Centauri Dreams
Imagining and Planning Interstellar Exploration
Ultrahigh Acceleration Neutral Particle Beamer: Concept, Costs and Realities
The advantages of neutral particle beam propulsion seem clear: Whereas a laser’s photon beams can exchange momentum with the sail, neutral particle beams transfer energy and are considerably more efficient. In fact, as we saw in the first part of this essay, that efficiency can approach 100 percent. A mission concept emerges, one that reaches a nearby star in a matter of decades. But what about the particle beam generators themselves, and the hard engineering issues that demand solution? For that matter, how does the concept compare with Breakthrough Starshot? Read on as James Benford, working in collaboration with Alan Mole, describes the salient issues involved in building an interstellar infrastructure.
By James Benford and Alan Mole
We discuss the concept for a 1 kg probe that can be sent to a nearby star in about seventy years using neutral beam propulsion and a magnetic sail. We describe key elements of neutral particle beam generators, their engineering issues, cost structure and practical realities. Comparison with the Starshot laser beam-driven concept gives roughly similar costs.
Beam Generator Concept
Figure 1 Block diagram of early neutral particle beam generator [1]. Drift-Tube Linac is not shown.
Creation of the neutral particle beam begins with
1. Extraction of a negative ion beam (negative ion with attached electrons) from a plasma source; it then drifts into the first acceleration stage, the RFQ. The first element of the accelerator will appear much like the geometry shown in figure 2. Here ions are extracted from the plasma source on the left by electrostatics and brought by a converging magnetic field to the linear accelerator.
Figure 2. Ion beam on left is propagated along converging magnetic field to the linac.
2. The ion beam enters a radiofrequency quadrupole (RFQ) accelerator, a vane-like structure where the application of radiofrequency power produces a continuous gentle acceleration much like a surfer riding a wave. It also provides strong electrostatic focusing to prevent divergence growth. The structure bunches the particles in phase space.
The RFQ fulfils at the same time three different functions:
- focusing of the particle beam by an electric quadrupole field, particularly valuable at low energy where space charge forces are strong and conventional magnetic quadrupoles are less effective;
- adiabatic bunching of the beam: starting from the continuous beam produced by the source it creates with minimum beam loss the bunches at the basic RF frequency that are required for acceleration in the subsequent structures;
- acceleration of the beam from the extraction energy of the source to the minimum required for injection into the following linac structure.
3. After the ions exit the RFQ at energies of a few MeV, further acceleration to increase the particle energy is done with a drift-tube linac (DTL), which consists of drift tubes separated by acceleration regions, as shown in Figure 3. Particles arriving at the gaps at the proper phase in the radiofrequency waves are given acceleration impulses. When the electric field of the wave reverses, the particles are shielded from being accelerated by passing through the drift tubes. The typical accelerating gradient is a few MeV/m.
Figure 3. Drift-Tube Linac, which consists of drift tubes separated by acceleration regions.
4. In order to maintain low emittance and produce the microradian divergence we desire, the beam is expanded considerably as it exits the accelerator. Beam handling elements must have minimal chromatic and spherical aberrations.
5. Beam pointing to be done by bending magnets with large apertures.
6. Finally, the extra electrons are stripped from the beam, making a neutral particle beam. This can be done with by stripping the electrons in a gas neutralization cell or by photodetachment with a laser beam. It may be possible to achieve 100% neutralization by a combination of methods. Thus far this high-efficiency neutralization has not been demonstrated.
Beamer Engineering
There are several possible schemes for building the beam generator. Both electrostatic and electromagnetic accelerators have been developed to produce high power beams. The most likely approach is to use linear accelerators. In the past, the cost of an electromagnetic accelerator is on the order of a person year per meter of accelerator (~1 man-year/m) but this could be larger for the more sophisticated technologies.
The power system to drive such accelerators could come from nuclear power (fission or fusion) or solar power. Furthermore, if it were to be space-based, the heavy mass of the TW-level high average power required would mean a substantially massive system in orbit. Therefore Mole’s suggestion, that the neutral beam be sited on Earth, has its attractions. There is also the question of the effects of propagating in the atmosphere, on both beam attenuation and on divergence.
If the beam generator were to be on Earth, it should all be at the highest altitude for practical operations. The Atacama Desert, for example, would offer very low humidity and half of sea level pressure. In addition, a way to reduce beam losses in the atmosphere would be to launch a hole-boring laser beam in advance just before the neutral beam. This laser would heat up a cylinder of atmosphere to lower the pressure, allowing the neutral beam to propagate with less loss. Such hole-boring exercises have been conducted in laser weapon studies and does appear to be a viable technique.
The final neutral beam can be generated by many small beam drivers or a single large beam driver. If a great number of driver devices and their associated power supplies are required, increasing the construction and maintenance expense of this portion. Of course, economies of scale will reduce the cost of individual segments of the Beamer by mass production of the system modules. Making such choices is an exercise for future engineers and designers.
Neutral particle beam generators so far have been operated in pulsed mode of at most a microsecond with pulse power equipment at high voltage. Going to continuous beams, which would be necessary for the seconds of beam operation that are required as a minimum for useful missions, would require rethinking the construction and operation of the generator. The average power requirement is quite high, and any adequate cost estimate would have to include substantial prime power and pulsed power (voltage multiplication) equipment, the major cost element in the system. Of course, it will vastly exceed the cost of the Magsails, which is an economic advantage of beamed propulsion.
However, this needs economic analysis to see what the cost optimum would actually be. Such analysis would take into account the economies of scale of a large system as well as the cost to launch into space versus the advantages of beaming from Earth.
Beamer Cost Estimates
The interstellar neutral particle beam system described here is a substantial extrapolation beyond the present state-of-the-art. Nevertheless, estimates can be made of both the capital and operating costs.
The cost of the Beamer is divided between the cost of the accelerator structure (RFQ and DTL) and the power system that drives it. For a cost estimate for the Mercury system, we assume that the present day accelerating gradient is maintained for this very high-power system. That gradient is ~ 2 MeV/m. For the mercury neutral particle beam the length of the 1.35 GeV accelerator would be 675 m.
There is an extensive technology base for drift-tube linacs; there are many in operation around the world [2]. We use as a model the well-documented 200 MeV Brooklyn National Laboratory 200 MeV ion beam system, which was completed in 1978 at a cost of $47M. It used 22 MW of radiofrequency power and was 145m long. In that era, the cost of microwave equipment was ~$1/W. The cost today is ~$3/W, so the 22 MW would cost 22 M$ then and 66 M$ today. Since the total cost of accelerator was $47 M$, the Accelerator structure would cost 47 M$ -22 M$ = $25 M$. Thus at this level the two cost elements are roughly equal. The accelerator structure then costs $25 M$/145 m = $0.17 M$ per meter in 1978. We multiply all costs by a factor of three to account for inflation to get today’s costs.
To estimate the capital cost of the mercury in NPB described here, we have the following relations:
Caccl= 0.5 M$/m x 675 m = 350 M$
Cmicrowave= 3$/W x 18 TW = 5.47 B$
Therefore the dominant cost element would be the microwave system driving the accelerator.
However, high-volume manufacturing will drive costs down. Such economies of scale are accounted for by the learning curve, the decrease in unit cost of hardware with increasing production. This is expressed as the cost reduction for each doubling of the number of units, the learning curve factor f. This factor typically varies with differing fractions of labor and automation, 0.7 < f < 1, the latter value being total automation.
It is well documented that microwave sources have an 85% learning curve, f = 0.85 based on large-scale production of antennas, magnetrons, klystrons, etc [3]. Today’s cost is about $3/W for ~1 MW systems. Note that this includes not only the microwave generating tube, but also the power system to drive that continuous power. The 18 TW power needed would require 18 million such units. Therefore the cost is ~1.1 B$. Adding together the accelerator and microwave power system, the cost will be 1.45 B$.
The electrical power to drive this large system cannot possibly come from the electrical grid of Earth. Therefore a large cost element will be the system that stores the 162 TJ of energy. (Note that the beam power starts at zero and rises with time (t2) to 18 TW at the end.) From Parkin’s estimates of the Starshot energy storage system [10], based on Li-ion batteries, we take the storage cost to be $50 per kilowatt-hour, which is $13,900 $/TJ. Consequently the cost for the energy store is ($13,900 $/TJ) 162 TJ = 2.25 B$. So the energy stores cost is comparable to that of the accelerator.
The total capital cost is
Caccl= 350 M$
Cmicrowave = 1.1 B$
Cstore= 2.25 B$
Total accelerator capital cost is 3.7 B$.
The operating cost to launch a single Magsail is of course far smaller. It is simply the cost of the spacecraft and the energy to launch it. We will assume that the cost of the spacecraft will be on the order of $10 million. The cost of the electricity at the current rate of $.10 per kilowatt-hour is $4.5 million.
Total operating cost for a single launch is ~15M$.
Comparison with Starshot
The neutral particle beam approach is conceptually similar to photon beams such as the laser-driven Starshot project. A disadvantage of reflecting photons from the sail will be that they carry away much of the energy because they exchange only momentum with the sail. Neutral particle beams transfer energy, which is much more efficient. The reflecting particles may in principle be left on moving in space after reflection and thus the efficient energy efficiency can approach 100%.
The Starshot system, a laser beam-driven 1 gram sail with the goal of reaching 0.2c, has been quantified in a detailed system model by Kevin Parkin [4]. Since both the high acceleration neutral particle beam described here and Starshot are both beam-driven high-velocity systems, we make the following comparison between their key parameters and cost elements:
Physical parameters and cost elements of beam-driven probes
| Mercury Neutral Particle Beam System | Starshot | |
|---|---|---|
| Sail mass | 1 kg | 1 g |
| Velocity | 0.06 c | 0.2 c |
| Beamer capital cost | 1.45 B$ | 4.9 B$ |
| Energy store cost | 2.25 B$ | 3.4 B$ |
| Total capital cost | 3.7 B$ | 8.3 B$ |
| Energy cost/launch | 4.5 M$ | 7 M$ |
| Kinetic energy | 1.6 1014 J | 1.8 1012 J |
| Kinetic energy/ capital cost | 43.2 kJ/$ | 0.2 kJ/$ |
Here we have summed the accelerator and microwave power system costs for the neutral Beamer and the laser and optics cost for Starshot. A major caveat is that Parkin’s estimates have realistic efficiencies of the systems of Starshot, but our costs assume unrealistically high efficiencies.
Although they differ in detail, the two concepts give the same order of magnitude cost. However, the kinetic energy in the NPB-driven probe is 90 times that of the Starshot probe. This shows the disadvantage of reflecting photons from the sail: they carry away much of the energy because they exchange only momentum with the sail. Neutral particle beams transfer energy, which is much more efficient. The kinetic energy/capital cost ratio is 200 times greater in the NPB case.
It is instructive that the high-energy requirement of interstellar probes drives the existence of a stand-alone storage system, which is a major element in the total cost of both systems. The similarity of costs for these rather different beam- driven systems gives us some confidence that these rough estimates in this paper are credible.
Neutral Particle Beam Realities
Practical realities are always bad news. Performance of most systems degrades to below their design points because of inefficiencies of processes. Note that the beam systems described here are perfectly efficient, as determined from equation 5. That is, the beam reflects from the sailcraft with perfect efficiency, so as to stop dead, transferring all the energy to the spacecraft. The realities of neutral particle beams in the present day are substantially poorer.
To see where the problems lie, we consider a daring experiment called BEAR, conducted 30 years ago [1, 5]. A neutral particle beam generator was actually deployed and operated in space and its performance was measured.
On July 13, 1989 the Beam Experiment Aboard Rocket (BEAR) linear accelerator was successfully launched and operated in space by Los Alamos National Laborotory. The rocket trajectory was sub-orbital, reaching altitude of 220 km. The flight demonstrated that a neutral hydrogen beam could be successfully propagated in an exoatmospheric environment. The cross-section of the rocket is shown in figure 4.
Figure 4. Beam Experiment Aboard Rocket (BEAR) [1].
The accelerator, which was the result of an extensive collaboration between Los Alamos National Laboratory and industrial partners, was designed to produce a 10 rnA, 1 MeV neutral hydrogen beam in 50 microsecond pulses at 5 Hz. The major components were a 30 kev H- injector a 1 MeV radio frequency quadrupole, two 425 MHz RF amplifiers, a gas cell neutralizer, beam optics, vacuum system and controls. The beam extracted was 1 cm in diameter with a beam divergence of 1 mradian. There was no unexpected behavior such as beam instability in space.
The design was strongly constrained by the need for a light- weight rugged system that would survive the rigors of launch and operate autonomously. The payload was parachuted back to Earth. Following the flight the accelerator was recovered and successfully operated again in the laboratory.
From the paper and report describing this experiment we see substantial inefficiencies, which should guide our future expectations.
The input power to the accelerator was 620 kW for 60 µs, a 7.2 J energy input. The beam as extracted was 27 mA at 1 MeV for 50 µs, which gives 1.35 J. The efficiency therefore is 19%, so approximately 4/5 of the energy supplied was lost in the beamline shown in figure 1. The major loss was in the neutralizer which was a xenon gas injected into the beamline. The efficiency of the neutralizer was changed by varying the amount of gas injected. They obtained 50% neutral hydrogen and 25% each of negative and positive hydrogen. Therefore the neutralization process was only 50% efficient in producing a neutral beam. This accounts for most of the loss. The other losses can be accounted for by inefficiencies in the optics of the low-energy beam region and the high-energy beam region.
In the 30 years since the flight, little work on particle beams has occurred at high power levels, because of the termination of the Strategic Defense Initiative. Doubtless substantial improvements can be made in the efficiency of NPB’s, given substantial research funding. Therefore the concept in this paper, with its hundred percent efficiency of energy transfer from the electrical system to the sail, is an upper bound on the performance. Consequently the parameters in Table 1 and the capital and operating cost estimates given here are lower bounds on what would actually occur.
Conclusions
The cost model presented here is lacking in realistic efficiencies. The next level of analysis should address this lack.
We can forsee a development path: a System starts with lower speed, lower mass Magsails for faster missions in the inner solar system. As the system grows, the neutral beam System grows and technology improves. Economies of scale lead to faster missions with larger payloads. As interplanetary commerce begins to develop, making commerce operate efficiently, outcompeting the long transit times of rockets between the planets and asteroids, the System evolves [6]. Nordley and Crowl describe such a development scenario [7]. We conclude that this concept is a promising method for interstellar travel.
References
1. P. G. Oshey, T. A. Butler, M. T. Lynch, K. F. McKenna, M. B. Pongratz, T. J. Zaugg, “A Linear Accelerator In Space-The Beam Experiment Aboard Rocket”, Proceedings of the Linear Accelerator Conference 1990.
2. H. B. Knowles, “Thirty-Five Years of Drift-Tube Linac Experience” Los Alamos Scientific Laboratory Report, LA-10138-MS, 1984. See also reference 4, pg. 81.
3. J. Benford, J. A. Swegle and E. Schamiloglu, High Power Microwaves, Third Edition, pg. 77, Taylor and Francis, Boca Raton, FL, (2015).
4. K. L. G. Parkin, “The Breakthrough Starshot System Model”, Acta Astronautica 152, 370-384, 2018.
5. G. J. Nutz, “Beam Experiments Aboard a Rocket (BEAR) Project Summary’, LA-11737, 1990.
6. J, Benford, “Beam-Driven Sails and Divergence of Neutral Particle Beams” JBIS 70, pg. 449-452, 2017.
7. G. Nordley and A. J. Crowl, “Mass Beam Propulsion, An Overview”, JBIS 68, pp. 153-166, 2015.
Ultrahigh Acceleration Neutral Particle Beam-Driven Sails
Beamed propulsion has clear advantages when it comes to pushing a payload up to interstellar flight speeds, which is why Breakthrough Starshot is looking at laser strategies. But what about a neutral particle beam in conjunction with a magnetic sail? We’ve discussed the possibilities before (see Interstellar Probe: The 1 KG Mission), where I wrote about Alan Mole’s paper in JBIS, followed by a critique from Jim Benford. Mole, a retired aerospace engineer, is now collaborating with plasma physicist Benford (CEO of Microwave Sciences) to examine a solution to the seemingly intractable problem of beam divergence. Getting around that issue could be a game-changer. Read on for the duo’s thoughts on sending a 1 kg probe to a nearby star system with a flight time in the range of 70 years. Part 2 of this study, outlining engineering issues and the practical realities of cost, will follow.
by James Benford and Alan Mole
We advance the concept for a 1 kg probe that can be sent to a nearby star in about seventy years using neutral beam propulsion and a magnetic sail. The concept has been challenged because the beam diameter was too large, due to inherent divergence, so that most of the beam would miss the sail. Increasing the acceleration from 1000 g’s to 100,000 g’s along with reducing the final speed from 0.1 c to 0.06 c redeems the idea. Such changes greatly reduce the acceleration distance so that the mission can be done with realistic beam spread. Magsail-beam interaction remains an aspect of this concept that needs further study, probably by simulations.
Central features of Neutral Particle Beam Propulsion
Use of a neutral particle beam to drive a Magsail was proposed by Geoffrey Landis as an alternative to photon beam-driven sails [1]. Compared to beam-driven propulsion, such as Starshot, particle beam propelled magnetic sails, Magsails, substitute a neutral particle beam for the laser and a Magsail for the ‘lightsail’, or ‘sailship’. The particle beam intercepts the spacecraft: payload and structure encircled by a magnetic loop. The loop magnetic field deflects the particle beam around it, imparting momentum to the sail. The general ‘mass beam’ approach has been reviewed by Nordley and Crowl [2].
Particle beam propelled Magsails require far less power for acceleration of a given mass. There’s also ~ 103 increase in force on the sail for a given beam power. Deceleration at the target star is possible with the Magsail but not with a laser driven sail.
The neutral particle beam approach is conceptually similar to photon beams such as the laser-driven Starshot project. A disadvantage of reflecting photons from the sail will be that they carry away much of the energy because they exchange only momentum with the sail. Neutral particle beams transfer energy, which is much more efficient. The reflecting particles may in principle be left unmoving in space after reflection and thus the efficient energy efficiency can approach 100%.
The thrust per watt beam power is maximized when the particle velocity is twice the spacecraft velocity. The Magsail, with a hoop force from the magnetic field, is an ideal structure because it is under tension. High-strength low-density fibers make this lightweight system capable of handling large forces from high accelerations. The rapidly moving magnetic field of the Magsail, seen in the frame of the beam as an electric field, ionizes the incoming neutral beam particles. Nordley and Crowl discuss on-board lasers to ionize the incoming beam, although this adds additional on-board mass and power [2]. When the dipole field of the Magsail is inclined to the beam vector the Magsail experiences a force perpendicular to the beam vector, which centers it on the particle beam, perhaps providing beam-riding stability.
Ultrahigh Acceleration
Alan Mole proposed using it to propel a lightweight probe of 1 kg [3]. The probe was accelerated to 0.1 c at 1,000 g by a neutral particle beam of power 300 GW, with 16 kA current, 18.8 MeV per particle. The particle beam intercepts a spacecraft that is a Magsail: payload and structure encircled by a magnetic loop. The loop magnetic field deflects the particle beam around it, imparting momentum to the sail, and it accelerates.
Benford showed that the beam divergence is fundamentally limited by the requirement, at the end of the acceleration process, to strip electrons from a beam of negative hydrogen ions to produce a neutral beam [4,5]. Therefore neutral beam divergence is typically a few microradians. Mole’s beam had an inherent beam divergence of 4.5 µradians.
In Mole’s work, the neutral hydrogen beam at 18.8 MeV per particle and inherent beam divergence of 4.5 µradians accelerated to two-tenths of the speed of light (0.2 c) had acceleration of 103 g’s for 50 minutes [3]. This resulted in a 411 km diameter beam spot, far larger than the Magsail diameter, which was 0.27 km. So most of the beam missed the sail.
But if we use much higher acceleration, the sail will stay within the beam until it reaches the desired final velocity, even with microradian divergence. We choose 105 g’s, 106 m/s2 to accelerate to 0.06 c, 1.8 x 107 m/s.
Numerical experiments with the model developed by Nordley [6], and later replicated by Crowl, showed that the greatest momentum delivery efficiency is when the velocity of the neutral beam is twice the sail velocity. The physics of this is straightforward: Maximum energy efficiency comes when all of the energy goes to the sail and none of it remains in the beam. For a sail that is perfectly reflective, the beam bounces off the sail at the same velocity it impinges the sail. If after reflection it is moving at zero velocity (so none of the energy is left in the beam), the initial beam velocity must be twice the sail velocity, so that it impinges on the sail at a relative velocity equal to the sail velocity.
We take the beam velocity at the end of acceleration to be the twice the final sail velocity, 0.06c The energy of a hydrogen atom is imparted by accelerating through a voltage of 6.6 MeV. The mission parameters for a hydrogen beam then become those shown in Table 1.
The lighter the particle to be accelerated, the shorter the beam driver can be at a fixed field gradient. However, lighter-particle shorter beam drivers, while they may cost less, would require a larger sail due to the higher divergence of the beam.
For a second example, a mercury beam has a minimum divergence of 0.8 µradians, but must use far higher voltage because of the larger mass [4]. Mercury beam parameters are also given in Table 1.
Table 1 Parameters of neutral particle beam-driven sail probes
| Beam and Sail Parameters | Hydrogen Beam | Mercury Beam |
|---|---|---|
| Beam Divergence | 4.5 microradian | 0.8 microradian |
| Acceleration | 105 g’s=106 m/sec2 | 105 g’s=106 m/sec2 |
| Sail diameter | 1.46 km | 260 m |
| Sail final velocity | 0.06 c, 1.8 x 107 m/s | 0.06 c, 1.8 x 107 m/s |
| Acceleration distance | 1.6 x 105 km, 10-3 AU | 1.6 x 105 km, 10-3 AU |
| Acceleration time | 18 sec | 18 sec |
| Magsail mass | 1 kg | 1 kg |
| Kinetic energy | 1.6 1014 J | 4 1014 J |
| Beam peak power | 1.8 1013 W, 18 TW | 1.8 1013 W, 18 TW |
| Beam voltage | 6.76 MeV | 1.35 GeV |
| Beam current | 2.66 MA | 1.33 kA |
We will see that when the beam divergence is in reality roughly 3 orders of magnitude higher than previous studies have assumed, from a nanoradian to microradian, rapidly moves the beam generator regime toward being very large and expensive.
Because in Table 1 the hydrogen beam sail diameter is so large, we will focus the rest of this discussion on the mercury beam. Even so, the mercury beam Magsail has a 260 m diameter and 1 kg mass, if the superconducting hoop has a density of steel, the thickness must be no larger than 0.44 cm, if the density of carbon, 0.8 cm.
Magsail-Beam Interaction
Note that the sail diameter given in Table 1 is taken to be simply the diameter of the divergent beam encountering the Magsail. The diameter of the reflection region produced by the magnetic field of the sail could well be somewhat larger than the superconducting hoop diameter. (Of course, early in the acceleration, the beam will hit it at the axis where the magnetic field is greatest.)
When a Magsail driven by a neutral particle beam is at the early stages of the acceleration, the beam will have a considerably smaller spot size on the Magsail than it will later and will hit it at the axis where the magnetic field is greatest. Later on, as the Magsail flies away, the beam will reach a size dictated by its divergence. A question is: does the initial beam high intensity of the beam on the magnetic field tend to push the sails magnetosphere outward radially and make the effective diameter of the Magsail larger? If it does, then the beam divergence can be a bit larger and still strike the Magsail. Or, conversely one could accelerate the Magsail for a longer time because some of the beam would still be captured.
Simulations show the field being compressed; but they are of solar wind, which is taken to be uniform across a magnetic dipole. There are no simulations of the beam smaller than the sail. One would expect the loop generated field to be compressed in the direction of motion, but it seems reasonable for it to be inflated radially, especially if charged particles are trapped in it for significant periods of time.
Andrews and Zubrin have done single particle numerical calculations that do not include modeling dynamic effects (such as field distortions from magnetic pressure) and do not include any such “inflation” of the mirror due to trapped beam ions [7].
Figure 1 is taken from the late Jordan Kare’s NIAC report [8]. (From his figure, he considered using a nuclear detonation to accelerate a Magsail, which is not relevant to our discussion.) From the left a uniform solar wind strikes the Magsail, which in our case would be a non-uniform neutral particle beam. The beam encounters the peak of magnetic field along the axis of the sail. On the right of the figure, the field is distorted, producing a plasma interface shock against the magnetic field of the Magsail. Inflation of the magnetic field due to a particle beam pressure could occur. However, the effect would be to allow the beam divergence to be only a bit larger.
Note also that in this diagram the sail is shown as dragging the payload behind it as it accelerates. If part of the particle beam reaches the payload it could create substantial damage. Consequently, it might it be better to distribute the payload around the superconducting hoop where it would have the most protection against incoming charged particles. Note also the stability of the superconducting loop on a beam of finite width has not been investigated to date. However, the Starshot program is looking at this issue extensively.
Figure 1: Interaction of streaming plasma flow with a Magsail. From Jordan Kare NIAC report [8].
The assumption that the moving magnetic field of the Magsail, seen in the frame of the beam as an electric field, ionizes the incoming neutral beam particles must be quantified.
Conclusions
Since beam divergence is fundamentally limited, high accelerations can be used to insure the sail will stay within the beam until it reaches the desired final velocity, even with microradian divergence. This leads to ultrahigh, 105 g’s, 106 m/s2 to accelerate to 0.06 c. The Starshot system, a laser beam-driven 1 gram sail with the goal of reaching 0.2c, has been quantified in a detailed system model by Kevin Parkin [9]. It too uses 105-106 g’s. Magsail-beam interaction remains an aspect of this concept that needs further study, probably by simulations. This promising method for interstellar travel should receive further attention.
References
1. G.A. Landis, “Optics and Materials Considerations for Laser-Propelled Lightsail,” IAA-89-664, 1989.
2. G. Nordley and A. J. Crowl, “Mass Beam Propulsion, An Overview”, JBIS 68, pp. 153-166, 2015.
3. Alan Mole, “One Kilogram Interstellar Colony Mission”, JBIS, 66, pp.381-387, 2013.
4. J, Benford, “Beam-Driven Sails and Divergence of Neutral Particle Beams” JBIS 70, pg. 449-452, 2017.
5. Report to the APS of the study on science and technology of directed energy weapons, Rev. Mod. Phys 59, number 3, part II, pg. 80,1987.
6. G. D. Nordley, “Relativistic Particle Beams for Interstellar Propulsion,” JBIS 46, pp 145-150,1993
7. Andrews, D. G. and R. M. Zubrin, “Magnetic Sails and Interstellar Travel”, JBIS 43, pp. 265-272, 1990
8. J. T. Kare, “High-acceleration Micro-scale Laser Sails for Interstellar Propulsion,” Final Report NIAC RG#07600-070, 2002.
www.niac.usra.edu/files/studies/final_report/597Kare.pdf. Accessed 03 Dec 2018.
9. K. L. G. Parkin, “The Breakthrough Starshot System Model”, Acta Astronautica 152, 370-384, 2018.
A Closer Look at Ultima Thule
“We think we are looking at the most primitive object ever imaged by a spacecraft,” said Jeff Moore (NASA Ames) at today’s Ultima Thule press conference. Moore, New Horizons geology and geophysics lead, went on to describe the process of innumerable particles growing into nodes amidst growing low velocity collision and interaction. We are truly looking at primordial materials with Ultima Thule, which is now revealed as a contact binary. Have a look.
Image: This image taken by the Long-Range Reconnaissance Imager (LORRI) is the most detailed of Ultima Thule returned so far by the New Horizons spacecraft. It was taken at 5:01 Universal Time on January 1, 2019, just 30 minutes before closest approach from a range of 18,000 miles (28,000 kilometers), with an original scale of 730 feet (140 meters) per pixel. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
Bear in mind that New Horizons was working with a Sun 1,900 times fainter than a sunny day on Earth, as mission principal investigator Alan Stern reminded the audience when he unveiled the image above. “It’s a snowman, not a bowling pin,” joked Stern as the image was displayed. Bear in mind as well that these early images are just the beginning. The mission team has now downloaded less than 1 percent of the data available on the spacecraft’s solid state recorders.
One of Jeff Moore’s slides:
And here’s the slide Moore showed to illustrate the process of accretion:
Putting these two lobes together would, Moore said, be gentle enough that “…if you were in a car collision at this speed you wouldn’t bother to fill out the insurance forms.” These are high-Sun images, meaning we see little shadow, but the Sun angle will change as we move into later views at higher resolution. Even so, note the absence of obvious impact craters, and the mottled suggestions of hills and ridges. Also note the brightness of the ‘neck’ between the lobes.
Image: The first color image of Ultima Thule, taken at a distance of 85,000 miles (137,000 kilometers) at 4:08 Universal Time on January 1, 2019, highlights its reddish surface. At left is an enhanced color image taken by the Multispectral Visible Imaging Camera (MVIC), produced by combining the near infrared, red and blue channels. The center image taken by the Long-Range Reconnaissance Imager (LORRI) has a higher spatial resolution than MVIC by approximately a factor of five. At right, the color has been overlaid onto the LORRI image to show the color uniformity of the Ultima and Thule lobes. Note the reduced red coloring at the neck of the object. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
Ultima Thule’s rotation period is currently pegged at 15 hours, plus or minus an hour. The object turns out to be red, as expected. As to reflectivity, deputy project scientist Kathy Olkin (SwRI) pointed out that the brightest areas reflect about 13 percent of incident sunlight, the darkest areas only 6 percent. Ultima Thule is, in other words, very dark, as dark as potting soil, Olkin added, with significant variation across the surface.
OSIRIS-REx: Orbital Operations at Bennu
Sometimes one mission crowds out another in the news cycle, which is what has happened recently with OSIRIS-REx. The study of asteroid Bennu, significant in so many ways, continues with the welcome news that OSIRIS-REx is now in orbit, making Bennu the smallest object ever to be orbited by a spacecraft. That milestone was achieved at 1943 UTC on December 31, which in addition to the upcoming New Year’s celebration was also deep into the countdown for New Horizons’ epic flyby of MU69, the Kuiper Belt object widely known as Ultima Thule.
Image credit: Heather Roper/University of Arizona.
I suppose the classic case of mission eclipse was the Voyager flyby of Uranus, which occurred on January 24, 1986. I was flying commercial students in a weekend course four days later in Frederick, MD and anxious to hear everything I could about the flyby, its images and their analysis, but mid-morning between flights I learned about the Challenger explosion, and the news for days, weeks, was filled with little else. Now, of course, we can study the striking images of Uranus’ rings and the tortured moon Miranda, putting them in the great context of Voyager exploration, but for a time the story was mute.
OSIRIS-REx has a long period of mapping and sampling ahead of it, with the sample site selection gearing up, and we’ll have plenty to say about it in coming weeks. Ponder that the spacecraft orbits Bennu from a distance just 1.75 kilometers from its center, a tighter value even than Rosetta’s, when it orbited 7 kilometers from the center of comet 67P/Churyumov-Gerasimenko.
From OSIRIS-REx flight dynamics system manager Mike Moreau (NASA GSFC):
“Our orbit design is highly dependent on Bennu’s physical properties, such as its mass and gravity field, which we didn’t know before we arrived. Up until now, we had to account for a wide variety of possible scenarios in our computer simulations to make sure we could safely navigate the spacecraft so close to Bennu. As the team learned more about the asteroid, we incorporated new information to hone in on the final orbit design.”
Using 3-D models of Bennu’s terrain created from OSIRIS-REx’s recent global imaging and mapping campaign, the mission team intensifies its navigation survey, and will analyze changes in the spacecraft’s orbit to study the minute gravitational pull of the object, which should tighten up existing models of not just the gravity field, but Bennu’s thermal properties and spin rate. By the summer of 2020, controllers will be ready for the spacecraft to touch the surface for sampling operations, with the sample scheduled for return to earth in September of 2023.
New Horizons Healthy and Full of Data
We’ve just learned that New Horizons is intact and functional, with a ‘phone home’ message at about 1530 UTC that checked off subsystem by subsystem — all nominal — amidst snatches of applause at the Johns Hopkins Applied Physics Laboratory. The solid state recorders (SSR) are full, with pointers indicating that flyby information is there for the sending, even as the spacecraft continues with outbound science. New Horizons will pass behind the Sun in early January, giving us a break in communications for a few days this weekend. Over the next 20 months we will get the entire package from Ultima Thule. Patience will be in order.
Here’s the approach image that was released yesterday.
Image: Just over 24 hours before its closest approach to Kuiper Belt object Ultima Thule, the New Horizons spacecraft has sent back the first images that begin to reveal Ultima’s shape. The original images have a pixel size of 10 kilometers (6 miles), not much smaller than Ultima’s estimated size of 30 kilometers (20 miles), so Ultima is only about 3 pixels across (left panel). However, image-sharpening techniques combining multiple images show that it is elongated, perhaps twice as long as it is wide (right panel). This shape roughly matches the outline of Ultima’s shadow that was seen in observations of the object passing in front of a star made from Argentina in 2017 and Senegal in 2018. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
And here’s the best approach image, released a few minutes ago at the press briefing.
The bi-lobate structure is obvious, but is it a single object or two in tight orbit of each other? We should have the answer to that question in an image that will be released tomorrow. Project scientist Hal Weaver displayed the slide below showing the shape and spin of Ultima. The lack of lightcurve is explained by New Horizons approaching along the line of polar rotation.
Some background thoughts:
Ultima Thule has pushed New Horizons to its limits. Mission principal investigator Alan Stern put it best at yesterday’s mid-afternoon news conference when he noted “We are straining at the capabilities of this spacecraft. There are no second chances for New Horizons.”
If the primary mission had been the long-studied flyby of Pluto/Charon, whose orbit had the benefit of decades of analysis, Ultima Thule presented controllers with an object not known until 2014, when it was discovered as part of the deliberate hunt for a Kuiper Belt object within range. Thus much about the orbit was unknown, making for what Stern described as a ‘tough intercept.’ Factor in the increased distance from the Sun far beyond Pluto and its effects on lighting conditions, as well as a power generator now producing less wattage because of its age.
Fortunately, LORRI, the Long Range Reconnaissance Imager, had spotted Ultima as far back as August 16 and the spacecraft had been imaging the object ever since, using long exposure times and co-adding procedures in which multiple optical navigation images are layered over each other, until in the last month of the approach the motion of the target could be seen, as mission project manager Helene Winters showed graphically at the same news event. Hazards like moons and rings were ruled out and the optimal trajectory, with approach to within 3500 kilometers, was available. If all went well, the early imagery will give way to fine detail.
1.6 billion kilometers beyond Pluto, New Horizons needed to hit a 40 square mile box with a timing window of 80 seconds, an epic feat of navigation that will surely wind up discussed in the next edition of David Grinspoon and Alan Stern’s book Chasing New Horizons (Picador, 2018), unless the duo decide to spin Kuiper object exploration into a book of its own. But I think not. New Horizons’ story should be seen whole, a continuing story pushed to its limits and, like the Voyagers that preceded it to system’s edge, still returning priceless data.
Ultima Thule Flyby Approaches
Despite the various governmental breakdowns attendant to the event, the New Horizons flyby of Ultima Thule is happening as scheduled, the laws of physics having their own inevitability. Fortunately, NASA TV and numerous social media outlets are operational despite the partial shutdown, and you’ll want to keep an eye on the schedule of televised events as well as the New Horizons website and the Johns Hopkins Applied Physics Laboratory YouTube channel.
Image: New Horizons’ path through the solar system. The green segment shows where New Horizons has traveled since launch; the red indicates the spacecraft’s future path. The yellow names denote the Kuiper Belt objects New Horizons has observed or will observe from a long distance. (NASA/JHUAPL/SwRI).
We’re close enough now, with flyby scheduled for 0533 UTC on January 1, that the mission’s navigation team has been tightening up its estimates of Ultima Thule’s position relative to the spacecraft, key information when it comes to the timing and orientation of New Horizons’ observations. Raw images from the encounter will be available here. Bear in mind how tiny this object is — in the range of 20 to 30 kilometers across — so that we have yet to learn much about its shape and composition, though we’ve already found that it has no detectable light curve.
On the latter point, mission principal investigator Alan Stern (SwRI):
“It’s really a puzzle. I call this Ultima’s first puzzle – why does it have such a tiny light curve that we can’t even detect it? I expect the detailed flyby images coming soon to give us many more mysteries, but I did not expect this, and so soon.”
Thus the mission proceeds in this last 24 hours before flyby with grayscale, color, near-infrared and ultraviolet observations, along with longer-exposure imaging to look for objects like rings or moonlets around Ultima. Closest approach is to be 3,500 kilometers at a speed of 14.43 kilometers per second. JHU/APL is reporting that the pixel sizes of the best expected color and grayscale images and infrared spectra will be 330 meters, 140 meters and 1.8 kilometers, respectively, with possible images at 33-meter grayscale resolution depending on the pointing accuracy of LORRI, the Long Range Reconnaissance Imager.
Image: New Horizons’ cameras, imaging spectrometers and radio science experiment are the busiest members of the payload during close approach operations. New Horizons will send high-priority images and data back to Earth in the days surrounding closest approach; placed among the data returns is a status check – a “phone home signal” from the spacecraft, indicating its condition. That signal will need just over 6 hours, traveling at light speed, to reach Earth. (NASA/JHUAPL/SwRI).
Post flyby, New Horizons will turn its ultraviolet instrument back toward the Sun to scan for UV absorption by any gases the object may be releasing, while simultaneously renewing the search for rings. Scant hours after the flyby, New Horizons will report back on the success of the encounter, after which the downlinking of approximately 7 gigabytes of data can begin. The entire downlink process, as at Pluto/Charon, is lengthy, requiring about 20 months to complete.
Let’s keep in mind that, assuming all goes well at Ultima Thule, we still have a working mission in the Kuiper Belt, one with the potential for another KBO flyby, and if nothing else, continuing study of the region through April of 2021, when the currently funded extended mission ends (a second Kuiper Belt extended mission is to be proposed to NASA in 2020). The Ultima Thule data return period will be marked by continuing observation of more distant KBOs even as New Horizons uses its plasma and dust sensors to study charged-particle radiation and dust in the Kuiper Belt while mapping interplanetary hydrogen gas produced by the solar wind.
So let’s get this done, and here’s hoping for a successful flyby and continued exploration ahead! It will be mid-afternoon UTC on January 1 (mid-morning Eastern US time) when we get the first update on the spacecraft’s condition, with science data beginning to arrive at 2015 UTC, and a first 100 pixel-across image (and more science data coming in) on January 2 at 0155 UTC. The best imagery is going to take time to be released, perhaps becoming available by the end of February. We’ll be talking about Ultima Thule a good deal between now and then.
Today is the day we explore worlds farther than ever in history!! EVER. #NASA #Space #Science #UltimaFlyby #ULtimaThule pic.twitter.com/3Stdh2JrMM
— Alan Stern (@AlanStern) December 31, 2018
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