planetary science with space telescope

Adv. Space Res. Vol. 1, pp. 199213. 0273—1177/81/0301—0199$05.OO/O©COSPAR, 1981. Printed in Great Britain.

PLANETARY SCIENCE WITHSPACE TELESCOPE

JohnCaidwell

DepartmentofEarth andSpaceSciences,StateUniversityofNew York, StonyBrook, NY11794, USA

ABSTRACT

A discussion is given of the various capabilities and advantagesof the Space Tele-scope observatory, scheduled for launch into Earth—orbit in 1984. The firstgeneration instruments are described, and a detailed example for one specificobserving program, an intercomparison of Uranus and Neptune, is made. Theimportance of an extra—solar planet search is emphasized.

INTRODUCTION

The Space Telescope observatory (ST) will comprise a 2.4 m primary mirror, schedul-ed for launch into low Earth orbit in late 1983 by the Space Transportation System(Shuttl~ c~fthe N.A.S.A. An image of Jupiter from the recent Voyager mission,which corresponds to the expected spatial resolution that will be routinelyachievable by ST, will be used to introduce the capabilities of ST in section II.Advantages of ST over ground—based instruments with respect to spatial and tem-poral resolution, wavelength range, and sky background limits are discussed, aswell as plans for on—orbit refurbishment and possible replacement of instruments.Some of the relatively minor systematic performance limits are also discussed.

In section III, the first generation of science instruments, including two imagingsystems, two complementary spectrographs, a high speed photometer and the fineguidance/astrometry system are described. Section IV includes a suggestion forprioritorizing planetary science objectives, namely weighting them in favor ofthose objects about which least is known at present, in order to deal with theexpected oversubscription of observing proposals to use ST. Consistent with thissuggestion, a summary is given of some current outstanding problems and intrigu-ing results for Uranus and Neptune, and a general outline of useful ST observationsfor these planets is presented. The section concludes with a brief statement onthe desirability of investing significant time on the search for extra-solarplanets, and a Centauri is suggested as a good candidate for exploratory imaging.No attempt is made to provide a comprehensive list of all possible planetaryprograms for ST.

In section V, the conclusion, it is stated that it now is timely for the generalcommunity to begin making plans for using ST.

One subject has not been addressed in this paper--the planning that has gone into

199

200 J. Caidwell

the post-launch operational phase of the mission. The data rates will be so highcompared to previous space observatories that a major effort has been expended inthis area to prepare for operations. Discussion of such plans, including theestablishment of an independent Science Institute to manage the ST, has been omit-ted here to keep the paper to a manageable length. Its absence here does notreflect a lack of attention to this important aspect of the Observatory by theproject.

To conclude the introduction, it is noted that the project scientist is C. R. O’Dellof the Marshall Space Flight Center. Among his many other responsibilities, O’Dellchairs meetings of the Science Working Group, an assemblage of scientists chargedwith the task of providing science advice to the project on all matters. Theauthor of this paper serves the project and attends working group meetings as anInterdisciplinary Scientist, with particular responsibility for planetary science.

OBSERVATORYCAPABILITIES

Figure 1 is a series of images of Jupiter obtained from the ground over severalyears. Each image represents a composite of many individual pictures; suchprocessing is done to offset partially the well known distortion due to the imper-fect transmission of the Earth’s atmosphere.

There are two points to be made in connection with Figure 1. The first is obvious.Jupiter exhibits distinct changes in its classical belt—zone structure on a timescale comparable to a terrestrial year. These changes, together with the in-triguing differences between Voyager 1 and 2 results at high resolution [Smith etal., 1] indicate that imaging Jupiter over a time scale comparable to that in Fig-ure 1, but at higher spatial resolution, would clearly produce important science.

The second point about Figure 1 is that these images are by no means typical. Theycomprize the very best ground—based work available, from one of the best sites,from an entire apparition.

Figure 2 is more representative of typical ground—based imaging. This ultravioletimage was processed in the same way as the images of Figure 1, except that it wasselected on the basis of temporal proximity to some infrared observations. TheGreat Red Spot was not visible, as it happened. Figure 2 therefore representsthe quality one may expect from day-to-day ground—based work.

This may be compared with Figure 3, which is a mosaic of Voyager images, taken witha violet filter about six days before closest approach, and selected as representa-tive of the quality that will be available from ST imaging on a routine basis,[E. Danielson, private communication]. An intercomparison of Figures 2 and 3 isas good an illustration as possible of the improved potential for studying thedynamics and morphology of planetary atmospheres with ST.

The optical properties of ST that will produce such data are close to but nottruly diffraction—limited. As with any real telescope, there must be some finiteimperfections. The performance specifications of ST are that its Ritchey—Chretiendesign optics will produce a point—source image radius of 0.1 arc sec at 6328 ~.

Inside this radius will be included 70% of the point source energy.

Toward longer wavelengths, the performance will rapidly approach the diffractionlimit. Toward shorter wavelengths, the absolute performance will improve some-what as the effects of diffraction decrease; however, the gain is not as good asfor an ideal telescope, because small scale telescope imperfections become rela-tively more important. The actual imaging characteristics at short wavelengthswill be determined after launch,in orbit. In any case, its optical performancewill b~unprecedented in the history of astronomy.

Planetary Science with Space Telescope 201

CHANGES ON JUPITER SINCE 1973

25JULYD 2ONOV.~

5OCT. 28 JAN. rw~~~n1T1974 1978 Lf’1’~

~~‘El ‘~t~~•Blue filter photographs from the Planetary Patrol Telescope at the

Mauno Kea Observatory, Hawaii.LOWELL OBSERVATORY, PLANETARY RESEARCH CENTER

Figure 1.

202 J. Caidwell

Figure 2 (above) Figure 3 (below)


Freedom from scintillation in space provides another advantage in addition tospatial resolution, and that is temporal resolution. Many astronomical phenomenaare photometrically variable on time scales less than one second. Such eventstypically have their information content reduced severely or even obliterated bythe rapid fluctuations in brightness produced by the defocussing effect of atmos-pheric cells.

Of particular interest to planetary science is the observation of occultations ofstars by Solar System objects. If the occulting body has an atmosphere, numericalinversion of the light curve can be used to infer its refractivity, and thencedensity as a function of altitude. The latter leads to a complete thermal modelatmosphere. The deleterious effects of terrestrial scintillation on the inver-sion process have been discussed by Groth et al. [2).

Another advantage of space, and one which has previously been only partially ex-ploited by ST’s orbital predecessors, is the greatly increased range of accessiblewavelengths, due to the absence of atmospheric absorption. ST will be capableof operating efficiently (that is, with system throughput (excluding detectors) oforder 50%) over the wavelength range from 1150 ~ to 1 mm. The lower limit isimposed by the magnesium fluoride coatings on optical surfaces. The upper limitis somewhat loosely determined by the realization that at very long wavelengths,many terrestrial telescope will do better than ST.

Within the ST range lie many features of great interest to planetary astronomy,including electronic, vibrational and rotational transitions of molecules inplanetary atmospheres. Of course, it is not a coincidence that such transitionsare difficult or impossible to observe through the Earth’s atmosphere, since manymolecules have grossly similar spectral characteristics.

As will be discussed below, the first generation of science instruments aboard STwill exploit only the range from 1150 ~ to 11000 ~, although future instrumentscould, in principle, operate in the middle and far infrared.

A further benefit from operating in space, and one of the primary purposes inbuilding ST, is that it is possible to reach fainter objects than can be done fromthe ground because of the brightness of the sky. Throughout the entire range from1150 to 100 ~m, ST will outperform any previous ultraviolet, infrared or visualwavelength telescope, by from 4 to 8 stellar magnitudes. It is planned, however,for another future NASA space observatory, the Shuttle Infrared Telescope Facility(SIRTF) to have the capability of outperforming the ST significantly with respect

to faint objects in the infrared.

Limiting magnitudes for ST have been estimated on the basis of a nominal ten hourexposure limit. However, many Solar System observations will require exposuresonly of the order of seconds or minutes, because of the target brightness. Anobserver would have to choose the highest available spectral resolution and alsobypass many bright Solar System objects to approach the exposure limit. Theinitial scientific justification for doing so may be weak, in view of the presentabundance of unstudied bright objects, such as Neptune! In fact, many planetaryobservations will expend more time slewing to the target, including fine pointingwithin a planetary disk, than they will expend during exposures.

A general property of space experiments is that, once they successfully begin oper-ations, they usually work well for extended periods of time. With Shuttle—launched packages, such reliability should increase significantly. Revisits tothe ST are anticipated approximately every 2.5 years for observatory maintenance,and the possible replacementof scientific instruments, which will be modular.Return to the ground for major refurbishment may occur approximately every five

204 J. Caldwell

years. It is expected to continue operations for about two decades, at least. Itshould be the most reliable of a very reliable class of observatories.

Its long lifetime will be most opportune for many planetary projects, such asstudying the year—to—year changes seen in Figure 1.

For completeness, it should be noted that there are some operational limitations

on potential planetary work. One is that pointing near the Sun will be difficult.The problem is sufficiently serious for Mercury that it may only be possible toobserve the planet before (not after!) sunrise and only at extreme elongationand Only for an interval of several minutes, with each such interval separatedby tens of minutes of slewing for spacecraft safety. Venus is a less extremecase, but it will be sufficiently difficult to require a major scheduling effortto obtain a meaningful series of observations showing change in the ultravioletalbedo features on that planet. Comets at perihelion will also present problemsof varying severity.

Guiding errors will also be appreciable for very fast moving objects, such asApollo asteroids. ST should be able to track Halley’s comet at its greatest appar-ent motion (0.21 arc sec/sec) but significant errors will be introduced at higherrates. However, a reasonable assessment of the overall expected ST performance,including its limitations, is that it will be an exceptionally useful tool forplanetary astronomy for the remainder of this century.

FIRST GENERATION SCIENTIFIC INSTRUMENTS

This section will describe the first generation of scientific instruments (SI) thatwill be aboard ST. It will be a brief summary only, intended to provide an indi-cation of their capabilities. For more extensive discussions of the SIs, see thearticles by Leckrone [3] and by Bahcall and O’Dell [4].

A full complement of instruments on ST consists of five SIs, and three fine guid-ance sensors (FGS) . They are modular and replaceable in orbit. They share dif-ferent parts of the focal surface, with instrument selection being achieved byadjustments to the fine pointing of the telescope. All SI entrance apertures arelocated within the equivalent of several arc minutes from the optical axis.

There are two kinds of SI, respectively denoted “axial” and “radial”. There isroom for four axial instruments, each sharing a cuadrant of the focal surface,each with one corner touching the optical axis. There is one radial instrumentwhich, together with the three FGSs, forms a quartet symmetrically mounted withrespect to the optical axis. The radial instrument samples the central 3 arcminutes of the focal surface, where the effects of astigmatism and focal surfacecurvature are very small. It receives light by means of a flat pick-off mirrorthat blocks the central parts of the focal quadrants of the four axial SIs.Since the entrance apertures of the latter are necessarily off—axis, they mustcompensate internally for field distortion. The three FGSs share an annuluswhich surrounds the fields of view of the axial SIs, 9 arc minutes from the opticalaxis. The focal plane format is shown in Figure 4.

The radial SI and the 3 FGSs are located between the back of the primary mirrorand the four axial SI5.a) Wide Field/Planetary Camera. This is the first generation radial instrument.Its development team is led by Professor James A. Westphal of the California Insti-tute of Technology. Its primary purpose is imaging, with some low dispersionspectroscopic capability. It has the widest field of view of any SI, and thebroadest spectral range.


SI FIEID FORMAT

~22 4

RADIAL BAYMIRROR Is,..,,,VIGNETTING B

+

Figure 4.

The camera has two modes of operation, selectable by a rotation of an internal re-flecting pyramid. These modes are named “wide field” (f/l2.9; 2.7 x 2.7 (arcmm)

2 and “planetary” (f/3O; 1.2 x 1.2 (arc mm)2). In each mode, the detectorconsists of 4 silicon charge—coupled devices) . There are 8 such CCD5 in thecamera, each including 800 x 800 pixels. For each mode, the image is reassembledin the data processing stage into an equivalent 1600 x 1600 pixel image, with noloss of information at the internal boundaries.

Its wide spectral range is made possible by an organic phosphor coating, whichconverts ultraviolet photons to visual ones, where the intrinsic CCD efficiencyis high. The net quantum efficiency exceeds 1% from 1150 ~ to 11000 ~, peakingat 55% near 5500 L It has the only near infrared imaging capability of thefirst generation SIs.

Its photometric accuracy is - 1%. Its dynamic range is l0~, permitting the simul-taneous recording of widely different brightness levels in the same field. It isplanned to include a low reflection point on the reflecting pyramid to facilitateastrometric observations in which bright star positions, centered in the spot, canbe measured precisely with respect to faint background stars. This will have manyapplications, including a search for extra—solar planets.

In normal imaging, spectral definition comes from 48 filters. These have gener-ally been optimized for stellar astronomy research, but there will be many thatare useful for planetary work, including methar~ (CH

4) band filters for imagingthe giant planets. There will also be prisms and transmission gratings for lowdispersion objective spectra, and polarizers at several wavelengths.b) Faint Object Camera. This axial instrument, designed and built by the EuropeanSpace Agency, also has two modes: f/96 (11 x 11 (arc sec)

2) and f/48 (22 x 22(arc sec)2). Its field of view is actually limited by the observatory data hand-ling capacity. It will achieve the highest possible spatial resolution and will

206 J. Caidwell

reach the faintest objects for the first generation SIs. It has much greater speedthan the WF/PC in the ultraviolet.

It will contain approximately 48 selectable filters, including neutral density,and special purpose ones, objective prisms and polarizers. It will also have acoronographic capability, featuring a 0.6 arc sec occulting disk that will permitrecording objects 1 arc sec apart which differ in brightness by - 10 stellarmagnitudes.

There will also be a fixed grating spectroscopic mode, permitting observations inthree orders: 3600 — 5400 ~ 1800 — 2700 ~ and ~l20O — 1800 ~ at a resolution of

2 x lOs. This performance is less than those of the spectrographs discus-sed below, but this instrument has the unique feature among first generation SIs

of providing spatial information perpendicular to the dispersion direction. Itsentrance slit is 10 x 0.1 (arc sec)

2.

The detectors are three stage image intensifiers coupled to a high performancetelevision tube. The first stage photocathode is a •‘hot bialkali”. The peakquantum efficiency is - 26% at 3000 ~.

c) Faint Object Spectrograph. This axial instrument is being developed by a teamled by Dr. Richard Harms of the University of California at San Diego. It willobtain low and moderate resolution spectra of objects fainter than those accessiblefrom the ground. It has two operational modes, with resolutions respectively ofX/t~X - lO~ and 102.

It will have a useable spectral range from 1150 ~ to 9000 ~, although its quantumefficiency drops to 1% near 7000 ~ and decreases rapidly toward longer wavelengths.It will also exploit the ST spatial resolution capability on extended sources, with10 entrance apertures ranging in size from 0.1 to 4.3 arc sec. These propertieswill make it a valuable instrument for investigating the near infrared CH

4 bandsin the giant planets. The importance of such observations will be discussed belowin more detail.

The FOS has two identical internal light paths, one for a red sensitive detectorand one for a blue sensitive detector. The detectors are called “Digicons” anddiffer only in their photocathodes. They operate by focussing photoelectronsonto a linear array of 512 silicon diodes, which is arranged to receive spectrallydispersed light. Dispersion is achieved by selectable concave gratings mountedon a rotating carousel, having 10 positions. The gratings, plus one thin prism/mirror combination provide overlapping spectral intervals which together comprisethe spectrograph’s useful wavelength range, as described above.

Other features of the FOS include polarization capability below 3000 ~., and a timeresolution for sufficiently bright objects of 50 ii sec. The observatory datahandling capacity will permit up to 100 exposures/sec for 512 diode spectra.d) High Resolution Spectrograph. This instrument is being developed by a teamdirected by Dr. John C. Brandt of the Goddard Space Flight Center. It will pro-vide the highest spectral resolution of any of the first 9eneration SIs. Therewill be three modes, with A/LA - lOs, 2 x lOg, and 2 x lOs. Dispersion is achievedwith one of six selectable gratings, including an echelle for the high dispersionmode, mounted on a carousel. Its detectors are two digicons, similar in conceptto those used in the FOS, including the 512 diode array. Photocathodes will bedifferent.

Its high resolution capability is achieved at some cost in other parameters. Forexample, it will operate only below 3200 ~, and it will have only two selectablespatial resolutions, 0.25 and 2.0 arc sec. However, it will have high time reso-lution capability ( 25 m sec), and it will also have a finite response below1150 ~., both useful for very bright sources.


Its highest resolution observations will be totally without precedent in spaceastronomy. Therefore, astronomical objects themselves will not be totally adequateas calibrators, and the MRS will rely on its own internal light souces for spectraland photometric calibration.e) High Speed photometer. Dr. Robert C. Bless of the University of Wisconsinis the leader of the development team for this, the mechanically most simple SIof the first generation. It has two basic purposes: high time resolution (up to16 ~i sec) and high photometric precision ~J.2%). In both these respects, it willbe the best performer among first generation SIs. It can be related to a ground-based clock with an accuracy of 10 m sec.

Its detectors include four image dissectors which together cover the range from1150 to 6500 ~. The image dissectors are magnetically focussed single beamphotometers that permit aperture/filter combination selection by meansof deflec-tion of electron beams rather than by mechanical moving parts. Apertures of 0.4,1.0 and 10 arc sec will be available.

A fifth detector, a side—looking gallium arsenide photomultiplier tube with peaksensitivity from 6000 to 9000 ~, was added specifically to enhance the observa—bility of stellar occultations by planets. The red—sensitive tube is desirablebecause most stars in the sky are red, and the light from giant planets can begreatly suppressed in the red by choosing a filter in a strong methane band.Since it is the starlight which is the useful signal in such an observation, thebest signal to noise therefore typically occurs in the near—infrared CM

4 bands.Of course, if a very blue star were to be occulted, such as happened in 1971 whenJupiter occulted B Sco, it would be possible to use an ultraviolet image dissectorto observe it.

The HSP will also be able to measure linear polarization below 4000 ~. Its dynamicrange will be - 108.f) Fine Guidance System. There are three fine guidance sensors, which sample thefocal plane outside the SI fields of view. The light detector in each sensor isan image dissector/interferometer combination. The system is used for guiding theobservatory, with 2 FGSs required at any time for this purpose. The third is free

to perform astrometric observations.

Each sensor will measure relative positions within its own field of view (69 (arcmm)

2) to within 0.002 arc sec. The magnitude range for astrometric targets (in-cluding those observed through neutral density filters) is 4 to 20 my. It willbe able to track moving targets.

The ST astrometry science team leader is Dr. W. H. Jefferys of the University ofTexas at Austin. His team has identified many astrometric problems suitable forST, including refining the orbits of faint satellites of the outer planets. Oneof the most exciting prospects for its use will be the search for extrasolarplanets through their perturbations on positions of nearby stars.

PLANETARY SCIENCE WITH SPACE TELESCOPE

Broad reviews of this subject have been given by Belton (51 for major planets and

by Morrison [6] for smaller objects. The present paper will not attempt to dupli-cate those efforts in scope.

From those papers, or from any brief consideration of the future of planetaryscience, it is quite apparent that planetary work alone could oversubscribe avail-able observing time for ST for years, with uniformly good science. Therefore thereseems to be no viable alternative to the painful necessity for the planetarycommunity to discipline itself and make priority judgements about the various

208 J. Caidwell

programs. We must therefore adopt a workable criterion for rating ST solar systemresearch.

Currently, the quality of knowledge about the planets is excessively heterogeneous,with someplanets known in intimate detail (Venus, Mars, Jupiter) and someonlypoorly known (Mercury, Neptune, Pluto). Moreover, those planets which are wellknown are such becauseof their location, not their intrinsic interest. In fact,it is not possible to decide now, in our current state of ignorance, which planetsare in fact most interesting.

One of the basic aims of planetary research is to provide an understanding of theorigin and evolution of the Solar System. To attempt this, it would be desirableto establish data of more nearly uniform quality on the extremes of the system.To some extent, any comprehensive theory must be limited by the least precise dataupon which it is based.

It is therefore hereby proposed that planetary work on ST should be assigned apriority according to current ignorance of the target. In order to be consistentwith this suggestion, which is only the author’s personal opinion, and not of fic-ial policy, the present paper will consider only two topics, both of which meetthe criterion of obscurity: an intercomparison of Uranus and Neptune, and asearch for extrasolar planets.

a) An intercomparison of Uranus and Neptune. Table I summarizes some bulk proper-ties of the four giant planets. When central compression is taken into account,it is apparent that there are two categories of giant planets: Jupiter and Saturn,the composition of which may be mostly hydrogen and helium, with an admixture ofheavier elements in solar proportions or with a slight enhancement with respect to

solar; and Uranus and Neptune, which have a significant enhancement of the heavierelements. The densities and masses for the outer two giants are consistent witha similar composition for both.

TABLE 1 Bulk Properties of the Giant Planets

Mass Density Ratio of Thermal(Earth=l) (GM CM

2) Radiation to *

Absorbed Sunlight

Jupiter 317.9 1.31 2.0 + 0.4

Saturn 95.2 0.70 2.6 ±0.9

Uranus 14.6 1.21 < 1.3

+2.4Neptune 17.2 1.31 2.6 —0 ~

*Gautier and Courtin [7]

However, the column listing the magnitudes of the internal heat sources of theseplanets clearly shows that Uranus and Neptune themselves are distinctly different.This difference constitutes a major problem in planetary science, and there havebeen several proposed explanations for it.

Traf ton [8] suggested that the excess energy for Neptune originates in the orbital

Planetary Sc ience with Space Telescope 209

energy of the large retrograde satellite, Triton. However, this suggestion hasbeer criticized because the time scale for orbital decay necessary to generate thismuch power is less than the age of the Solar System. Hubbard [9] has presentedconvective—cooling models of these planets in which heat flow from the interiorof Uranus is relatively inhibited by the higher insolation of that planet (by a

factor of 2.4) . The net result is to have very similar effective temperatures(57.3 ± 2.6 K for Uranus; 38.4 for Neptune) while Neptune cools internallyfaster than Uranus does. The ultimate source of Neptune’s observable energy

source is then the primordial energy associated with its gravitational collapse.

A third possibility is that Uranus currently represents a net sink of energy,

rather than a source [Danielson, 10] . The polar axis of Uranus is virtually in

its orbital plane, a unique obliquity in the Solar System, and its northern hemi-sphere is now turning earthward after - 40 years of continuous night. Danielsonproposed that the apparent absence of an internal heat source on Uranus might benothing more than a seasonal phase lag in response to changing Uranographic ~solation. A counter argument to this suggestion was raised by Fazio et al. [liiwho claimed that the time scale for significant thermal change in the deep atmos-phere of Uranus is too long to permit observable thermal seasonal variations.

However, Klein and Turegano [12] demonstrated another unique feature of Uranus -

its microwave brightness temperature is both higher than for other giant planetsand currently increasing in a remarkable secular manner. This work has recently

been confirmed by Briggs and Andrew [12] using the Very Large Array radio inter-ferometer, which provides spatial resolution of several tenths of an arc second.They find that the emission is confined to the planetary disk, rather than therings or an extended magnetosphere, and that it is not uniformly distributedacross the disk. They find a polar region with TB - 275 K and that the equator

has T8 - 220 K.

Brigys and Andrew point out that simple orientation changes cannot produce thesecular increase, because the bright polar area is too small. It is thereforenecessary to invoke a dissipation or redistribution of the microwave absorber,essentially proving that Uranus has seasons. The identity of the microwaveabsorber itself on Uranus is uncertain. Gulkis et al. [14] point out that theprobable microwave absorber on other planets, ammonia (NH3) must be relativelydepleted on Uranus to permit the observation of such high brightness temperatures.

On the other planets, NH3 blocks microwave radiation from the deeper, warmer layers.Gulkis et al. suggested that the chemical reaction with hydrogen sulfide (H2S)NH3 + H2S ~ NH4SH; could explain the depletion of NH3, but this still leaves openthe question of why Uranus is so remarkably unusual.

Fink and Larson [15] have observed both planets from 1 to 2.5 .~m at high spectralresolution. Their results show that the two planets have distinctly differentatmospheric properties. In particular on Uranus, the very strong CH4 bands arealmost completely black throughout their cores, whereas they are noticeably filledin in the spectrum of Neptune. The difference is explained by Fink and Larson asresulting from a totally clear Uranian atmosphere (down to a pressure level of- 1 bar) and from particles which scatter light in the Neptunian atmosphere at apressure level of - 0.7 bar.

A partial contradiction to the Uranus findings comes from Price and Franz [16] whoreconstructed an image of Uranus in the 7300 ~ CH4 band from data obtained with anarea scanning slit photometer. In the band, reflection from the general cloudlayer is strongly suppressed. Their image is reproduced here as Figure 5.

They found general limb brightening, which is consistent with Rayleigh scatteringfrom a clear atmosphere which is hundreds of km—atmospheres deep, as expected from

210 J. Caldwell

(i/I

:i

N

Figure 5, reproduced from Price and Franz (16],

the results of Fink and Larson (15] at longer wavelengths,and if CH4 is frozen

out of the upper atmosphere [Danielson, 10]. However, they found even greaterbrightening near the pole (which is not now at the limb!), which they attributeto scattering from a local high—altitude haze layer, well above the main clouds.Furthermore, the region of excess brightening is not coincident with the pole.The discrepancy of this result of Price and Franz with the spectrum of Fink and

Larson may be due to temporal change on Uranus or to the relatively small areaof the bright spot.

The relationship, if any, between this polar brightening and the warm pole obser-ved by Briggs and Andrew [13] is unclear. It is certain, however, that theradiation at 7300 ~ originates from significantly higher altitude on Uranus thandoes the microwave radiation.

Neptune also exhibits its own characteristic photometric behavior in CH4 bands.In a two paper series, Joyce et al. [17] and Pilcher [18] showed that the 1—4 pmspectrum of Neptune was variable over a one year period from 1975 to 1976. Ananomalous brightening was detected and attributed to the formation of a high alti-tude scattering haze layer, which subsequently partially dissipated during theirobservations. Since the reflectivity of Neptune is typically very low in thecenter of the strong CH4 bands in this region [15] the effect of a small abundanceof a scattering substance occurring above the bulk of the absorber is readilyobservable.

Joyce et al. [17] did not have the capability to resolve Neptune spatially. It is


9:40 10:19 UT

8900 A

NE PTUNE10:07

Figure 6, reproduced from original work

7550 A by Reitsema, Smith and Larson [19].

intuitively implausible that such an effect would occur uniformly over the planet,so that the prospect of ST spatial resolution is most intriguing. A glimpse ofwhat is in store is shown in F~gure 6, which was obtained by Reitsema et al. [19]with a ground-based CCD. The images at 8900 ~, in a strong CH

4 band, show evidenceof a high altitude haze in both the southern and northern hemispheres. The equatoris relatively clear, and therefore darker, as we see to lower levels where the CM4absorption is greater. The continuum image at 7550 ~ has very little contrast,with light being strongly reflected everywhere from the main cloud deck, withoutabsorption.

212 J. Caldwell

A final example of the differences between Uranus and Neptune was shown by Gillettand Rieke [20]. Neptune is readily detectable in the 8—14 jim terrestrial atmos-pheric window, particularly at 8 and at 12 jim, where emission peaks respectivelydue to CH

4 and ethane (C2H6) originating in a relatively warm stratosphere areseen. Uranus is not even detected here, and subsequent attempts to do so have notyet succeeded. The difference could be due to two effects, or a combination ofthem: either the stratosphere of Uranus is not as warm as that of Neptune, or thegases CH4 and C2H6 are not present. As yet there is no satisfactory explanationof why the more distant planet has the warmer stratosphere.

One interesting observational result is that wherever C2H6 is seen in infraredemission (Jupiter, Saturn and Titan being other examples) there is either directevidence or a strong suggestion that acetylene (C2H2) is also present [e.g. Ortonand Aumann, 21]. C2H2 is a convenient molecule for diagnosing planetary tracegases, becauseit also has a distinctive ultraviolet signature. Owen et al. [22]have described its influence on Jupiter, as observed with the IUE. For ST, theouter planets are bright in the region where the C2H2 bands occur. It would beinteresting to compare the far ultraviolet spectra of Uranus, where C2H2 is notexpected [10] and of Neptune, where it may well be present.

With the above discussion, it is obvious that excellent scientific potential existsin a comparative study of Uranus and Neptune with the first generation SIs of ST.A few useful observations are now summarized: i) imaging both planets with theWF/PC in filters at 4860 ~, 6190 ~ and 8890 ~, which correspond to weak, mediumand strong CM4 bands in both planets, and complementary images in adjacent continua.With such images, one could probe various levels of the atmospheres, down to themain clouds, and study the temporal properties of the features which almost cer-tainly will be found; ii) supplementary FOS spectra of interesting regions athigh spatial resolution wou]d improve on the necessarily abridged spectral cover-age of the imaging system; iii) HSP observations of stellar occultations willsample higher altitudes. There is a high statistical probability of favorableoccultations occurring frequently. iv) ultraviolet spectra of both planets shouldbe examined for compositional studies, such as C2H2 presence, distribution andvariability.

b) Extra—solar planets. Two systems aboard the ST will be used for astrometricsearches of candidate stars: the FGS and the WF/PC, both of which have the capa-bility of doing astrometric work at the 0.002 arc sec level. K. A. Baum (23] hasdetermined that there are about 200 candidate stars brighter than visual magni-tude 16 which would exceed this detection threshold, if such stars had a planet

equivalent to Jupiter orbiting them. If such a planet were detected astmnretrically,it would certainly become one of the highest priority single objects in the uni-verse for detailed study.

The temptation to do exploratory imaging may prove to be irresistible, however,before the astrometric results are in. In several cases, the ST should be capableof imaging hypothetical extrasolar planets if the actual scattered light levelsare as low as the most optimistic estimates. The FOC, with its occulting disk,may be the best SI for this purpose.

The nearest stellar system, a Centauri, which is multiple, may be the best targetfor such imaging. Harrington [24] has claimed that stable orbits jr a binarysystem can exist if the planetary semi—major axes are less than one-third of ormore than three times the stellar semi—major axis. This means that a potential Sarc sec radius around a Cen A and a Cen B could contain planets, that is within6 a.u. of the stars. There is one Jupiter—like planet within this range of theSun, and it is not unreasonable to inquire if there might also be one at a Cen.If so, and if it is at 5 AU from a Cen A, it would be a 22 visual magnitudeobject, about 1000 times brighter than the clear sky detection limit for the FOC.


CONCLUSION

This paper was written in an attempt to convince planetary astronomers that anextraordinarily valuable observational tool is about to become available. TheN.A.S.A. is currently encouraging the highest possible level of scientific parti-cipation in the project. In particular, international participation is mostwelcome, as witnessed by the E.S.A. involvement through the FOC.

Participation is possible in two ways. First, general observers with specificscientific proposals will be eligible to compete for observing time with the tele-scope. And second, on a more involved level, there will be future SIs to bedeveloped. General areas for such opportunities include infrared spectrophoto-meters, and imaging devices with higher spectral selectivity than the firstgeneration FOC and WF/PC.

Becausecompetition probably will be strong, it cannot be stressed too highly thatnow is an excellent time to start planning for such participation. There isclearly some personal risk involved in committing one’s time to a satellite forwhich the launch vehicle has never flown. However, given the enormous potentialof this system, the converse risk must now also be addressed by individuals: canone now afford not to be prepared to use ST.

AC~jOWLEDGEMENTS

This review was written with the support of N.A.S.A. contract NAS 8-32904. B.A.Smith, W.A. Baum, G.E. Danielson, and M.J. Price kindly provided figures.

REFERENCES

1. Smith, B.A., and 21 others, Science 206, 927, 1979.2. Groth, E.J., Klopfenstein, J.B., Wickes, W.C. and Caldwell, J., Astron.J. 83,

442, 1978.3. Leckrone, D.S., P.A.S.P. 92, 5, 1980.4. Bahcall, J.N., and O’Dell, C.R. NASA CP 2111, Scientific Research with the

Space Telescope, M.S. Longair and J.W. Warner, editors. (lAP Coll. 54) 1979.5. Belton, M.J.S., NASA C? 2111. 6. Morrison, D. NASA CP 2111.7. Gautier, D. and Courtin, R. Icarus 39, 28, 19798. Trafton, L., Astrophys. J. 193, 477, 1974.9. Hubbard, W.B., Icarus 35, 177, 1978. 10. Danielson, R.E., Icarus 30, 462, 1977.

11. Fazio, C.G., Traub, W.0., Wright, E.L., Low, F.J. and Trafton, I., Astrophys.J. 209, 633, 1976.

12. Klein, M.J. and Turegano, J.A., Astrophys. J. 224, L3l, 1978.13. Briggs, F.M., and Andrew, B.H., Icarus 40, in press, 1980.14. Gulkis, S., Janssen, M.A. and Olsen, E.T., Icarus 34, 10, 1978.15. Fink, U. and Larson, H.P., Astrophys. J. 233, 1021, 1979.16. Price, M.J. and Franz, O.G., Icarus 39, 459, 1979.17. Joyce, R.R., Pilcher, C.B., Cruikshank, D.P., and Morrison, D. Astrophys. J.

214, 657, 1977. 18. Pilcher, C.B., Astrophys. J. 214, 663, 1977.

19. Reitsema, N.J., Smith, B.A. and Larson, S.M. Private communication, to bepublished. (1980).

20. Gillett, F.C. and Rieke, G.H., Astrophys. J. 218, Ll44, 1977.21. Orton, G.S. and Aumann, H.H.,Icarus 32, 431, 1977.22. Owen, T., Caldwell, J., Rivolo, A.R., Moore, V., Lane, A.L., Sagan, C., Hunt,

G. and Ponnamperuma, C., Astrophys. J. 236, L39, 1980.23. Baum, W.A., paper presented at the Montreal, Canada, meeting of the I.A.U.,

August, 1979.24. Harrington, R.S. Astron. J. 82, 753, 1977.

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