Ground-Based Observational Support for Spacecraft Exploration of the Outer Planets
- First Online:
- Cite this article as:
- Orton, G.S. Earth Moon Planet (2009) 105: 143. doi:10.1007/s11038-009-9295-x
- 57 Views
This report presents both a retrospective of ground-based support for spacecraft missions to the outer solar system and a perspective of support for future missions. Past support is reviewed in a series of case studies involving the author. The most basic support is essential, providing the mission with information without which the planned science would not have been accomplished. Another is critical, without which science would have been returned, but missing a key element in its understanding. Some observations are enabling by accomplishing one aspect of an experiment which would otherwise not have been possible. Other observations provide a perspective of the planet as a whole which is not available to instruments with narrow fields of view and limited spatial coverage, sometimes motivating a re-prioritizing of experiment objectives. Ground-based support is also capable of providing spectral coverage not present in the complement of spacecraft instruments. Earth-based observations also have the capability of filling in gaps of spacecraft coverage of atmospheric phenomena, as well as providing surveillance of longer-term behavior than the coverage available to the mission. Future missions benefiting from ground-based support would include the Juno mission to Jupiter in the next decade, a flagship-class mission to the Jupiter or to the Saturn systems currently under consideration, and possible intermediate-class missions which might be proposed in NASA’s New Frontiers category. One of the principal benefits of future 30 m-class giant telescopes would be to improve the spatial resolution of maps of temperature and composition which are derived from observations of thermal emission at mid-infrared and longer wavelengths. In many situations, this spatial resolution is competitive with those of the relevant instruments on the spacecraft themselves.
Astronomical observations from ground-based platforms have provided support for outer-solar-system spacecraft missions for several decades. The usefulness of these observations has ranged from merely providing incidental information to enabling the success of mission experiments. Several categories of ground-based support are reviewed as case studies, and an indication of the types of support that might be useful for future missions to the outer solar system are discussed. These case studies are drawn from personal experience and should be only be considered as representative of a broader class of observations which have been or could be obtained by the wider community.
2 Case Studies from Past Missions
2.1 Essential Support
Several examples of spacecraft support arise from the Galileo mission, which suffered a number of hardware setbacks en route to Jupiter and during its orbital tour of the planet and the Galilean satellites, partially mitigated by ground-based astronomical support. One type of support was essential; without it the remote-sensing science goals for the neutral atmosphere would not have been possible. The failure of Galileo’s high-gain antenna to deploy properly forced the use of a low-gain antenna with a very low data rate, requiring all science objectives to be diminished. Atmospheric science was optimized by observing selected features in Jupiter’s atmosphere. Because the pointing needed to be done well in advance, it was important to predict where discrete features in Jupiter would be located several months in the future. Discrete features do not flow uniformly in Jupiter’s mean zonal (east–west) winds because of migration in latitude or interaction with other features. An observational program tracked the longitudinal positions of selected features which could be used to predict their location. Galileo’s imaging experiment was too data-limited to do this, so Jupiter was observed at key wavelengths from NASA’s Infrared Telescope Facility (IRTF) using near-infrared imaging, allowing Jupiter to be observed during the daytime, with many features prominent in the near infrared. These observations also provided a record of the “life cycles” of some features, particularly relatively cloudless regions known as “5-μm hot spots” whose importance is described below. Some examples of the 5-μm images taken in the early stages of this program are shown by Ortiz et al. (1998).
2.2 Critical Support
2.3 Experiment-Enabling Support
Other ground-based observations have enabled individual experiments. For example, the first missions to the outer solar system, Pioneers 10 and 11 contained an Imaging Photo Polarimeter (IPP) experiment. Although the relative observations of the IPP were trustworthy, the absolute calibration was not known. Ground-based observations provided that information, using spatially resolved observations of specific regions of Jupiter’s (Orton 1975) and Saturn’s (Bergstralh et al. 1981) atmosphere. These enabled the analysis of IPP observations to determine properties of clouds in Jupiter (e.g. Tomasko et al. 1978) and in Saturn (e.g. Tomasko and Doose 1984).
2.4 Observational Reprogramming
2.5 Complementary Spectral Coverage
Mass- and budget-constrained missions cannot include all remote-sensing experiments that might be desired, and another potential role for ground-based observations is to fill in spectral coverage missing from on-board experiments. NASA’s New Horizons mission encountered Jupiter in 2007 on its way to Pluto. Its payload included LEISA which returned near-infrared spectral images and LORRI which returned broadband visible images, returning information about horizontal winds from cloud tracking and the vertical distribution and microphysical properties of clouds particles. New Horizons instruments did not observe Jupiter at wavelengths greater than 2.5 μm. Ground-based observations returned information on the tropospheric temperature field (from 13 μm to 25 μm imaging), and the distribution of deep clouds (from 5-μm imaging) and of ammonia gas (from 10-μm imaging) around Jupiter’s Great Red Spot (GRS) and the smaller anticyclonic vortex known as Oval BA, both targets of New Horizons remote sensing (Baines et al. 2007; Cheng et al. 2008). Using the geostrophic thermal wind equation, knowledge of the 3-dimensional temperature system allows the upward extrapolation of the horizontal wind field measured at the cloud-top level.
Ground-based images also included Jupiter’s auroral H3+ emission at 3.43 μm which could be related to auroral emission from the New Horizons ultraviolet experiment (Gladstone et al. 2007). Similar images before and during the Galileo mission were taken by Sato and Connerney (1996, and references therein). At 7.8 μm, these images also tracked the shape and amplitude of a compact region of warming in Jupiter’s upper stratosphere from bombardment by auroral-related charged particles. During the Galileo mission, a similar set of mid-infrared images supplemented the thermal mapping in the mid- through far-infrared provided by the Photopolarimeter-Radiometer (PPR), whose filter wheel suffered restricted movement.
2.6 Continued Surveillance
Ground-based observations which provide regular, continued surveillance support spacecraft experiment data which are limited in their spatial or temporal coverage. These observations provide context for intensive studies of narrow spatial regions and fill in gaps in time to monitor the evolution of the atmosphere when this is not possible by the relevant spacecraft experiments. Ground-based observations at 5 μm illustrated the spatial variability of the atmosphere in a region sampled by several spectra taken by Galileo’s Near-Infrared Mapping Spectrometer (NIMS) (see Figs. 1 and 2 of Carlson et al. 1996), allowing them to interpret their very heterogeneous spectra. Middle-infrared maps of Jupiter from the IRTF provided a spatial and temporal context for thermal studies of particular regions by the PPR experiment. Similar observations of Jupiter supported joint Cassini and Galileo remote-sensing of the atmosphere during the late 2000/early 2001 Cassini encounter, independently verifying the first Cassini CIRS maps of Jupiter’s temperature field, filling in gaps of their spatial coverage, and supplementing the sparser time sequence of CIRS maps. Continued observations of Saturn taken before and during the Cassini nominal mission verify the disappearance of zonal temperature waves, supplementing CIRS observations by surveying the entire planet (CIRS only maps a narrow latitude bands over all longitudes at a time). Similarly support is provided to the Cassini Visible/Near Infrared Mapping Spectrometer (VIMS) by mapping Saturn’s remarkably heterogeneous thermal emission near 5 μm, during periods of several months at a time when VIMS is not scheduled to observe Saturn’s atmosphere. Gathering contextual information was also one component of extensive support provided for the Huygens entry probe into Titan’s atmosphere (Witasse et al. 2006).
2.7 Long-Term Behavior
Surveillance over relatively long periods of time, particularly preceding a spacecraft mission, provides contextual information on slower variability. For example, long-term observations of Saturn’s temperature structure from the IRTF and other telescopes track its seasonal variability and distinguish it from non-seasonal influences, such as the low-latitude wave structure in Saturn’s atmosphere over 22 years (Orton et al. 2008), whose spatial manifestation was mapped in considerable detail over a much narrower time frame from limb-sensing CIRS spectra (Fouchet et al. 2008). The ground-based data determined that the stratospheric structure which looks like the Earth’s quasi-biennial oscillation (QBO) actually had a repeatability of 14.8 ± 1.2 years, one half of Saturn’s year, relating it to another terrestrial phenomenon known as the semi-annual oscillation (SAO) because this period is nearly exactly one half the period of Saturn’s revolution around the sun. A similar phenomenon is observable in Jupiter’s atmosphere, known as the Quasi-Quadrennial Oscillation (QQO) which was measured by the long-term study of Orton et al. (1991), identified by Leovy et al. (1991) and subsequently modeled by Friedson (1999).
3 Support for Future Missions
3.1 Cassini Extended Mission
The nominal Cassini mission ended in mid-2008 and will continue in more or less the same mode for two more years. A subsequent reduced level of operations has been proposed to extend the mission through Saturn’s and Titan’s northern solstice in 2017. This mission extension will necessarily provide only limited opportunities to sample the atmosphere as a function of time, as well as limited spatial coverage, both of which should be supplemented by ground-based studies. These include surveying the morphology and evolution of deep atmospheric clouds by imaging Saturn’s 5-μm thermal emission. In the mid-infrared, observations would track the ephemeral amplitudes of zonal thermal waves, verify Saturn’s stratospheric oscillations for consistency with a 14.8-year semi-annual period, and examine closely the rise of temperatures in the north polar region and the simultaneous drop of temperatures in the south polar region to examine consistency with the behavior of polar vortex phenomena and seasonal forcing.
JUNO is scheduled to launch in 2011 and begin orbiting Jupiter 6 years later. JUNO’s science objectives include determining the O/H ratio through the abundance of H2O in the deep atmosphere and constraining its core mass. Doppler tracking of JUNO’s high-gain antenna carrier signal will map Jupiter’s gravity field and its in situ particle and field experiments will constrain Jupiter internal dynamics, by providing simultaneous mapping of its gravity and magnetic fields. These in situ experiments, coupled with tracking of Jupiter’s UV and near-IR aurora through spectral imaging, will characterize and explore the 3-dimensional structure of Jupiter’s polar magnetosphere and auroras. Its MicroWave Radiometer (MWR) and Jupiter InfraRed Auroral Mapper (JIRAM) will map variations in atmospheric composition, temperature, cloud opacity and dynamics to depths greater than 100 bars at all latitudes. However, JUNO will make 31 highly eccentric orbits of 11 days each, only the first few of which are dedicated to remote sensing. Furthermore, the atmosphere will be mapped only in narrow longitude swaths by the spin-stabilized spacecraft. It has no mid-IR instruments, and it will thus return no information on the response of the neutral atmosphere to heating by the auroral related charged particles which are normally tracked via the emission of neutral CH4 at 7 μm and other hydrocarbons at longer wavelengths. Nor does it have a narrow-angle camera capable of tracking winds using high-resolution images of Jupiter’s cloud field. In addition to expanding the wavelength range, ground-based support observations should effectively expand coverage in the near-infrared for global context, as well as providing a longer baseline for characterization of atmospheric properties. Near-infrared imaging and spectroscopy should provide sensitivity to particulate reflection from clouds at a variety of altitudes in Jupiter, as well as signatures of NH3 gas and ice, supporting the JIRAM investigation. Near-infrared observations, which resolve features across the entire Earth-facing disk of Jupiter at scales as small as 500 km (0.14 arcsec), and mid-infrared observations would provide the global context for JIRAM (and MWR) investigations, as well as providing a long-term baseline of observations. These extensions of the wavelength grasp of JUNO instruments would provide a fundamental characterization of the atmospheric meteorology in order to aid the interpretation of JIRAM and MWR results for clouds and constituents in the deep atmosphere.
3.3 Flagship-Class Missions
Currently NASA and ESA are considering two flagship-class missions to the outer solar system, one to study Jupiter and Europa/Jupiter and Ganymede (if combined), and another to the Saturn system to study Titan and Enceladus. A mission to the Jovian system would include a great deal of atmospheric-intensive remote-sensing investigations. Surveillance of the atmosphere in the long hiatus between such missions, particularly in the mid infrared, would be important to track long-term changes to the atmosphere which would not be detected by the spacecraft during its nominal operations. Whether an intensive ground-based surveillance would be useful would depend on the extent to which the mission itself would make regular observations of the atmosphere. Surveillance of activity in Titan’s atmosphere should take place, emphasizing the near-infrared, where atmospheric cloud activity could be tracked using facilities with adaptive optics (e.g. Roe et al. 2005 and references therein). Such monitoring, following the Cassini mission, would provide a similar record of long-term events which differentiate between seasonal and non-seasonal phenomena.
3.4 Future Missions to Uranus and Neptune
It is not difficult to imagine that future missions may be directed to Uranus and Neptune, the “icy giants” of our solar system, representing a class of planets which are now just beginning to be characterized among extrasolar planets. Together with future flagship-class missions, studies are underway to determine the viability of New-Horizons-class missions to both planets, although with limited scientific scope and power. The 98° obliquity of Uranus provides an opportunity to study the reaction of the atmosphere to extremes of seasonally dependent solar irradiance. Current observations from the VLT/VISIR and Subaru/COMICS imply, for example, that there is a several-degree decrease in tropospheric temperatures in the northern hemisphere with respect to the measurements made in the mid-1980s by the Voyager IRIS experiment (Conrath et al. 1998). A variety of observations attests to the strength of Neptune’s dynamical system compared with that of Uranus. Seasonal forcing is even more important on Neptune than on Saturn, even though their obliquities are similar, because seasonally driven south polar temperatures in Neptune are warm enough to provide a “leak” in an otherwise efficient tropospheric cold trap for methane and into the stratosphere (Orton et al. 2007). Long-term observations of both planets in the visible and near-infrared for cloud variability using near-Earth space-borne and ground-based adaptive-optics facilities together with large-aperture telescopes which minimize diffraction-limited angular resolution in the mid infrared are capable of mitigating uncertainties regarding the length of atmospheric phenomena observed by future flyby or orbiting spacecraft at either planet.
The International Outer Planet Watch, a self-organized group of planetary astronomers, has succeeded in maintaining a communications between several groups addressing time-dependent changes in the outer solar system, as well as with the amateur community tracking changes in Jupiter’s and Saturn’s atmospheres. With groups specializing in magnetospheres, Jupiter’s volcanic Io, Saturn’s frozen-earthlike satellite Titan, the atmospheres of Jupiter and Saturn, the atmospheres of Uranus and Neptune, and requisite laboratory support, this organization provides a means of communication not only among planetary scientists, but also the amateur community. This is an important point for the atmospheres of Jupiter and Saturn, in an age where amateur observers frequently post stunning images, e.g., the amateur Christopher Go’s detection that a large white vortex in Jupiter had changed to a color similar to Jupiter’s Great Red Spot. The role of this organization as a whole could also be important in promoting to the public the importance of ground-based observations in space exploration. The web site for the IOPW is: http://www-ssc.igpp.ucla.edu/IJW/, and the author is the current chair of its Steering Committee.
3.6 The Role of Giant Telescopes
The role of extremely large telescopes, 30 m and larger, could be significant in the exploration of the outer planets. One of the principal advantages of such telescopes is in the mid-infrared and longer wavelengths, where even 8–10 m telescopes are still diffraction limited in their spatial resolution. For example, the spatial resolution of the IRTF in mapping Jupiter’s stratospheric temperature field at 7.8 μm is diffraction-limited to no better than 2,500 km and Jupiter’s tropospheric temperature field at 18 μm no better than 5,700 km, length scales that are larger length scale over which vortex interaction takes place—the radius of deformation. For the VLT and other ~8-m telescopes, these values become 920 km in the stratosphere and 2,100 km in the troposphere, commensurate with the radius of deformation. For a 30-m telescope, these values shrink to 250 km in the stratosphere and 570 km in the troposphere, small enough to detect structure within not only large vortices, such as the Great Red Spot, but also in smaller ones, and to characterize the thermal and compositional properties of small-scale, short-lived storms across the planet, as are currently resolved by Hubble Space Telescope visible and near-infrared instruments and wide-angle imaging of orbiting spacecraft. This resolution is competitive with the expected spatial resolutions of notional infrared instruments on future missions. For Saturn, similar resolution improvements, down to 550 km in the stratosphere and 1,260 km in the troposphere, would allow resolution of the temperature and compositional fields within Saturn’s polar hot spots (Dyudina et al. 2008, Fletcher et al. 2008) and possibly associated with storms which have been detected in the visible and near-infrared in reflected sunlight but not in the thermal. For some instances, then, such ground-based observations might well improve spacecraft-instrument spatial resolution, although only for Earth-facing geometries.
For the small and thermally faint disks of Uranus and Neptune, improvements in both the elements of spatial resolution and sensitivity are possible with giant-telescope observations. The IRTF cannot resolve either planet in the mid-infrared. Diffraction-limited resolution is 0.3 arcsec at 7.8 μm and 0.6 arcsec at 18 μm for an 8 m telescope versus 0.07 and 0.16 arcsec, respectively, for a 30-m telescope. The former are nominally sufficient to resolve the 3.8-arcsec disk of Uranus and the 2.4-arcsec disk of Neptune (e.g., Orton et al. 2007), but with few resolution elements. Observations using a 30-m telescope, on the other hand, could resolve thermal emission from Neptune with about the same resolution as the 3-m IRTF can resolve it in Saturn at the same wavelength or emission from Uranus with the same resolution as an 3-m telescope could resolve it in Saturn. This would provide a breakthrough in discriminating between radiative and dynamical phenomena observed at the poles of Saturn, for example. In general, it would determine the extent of similarities or differences between meteorological phenomena among the entire range of outer planets, providing a test bed for models of a range of extrasolar planetary atmospheres. It is important to note that descriptions of temperature for the dynamically active troposphere’s of Jupiter, Saturn and Neptune rely on observations of the planet in the 17–25 μm region where opacity is dominated by well-mixed H2 (for Uranus, H2 opacity alone is also the dominant opacity at 9–11 μm). Thus, it is important to devise means for operating effectively in the Q-band region for observations of the tropospheric emission from these planets. Improvements in sensitivity, particularly for Uranus toward the shorter wavelengths, would be important in order to begin understanding the structure of clouds in the deeper atmosphere, and to differentiate between reflected sunlight and non-thermal emission, such as H3+. In addition to the improvement of mid-infrared angular resolution, such a telescope would also possess greatly enhanced sensitivity for detection of faint features, such as searching for trace species in the 5-μm spectral window in Uranus and Neptune, or for H3+ emission in Uranus. For Titan, a 30-m telescope could detect weak spectral lines in the mid-infrared with spatial resolution, as well as vastly improved sensitivity to both the horizontal and vertical distribution of clouds across Titan’s disk.
Ground-based support for future missions to the outer planets would benefit from extensions of their spatial, spectral and temporal coverage using ground-based platforms. One of the biggest impacts that giant telescopes would have on outer-planet mission support would be a vast improvement in diffraction-limited thermal emission. A second impact which is important for Uranus and Neptune would be to provide increased sensitivity to faint emission. Filtered or spectroscopic measurements in the Q band would be important for determining spatially resolved tropospheric temperatures in most of the outer planets.
This work was supported by grants from NASA to the Jet Propulsion Laboratory, California Institute of Technology. The observing, reduction and analysis contributions of several colleagues are greatly appreciated, particularly those of co-investigators Kevin Baines, Brendan Fisher, Leigh Fletcher, Paul Parrish, John Spencer and Padma Yanamandra-Fisher. The staff and management personnel of the Gemini South Telescope, Infrared Telescope Facility, W. M. Keck Telescope, Subaru Telescope, and Very Large Telescope are greatly appreciated. Copyright, California Institute of Technology, 2009.