PLANETARY DYNAMICS EXPLORER: A Space Telescope for Time-Domain Solar System Studies. Point of contact: Michael H. Wong1,2, [email protected], +1-510-224-3411. Contributing authors: John Clarke3, Amanda Hendrix4, Amy Simon-Miller5, Keith Noll5, Walter Harris6, Kunio Sayanagi7, Heidi Hammel8, David Choi5, Jim Bell9, Imke de Pater1, Glenn Orton10. 1UC Berkeley, 2University of Michigan, 3Boston University, 4PSI, 5NASA GSFC, 6UC Davis, 7Hampton University, 8AURA, 9Arizona State University, 10JPL. Primary concept goal category: planetary science Additional concept goal categories: human spaceflight, space technology Concept summary: The Planetary Dy- ducing time-domain data sets that no other namics Explorer (PDX) will observe time- current or planned facility will be able to variable phenomena in planetary atmospheres match. Animated PDX data will have very and surfaces, creating major advances in high value for education/outreach efforts, planetary science in the tradition of other where video already is a common method of NASA efforts like SOHO in heliophysics or presentation. The mission’s rapid science re- UARS and EOS in Earth science. From a po- turn will give a near-immediate payoff on the sition beyond low Earth orbit, the observatory investment. will achieve continuous views of planetary Design: The driving design requirements targets on rotational timescales, with cam- are sampling rate and campaign duration, paign durations limited only by the mission which strongly favor an orbit beyond low lifetime: ~5 years, or 12–15 years with Earth orbit (BLEO) or around a Lagrange manned servicing. point. A workhorse instrument suite will pro- Goal: PDX will address goals in a wide vide imaging, spectroscopic, and photometric range of planetary science areas, by acquiring capability using highly mature technology, data with the ideal sampling rates and cam- such as a WFPC2/ACS clone and mini-STIS paign durations. In particular, studies of (without coronography and FUV capability). transport, composition, and temperatures—on The quadchart gives general instrument speci- seasonal timescales—will give insight into fications, although mission success depends energy transport and climate in planetary at- much more on temporal coverage than on tra- mospheres. Transient events—such as impact ditional drivers like spectral resolution or sen- scars, eruptions, and stellar occultations— sitivity. Moving object tracking is needed, address wide ranging topics from small body along with a spacecraft design permitting ex- populations, planetesimal composition, plane- tended operation with anti-Sun pointing. tary interiors and tidal heating, thin atmos- The BLEO location would solve the issue pheres, exoplanet characterization, and small of frequent Earth occultations that stop facili- body physical studies. ties like HST from observing a complete Benefits: Earth-observing satellites (and outer planet rotation period of ~10 hours. A missions like SOHO) ushered in a new era of BLEO location would be a very productive time-domain science, akin to the transition target for deep space human spaceflight. A from still photography to the first motion pic- serviced mission, as with HST, would enable tures. PDX will follow in this tradition, pro- instrument upgrades, as well as a longer mis- Wong et al. / Time-domain solar system science / NASA SALSO RFI: Jan 2013 - 2 - sion via reaction wheel replacement and pro- A participating scientist call, or guest ob- pellant recharge. A mission lifetime of 11–15 server program, will accommodate commu- years would enable coverage of half of the nity investigations from outside the science solar cycle or two Saturn seasons. There is no team. Exoplanet observations will also be ac- strict dependency on human spaceflight, be- commodated, within the constraints of the cause even an unserviced mission lifetime of overall observing program design. ~5 years would be of great science value. Results: The PDX mission is very ex- High data volumes will be needed, with ploratory by nature, opening a new avenue of the potential for communications solutions investigation that is currently limited by a that could enhance NASA’s capabilities in number of factors. The Cassini mission’s full space technology. For example, laser com- campaign duration will be the first to cover an munication could be utilized, with PDX per- entire giant planet season, but PDX will ac- haps even serving as a laser relay station for quire larger volumes of data at higher sam- other deep space missions. pling rates. The advanced instruments on Operation: The mission science team will HST are rarely trained on solar system targets design a set of long-term observing programs due to high oversubscription and an increas- to address major science questions as outlined ing focus on deep field observations. Rare in Table 1. A scheduling system will optimize HST solar system observations must be care- the telescope pointing sequence according to fully planned for 50-minute windows be- the temporal needs (sampling rate and cam- tween Earth occultations (not a concern for paign duration) of the full set of programs. PDX at HEO or beyond). Table 1. Examples of time-domain solar system science investigations. Wavelength regimes are ultraviolet (UV), optical (O), and near infrared (IR). Resolution: Data type Campaign R = spectral, Investigation Category (wavelength regime) Sampling scales duration θ = spatial Hours, single Atmospheric Imaging Atmospheres target rotation Years θ ≤ 0.05" flow (O) period Cloud/storm Imaging, spectroscopy R ≥ 2500 evolution and Atmospheres Hours, days Days, years (O, IR) θ ≤ 0.05" variability Photometry, spectroscopy Occultations Atmospheres Milliseconds Hours R ≥ 100–1000 (UV, O, IR) Aurorae, Atmospheres/ Imaging, spectroscopy R ≥ 15000 Minutes, hours Years, hours magnetospheres space science (UV) θ ≤ 0.05" Volcanic trace Atmospheres/ Spectroscopy, imaging Days Years R ≥ 500–10000 gases geology/ (UV, O, IR) astrobiology Volcanism, Geology/ Imaging, spectroscopy R ≥ 2500 Days, hours Years cryovolcanism astrobiology (UV, O, IR) θ ≤ 0.025" Mutual events, Photometry Milliseconds, Hours, R ≥ 5 Small bodies lightcurves (O) minutes months θ > 10" Cometary Imaging, spectroscopy R ≥ 30000 Small bodies Hours Months evolution (UV, O, IR) θ ≤ 0.05" Wong et al. / Time-domain solar system science / NASA SALSO RFI: Jan 2013 - 3 - The anticipated results cannot be fully past studies, ideally spanning an 11-year predicted, because data with the right sam- solar cycle. pling rate and campaign duration have never • Volcanism/cryovolcanism. Volcanic ac- been acquired in most cases. However, some tivity in the outer solar system has been highlights can be discussed, touching on the discovered to date only by spacecraft mis- areas of planetary science in Table 1: sions: sulfurous eruptions on Io and gey- • Atmospheric flow. Giant planet winds are sers on Triton by Voyager, and plumes on one of the biggest constraints on atmos- Enceladus by Cassini. Although the resolu- pheric dynamics. PDX will open up new tion requirement of 0.025" or better (Table investigations into zonal winds, vortex in- 1) needed to resolve large volcanic/cryo- teractions, and conditions in the deeper volcanic eruptions is beyond the NRO atmosphere (see SALSO abstract by David telescope diffraction limit, PDX will ap- Choi). proach the problem from another angle, • Cloud/storm evolution. Storms are mark- with high temporal coverage. Search cam- ers of energy transport in the atmospheres paigns will be designed to find unresolved of Mars, Jupiter, Saturn, Titan, Uranus, volcanism/cryovolcanism on both known and Neptune. No other mission could ob- and unknown active bodies. tain high temporal resolution data over a • Small bodies. In the time domain, rota- long campaign duration for all of these tar- tional and eclipsing light curves are great gets together. Detailed storm movies from tools for determining basic physical prop- PDX will provide global views of the erties of small bodies. PDX will be able to genesis and evolution of dust storms on characterize even slow rotators, and char- Mars, enormous convective energy re- acterize faint objects with high photomet- leases on Jupiter and Saturn, impact scars, ric stability in the optical range. High- the simultaneously Earth-like yet foreign resolution spectroscopy allows measure- methane cycle on Titan, and convection on ment of cometary outflow velocities and the distant blue ice giants (see SALSO ab- isotopic ratios. PDX would characterize stract by David Choi). In fact, the Great cometary evolution over a full apparition Dark Spots on Neptune and Uranus are for many targets, and even over multiple only visible at optical wavelengths inac- solar encounters for some. cessible to JWST. • Planetesimal populations. Populations of • Occultations. Stellar occultations by plan- small bodies are perhaps more informative ets, small bodies, and now exoplanets is a than the individual members. Wide field trusted method to constrain size, shape, capability on PDX, combined with point- and atmospheric structure with unparal- ings almost exclusively in the ecliptic leled vertical resolution. Occultations can plane, will enable searches for new popula- be observed from the ground, but PDX’s tion members, in groups such as giant optimal BLEO location will enable unin- planet Trojans, Kuiper belt objects, and terruped records of the entire occultation comets. event, increased photometric stability (for Other SALSO concepts: It cannot