6757( 20) - 09/03/96 10:00 - [ 0] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6757 Version: 20 Check-in Date: 03-Sep-1996 10:57:09 1.Proposal Title: Transit Observations of Io's Atmosphere ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle SOLAR SYSTEM GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Doyle Hall THE JOHNS HOPKINS UNIVERSITY CoI: Paul Feldman The Johns Hopkins University Y CoI: Darrell Strobel The Johns Hopkins University N CoI: Xun Zhu The Johns Hopkins University N ------------------------------------------------------------------------------------ 6. Abstract We propose to use the HST GHRS to observe Io while in transit of Jupiter to determine the abundance and distribution of SO_2 in Io's atmosphere. Because Jupiter's atmosphere reflects more sunlight in the UV than Io's solid surface, during transit events Io appears in silhouette in front of Jupiter's bright disk. Though extremely thin, Io's predominantly SO_2 atmosphere is thick enough to absorb a significant number of photons in UV absorption bands. Consequently, much like in occultation observations of planetary atmospheres, spectroscopic observations of Io in transit should reveal atmospheric photoabsorption structure. The amplitude of detected spectral features depends on the SO_2 photoabsorption cross section as well as the abundance and temperature of SO_2 in Io's atmosphere. The HST GHRS is uniquely well-suited for this study because it is relatively free of scattered-light in the spectral range of interest, and Io's disk (including its highest-altitude atmospheric absorption level) fit nicely into the Large Science Aperture (LSA). Also, by placing the LSA at a fixed point on Jupiter's disk in the direct path of Io during a transit event, the GHRS RAPID readout mode allows the acquisition of time- resolved data that are sensitive to the abundance and vertical distribution of SO_2 in Io's atmosphere as well as to the existence of leading/trailing and north/south hemisphere atmospheric asymmetries. ------------------------------------------------------------------------------------ Observations Description ------------------------ We propose to use the GHRS to observe four transit events using two different methods. The first simpler technique, method 1, involves centering the GHRS LSA on Io while it is in front of Jupiter, and obtaining relatively long-duration ACCUM mode spectra while tracking the satellite to keep it centered in the aperture. Acquired in a wavelength range where the SO_2 photoabsorption cross section is known to have large peaks and significant variations, these in- transit spectra are predicted to have spectral features that are sensitive to Io's atmospheric SO_2 abundance and temperature. Method 2 is more complicated but promises to yield valuable information on the leading/trailing and north/south hemisphere atmospheric asymmetries. It involves pointing the LSA at a fixed point on Jupiter's disk in the direct path of Io, and using the RAPID readout mode to monitor the degradation of Jupiter's UV flux as Io's dark disk traverses the aperture. The proposed observations rely on three features of the GHRS instrument: 1) GHRS spectra do not suffer from scattered-light (like FOS spectra) that might compromise the proposed UV observations, 2) the size of the LSA is uniquely well-suited to observe Io and its atmosphere, and 3) the RAPID readout mode naturally provides time-series data that (as in occultation experiments) yield a great deal of information on atmospheric structure. Each transit observation requires two HST orbits, the first to observe Io before it begins its transit (i.e., without Jupiter in the background) and the second to perform the in-transit observation. Figure 1 illustrates the proposed observations. We begin by explaining the second type of transit observation, method 2. At the top left, Io is shown as a shaded disk at two different positions as it transits Jupiter; the right-hand disk shows Io centered in the LSA, and the left-hand disk shows its position about 5 minutes earlier. Because Io's UV reflectivity is much lower than Jupiter's (see below), Io's disk appears in silhouette during transit events. Therefore, as Io passes through the LSA the detected flux should decrease relative to Jupiter's on-disk flux. If Io had no atmosphere and its emissions were negligible relative to Jupiter's, then the flux should decrease by the fraction of the aperture that Io's opaque disk fills at any given time. However, molecules in Io's atmosphere will absorb additional photons. In spectral regions where the atmosphere is optically thick the effective diameter of Io's absorbing disk can be increased significantly by the atmosphere, and spectroscopic observations of transit events should provide the spectral signature of optically thick atmospheric constituents. To illustrate the feasibility of the proposed observations, we have performed simulations using two representative Io model SO_2 atmospheres: model A, a ``cold and thick'' atmosphere, using a vertical SO_2 column density of N_v,= 10^18 with temperature T = 250 K, and model B (``hot and thin'') that has N_v,= 10^17 and T =1000 K@. For each we use an isothermal, barometric gas distribution that includes the variation of Io's gravitational acceleration (see Chamberlain & Hunten 1987). While not as detailed as the models developed by Strobel et al. (1994), these relatively simple models are adequate to establish the feasibility of the proposed technique. We choose to search for SO_2 absorption features because SO_2 is a confirmed dominant constituent of Io's atmosphere and has several strong absorption bands in the 1970--2400 Angstrom range (Martinez & Joens 1992). For this study we will use the GHRS G270M grating to sample the ~ 44 Angstrom interval centered on 2115 Angstrom, because in this wavelength range the SO_2 photoabsorption cross section is near its peak magnitude and varies by almost a factor of 100 in banded absorption features separated by 15 Angstrom (see the upper right panel of Figure 1). As explained below, these large cross section variations will produce detectable variations in the observed spectra. The specific intensity (erg,cm^-2 s^-1 Angstrom^-1 arcsec^-2) of the Jupiter/Io system during transit events is given beginequation I(Lambda) = I_Io(Lambda) + I_Jup(Lambda) e^-tau_Lambda, endequation where I_Io(Lambda) is Io's surface intensity along a particular sight-line, I_Jup(Lambda) is the intensity of Jupiter's disk (nearly constant over the aperture), and tau_Lambda is Io's photoabsorption optical depth for photons originally emitted from Jupiter. Here, tau_Lambda = infty for sight- lines that intersect Io's solid surface. For sight- lines traversing Io's atmosphere, tau_Lambda = sum_i N_i sigma_i(Lambda), the product of the column density and photoabsorption cross section summed over all atmospheric species. Previous HST observations indicate that near 2100 Angstrom, I_Jup(Lambda) = 2.2,* 10^-13 and I_Io(lambda) = 1.2 * 10^-14 ergs cm^-2 s^-1 Angstrom^-1 arcsec^-2, (Noll et al. 1995; Ballester et al. 1994). Note that the disk-center intensity of Jupiter is ~ 20 times that of Io. In other words, Jupiter has a much greater geometric albedo than Io at these wavelengths, and Io will naturally appear in silhouette during the transit events. Detected fluxes are modeled by integrating the specific intensity over the entrance aperture, beginequation F(Lambda) = F_Io(Lambda) + F_Jup(Lambda) <=ft frac1Omega_LSA int dOmega e^-tau_Lambda right = F_Io(Lambda) + F_Jup(Lambda) <=ft T_Lambda right, endequation where T_Lambda is the effective transmission of Io and its atmosphere to light originally emitted from Jupiter. Observational method 1 relies on accurately measuring T_Lambda to reveal atmospheric absorption spectral structure. In the upper left panel of Figure 1, dotted circles around the right-hand Io disk illustrate the altitudes where the optical depth of the strong SO_2 absorption band centered at Lambda = 2104 Angstrom reaches tau_Lambda = 0.1 in the two models. The larger circle applies to the hot atmospheric model B and the inner circle to the cooler model A. This illustrates the strong dependence of effective absorption altitudes on Io's atmospheric temperature, as well as how nicely the atmospheric absorption region fits inside the GHRS LSA. In the lower-left panel of Figure 1, the predicted flux at the 2104 Angstrom SO_2 absorption peak is plotted as a function of time (or Io radii) during the observation, and shows that atmospheric model B (lower solid line) absorbs more strongly than model A (upper solid line). (For this part of the feasibility calculation, we approximate Io's emission as that of a Lambertian reflecting sphere the size of Io's solid surface, using the geometric albedo spectrum measured by Ballester et al. 1994.) A hybrid model that arbitrarily uses model B on Io's trailing hemisphere and model A on the leading hemisphere is shown as the dotted line, illustrating the effects leading/trailing atmospheric asymmetries. The predicted difference between the two atmospheric models is in the neighborhood of 10--20 %, and discriminating between the two requires a signal-to-noise ratio (SNR) of about 10. The left side axis of the plot shows the predicted signal acquired using the GHRS G270M grating that has sensitivity of about 17 * 10^11,(count s^-1)/(erg cm^-2 s^-1 Angstrom^-1) near 2100 Angstrom@. Typical signals are approx 10 count s^ -1 Angstrom^-1, so binning the spectra at 1 Angstrom intervals (about 11 diodes) and in 10 second periods yields the required SNR. We illustrate the spectral signatures of the two model atmospheres by plotting the predicted transmission spectra, T_Lambda, in the lower right panel of Figure 1, using the approximation I_Jup(Lambda) >> I_Io(Lambda) for simplicity. As expected, model B (short dashed line) absorbs more strongly than model A (solid line). In the case of no atmosphere, no spectral signature should be visible (in this approximation) and the transmission should be flat as a function of wavelength (long -dashed line). For the hotter model, spectral variations at the 10--15 % level are indicated, but the cooler model produces smaller amplitude variations of about 5 %. By accumulating signal from the the entire ~3 minute period while Io's disk in completely inside the LSA, spectra with approx 1800 count Angstrom^-1 will be available. Since the unattenuated flux spectrum of Jupiter (measured before or after Io passes through the aperture) will have comparable statistical quality, then the empirically-derived transmission spectrum will have a SNR ratio of roughly 30, adequate to detect the predicted variations for model B, but only marginally adequate for model A. To improve this situation, the total exposure time when Io is in the center of the LSA needs to be increased. This is the reasoning behind observational method 1, where the LSA is centered on Io early during a transit event, and the GHRS acquires a long -duration ACCUM mode spectrum while tracking the the satellite as it traverses Jupiter's disk. We estimate that method 1 will allow exposure times with Io centered in the aperture 5 to 7 times longer than method 2, and will allow the transmission spectra T_Lambda to be empirically determined to an accuracy of 1--2 %, adequate to detect the predicted variations for even the cooler atmospheric model. Io's emission, I_Io(Lambda), while treated in an approximate fashion in this feasibility analysis, must be empirically constrained in the real transit study. When observational method 1 is used, ACCUM mode spectra of Io centered in the LSA will be measured at a time when Jupiter is not behind Io, most likely during the HST orbit that immediately precedes the orbit used for the transit observation. For method 2, the spatial variation of Io's emission will be measured using RAPID readout mode in precisely the same manner as for the transit observation, but during the HST orbit preceding the transit event. In addition to its role in the transit observation, this measurement will provide a spatial profile of Io's albedo spectrum that will be sensitive to the existence of any major albedo features on Io's surface. We propose four separate Io transit observations, the first using the simpler method 1, and three others using method 2. Each observation requires two HST orbits, one for the transit observation and the preceding orbit to measure Io's emission. All four also use the GHRS G270M grating sampling a wavelength band centered at 2115 Angstrom (the same shown in Figure 1). The first observation employs method 1 in order to obtain long-duration ACCUM mode exposures of Io before and during transit. This method requires three exposures: one to measure Io's flux, F_Io(Lambda) in the orbit preceding the transit event, another during the transit event centered on Io to measure the attenuated in-transit flux, F(Lambda), and a final exposure to measure Jupiter's unattenuated flux, F_Jup(Lambda), placing the LSA aperture along the same path across Jupiter's disk as in the previous observation, but at a later time during the orbit when Io is not in the aperture. From equation (2) it is apparent that, when combined, these three measurements provide an empirical measure of the transmission spectrum, T_Lambda, that is sensitive to the abundance and temperature of SO_2 in Io's atmosphere. The three other observations will employ the same GHRS grating and wavelength interval, but will use method 2 in order to obtain time-series data as illustrated in Figure 1. The second observation will be performed as explained above: point the LSA at a fixed point directly in Io's transit path and record GHRS RAPID mode spectra as Io passes through the center of the aperture. In the preceding orbit, Io's emission flux will be measured in exactly the same manner, but at a time when Jupiter is not behind Io. The third and fourth observations use the same basic strategy, but pointing the LSA such that only Io's northern or southern hemisphere pass through the aperture. These observations will provide information on north/south hemisphere asymmetries. Predicted fluxes and signal levels for these observations are given in Figure 1 and, as explained earlier, are adequate to achieve the stated scientific objectives if the spectra are binned in 1 Angstrom intervals and the RAPID mode readout time is ~ 10 seconds. Since RAPID mode will be used for at most two consecutive HST orbits, the number of GHRS recorded in real time in should not be prohibitively large. , , Ballester, G. E., et al. (1994) Icarus 111 2. Broadfoot, A. L., et al. (1989) Science 246 1459. Chamberlain, J. W. & Hunten, D. M. (1987) Theory of Planetary Atmospheres, 2nd Ed. Clarke, J. T., et al. (1994) JGR 99 8387. Lellouch, E., et al. (1992) Icarus 98 271. Martinez, R. D. & Joen, J. A. (1992) GRL 19 277. Noll, K. S. et al. (1995) Science 267 1307. Pearl, J. C., et al. (1979) Nature 280 755. Sartoretti, P., et al. (1994) Icarus 108 272. Schneider, N. M., et al. (1989) Time- Variable Phenomenon in the Jovian System, Belton et al. Ed., NASA SP 494, p. 75. Smith, G. R., et al. (1982) JGR 87 1351. Strobel, D. F., et al. (1994) Icarus 111 18. Real Time Justification ----------------------- Each transit observation requires two consecutive HST orbits, timed such that during the second Io will be in transit of Jupiter's disk within ~ 13 arcsec of the central meridian longitude (CML) and observable with the GHRS LSA. We constrain the in-transit exposures to be within ~ 13 arcsec of the CML to ensure that the observation position is not too close to Jupiter's limb (Jupiter's equatorial radius is typically ~ 22 arcsec). During Cycle 6, Io transit events occur every 1.77 days, and during each Io is adequately close to the CML for about 1.2 hours. For the last three observations, we prefer aperture position-angles that align one of the sides of the LSA as closely as possible with the direction Io's transit motion, in order to provide the best resolution as Io passes into the aperture. However, this latter constraint is not absolutely necessary to achieve the stated scientific objectives. For both proposed methods of observation, we will use established tracking modes for Solar System objects. Previous GHRS observations of the Galilean satellites indicate that Io can be accurately centered in the LSA and tracked, allowing the ACCUM mode exposures required for method 1. The OFFSET solar system target mode will be used for the RAPID mode observations of method 2. Calibration Justification ------------------------- Additional Comments ------------------- NOTES ON OBSERVING STRATEGY. This type of observation has never been attempted with HST before, and is likely to present some difficulties. Each observation sequence requires two HST orbits, one to acquire Io and obtain exposures before the transit event begins, and one to obtain an in-transit spectrum of Io as well as a reference spectrum of the region of Jupiter behind Io. The most reasonable order to perform these is as follows: Orbit-1) HRS ACQ and ACCUM observations of Io, Orbit-2) HRS ACCUM observation of Jupiter's disk for reference followed by ACCUM observation of Io in-transit. Note that it is important to try to obtain the Jupiter disk spectrum along the same path on Jupiter's disk as the in-transit spectrum. I have developed a MOSS system PERCY script to calculate acceptable times for the in-tranit observations. For the first observation this is: SET BOUNDS FROM 01 JUN 1996 TO 01 AUG 1996; SET STEPSIZE 5 MINUTES; FIND IO-TRANSIT TRANSIT OF IO ACROSS JUPITER FROM EARTH; FIND IO-INSIDE SEPARATION OF IO JUPITER FROM EARTH LESS THAN -7 ARCSECONDS WITHIN IO-TRANSIT; This produces a window IO-INSIDE specifying satisfactory observing intervals. Note that the Jupiter disk refernce spectrum can be obtained at times outside this window. The second observation should be performed in the interval 10 SEP 1996 TO 17 NOV 1996. A very tricky part of this program will be ensuring that the in-transit and Jupiter reference spectrum are exposed on very nearly the same swath of Jupiter's disk. I have used the POS_ANGLE target reference system to specify this, because it seemed like it was the most reasonable method. I specify an offset position from Io, along its leading orbital path, and the offset must be adjusted to match the separation between the position of Io at the beginning of the in-transit observation and the beginning of the Jupiter disk observation. Since the drift rates of Io acrross Jupiter's disk (arcsec/hour) vary as a function of time (they are generally largest near opposition), this offset cannot be calculated precisely until the specific orbit for the observations are chosen. Below, a nominal offset is used, but this must be adjusted before the final scheduling can be performed. ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6757( 20) - 09/03/96 10:00 - [ 1] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: IO-PRETRANSIT-1 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE IO ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=JUPITER | STD=IO | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | SEP OF IO JUPITER FROM EARTH GT 5" | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 5.02 | Io one orbit before the in-transit B-V = 1.17 | observation. | ------------------------------------------------------------------------------------------------------------------------------------ 6757( 20) - 09/03/96 10:00 - [ 2] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 2 | TARGET NAME: JUP-DISK-1 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: OFFSET IO ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=JUPITER | STD=IO | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | TYPE=POS_ANGLE,RAD=7.54,ANG=265.32,REF=N | SEP OF JUP-DISK-1 JUPITER FROM EARTH LT ORTH,EPOCH=29-May-1996 | -7" | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 5.02 | This target is the swath accross B-V = 1.17 | Jupiter's disk that will be observed | for target IO-IN-TRANSIT-1 but | without Io in the aperture. See | "additional comments" section above | for more information. Note the | position angles of the leading | segment of Io's orbit are 261.49 | degrees on 96 MAY 29 00:00 UT and | 264.72 degrees on 96 AUG 01 00:00 | UT. The offset RAD=8.3 arcsec was | calculated assuming a nominal | angular rate of 20 arcsec/hour | between Io and Jupiter, and a | nominal time interval of 25 min | between the beginning of the | exposure for target JUP-DISK and the | beginning of the exposure for target | IO-IN-TRANSIT-1. | ------------------------------------------------------------------------------------------------------------------------------------ 6757( 20) - 09/03/96 10:00 - [ 3] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 3 | TARGET NAME: IO-IN-TRANSIT-1 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE IO ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=JUPITER | STD=IO | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | SEP OF IO JUPITER FROM EARTH LT -7" | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 5.02 | Io while in transit of Jupiter's B-V = 1.17 | disk. The intention here is to | expose on Io while it is inside of | Jupiter's disk by at least 6". See | "additional comments" section above | for more information. | ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 4 | TARGET NAME: IO-PRETRANSIT-2 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE IO ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=JUPITER | STD=IO | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | SEP OF IO JUPITER FROM EARTH GT 5" | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 5.02 | Io one orbit before the in-transit B-V = 1.17 | observation. | ------------------------------------------------------------------------------------------------------------------------------------ 6757( 20) - 09/03/96 10:00 - [ 4] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 5 | TARGET NAME: JUP-DISK-2 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: OFFSET IO ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=JUPITER | STD=IO | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | TYPE=POS_ANGLE,RAD=6.18,ANG=264.12,REF=N | SEP OF JUP-DISK-2 JUPITER FROM EARTH LT ORTH,EPOCH=10-Sep-1996 | -7" | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 5.02 | This target is the swath accross B-V = 1.17 | Jupiter's disk that will be observed | for target IO-IN-TRANSIT-2 but | without Io in the aperture. See | "additional comments" section above | for more information. Note the | position angles of the leading | segment of Io's orbit are 265.48 | degrees on 96 SEP 10 00:00 UT and | 261.98 degrees on 96 NOV 17 00:00 | UT. The offset RAD=7.1 arcsec was | calculated assuming a nominal | angular rate of 17 arcsec/hour | between Io and Jupiter, and a | nominal time interval of 25 min | between the beginning of the | exposure for target JUP-DISK and the | beginning of the exposure for target | IO-IN-TRANSIT-2. | ------------------------------------------------------------------------------------------------------------------------------------ 6757( 20) - 09/03/96 10:00 - [ 5] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 6 | TARGET NAME: IO-IN-TRANSIT-2 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE IO ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=JUPITER | STD=IO | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | SEP OF IO JUPITER FROM EARTH LT -7" | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 5.02 | Io while in transit of Jupiter's B-V = 1.17 | disk. The intention here is to | expose on Io while it is inside of | Jupiter's disk by at least 6". See | "additional comments" section above | for more information. | ------------------------------------------------------------------------------------------------------------------------------------ 6757( 20) - 09/03/96 10:00 - [ 6] Visit: 01 Visit Requirements: ON HOLD BETWEEN 14-Aug-1996:22:30:00.0 AND 14-Aug-1996:23:00:00.0 On Hold Comments: On-hold because the magnitude of the offset for the POS_ANGLE target reference system needs to be modified. Additional Comments: BETWEEN is for current JUP-DISK-1 specification. The BETWEEN may be changed to any time from 1 June 1996 to 1 August 1996, but the specification of JUP- DISK-1 must also be modified accordingly. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 111 IO-PRETRANS HRS ACQ 2.0 MIRROR-N2 BRIGHT=RETURN,LOCATE=EXTENDED, 1 5.0 S ONBOARD ACQ FOR 112-122 IT-1 SEARCH-SIZE=5 Comments: An acquisition with STEP-TIME=0.2S using the GHRS Side-2. ------------------------------------------------------------------------------------------------------------------------------------ 112 IO-PRETRANS HRS ACCUM 2.0 G270M 2110. 1 571.2 S IT-1 Comments: Io exposure obtained the orbit immediately preceeding the exposures for targets IO-IN-TRANSIT-1 and JUP-DISK. ------------------------------------------------------------------------------------------------------------------------------------ 113 IO-PRETRANS HRS ACCUM 2.0 G270M 2110. 1 652.8 S IT-1 Comments: Io exposure obtained the orbit immediately preceeding the exposures for targets IO-IN-TRANSIT-1 and JUP-DISK. ------------------------------------------------------------------------------------------------------------------------------------ 121 JUP-DISK-1 HRS ACCUM 2.0 G270M 2110. 1 952.0 S SEQ 121-122 NON-INT Comments: This exposure is designed to begin at the location where exposure 121 will begin, and expose over the same swath on to Jupiter's disk center that will exposed during the IO-IN-TRANSIT-1 exposure. As explained in the comments for target JUP-DISK, this position leads the location of Io in its orbit by approximately 8.3 arcsec, but this must be adjusted when the specific orbits are choosen. ------------------------------------------------------------------------------------------------------------------------------------ 122 IO-IN-TRANS HRS ACCUM 2.0 G270M 2110. 1 952.0 S IT-1 Comments: Io while transiting Jupiter's disk. The intention here is to expose on Io while it is inside of Jupiter's disk by at least 6". See "additional comments" section above for more information. ------------------------------------------------------------------------------------------------------------------------------------ 6757( 20) - 09/03/96 10:00 - [ 7] Visit: 02 Visit Requirements: ON HOLD BETWEEN 19-Oct-1996:09:30:00.0 AND 19-Oct-1996:10:30:00.0 On Hold Comments: On-hold because the magnitude of the offset for the POS_ANGLE target reference system needs to be modified. Additional Comments: BWTEEN is for current specification of JUP-DISK-2. BETWEEN may be changed to any time from 10 September 1996 to 15 November 1996, but the JUP-DISK-2 specification must be modified accordingly. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 211 IO-PRETRANS HRS ACQ 2.0 MIRROR-N2 BRIGHT=RETURN,LOCATE=EXTENDED, 1 5.0 S ONBOARD ACQ FOR 212-222 IT-2 SEARCH-SIZE=5 Comments: An acquisition with STEP-TIME=0.2S using the GHRS Side-2. ------------------------------------------------------------------------------------------------------------------------------------ 212 IO-PRETRANS HRS ACCUM 2.0 G200M 2110. 1 571.2 S IT-2 Comments: Io exposure obtained the orbit immediately preceeding the exposures for targets IO-IN-TRANSIT-2 and JUP-DISK. ------------------------------------------------------------------------------------------------------------------------------------ 213 IO-PRETRANS HRS ACCUM 2.0 G200M 2110. 1 652.8 S IT-2 Comments: Io exposure obtained the orbit immediately preceeding the exposures for targets IO-IN-TRANSIT-2 and JUP-DISK. ------------------------------------------------------------------------------------------------------------------------------------ 221 JUP-DISK-2 HRS ACCUM 2.0 G200M 2110. 1 952.0 S SEQ 221-222 NON-INT Comments: This exposure is designed to begin at the location where exposure 221 will begin, and expose over the same swath on to Jupiter's disk center that will exposed during the IO-IN-TRANSIT-2 exposure. As explained in the comments for target JUP-DISK, this position leads the location of Io in its orbit by approximately 7.1 arcsec, but this must be adjusted when the specific orbits are choosen. ------------------------------------------------------------------------------------------------------------------------------------ 222 IO-IN-TRANS HRS ACCUM 2.0 G200M 2110. 1 952.0 S IT-2 Comments: Io while transiting Jupiter's disk. The intention here is to expose on Io while it is inside of Jupiter's disk by at least 6". See "additional comments" section above for more information. ------------------------------------------------------------------------------------------------------------------------------------ 6757( 20) - 09/03/96 10:00 - [ 8] Summary Form for Proposal 6757 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations HRS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BRIGHT=RETURN LOCATE=EXTENDED SEARCH-SIZE=5 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category SOLAR SYSTEM ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ON HOLD BETWEEN 14-Aug-1996 22 30 00.0 AND 14-Aug-1996 23 00 00.0 ONBOARD ACQ FOR 112-122 SEQ 121-122 NON-INT ON HOLD BETWEEN 19-Oct-1996 09 30 00.0 AND 19-Oct-1996 10 30 00.0 ONBOARD ACQ FOR 212-222 SEQ 221-222 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR-N2 G270M G200M ------------------------------------------------------------------------------------------------------------------------------------ Target Names IO-PRETRANSIT-1 JUP-DISK-1 IO-IN-TRANSIT-1 IO-PRETRANSIT-2 JUP-DISK-2 IO-IN-TRANSIT-2 ------------------------------------------------------------------------------------------------------------------------------------