Centauri Dreams
Imagining and Planning Interstellar Exploration
Ross 128 b: A ‘Temperate’ Planet?
At 10.89 light years from Earth, Ross 128 is the twelfth closest star to the Solar System, a red dwarf (M4V) first cataloged in 1926 by astronomer Frank Elmore Ross. Now we have news that a team working with the European Southern Observatory’s HARPS spectrograph (High Accuracy Radial velocity Planet Searcher) at the La Silla Observatory in Chile has discovered an Earth-sized planet orbiting Ross 128 every 9.9 days, a world whose orbit could conceivably place it in the habitable zone, where liquid water could exist on the surface. That gives us a second nearby world in an interesting orbit, the other of course being Proxima Centauri b.
What gives the Ross 128 b detection a wrinkle of astrobiological interest is that the star the planet orbits is relatively inactive. Red dwarfs are known for the flares that can flood nearby planets with ultraviolet and X-ray radiation. Compounded with the fact that habitable zone planets must orbit quite close to a parent M-dwarf (given the star’s small size and low temperature compared to the Sun), such flares could act as a brake on the development of life.
Ross 128 b may thus have a higher likelihood for astrobiological activity than Proxima b, assuming that it actually is in the habitable zone. Right now the team behind this work, led by Xavier Bonfils (Université Grenoble Alpes) hedges its bets by referring to the planet as ‘temperate’ and ‘close to the inner edge of the conventional habitable zone.’
Image: This artist’s impression shows the temperate planet Ross 128 b, with its red dwarf parent star in the background. This planet, which lies only 11 light-years from Earth, was found by a team using ESO’s unique planet-hunting HARPS instrument. The new world is now the second-closest temperate planet to be detected after Proxima b. It is also the closest planet to be discovered orbiting an inactive red dwarf star, which may increase the likelihood that this planet could potentially sustain life. Ross 128 b will be a prime target for ESO’s Extremely Large Telescope, which will be able to search for biomarkers in the planet’s atmosphere. Credit: ESO/M. Kornmesser.
Let’s dig into ‘habitability’ a bit more. Ross 128 is a star with about half the surface temperature of the Sun. The newly discovered planet orbits it some twenty times closer than the Earth to the Sun, while receiving 1.38 times the irradiation the Earth receives. The researchers derive an equilibrium temperature between -60 and 20°C, equilibrium temperature being determined without regard to any atmosphere. An atmosphere would substantially affect surface temperature, as the example of Venus reminds us all too well in our own system. With an equilibrium temperature of 227 K, Venus’ actual surface temperature is around 740 K.
Thus trying to figure out Ross 128 b’s placement in or near the habitable zone is a tricky proposition. Consider this from the paper:
For assumed albedos of 0.100, 0.367, or 0.750, its equilibrium temperature would thus be 294, 269, or 213 K. Using theoretically motivated albedos, the Kopparapu et al. (2017) criteria place the planet firmly outside the habitable zone, while Kopparapu et al. (2013), Yang et al. (2014), and Kopparapu et al. (2016) find it outside, inside and just at the inner edge of the habitable zone.
The key point here is that the habitable zone will take a good deal of future work to pin down. The paper resumes with this statement explaining the choice of the word ‘temperate’:
The precise location of the inner edge is therefore still uncertain, as it depends on subtle cloud-albedo feedbacks and on fine details in complex GCM [global climate] models. The habitable zone most likely will not be firmly constrained until liquid water is detected (or inferred) at the surface of many planets. Meanwhile, it is probably preferable to refer to Ross 128 b as a temperate planet rather than as a habitable zone planet.
Image: This image shows the sky around the red dwarf star Ross 128 in the constellation of Virgo (The Virgin). It was created from images forming part of the Digitized Sky Survey 2. Ross 128 appears at the centre of the picture. Close inspection reveals that Ross 128 has a strange multiple appearance as this image was created from photographs taken over a more than forty year period, and the star, which is only 11 light-years from Earth, moved across the sky significantly during this time. Ross 128 is a “quiet” red dwarf star and is orbited by Ross 128 b, an exoplanet with a similar mass and temperature to the Earth. Credit: Digitized Sky Survey 2. Acknowledgement: Davide De Martin.
Whether or not Ross 128 b is in the classic habitable zone, it presents an appealing target for future observation with instruments like the European Extremely Large Telescope. Low stellar activity bodes well for the survival of planetary atmospheres and the potential emergence of life on planets orbiting M-dwarfs. Thus this interesting planet will appear high on our target lists for future atmospheric characterization and the search for potential biomarkers.
“New facilities at ESO will first play a critical role in building the census of Earth-mass planets amenable to characterisation. In particular, NIRPS, the infrared arm of HARPS, will boost our efficiency in observing red dwarfs, which emit most of their radiation in the infrared. And then, the ELT will provide the opportunity to observe and characterise a large fraction of these planets,” concludes Bonfils.
The paper is Bonfils et al., “A temperate exo-Earth around a quiet M dwarf at 3.4 parsecs,” accepted at Astronomy & Astrophysics. I’ll post a link to the online abstract and preprint as soon as they become available.
An Origin for a Far Traveling Asteroid
I used to think the Kuiper Belt object Quaoar was hard to pronounce (“Kwawar”), and even muffed it despite having plenty of time to practice before the recent Tennessee Valley Interstellar Workshop. Pontus Brandt (JHU/APL) had mentioned Quaoar in his talk in Huntsville as a target that lined up in useful ways with a proposed interstellar precursor mission he was presenting, one designed to examine dust distribution from within the system by looking back at our heliosphere at distances up to 1000 AU, seeing it as we see other stars’ dust environments.
So I summarized Brandt’s ideas in my wrap-up talk and couldn’t get Quaoar pronounced properly without multiple tries. But even Quaoar pales into the realm of everyday lingo when compared to 1I/’Oumuamua. Please tell me how to do this. The word is a Hawaiian term for ‘scout,’ and the Ulukau: Hawaiian Electronic Library’s online dictionary tells me it’s pronounced this way: ?’u-mu’-a-mu’-?. I could work with that and maybe get it right in a talk, with extra practice. At least until I look at it — all those vowels defeat me.
The object originally tagged A/2017 U1, then, is now tagged as interstellar in the combined 1I/’Oumuamua, the 1 indicating it is the first such object to be observed, the I indicating interstellar. It is fitting that 1I/’Oumuamua was the name chosen by the Pan-STARRS team in Hawaii that first brought this object to our attention.
Image: The trajectory of 1I/ ‘Oumuamua, which made its closest approach to Earth on October 14, coming within 24,000,000 km, or about 60 times the distance to the moon. Credit: NASA/JPL-Caltech.
As far as the sheer number of objects making long interstellar journeys, consider what New Horizons PI Alan Stern recently told Astronomy Magazine (see The First Known Interstellar Interloper):
According to Stern, Jupiter, Saturn, Uranus, and Neptune combined probably ejected 1013 to 1014 objects larger than 1 km early in our solar system’s history, when it was still cluttered with debris left over from the planet-formation process. Multiply that by the 1011 stars in the Milky Way, and one comes up with numbers like 1024 to 1025 objects larger than a kilometer. Smaller objects like 1I/‘Oumuamua must be orders of magnitude more plentiful.
We’ve just seen, in the work of members of the Initiative for Interstellar Studies, how reaching such an object might be attempted. Now the question becomes, where did it come from? On that score, we have quick work indeed from Eric Gaidos (University of Hawaii at Manoa), working with Jonathan Williams at the same institution and Adam Kraus (University of Texas at Austin). The trio believes it has traced 1I/‘Oumuamua’s origins and presents its case in a paper submitted to Research Notes of the American Astronomical Society.
Gaidos and team traced the path of 1I/’Oumuamua backwards along its route and took into account the numerous variables along the way in so extended a journey. The likelihood, the researchers believe, is that the object originated in a nearby young stellar cluster. They point to the Carina and Columba Associations, the word ‘association’ referring to stellar associations, which are loosely bound star clusters whose stars share a common origin. They still move together through space but are at this point gravitationally unbound. The estimated distances to the Carina and Columba Associations range from 50 to 85 parsecs (163 to 277 light years); the age of stars within these groups is on the order of 45 million years. An object ejected at 1-2 kilometers per second soon after star formation would thus have had time to reach the Sun.
From the paper:
We suggest that A/2017 U1 formed in a protoplanetary disk in the Carina/Columba associations and was ejected by a planet ?40 Myr ago. The absence of ice indicates an origin inside the “ice line” of the disk plus an ejection velocity of 1-2 km sec?1 (assuming the cluster was already unbound), constrain the mass mP and semi-major axis aP of the planet.
What kind of planet could have ejected this object? The paper examines the issue both for solar mass stars as well as M-dwarfs:
Permitted values (grey zone) center around a 20-30M? planet forming by a few Myr within a few AU, reminiscent of the core accretion scenario for giant planet formation. In contrast, a “super-Earth” at ?1 AU could eject ice-free planetesimals from a lower-mass M dwarf.
Image: This is half of the paper’s Figure 1, showing only the projection from a protoplanetary disk around a solar mass star. The paper’s second chart, not shown here, gives equivalent information for an M-dwarf. The caption continues: “Below the red line planets accrete rather than scatter planetesimals. Above the green line planets eject planetesimals at > 1 km sec?1. Below the purple line planetesimals are captured into clouds by the cluster tide. Below the black line planetesimals require > 10 Myr to escape. To the right of the blue lines planetesimals contain ices.” Credit: Gaidos, Williams & Kraus.
As the paper notes, future interstellar interlopers may well have radiants similar to 1I/’Oumuamua. Practicing our skills on this celestial wanderer may thus tune us up for another.
The paper is Gaidos, Williams and Kraus, “Origin of Interstellar Object A/2017 U1 in a Nearby Young Stellar Association?” submitted to Research Notes of the American Astronomical Society (abstract). Our interstellar wanderer seems to be spawning a growth industry in these early days following its detection. See also Zwart et al., “The origin of interstellar asteroidal objects like 1I/2017 U1,” submitted to Monthly Notices of the Royal Astronomical Society (preprint). I haven’t read this one yet and thus won’t comment.
Project Lyra: Sending a Spacecraft to 1I/’Oumuamua (formerly A/2017 U1), the Interstellar Asteroid
Now that we have determined that the object now known as 1I/’Oumuamua is indeed interstellar in origin, is there any way we could launch a mission to study it? The study below, written by key players in the Initiative for Interstellar Studies (i4is), examines the possibilities. Andreas Hein is Executive as well as Technical Director of i4is, while Nikolas Perakis, a graduate student at the Technical University of Munich, serves as Deputy Technical director. Kelvin Long is president and co-founder of i4is; Adam Crowl, a familiar figure to Centauri Dreams readers, is active in its technical programs. Physicist and radio astronomer Marshall Eubanks is the founder of Asteroid Initiatives; systems engineer Robert Kennedy is president of i4is-US and general chair of the Asilomar Microcomputer Workshop. Propulsion scientist Richard Osborne serves as i4is Director of Technology & Strategic Foresight. Their plan for 1I/’Oumuamua follows. For a more in-depth look, view the paper just released on arXiv at https://arxiv.org/abs/1711.03155.
by Andreas M. Hein, Nikolas Perakis, Kelvin Long, Adam Crowl, Robert G. Kennedy III, Marshall Eubanks and Richard Osborne
A mysterious visitor from our galaxy has entered our solar system. On October 19th 2017, the University of Hawaii’s Pan-STARRS 1 telescope on Haleakala discovered a fast-moving object near the Earth, initially named A/2017 U1, but now designated as 1I/’Oumuamua [1]. This object was found to be not bound to the solar system, with a velocity at infinity of ~26 km/s and an incoming radiant (direction of motion) near the solar apex in the constellation Lyra [2]. Due to the non-observation of a tail in the proximity of the Sun, the object does not seem to be a comet but an asteroid. More recent observations from the Palomar Observatory indicate that the object is reddish, similar to Kuiper belt objects [3]. This is a sign of space weathering. Its orbital features have been analyzed by [2,4].
When will such an object visit us again? In 10 years, 100 years, 1000? We do not know. This could be the only opportunity in a lifetime, or even in a 100 lifetimes to observe an interstellar visitor close by. As 1I/‘Oumuamua is the nearest macroscopic sample of interstellar material, likely with an isotopic signature distinct from any other object in our solar system, the scientific returns from sampling the object are hard to overstate. Detailed study of interstellar materials at interstellar distances are likely decades away, even if Breakthrough Initiatives’ Project Starshot, for example, is vigorously pursued. Hence, an interesting question is if there is a way to exploit this unique opportunity by sending a spacecraft to 1I/’Oumuamua to make observations at close range.
To answer these questions, the Initiative for Interstellar Studies, i4is, has announced Project Lyra on the 30th of October. The goal of the project is to assess the feasibility of a mission to 1I/’Oumuamua using current and near-term technology and to propose mission concepts for achieving a fly-by or rendezvous. The challenge is formidable: 1I/’Oumuamua has a hyperbolic excess velocity of 26 km/s, which translates to a velocity of 5.5 AU/year. It will be beyond Saturn’s orbit within two years. This is much faster than any object humanity has ever launched into space. Compare this to Voyager 1, the fastest object humanity has ever built, which has a hyperbolic excess velocity of 16.6 km/s. As 1I/’Oumuamua is already on its way of leaving our solar system, any spacecraft launched in the future needs to chase it. However, besides the scientific interest of getting data back from the object, the challenge to reach the object could stretch the current technological envelope of space exploration. Hence, Project Lyra is not only interesting from a scientific point of view but also in terms of the technological challenge it presents.
Figure 1: Logo for the i4is initiative Project Lyra
After days of intense work, we are now able to present some preliminary results for reaching the object within a timeframe of a few decades.
Trajectory analysis
Given the hyperbolic excess velocity and its inclination with respect to the solar system ecliptic, the first question to answer is the required velocity increment (DeltaV) to reach the object, a key parameter for designing the propulsion system. Obviously, a slower spacecraft will reach the object later than a faster spacecraft, leading to a trade-off between trip duration and required DeltaV. Furthermore, the earlier the spacecraft is launched, the shorter the trip duration as the object’s distance increases with time. However, a launch date within the next 5 years is likely to be unrealistic, and even 10 years could be challenging, in case new technologies need to be developed. Hence, a third basic trade-off is between launch date and trip time / characteristic energy C3. The characteristic energy is the square of the hyperbolic excess velocity, which can be understood as is the velocity at infinity with respect to the Sun. Nikolaos Perakis (i4is) has captured these trade-offs in Figure 2. The figure plots the characteristic energy for the launch with respect to mission duration and launch date. An impulsive propulsion system with a sufficiently short thrust duration is assumed. No planetary or solar fly-by is assumed, only a direct launch towards the object. The deformations of the velocity curves is due to the Earth’s orbit around the Sun, which results in a more or less favorable position for a launch towards the object. It can be seen that a minimum ?3 exists, which is about 26.5 km/s (703km²/s²). However, this minimum value rapidly increases when the launch date is moved into the future. At the same time, a larger mission duration leads to a decrease of the required ?3 but also implies an encounter with the asteroid at a larger distance from the Sun. A realistic launch date for a probe would be at least 10 years in the future (2027). At that point, the hyperbolic excess velocity is already at 37.4km/s (1400km²/s²) with a mission duration of about 15 years, which makes such an orbital insertion extremely challenging with conventional launches in the absence of a planetary fly-by.
Figure 2: Characteristic energy C3 with respect to mission duration and launch date.
Apart from the hyperbolic excess velocity at launch, the excess velocity relative to the asteroid at encounter (??,2) has to be taken into account since it defines the type of mission that is achievable. A high excess velocity with respect to the asteroid reduces the flight duration but also reduces the time available for the measurements close to the interstellar object. On the other hand, a low value for ??,2 could even enable orbital insertion around the asteroid with an impulsive or low thrust maneuver to decelerate the probe. The excess velocity at arrival is plotted in Figure 3 as a function of the launch date and the flight duration. It can be seen that a minimum excess velocity of about 26.75 km/s implies a launch in 2018 and a flight duration of over 20 years. Such value for excess velocity does not prohibit an orbital insertion around ‘Oumuamua. However, this minimum value rapidly increases for later launch dates. A realistic launch date for a probe would be between 5 to 10 years in the future (2023 to 2027). At that point, the required hyperbolic excess velocity for the mission is between 33 to 76 km/s, for mission durations between 30 to 5 years. These values highly exceed the current chemical and electric propulsion system capabilities for deceleration and orbital insertion, and hence a fly-by would be more reasonable.
Figure 3: Hyperbolic excess velocities with respect to mission duration and launch date
Figure 4 shows the approximate distance at which the spacecraft passes the object. For a realistic launch date of 2027 or later, the spacecraft flies past the object at a distance between 100 and 200 AU, which is similar to the distance to the Voyager probes today. At such a distance, obviously power and communication becomes an issue and nuclear power sources such as RTGs are required.
Figure 4: Launch date versus mission duration. Color code indicates the distance at which the spacecraft passes the object
Figure 5 shows a sample trajectory with a launch date in 2025. The orbit of Earth can be seen as a tiny ellipse around the Sun (indicated as a black circle) at the bottom right of the figure. The trajectories of the comet and the spacecraft are almost straight lines. 
Figure 5: Sample spacecraft trajectory for a launch in 2025 and an encounter with 1I/‘Oumuamua in 2055
Another thought by Robert Kennedy (i4is) is to not necessarily chase 1I/‘Oumuamua but to prepare for the next interstellar object to enter our solar system by developing the means to quickly launch a spacecraft towards such an object.
Two scenarios are analysed: First a mission with short duration of only a year, leading to an encounter only 5.8 AU from the sun. However the required hyperbolic excess velocity the current launcher capabilities at approximately 20 km/s. Finally, due to the angle of the encounter, a high velocity relative to the asteroid would be expected, amounting to 13.6 km/s, shown in Figure 6.
Figure 6: Trajectory for a launch in 2017 and an encounter in 2018
A mission on the same launch date but with a duration of 20 years is shown in Figure 7. At encounter, the relative velocity of the spacecraft with respect to the object is relatively low (about 600m/s for this specific case), which would be an opportunity for a deceleration maneuver.
Figure 7: Trajectory for a launch in 2017 and an encounter in 2037
To summarize, the difficulty of reaching 1I/‘Oumuamua is a function of when to launch, the hyperbolic excess velocity, and the mission duration. Future mission designers would need to find appropriate trade-offs between these parameters. For a realistic launch date in 5 to 10 years, the hyperbolic excess velocity is of the order of 33 up to 76 km/s with an encounter at a distance far beyond Pluto (50-200AU).
Concepts and technologies
As shown previously, chasing 1I/‘Oumuamua with a realistic launch date (next 5-10 years), is a formidable challenge for current space systems. Adam Crowl (i4is) and Marshall Eubanks (Asteroid Initiatives LLC) have pondered a single launch architecture. Nominally a single launch architecture, via the Space Launch System (SLS) for example, would simplify mission design. However other launch providers project promising capabilities in the next few years. One potential mission architecture is to make use of SpaceX’s Big Falcon Rocket (BFR) and their in-space refueling technique with a launch date in 2025. To achieve the required hyperbolic excess (at least 30 km/s) a Jupiter flyby combined with a close solar flyby (down to 3 solar radii), nicknamed “solar fryby” is envisioned. This maneuver is also known under “Oberth Maneuver” [5]. The architecture is based on the Keck Institute for Space Studies (KISS) [6] and the Jet Propulsion Laboratory (JPL) [7] interstellar precursor mission studies. Using the BFR however eliminates the need for multi-planet flybys to build up momentum for a Jupiter trajectory. Instead via direct launch from a Highly Eccentric Earth Orbit (HEEO) the probe, plus various kick-stages, is given a C3 of 100 km²/s² into an 18 month trajectory to Jupiter for a gravity assist into the solar fryby. A multi-layer thermal shield protects the spacecraft, which is boosted by a high-thrust solid rocket stage at perihelion. The KISS Interstellar Medium study computed that a hyperbolic excess velocity of 70 km/s was possible via this technique, a value which achieves an intercept at about 85 AU in 2039 for a 2025 launch. More modest figures can still fulfill the mission, such as 40 km/s with an intercept at 155 AU in 2051. With the high approach speed a hyper-velocity impactor to produce a gas ‘puff’ to sample with a mass spectrometer could be the serious option to get in-situ data.
The above architecture emphasizes urgency, rather than advanced techniques. Kelvin Long (i4is) has thought about using more advanced technologies, for example solar sails, laser sails, and laser electric propulsion could open up further possibilities to flyby or rendezvous with 1I/‘Oumuamua. In the following, first order analyses for solar and laser sail missions are given.
For the solar sail mission, Kelvin assumed a launch from Earth orbit, given a time to launch of 3 to 4 years. The velocity requirement is ~55 km/s, suggesting a lightness number for the mission of 0.15, and a characteristic acceleration of 0.009 m/s2. This requires a sail loading of 1 g/m², advanced materials with light payloads might achieve 0.1 g/m². Given this, for different spacecraft masses assuming a sail loading of ? = 1 g/m² sail design leads to the values shown in Table 1 for a circular and square-shaped sail.
Table 1: Solar sail parameters with respect to spacecraft mass
| Spacecraft mass [kg] | Sail area [m²] | Circular radius [m] | Square size [m] |
|
0.001 |
1 |
0.56 |
1 |
|
0.01 |
10 |
1.78 |
3 |
|
0.1 |
100 |
5.64 |
10 |
|
1 |
1000 |
17.84 |
32 |
|
10 |
10,000 |
56.42 |
100 |
|
100 |
100,000 |
178.41 |
316 |
The most appropriate and practical design would assume a launch in 4 years and a 1 kg spacecraft mass and lower.
Laser-pushed sail-based missions, based on Breakthrough Initiatives’ Project Starshot technology [8–10], would use a 2.74 MW laser beam, with a total velocity increment of 55 km/s, launched in 3.5 years (2021), accelerating at 1g for 3,000s, the probe size would be about 1 gram. It would reach 1I/‘Oumuamua in about 7 years. With a 27.4 MW laser then a 10 gram probe could be used. Higher spacecraft masses could be achieved by using different mission architectures, lower acceleration rates, and longer mission durations. However, with such a laser beaming infrastructure in place, hundreds or even thousands of probes could be sent, as illustrated in Figure 8. Such a swarm-based or distributed architecture would allow for gathering data over a larger search volume without the limitations of a single monolithic spacecraft.
Figure 8: Laser sail swarm (Image credit: Adrian Mann)
Another concept proposed by Streeman and Peck [11] is to send ChipSats into the magnetosphere of Jupiter, then using the Lorentz force to accelerating them to very high velocities of about 3,000 km/s [11–13]. However, controlling the direction of these probes might not be trivial.
An important implication is that once an operational Project Starshot beaming infrastructure has been established, even at a small scale, missions to interstellar objects flying through the solar system could be launched within short notice and could justify their development. The main benefit of such an architecture would be the short response time to extraordinary opportunities. The investment would be justified by the option value of such an infrastructure.
Regarding deceleration at the object, obviously existing propulsion systems could be used, e.g. electric propulsion, though limited by the low specific power of RTGs as a power source. With an intercept distance beyond the Heliosphere, into the pristine Interstellar Medium (ISM) more advanced technologies such as magnetic sails [14,15], electric sails [16], and the more recent magnetoshell braking system [17] are worth investigating. The Technological Readiness of these more advanced technologies is currently low, dependent on breakthroughs in superconducting materials manufacture, but they would multiply the scientific return by orders of magnitude.
The small size of the object and its low albedo will make it difficult to observe it once it has entered deep space again. This means the navigation problem of getting a sufficiently accurate fix on 1I/‘Oumuamua to get close enough to the object to send back useful data is considerable. Due to the positional uncertainty of such a difficult-to-track object, a distributed, swarm-based mission design that is able to span a large area, should be investigated.
Conclusions
The discovery of the first interstellar object entering our solar system is an exciting event and could be the chance of a lifetime or several lifetimes. In order to assess the feasibility of reaching this object, i4is has recently initiated Project Lyra. In this article, we identified key challenges of reaching 1I/‘Oumuamua and ballpark figures for the mission duration and hyperbolic excess velocity with respect to the launch date. In any case, a mission to the object will stretch the boundary of what is technologically possible today. A mission using conventional chemical propulsion system would be feasible using a Jupiter flyby to gravity-assist into a close encounter with the Sun. Given the right materials, solar sail technology or laser sails could be used.
An important result of our analysis is that the value of a laser beaming infrastructure from the Breakthrough Initiatives’ Project Starshot would be the flexibility to react quickly to future unexpected events, such as sending a swarm of probes to the next object like 1I/‘Oumuamua. With such an infrastructure in place today, intercept missions could have reached 1I/‘Oumuamua within a year.
Future work within Project Lyra will focus on analyzing the different mission concepts and technology options in more detail and to downselect 2-3 promising concepts for further development.
References
[1] The International Astronomical Union – Minor Planet Center, MPEC 2017-V17?: New Designation Scheme for Interstellar Objects, Minor Planet Electronic Circular. (2017). https://www.minorplanetcenter.net/mpec/K17/K17V17.html (accessed November 7, 2017).
[2] E. Mamajek, Kinematics of the Interstellar Vagabond A/2017 U1, (2017). http://arxiv.org/abs/1710.11364 (accessed November 5, 2017).
[3] J. Masiero, Palomar Optical Spectrum of Hyperbolic Near-Earth Object A/2017 U1, (2017). http://arxiv.org/abs/1710.09977 (accessed November 5, 2017).
[4] C. de la F. Marcos, R. de la F. Marcos, Pole, Pericenter, and Nodes of the Interstellar Minor Body A/2017 U1, (2017). doi:10.3847/2515-5172/aa96b4.
[5] R. Adams, G. Richardson, Using the Two-Burn Escape Maneuver for Fast Transfers in the Solar System and Beyond, in: 46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, American Institute of Aeronautics and Astronautics, Reston, Virigina, 2010. doi:10.2514/6.2010-6595.
[6] L. Friedman, D. Garber, Science and Technology Steps Into the Interstellar Medium, 2014.
[7] L. Alkalai, N. Arora, S. Turyshev, M. Shao, S. Weinstein-Weiss, A Vision for Planetary and Exoplanet Science: Exploration of the Interstellar Medium: The Space between Stars, in: 68th International Astronautical Congress (IAC 2017), 2017.
[8] P. Lubin, A Roadmap to Interstellar Flight, Journal of the British Interplanetary Society. 69 (2016).
[9] A.M. Hein, K.F. Long, D. Fries, N. Perakis, A. Genovese, S. Zeidler, M. Langer, R. Osborne, R. Swinney, J. Davies, B. Cress, M. Casson, A. Mann, R. Armstrong, The Andromeda Study: A Femto-Spacecraft Mission to Alpha Centauri, (2017). http://arxiv.org/abs/1708.03556 (accessed November 5, 2017).
[10] A.M. Hein, K.F. Long, G. Matloff, R. Swinney, R. Osborne, A. Mann, M. Ciupa, Project Dragonfly: Small, Sail-Based Spacecraft for Interstellar Missions, Submitted to JBIS. (2016).
[11] B. Streetman, M. Peck, Gravity-assist maneuvers augmented by the Lorentz force, Journal of Guidance, Control, and Dynamics. (2009).
[12] M. Peck, Lorentz-actuated orbits: electrodynamic propulsion without a tether, NASA Institute for Advanced Concepts, Phase I Final Report. (2006). http://www.niac.usra.edu/files/studies/abstracts/1385Peck.pdf (accessed April 18, 2016).
[13] J. Atchison, B. Streetman, M. Peck, Prospects for Lorentz Augmentation in Jovian Captures, in: AIAA Guidance, Navigation, and Control Conference and Exhibit, American Institute of Aeronautics and Astronautics, Reston, Virigina, 2006. doi:10.2514/6.2006-6596.
[14] D. ANDREWS, R. ZUBRIN, Magnetic sails and interstellar travel, British Interplanetary Society, Journal. (1990). http://www.lunarsail.com/LightSail/msit.pdf (accessed April 16, 2016).
[15] N. Perakis, A.M. Hein, Combining Magnetic and Electric Sails for Interstellar Deceleration, Acta Astronautica. 128 (2016) 13–20.
[16] P. Janhunen, Electric sail for spacecraft propulsion, Journal of Propulsion and Power. (2004). http://arc.aiaa.org/doi/abs/10.2514/1.8580 (accessed August 14, 2016).
[17] A. Shimazu, D. Kirtley, D. Barnes, J. Slough, Cygnus Code Simulation of Magnetoshell Aerocapture and Entry System, Bulletin of the American Physical Society. (2017).
Ceres: A Residual Ocean?
Given yesterday’s look at the ocean beneath Enceladus’ ice, it seems the right time to examine the recent work on Ceres. We know that the dwarf planet may have had a global ocean of its own, but as with Enceladus, questions abound. Is there still liquid within Ceres? We have two new studies from the Dawn mission to give us some insights. The upshot:
“More and more, we are learning that Ceres is a complex, dynamic world that may have hosted a lot of liquid water in the past, and may still have some underground,” said Julie Castillo-Rogez, Dawn project scientist and co-author of the studies, based at NASA’s Jet Propulsion Laboratory, Pasadena, California.
Anton Ermakov (JPL) is lead author of the first paper, published in the Journal of Geophysical Research, which examined gravity data measurements from Dawn to analyze the composition of Ceres. This is exceedingly fine-grained work, drawing not only on Dawn data but on Deep Space Network observations of tiny changes in the spacecraft’s orbit. We learn that the craters Occator, Kerwan and Yalode, along with the mountain Ahuna Mons, are all associated with gravity anomalies — differences between observed gravity and the values predicted by our best models of the dwarf planet’s gravitational field.
The variations from the scientists’ models of Ceres gravity and what Dawn actually observed at these four locations can tell us something about structure and composition beneath the surface. Both Ahuna Mons and Occator appear to be associated with cryovolcanism. We also learn that the density of the crust is closer to ice than rock, a puzzling finding given other Dawn studies showing that ice would be too soft to serve as the dominant component in Ceres’ crust.
But there is an explanation. From the paper:
Finite element modeling of Ceres’ topography [Fu et al., 2017] shows that the topographic power cannot be supported by a solely ice rheology [physics dealing with the deformation and flow of matter] over billion year timescales. Using a lower bound for crustal density based on rheology, we derive constraints on the crustal thickness using the assumption of hydrostatic equilibrium. A low-density, high strength mixture is required to explain the inferred crustal density and rheology. The latter does not allow more than 43 vol% silicates assuming 15% void porosity in the crust. Therefore, lower density materials, such as salt or gas (clathrate) hydrates, are required.
Image: This animation shows Ceres as seen by NASA’s Dawn spacecraft from its high-altitude mapping orbit at 1,470 kilometers above the surface. The colorful map overlaid at right shows variations in Ceres’ gravity field measured by Dawn, and gives scientists hints about the dwarf planet’s internal structure. Red colors indicate more positive values, corresponding to a stronger gravitational pull than expected, compared to scientists’ pre-Dawn model of Ceres’ internal structure; blue colors indicate more negative values, corresponding to a weaker gravitational pull. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
A second study, published in Earth and Planetary Science Letters, delved into that crust, its strength and its composition, by studying Ceres’ topography. We would expect that a crust laden with ices and salts would gradually deform over the age of the Solar System, whereas a crust dominated by rock could remain essentially unchanged. Flow models that Roger Fu (Harvard University) applied to the data show a crust that not only mixes ice, salts and rock, but is also composed of clathrate hydrate, as suggested in the paper above.
The latter is the key: Clathrate hydrate produces a structure far stronger than water ice, although maintaining nearly the same density. Fu and colleagues believe that Ceres once had more well defined surface features that have smoothed out over time. The process would require a deformable layer beneath a high-strength crust, and that deformable layer may well contain liquid. We have the possibility, therefore, of at least a small residual liquid ocean.
The Ermakov paper is “Constraints on Ceres’ internal structure and evolution from its shape and gravity measured by the Dawn spacecraft,” Journal of Geophysical Research: Planets, 18 October 2017 (abstract). The Fu paper is “The interior structure of Ceres as revealed by surface topography,” Earth and Planetary Science Letters, Vol. 476 (15 October 2017), 153-164 (abstract).
Heating Up Enceladus
How to explain the water vapor and ice blown off in the form of geyser-like jets from Enceladus? It’s a question we need to answer, because we’re learning just how interesting the icy moons of gas giants can be, with the potential for biological activity far from the Sun.
In the case of Enceladus, though, the average global thickness of the ice is thought to be 20 to 25 kilometers. What has thinned the ice at the south pole, where warm fractures expel mineral-rich water into space? An unusual amount of heat is demanded to sustain this ongoing activity, along with a mechanism to explain what is happening within the moon.
The heat in question is likely the result of friction, according to a new study published in Nature Astronomy. The work of Gaël Choblet (University of Nantes, France) and colleagues, the investigation involved modeling by Cassini researchers in Europe and the U.S., tapping the abundant data that the Saturn orbiter returned to Earth in its 13 years at the planet. It was Cassini that first showed us the jets of water vapor and simple organics.
Image: Dramatic plumes, both large and small, spray water ice out from many locations along the famed “tiger stripes” near the south pole of Saturn’s moon Enceladus. The tiger stripes are fissures that spray icy particles, water vapor and organic compounds. More than 30 individual jets of different sizes can be seen in this image and more than 20 of them had not been identified before. At least one jet spouting prominently in previous images now appears less powerful. This mosaic was created from two high-resolution images that were captured by the narrow-angle camera when NASA’s Cassini spacecraft flew past Enceladus and through the jets on Nov. 21, 2009. Credit: NASA/JPL/Space Science Institute.
The authors find evidence in the Cassini data that hydrothermal activity takes place on the seafloor under Enceladus’ ice. Rock grains thought to be the result of this activity show chemistry occurring at temperatures of at least 90 degrees Celsius. What’s intriguing here is that Choblet and team believe the energy required to produce these temperatures exceeds what could be expected from the decay of radioactive elements within Enceladus. Says Choblet:
“Where Enceladus gets the sustained power to remain active has always been a bit of a mystery, but we’ve now considered in greater detail how the structure and composition of the moon’s rocky core could play a key role in generating the necessary energy.”
The result: An Enceladus whose core is relatively loose and rocky, with 20 to 30 percent empty space. The orbiting moon would experience tidal forces that cause rock in the relatively porous core to flex and abrade against adjacent rock, generating the required heat. This internal scenario also accounts for the rise of heated water from the ocean, moving upward and interacting chemically with the rocks. From the paper’s abstract:
Water transport in the tidally heated permeable core results in hot narrow upwellings with temperatures exceeding 363?K, characterized by powerful (1–5?GW) hotspots at the seafloor, particularly at the south pole. The release of heat in narrow regions favours intense interaction between water and rock, and the transport of hydrothermal products from the core to the plume sources. We are thus able to explain the main global characteristics of Enceladus: global ocean, strong dissipation, reduced ice-shell thickness at the south pole and seafloor activity.
If Choblet’s models are correct, we should expect maximum activity from this process at the moon’s poles, with mineral-rich water thinning the ice shell to as little as 1 kilometer at the south pole, and being expelled into space through fractures in the ice. This analysis would explain why Enceladus has thinner ice at the south pole, but it does not explain why north and south poles are so utterly different. Unlike the south pole, Enceladus’ north pole is cratered and ancient.
The circulation of the global ocean over millions of years would mean that the entire volume of the ocean passes through the moon’s core, an amount estimated to equal 2 percent of the volume of Earth’s oceans. The new study goes past earlier work in modeling tidal friction at a higher level of complexity — earlier models found that tidal heating would be insufficient to keep the ocean liquid. The porous core model thus accounts for the plumes we see today, which may result from thinner ice in the south to begin with, causing runaway heating over time.
The paper is Choblet et al., “Powering prolonged hydrothermal activity inside Enceladus,” Nature Astronomy 6 November 2017 (abstract).
A Laser-Powered Ion Engine for Deep Space
When we kick around ideas for deep space propulsion, we have to keep in mind that the best solutions may involve hybrid technologies, leveraging the best of several methods. JAXA (Japan Aerospace Exploration Agency) demonstrates this with its ambitious plan to take a sail like IKAROS to Jupiter, for a study of the Jupiter trojan asteroids. Like the original, the upgraded IKAROS will use liquid crystal reflectivity control devices as a means of attitude control.
But operating at the limits of solar sail functionality, the new JAXA sail will also carry a high specific impulse ion engine to facilitate its maneuvers among the trojans. Here we have a mission that couldn’t be flown with just a sail, because the numerous trajectory changes required at destination demand a reliable thruster, one that in this case will be fed by some 30,000 solar panels in the form of thin film solar cells attached to the sail membrane.
What of missions into still deeper space? We’d like to return with robust technologies to the outer planets, not to mention the need to follow up the Voyagers and New Horizons with craft specifically designed to study the interstellar medium beyond the heliosphere.
For that matter, we’re seeing increased interest in exploring the Sun’s gravity lens, whose effects can be examined beginning at 550 AU. That last is a hot and controversial topic, as the spirited debate between Slava Turyshev (JPL) and Geoff Landis (NASA GRC) showed at the recent TVIW conference in Huntsville (keep an eye on the TVIW 2017 video page, where these discussions will soon be available). To resolve the matter, we need to actually go there.
JPL’s John Brophy has been exploring another form of hybrid technology to make such missions possible — I talked about this one when it surfaced last April (see NIAC 2017: Interstellar Implications). Now working under a Phase 1 grant from the NASA Innovative Advanced Concepts (NIAC) program, Brophy proposes using lasers to provide the power source for a spacecraft’s ion engines, beaming to solar panels on the craft. The idea here is to take advantage of a power source separate from the spacecraft in return for major gains in weight and efficiency.
Brophy’s ion engine infrastructure depends upon a 10-kilometer 100 MW laser array capable of beaming power across the Solar System. The beam would be captured by a 70% efficient photovoltaic array tuned to the laser frequency and producing power at 12 kV, according to this precis prepared for the NIAC program. As Brophy has noted, the array output here far surpasses our best solar arrays today, found on the International Space Station, which produce 160 volts. With an areal density of 200 grams per square meter, the 10-kilometer array would be heavier than today’s solar sails but substantially lighter than existing solar arrays.
The laser array in question has its roots in high-power laser concepts of the kind Philip Lubin has advocated for Breakthrough Starshot, only Brophy’s array is in space, avoiding the numerous issues raised by Starshot’s ground-based installation (and creating construction issues of its own). What you achieve with this kind of configuration is the ability to deploy powerful ion engines in the outer Solar System, as Brophy is quick to note in his NIAC precis:
Our innovation is the recognition that such an array increases the power density of photons available to a spacecraft illuminated by the laser beam by two orders of magnitude relative to solar insolation at all the solar system distances beyond 5 AU, and that this enormous power can then be used to great effect by driving a highly-advanced ion propulsion system.
Image: The laser-powered ion thruster concept as developed by John Brophy and colleagues.
The thruster being powered by the laser beam is a lithium-fueled gridded ion propulsion system that does away with what would otherwise be heavy power processing hardware and associated thermal radiators. The 58,000 second specific impulse — compare this to Dawn’s 3,000 seconds — goes well beyond current state of the art in spacecraft systems — about 20 times — and takes advantage of the fact that lithium is both easily ionized and easily stored. The result:
This allows the thruster to be operated with nearly 100% ionization of the propellant which effectively eliminates neutral gas leakage from the thruster and the production of charge-exchange ions that are responsible for thruster erosion and current collection on the photovoltaic arrays. This key benefit enables very long thruster life and facilitates the development of the 12-kV photovoltaic array.
Brophy believes such a system could achieve velocities of 260 kilometers per second, which would make missions to Jupiter feasible within one year of flight time, while reaching the gravity focus would be a matter of 10 to 12 years. If that isn’t tantalizing enough, he also talks about a Pluto orbiter mission with a travel time in the area of 4 years. Thus the hybrid concept ramps up the performance of ion engines in places far enough from the Sun to pose serious power issues, and also taps into a laser infrastructure that could one day drive missions system-wide.
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