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Planetary Science Vision 2050 Workshop 2017 (LPI Contrib. No. 1989)
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OBSERVING OUTER PLANET SYSTEMS IN THE MID-21ST CENTURY.
Matthew S. Tiscareno ([email protected]) and Mark R. Showalter ([email protected])
SETI Institute, 189 Bernardo Avenue #200, Mountain View CA 94043.
Executive Summary: We offer several ideas on
studying the outer solar system during the coming decades. Our particular interest is in ring systems and
satellite systems, though surfaces and interiors and
atmospheres are briefly touched upon.
The technology of tomorrow will likely make basic
hardware increasingly inexpensive, both for computing
and for rocketry. Data transmission and storage will
also become much more inexpensive, such that human
attention rather than data will be the limiting factors in
scientific endeavor. Automation and other data strategies will help us to grapple with this new reality [1].
These trends will enable better traditional spacecraft missions as well as entirely new kinds of spacecraft missions. We discuss some modest ideas for
spacecraft missions in the second part of this abstract.
On a parallel track, there is enormous potential in
taking existing technology and multiplying it as it becomes more inexpensive, especially in the realm of
space telescopes. We discuss some of the science return that could accrue in response to a major increase
in time-domain observations.
Finally, we close by discussing the scientific community of tomorrow, with hopes that it will be more
diverse and welcoming.
Space Telescopes: Studying the outer planets with
space-based and ground-based telescopes will be essential in the near-term future, as no spacecraft will be
operating in any outer planet system between the impending close of the Cassini and Juno missions and the
arrival of the Europa and Juice missions around 2030.
The Hubble Space Telescope continues to generate
critically important science, and the James Webb
Space Telescope will improve upon its capabilities in
several ways. Particularly in regard to rings and small
moons, JWST will discover new rings and moons that
are beyond the sensitivity of Hubble, will conduct unprecedented spectroscopy of rings and moons, and will
continue Hubble’s important work of time-domain
science [2].
Time-domain science: Many aspects of the solar
system are in constant flux, including planetary rings,
satellite systems, and atmospheres. Examples include
• Impacts and storms and cloud movements in giant
planet atmospheres, all of which are transient
events that are best studied frequently [3,4,5]
• Jupiter’s Great Red Spot, which may be in the
process of disappearing [6]
• Volcanic activity at Io, with correlated changes in
the Io Plasma Torus and in Jupiter’s aurorae [7]
• The plumes of Enceladus [8]
• “Propeller” moons embedded in Saturn’s rings [9]
• The ring arcs of Neptune, post-Voyager movements of which have invalidated the prevailing
model from the Voyager era [10]
• The F ring of Saturn [11]
• The spokes in Saturn’s rings; a seasonal pattern
has been discerned [12], but we do not know how
they vary on a climate-like scale from one Saturn
year to another [2].
• The ring-moon system of Uranus, which shows
signs of recent and frequent change [13]
Heretofore, the study of such systems has involved
periodically taking detailed snapshots and then using
theory to figure out how the snapshots fit together.
While that method has met with much success, the
enormous increase in understanding that has come
from Cassini’s extended time baseline in the Saturn
system demonstrates how often nature surprises us
when we fill in the gaps with more data, rather than
with our own surmises.
By the mid-21st century, it will have become vastly
more inexpensive to launch a space telescope incurporating Hubble-class optics and electronics. Data
transmission and storage will also become more and
more affordable, to the point that they no longer exert
limitations on our work. This will open an entirely
new horizon with regard to time-domain science. It is
not difficult to imagine a number of space telescopes,
each focused on extended observations of one or a few
targets. This will enable us to move beyond understanding basic structure and to focus our attention on
weather and climate (and their analogues in other types
of systems).
Spacecraft Mission Concepts:
Visiting Ocean Worlds. The currently planned
missions to Europa (NASA) and Ganymede (ESA) will
have concluded by 2050. As we continue to search for
evidence of habitability on the “ocean worlds” of the
solar system, perhaps the most compelling target is
Enceladus, whose subsurface water reservoirs were
recently shown by analysis of its rotation state to extend globally [14]. A more multifaceted target would
Planetary Science Vision 2050 Workshop 2017 (LPI Contrib. No. 1989)
be one or both of the Ice Giant planets, which feature
complex ring systems wholly unlike those of Saturn,
nearly unexplored icy moons that may well harbor
oceans, and planets of a size class that is poorly understood yet known to be plentiful among exoplanets. Ice
Giant orbiter missions have been proposed to ESA [15]
and are under study by NASA [16]; a joint effort between the two agencies is likely the best solution.
Returning to Saturn’s rings. Ring systems furnish
the only accessible natural laboratory in which we can
study disk processes, which are of high importance for
understanding the origins of planetary systems [5,17].
The Cassini mission has returned a wealth of discoveries regarding Saturn’s rings and moons [18,19], opening a window onto a new set of more detailed science
questions. Are the rings much younger than the planet
and its moons, or indeed are many of the moons also
younger than 100 Myr? How do disk processes such
as self-gravity wakes and propeller moons shed light
on the workings of proto-planetary systems? Spacecraft that specifically study Saturn’s rings in more detail, whether as dedicated missions or opportunistically, have the potential for high science return.
Advances in propulsion technology may enable the
Saturn Ring Observer mission concept [20] to be realized. By exerting a low but constant vertical thrust,
such a spacecraft would “hover” over the rings and
take detailed movies of individual particle interactions
within the rings.
Also, Keplerian trajectories that repeatedly skim
above Saturn’s rings in a geometry similar to that of
Cassini’s Saturn Orbit Insertion are in development
and could be used for a variety of mission concepts,
including low-cost ones.
Cubesat/Chipsat concepts. As Moore’s Law runs
aground due to physical limitations associated with
heat flow and other factors [21], computer innovation
will increasingly shift towards parallelization and towards making small components that are more capable
and more affordable. Planetary scientists would do
well to consider the new vistas opened up by large
numbers of small components. Seeding Saturn’s rings
with a swarm of “chipsats” may be more affordable
than a hovering spacecraft as a way of studying the
inter-particle interactions within disk systems [22].
Other problems amenable to swarms of chipsats might
include mapping the magnetospheres, gravity fields,
and/or atmospheres of the giant planets [23]. When
they become sufficiently plentiful, “cubesats” may also
be deployed inexpensively for focused studies of targets such as Iapetus or Chariklo that raise compelling
science questions that are perhaps not broad enough to
justify missions of even Discovery-class expense [23].
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Workforce Makeup and Climate: No less important than the goals we will pursue in the coming
decades and the means we will use to get there is the
scientific community that will undertake the work.
Currently, several communities within the general
population are severely underrepresented among planetary scientists, as are women [24], and members of
those minoritized groups have reported various forms
of harassment and other difficulties that hamper the
advancement and flourishing of their careers [25]. To
give one example, a recent study showed that spacecraft science teams and other paths to career advancement commonly lack women, even compared only to
the pool of qualified applicants at the time the team
was formed [26]. Far more research is needed to illuminate the magnitude of the problem, and the community must forge a courageous consensus to implement
solutions.
We particularly endorse the abstract submitted to
this workshop by Rathbun et al. [24], which is dedicated to discussing this important topic.
References: [1] Showalter MR et al. (2016), this
workshop. [2] Tiscareno MS et al. (2016), Pub. Astron.
Soc. Pac. 128, 018008. [3] Hueso R et al. (2010) Astrophys. J. 721, L129 [4] Sanchez-Lavega A et al.
(2016), arXiv 1611.07669. [5] Sromovsky LA et al.
(2015) Icarus 58, 192–223. [6] Simon-Miller A et al.
(2002), Icarus 158, 249–266. [7] Yoshioka K et al.
(2014), Science 345, 1581–1584. [8] Hedman MM et
al. (2013), Nature 500, 182–184. [9] Tiscareno MS et
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CS et al. (2012), Exp. Astron. 33, 753–791. [16] Hofstadter MD et al. (2016), this workshop. [17] Burns JA
and Cuzzi JN (2006), Science 312, 1753–1755. [18]
Tiscareno MS and Murray CD, eds. (2017), Planetary
Ring Systems: Properties, Structure, and Evolution,
Cambridge UK: Cambridge Univ. Press, 650pp. [19]
Tiscareno MS (2013), arXiv 1112.3305. [20] Spilker
TR et al. (2010), J. Brit. Interplanet. Soc. 63, 345–350.
[21] Waldrop MM (2016), Nature 530, 145–147. [22]
Hedman MM et al. (2012), iCubeSat 1, B.1.1. [23]
Tiscareno MS et al. (2012), iCubeSat 1, B.1.2. [24]
Rathbun JA et al. (2016), this workshop. [25] Diniega
S et al. (2016), Eos 97 (20), 11–13. [26] Rathbun JA et
al. (2016), DPS 48, 332.01.