosphere so we could study its structure and gain insights into other planetary systems. But there are other reasons that take us much farther afield. It’s worth remembering that within the heliosphere, we have to contend with the foreground infrared radiation from dust within the Solar System, known as the zodiacal cloud. Going beyond the heliosphere opens up the possibility of studying diffuse infrared radiation from other stars and galaxies that has been effectively blocked for us by that cloud.

We also get a look at the nature of the dust disk, one that we can observe around other stars but are unable to measure in terms of large-scale structure from within our own. Learning how the Sun affects the structure of the heliosphere will help us understand the dynamics of other stellar systems, and the data a probe like this will take will be crucial at defining the local interstellar medium, through which our much longer-range probes will eventually move.

Needless to say, a great deal of science can be accomplished along the way. Interstellar Probe would reach the Kuiper belt in a scant four years, where flybys of KBOs and long-range observation of the environment there would complement and extend what we are learning from New Horizons. The APL trade study is designed to craft “a realistic mission architecture that includes available (or soon-to-be available) launch vehicles, kick stages, operations concepts and reliability standards.” All of this produces the reference materials that will be needed for the science and technology definition team that will turn aspirations into hard designs.

We should always be thinking about the kinds of mission that might one day fly, the long-range improvements that can enable them, and the audacious targets we someday want to reach. But as we draw up these conjectures and think about eventually engineering them, we also must be thinking about the kind of missions that can fly today. An interstellar probe of the kind now under study at APL and other NASA centers was a part of the discussion for the last decadal survey, but only now are we reaching a technological level to make 1000 AU in 50 years possible.

We need these early steps to make the broader strides that will occur later, on a path toward a Solar System infrastructure that will eventually support probes into the Oort Cloud and one day beyond. So tracking the fortunes of Interstellar Probe will be a priority for Centauri Dreams in coming months.


{ 32 comments… add one }
  • Ron S. March 24, 2020, 14:58

    “Interstellar Probe (APL capitalizes its design) would go for 1000 AU, but at much improved speeds, reaching the distance in 50 years.”

    I see that the APL site is a little bit lean on detail at the moment. However this objective requires a launch velocity well over 100 km/s with a payload that includes all those experiments, nuclear electricity generator and large communication power and dish. That’s a tall order without significant breakthroughs of some kind and those cannot be scheduled.

    • Ashley Baldwin March 28, 2020, 11:35

      More detail ? The APL team published a recent comprehensive update in “Acta Astronautica ” – Full free text :

      ‘Near-term interstellar probe: First step’ , McNutt et al , Sept 2019

  • Dave Moore March 24, 2020, 16:01

    I have a few quick thoughts about this probe. It would seem to me that the best approach to propulsion would be a modest nuclear reactor (50-100 Kw) with an ion drive rather than stacks of chemical rocketry.

    The Astrometry could be really important for getting a very accurate distance measurement of nearby Cepheid variables. These are the basis for measuring the size of the Universe, and a really accurate measurement would help in refining the various Cosmological theories.

    Also, don’t forget gravitational lensing. Even if no specific observations are possible, proving out the concept would be very useful.

    • Andrei March 24, 2020, 17:52

      This is obviously a new proposal of TAU, but it is good.
      I agree with Dave Moore, NEP seem the most promising way to go.
      If it use the letters AEPS, NEXT, LiLFA, DUFF (beer) or KRUSTY (clown) can be decided later.
      Even if the reactor plus coolant system weight in at several hundred of kilos it’s still a weight saver compared to several tens of tons of upper stage conventional fuel. Yet the probe need power, both for systems, perhaps warming of some components instead of nuclear heaters as on the rovers.
      As a benefit then could have a powerful transmitter and save a bit mass on the dish by making the dish a part of the excess heat radiator.

  • Michael Fidler March 25, 2020, 10:22

    Laser communication would make for a much smaller and less energy intensive device. One instrument that could make many discoveries is a compact metalens microscope and an electron microscope. This in order to look closely at the dust that the craft encounters and the possibility of panspermia {viruses}. The Heliosphere may act as barrier to such material whereas interstellar space may have huge quantities of viruses. The long period comets and interstellar comets would be in this deep freeze for long periods before coming close to our sun and may bring in new types.

    Dr. Chandra Wickramasinghe makes a case for dust protecting microorganisms and viruses in space. When the earth was forming in the dust clouds of the infant solar system huge amounts of this material was brought in from comets. This is how life originated on earth from the cometary goo of small organisms and viruses.

    So imaging and chemical analysis of the dust and other material in the Heliosphere and beyond to interstellar space may help find where we originated from and just how common life may be in the universe.


    • Antonio March 25, 2020, 14:04

      Laser pointing would be very difficult at those distances. You would need a decent telescope to point the laser to Earth, but that would be too heavy for this kind of mission.

    • Antonio March 25, 2020, 14:30

      Adding to my previous reply, Earth at 1000 AU would have a diameter of 55 mas. Using Dawe’s limit formula, you would need a telescope of at least 2 meters only to know where to point the laser.

      • Michael Fidler March 25, 2020, 22:34

        There is an upgrade to the Australian tracking and communication radio telescope that include a optical mirror inside the radio dish. This would mean the spacecraft would need a similar setup to align the mirror for the laser. We would still have a large radio dish but the laser and small mirror would increase the data rate.

        • Antonio March 26, 2020, 9:59

          A radio dish in the probe doesn’t solve the problem. You would need a really huge radio dish to resolve the 55 mas of Earth (around 4,000 km diameter if I’m not mistaken in the computation).

        • Michael Fidler March 26, 2020, 10:01

          NASA prepares for Moon, Mars with new addition to Deep Space Network.
          Correction; it’s at NASA’s Deep Space Network complex in Goldstone, Calif.

          While DSS-23 will function as a radio antenna, it will also be equipped with mirrors and a special receiver for lasers beamed from distant spacecraft. This technology is critical for sending astronauts to places like Mars. Humans there will need to communicate with Earth more than NASA’s robotic explorers do, and a Mars base, with its life support systems and equipment, would buzz with data that needs to be monitored.

          “Lasers can increase your data rate from Mars by about 10 times what you get from radio,” said Suzanne Dodd, director of the Interplanetary Network, the organization that manages the DSN. “Our hope is that providing a platform for optical communications will encourage other space explorers to experiment with lasers on future missions.”



    • Robin Datta March 26, 2020, 17:55

      Viruses are metabolically inactive, lacking adequate molecular machinery. They hijack cellular machinery to serve their ends. For this they each have very specific requirements; not every cell type will do. To find an appropriate cell type on another world would be virtually impossible.

      Microbes are metabolically active, but such activity can be completely shut down by a deep freeze.

      • Michael Fidler March 26, 2020, 22:48

        We still have a lot to learn about viruses and they are part of life’s evolution then panspermia evolution on other world may be very similar to earth’s.

        Giant Viruses.

        Frozen Giant Virus Still Infectious After 30,000 Years.

        Newly discovered giant viruses have ‘the most complete translational apparatus of known virosphere’.

        Giant virus diversity and host interactions through global metagenomics.

        Hints of Life’s Start Found in a Giant Virus.
        Newly discovered specimens support a more ancient origin for viruses, perhaps all the way back to the origins of life.

        New giant virus may help scientists better understand the emergence of complex life.
        Large DNA virus that helps scientists understand the origins of DNA replication and the evolution of complex life.
        Virologists have discovered a giant virus that, much like the mythical monster Medusa, can turn almost amoeba to a stone-like cyst.


        • Alexander Tolley March 27, 2020, 13:14

          The issue with viruses as the original life forms is that they have no metabolisms. Hence the RNA/DNA codes they contain are useless without the means to replicate them, and part of that replication requires energy that must be acquired by metabolism, which is I think, Robin’s ultimate point.

          While DNA can act as a simple replication template, RNA cannot, as it folds up. RNA may act as a catalyst (hence the RNA World idea) but this appears limited although it might well be a start. But it is the creation of proteins that really gets life going, and viruses have protein coats (although the coronaviruses are far more complex).

          Viruses are clearly co-evolutionary, but I find it very difficult to think they predated life on Earth.

      • Michael Fidler March 27, 2020, 4:04

        Space weather and pandemic warnings?

        “There have been many claims that the occurrence of pandemic
        influenza and other viral outbreaks is correlated with the well-known 11-year sunspot cycle(1–3), although the precise mechanism for such a causative connection had remained unclear. Now, with space exploration and continuous monitoring of ‘space weather’, it is evident that the Earth’s magnetosphere, and the interplanetary magnetic field in its vicinity, are modulated by the solar wind that in turn controls the flow of charged particles onto the Earth. During times of sunspot minima, particularly deep sunspot minima, a general weakening of magnetic field occurs which would be accompanied by an increase in the flux of cosmic rays (GCR’s) and also of electrically charged interstellar and interplanetary dust particles. As there is growing evidence to suggest that the latter include biological entities, an increase in their incidence on the Earth is therefore to be expected at such times. Not only CR induced mutation events, but recombination events involving novel virion strains, would be the expected outcome of which we should be aware.
        The next minimum between the current cycle 24 and cycle 25 was predicted to occur between July 2019 and September 2020 (Figure 1). Perhaps, we have now approached the deepest sunspot minimum for a century, with more ‘spotless’ days per week than in previous minima. On the basis of this data, there appears
        to be a prima facie case for expecting new viral strains to emerge over the coming months and so it would be prudent for Public Health Authorities the world over to be vigilant and prepared for any necessary action. We need hardly to be reminded that the spectre of the 1918 devastating influenza pandemic stares us
        in the face from across a century.




        This was printed on November 25, 2019 and is legitimate.

        • Alexander Tolley March 27, 2020, 13:36

          “There have been many claims that the occurrence of pandemic
          influenza and other viral outbreaks is correlated with the well-known 11-year sunspot cycle(1–3)…

          The most recent reference is below. Yeung’s analysis shows that pandemics occur in both the peaks and troughs of the sunspot cycle. Because there are so many cycles compared to actual outbreaks, any prediction is akin to the famous “The Stock Market Has Predicted Nine Of The Past Five Recessions” (a joke that means that there are far more recession predictions than recessions.)

          Yeung JWK. A hypothesis: sunspot cycles may detect
          pandemic influenza A in 1700–2000 A.D. Med. Hypothesis

          Wickramasinghe has beaten this dead horse over his career and has singularly failed to show any concrete evidence for the hypothesis. Rather like UFO sightings and LGMs.

          • Michael Fidler March 27, 2020, 22:24

            You are smart but I think your logic is like this:

            Anyway, this would be a good instrument to send out on any probes to look for viruses or microorganisms, especially going t0 the plumes of Europa and Enceladus.

            Lab on a chip: Developing a tiny, super-resolution optical microscope.

            EU funded Research Project ChipScope: Overcoming the Limits of Diffraction with Super-Resolution Lighting on a Chip.

            • Alexander Tolley March 28, 2020, 12:49

              As someone who has just bought their 3rd lab microscope, I would welcome miniaturized scopes. There have been several novel approaches to cheap scopes, some made of paper and similar to Van Leeuvenhoek’s original microscope. I recall MIT had a design that seems familiar in concept to the ChipScope. I have used “toy” scopes that can magnify 20-100x when used with a computer, but I would love a design that would work with the screen of a smartphone so that images could be displayed and captured. Some years ago I bought a piece of kit so that I could display an image from a microscope to a lab class, obviating the need in my youth of needing to try to see what the instructor was seeing when peering into a microscope. The same applies to telescopes.
              In a similar vein, the ION DNA sequencer, about the size of a cigarette packet, promises to put DNA sequencing in the hands of anyone once the price falls. And yes, such technologies are potentially suitable for space probes. The probes just need to get smart enough so that they don’t get stuck like the Insight’s subsurface thermometer. ;-)

  • Antonio March 25, 2020, 10:59

    Mmm… if this probe is designed to reach 1000 AU in 50 years and will use the same kind of RTGs than the Voyagers, that have barely any power remaining now, 43 years after launch… how will this probe accomplish its mission?

    • Jens Knudsen March 25, 2020, 14:47

      I believe the Europeans are developing their own RTG using a material they have in abundance – Americium 241. It would be a heavier unit, but have a much longer decay time.

      • Brett P Bellmore March 26, 2020, 12:57

        Since an RTG is going to be producing power even if you don’t need it, it would only make sense to throw in an ion engine to use that power early in the mission.

        • Jens Knudsen March 26, 2020, 13:53

          Yes. The power levels in question are low, so the thrust level and engine size would be very small, but over the course of decades, the speed increase is significant. The advantage would be smaller with an americium RTG.

    • charlie March 25, 2020, 19:48

      if you vastly over power your probe with ALOT of RTGs initially, then at about 1000 AU you’ll be at normal power levels …

      • Andrei March 25, 2020, 20:42

        No need for a surplus of RTG’s if more long lived isotopes are used Americum 241 have a halflife of 432 years, so that solve the problem and would power the spacecraft even on one extended mission.
        Though not found in nature it is in wide use in smoke detectors.

        • charlie March 26, 2020, 13:13

          As 241Am has a roughly similar half-life to 238Pu (432.2 years vs. 87 years), it has been proposed as an active isotope of radioisotope thermoelectric generators, for use in spacecraft.[16][17] Even though americium-241 produces less heat and electricity than plutonium-238 (the power yield is 114.7 mW/g for 241Am vs. 390 mW/g for 238Pu)[16] and its radiation poses a greater threat to humans owing to gamma and neutron emission, it has advantages for long duration missions with its significantly longer half-life.

          • Ashley Baldwin March 26, 2020, 14:10

            And the half life isn’t even the main problem.

            RTGs use bimetallic thermocouples to convert the heat of radioactive decay into electrical energy. Exploiting the different conductive gradient between the hot end of the couple ( 1400K) the “cold” end ( 470K) to create AC current which is then converted into DC for use.

            To date the MOST efficient thermocouples have an efficiency of just 7.5 %. So most of the radioisotope decay , be it from Pu238 or Americium, will be waste heat, or at best used to maintain payload temperatures in the outer solar system.

            Apart from their very limited efficiency, thermocouples also degenerate with time as they are exposed to extreme chemical and thermal conditions . Their low efficiency drops at a greater rate than the half life of the radioisotope. So they, not the isotope half life , are the rate limiting factor for energy production .

            NASA are currently using MMRTGs ( in Curiosity and for Dragonfly) which are slightly more efficient than the older GPHS RTGs used in Cassini and New Horizons. ( thought less efficient , these older models last longer as their thermocouples are made of less degradable materials and more importantly – they contain several times more Pu238) . The MMRTGs were produced at a time when US DoE Pu238 has more or less ceased – to make wait limite supplies were left last longer. They contain just 4 Kgs of isotope. ( cf Cassini’s RTG contained about 20 Kgs) Should “Trident” be selected for the next Discovery round it will employ the last two MMRTGs. Which have a minimum “mission life” of 17 years. NASA do not plan to make more. DoE production has now slowly increased again .

            To this end NASA Glenn are working with the private sector to develop “next generation” ( make it so) modulated RTGs with the much more efficient and durable skutterudite alloy based thermocouples . It’s hoped their efficiency will approach 15 % and the current goal is to have a qualified unit by 2028 to replace the MMRTGs. Efficiency is sustained over time too and will be atleast 50 % better at the end of missions than with MMRTGs.

            As a point of note, some RTGs ( MMRTGs for instance ) work in both space and atmospheres – as with Curiosity on Mars and Dragonfly on Titan. Others however , just work in space. This includes the next gen RTGs. What that means in terms of any future Mars sample return mission is a moot point . Solar power obviously remains an option.

            Longer term, work continues on the “Krusty” U235 space adept reactor, which will eventfully produce up to 40 KWs of energy over a twenty year life span.

            From a 2 tonne reactor !

      • Ashley Baldwin March 26, 2020, 21:01

        That’s a lot of RTGs !

        NEP is the way forward though – for the next half a century or so at the least.

        Even Dawn’s first gen NSTAR Ion thruster required 3KWs to operate at maximum efficiency. ( using large solar arrays and thus limited to within just a few AU of the Sun ) NASA’s current state of the art NEXT ion thrusters require 7 KWs. The next gen Hall thruster currently being developed by NASA Glen and Aerodyne Rocketjet require 15 KWs.

        By way of comparison the most potent RTGs, past, present or planned produce just 0.6KWs. Fine for payload instruments , not fine for propulsion. Each RTG would also require 2-4 Kgs of precious Pu238 . NASA and the US DoE plan to reach a steady production of this isotope from the summer.

        At 0.4kgs – per year .

        So a non starter.

        As alluded to below NASA is now developing the space rated U235
        “Krusty” reactor. Capable of up to 40 KWs for a minimum of 20 years . Weighs in at 2 tonnes though. But more powerful still reactors are only a question of scale. Not new science.

        So why not a 4 MW reactor ? They already exist on the ground. Scale again – scaled up science to scale down size. To power a mighty plasmamagnetodynamic ion thruster. Just a sophisticated scaled up ion thruster really perhaps using a lithium propellant instead of a Nobie gas. Possessing a “ specific impulse” ( the rocket equivalent of a car’s mpg) ten times that of the most potent cryochemical rockets. Yet with near the same raw thrust. Wrap one of them with beefed up Parker Probe heat cladding and accelerate it hard around the Sun to within a couple of solar radii and you get an almighty Oberth manoeuvre-cum ( sort of) gravity assist . This will deliver a final delta v in excess of 100km/s . Further boosted by a similar but smaller interaction with Jupiter.

        Then fifty years of incessant and MW ion propulsion to build up serious additional delta v – over fifty years ! ( even humble Dawn added 11km/s over its decade long mission) . Off to the stars . Or 1000 AUs in a few decades.

        All the technology required is just about scaling and a bit of technological extrapolation. No exotic fragile gossamer thin fully reflective solar sail membranes. No unobtanium. Or phlogiston.

        Add in the shed load of propellant required and such a craft would be large. Seriously but obtainably large ( within the launch capacity of a SpaceX Super Heavy Starship ). Such a crafty would be ugly. Utilitarian for sure. But achievable and with currently available or near future technology and materials.

        No Starship Enterprise certainly, but like the Enterprise – bound for the stars.

        Engage-d ? !

        • Alexander Tolley March 27, 2020, 13:00

          It was the need for a 2MW power supply that ridiculed the VASIMR drive for a fast flight to Mars. If unshielded reactors delivering that power can be built (and assembled in space?) that might be a gamechanger. However, that power comes with a mass penalty that translates into a lower acceleration. For deep space flight, that mass penalty is probably worth it. For near space flight though?

          Electric engines have been the future of rocket engines for longer than I have been alive, yet only now are we getting the needed technology development that will eventually make them the standard once suitable power supplies have been developed too. Rather like battery technology is facilitating electric cars today. I don’t rule out beamed power either for some missions.

          • Ashley Baldwin March 28, 2020, 9:05

            In terms of the mass argument you are of course quite right. The major premise of your Q-drive.

            VASIMR was a case of too much , too soon . To this end it has now been largely been sidelined. As I understand it, coping with the large magnetic fields arising from its large 200KW plus power demands are what hamstrung it. Along with the ungainly huge solar arrays required to provide this power and their limited operational range from the Sun. But electric powered systems are here and expanding at last as you point out . Better late than never. Admittedly at a more realistic pace, but a pace that is now picking up . The question then, is not when but how ? Specifically in terms of power supply . Be it solar or otherwise.

            NASA and Aerojet have continued building the more modest but capable Hall thruster and NextStep Electric propulsion drives. The current Hall thrusters have already been developed to 13 KW capacity with high maturity. The Dawn probe underwent a delta V change of 11 Km/s over the decade operational lifetime of its humble 2.3 KW NSTAR electric engine . So a Hall thruster engine could increase the velocity of an erstwhile interstellar probe by atleast several AUs/ per year over the course of a similar period. With power. As you quite rightly point out the old mass versus power equation comes into action so any electric / nuclear powered system would need to be low mass. Imposing limitations on its power and so on. But is that the end ?

            The APL team recently published a detailed article “Near-term interstellar probe: The next step” in last September’s Acta Astronautica ( McNutt et al) . Free full text available. This focused on what could be achieved with current or planned technology .

            It considers all iterations and C3 of the SLS as potential launchers. ( just imagine the C3 required to launch a MW reactor and NEP propellant probe ! ) . The other big consideration was a realistic budget. Not in the realms of a “Project Orion” dimensions MW NEP probe either ! Though my proposal was only ever a thought experment to pique opinion and stimulate. discussion . Segueing nicely into your Q-Drive speculations .

            Anyway, the McNutt article explores two flight options based around a New Horizons massed probe ( presumably with a similar 30KG instrument payload) . Firstly “passive” (ballistic flyby) and active ( with one of two “kick start” solid fuel engines – the Star48b or Castor 30XL) ) gravity assists around Jupiter – which is next optimally placed following a 2034 launch window. They calculate the probe’s post Jupiter speed as up a respectable to 12 AU per year. No 1000 AUs in fifty years but still a respectable near 500.

            The second option involves performing an “Oberth manoeuvre” as described in my original post – around the Sun utilising SLS and the kick start engines above. The post encounter velocity is dependent on how close to the Sun the probe can get. In order to maximise its gravity well. For an Oberth manoeuvre to work as envisaged by the epynomous scientist in 1929 , any probe would need to get to or within 5 solar radii, R, of the corona. Phew ! The Parker Solar Probe gets to bout 9.5 R and its carbon foam insulation can be scaled up to get to about 5 R. Adding an extra layer of tungsten cladding ( increasing mass and cost of course) gets you to about 4 R. Getting closer than 3R is impossible with modern material science and any additional gains from within 4R are minimal. So 4R it is. The authors calculate the biggest iteration of SLS ( with cryogenic upper stages et L ) and the best kick start engine ( Castor 30XL) gives a post encounter velocity just shy of 15AU per year. Better than the Jupiter options ( but at much greater cost) and enough to reach the solar gravitational lensing zone around 500AU. But not much further and nowhere near a 1000 AU.

            After their encounters with the Sun and Jupiter all these Probe options “coast” on to their destination . If not continuous ,could electric propulsion provide “staging” to deliver the additional velocity via a time limited active “push” . Just enough to take these 12-15 AU/ year probes nearer to the 20AU/year required to reach the Nirvana of 1000AU in fifty years ?

            Beyond the asteroid belt SEP is useless or impractical. So what about a more modest NEP staging system post Jupiter ? Apart from the 20 tonne Krusty reactor, NASA have been developing the much smaller and practical reactor as part of the “SPEAR” concept mission. An NEP powered probe designed for active flight to Europa after a conventional EELV launch. It’s reactor has a mere 12 KW capacity . Utilising cheap “low enriched uranium ” ( as opposed to “high enriched uranium” which has huge production and security costs and can easily make nuclear weapons !) Within the same range as the imminent state of the art Hall thrusters described above and five times as powerful as NSTAR on Dawn. SPEAR – probe, reactor and propellant masses in at just 1300KG. With propellant capacity range for a Dawn length operation of 10 years. Well within the C3 of the SLS even with a 2 tonne solid stage gravity/Oberth assist ” kick” start engine bolted on . Ironically the probe would also require a number of RTGs to power instruments for a fifty years mission. These would only add a few tens of Kgs extra mass though.

            NONE of this last is considered in the McNutt article. Another one of my thought experiments . But could SPEAR become the 1000 AU Interstellar probe ?

            Just a thought.

            • Michael Fidler March 28, 2020, 23:34

              Looking for an article on magnetic reconnection thrusters I came across this, which is a review electrodeless plasma thrusters. They have many varieties and information on them which would be the form that would last on long duration flights.

              Electrodeless plasma thrusters for spacecraft: A review.
              The physics of electrodeless electric thrusters that use directed plasma to propel spacecraft without employing electrodes subject to plasma erosion is reviewed. Electrodeless plasma thrusters are potentially more durable than presently deployed thrusters that use electrodes such as gridded ion, Hall thrusters, arcjets and resistojets. Like other plasma thrusters, lectrodeless thrusters have the advantage of reduced fuel mass compared to chemical thrusters that produce the same thrust. The status of electrodeless plasma thrusters that could be used in communications satellites and in
              spacecraft for interplanetary missions is examined. Electrodeless thrusters under development or planned for deployment include devices that use a rotating magnetic field; devices that use a rotating electric field; pulsed inductive devices that exploit the Lorentz force on an induced current loop in a plasma; devices that use radiofrequency fields to heat plasmas and have magnetic nozzles to accelerate the hot plasma and other devices that exploit the Lorentz force. Using metrics of
              specific impulse and thrust efficiency, we find that the most promising designs are those that use Lorentz forces directly to expel plasma and those that use magnetic nozzles to accelerate plasma.


            • Michael Fidler March 29, 2020, 0:03

              The same author also did experiments on “A thruster using magnetic reconnection to create a high-speed plasma jet.” Under Section 6.1 Further investigations, Fig. 19 shows a simplified diagram of an experimental magnetic reconnection thruster using concentric solenoids. This design may have a high specific impulse of 16317 Isp as shown in table 6.

              A thruster using magnetic reconnection to create a high-speed plasma jet.
              Plasma thrusters propel spacecraft by the application of Lorentz forces to ionized propellants.
              Despite evidence that Lorentz forces resulting from magnetic reconnection in solar flares and Earth’s
              magnetopause produce jets of energetic particles, magnetic reconnection has only recently been considered as a means of accelerating plasma in a thruster. Based on theoretical principles, a pulsed magnetic reconnection thruster consisting of two parallel-connected slit coaxial tubes was constructed. The thruster was operated in argon plasma produced by RF energy at 13.56 MHz. A 1.0 ms current pulse of up to 1500 A was applied to the tubes. Three results provide evidence for magnetic reconnection. (1) The production of high-energy electrons resembling the outflow that is observed in the reconnection of field lines in solar flares and in laboratory experiments. (2) The high-energy electron current coincided with the rise of the magnetic field in the thruster and was followed by a large ion current. (3) In accordance with known physics of magnetic reconnection, ion currents were found to increase as the plasma became less collisional. The Alfvén speed of the outflowing ions was calculated to be 8.48x 10^3 m s^-1 corresponding to an Isp of 860 s.


              Have not seen any further work on the concentric solenoids design since this article…

  • charlie March 26, 2020, 13:32

    wouldn’t it be better to achieve an orbit at the Gravitational lensing limit for the sun ? an orbit there would allow continuous observations of distant stars which would give alot info payback.

  • Richard March 26, 2020, 15:06

    Hi Mr Gilster, that is quite interesting. The idea of having a probe that travels far beyond the solar system, for sometime. That is certainly the way to get to the outer reaches.

    Having mentioned the space launch system, considering with what is going at the moment, that has now slowed, so the first mission may be put off by a few months.

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