Venus Observations HST Program 8659

Objectives v Explain Venus observing strategy. v Review areas of special concern with Venus observations and explain how these concerns are being addressed. v Determine if the implementation is correct and complete.

Why Is Venus Special? v At its farthest, Venus gets to be about 47 degrees from the Sun. v HST is normally not permitted to point within 50 degrees of the Sun.

Venus Observing Program History v In cycle 4, HST observed Venus with GHRS and WFPC2 (GO programs 4518 and 5783). v In cycle 6, two GO Venus proposals (6607 and 6771) were approved by the TAC; and there was one GTO/WF2 program (6851) to observe Venus. All of these were cancelled by the Director shortly after phase 2 submission. v In cycle 7, there was a GTO/STIS program (7581) to observe Venus. Much implementation work was done, but the program was then withdrawn by the PI.

Venus Observing Program History v In cycle 9, the TAC approved GO program 8659 to observe Venus. This program was planned for June 2001 but did not execute due to the failure of STIS side 1 electronics. v Program 8659 is now re-planned for January 26, 2003.

Scientific Goals  High resolution STIS spectrum of Lyman-  v Assess the abundance of deuterium in the Venus atmosphere v Constrain models of Venus atmosphere and climate evolution

Observing Strategy v Observe while Venus is near maximum elongation ä In cycle 11, there are two 10 day long windows separated by 6 months. v Observe while HST is in Earth’s shadow ä A window of minutes per orbit v One HST orbit is sufficient to conduct observation. v Generic TDRSS scheduling will be employed.

Which Elongation? v Eastern: August 2002, Western: January 2003 v Both are possible, but Western is easier.

Target Visibility Shadow Target Visibility Shadow Eastern Elongation: Western Elongation: Eastern vs. Western Elongation

Target Visibility GS Pair Acq at Venus+5D Slew to Venus Target Visibility Shadow Single GS Acq at Venus Observe Venus Slew to Venus+5D Observe Venus+5D GS Pair Acq at Venus+5D Observing Strategy Details v Initial GS Acq at Venus+5D ä A pointing 5 degrees from Venus and degrees from the Sun. ä Minimizes slew distance to Venus ä Precautionary measure to protect against large pointing error after slew

Observing Strategy Details Target Visibility GS Pair Acq at Venus+5D Slew to Venus Target Visibility Shadow Single GS Acq at Venus Observe Venus Slew to Venus+5D Observe Venus+5D GS Pair Acq at Venus+5D v Single GS Acq at Venus ä Saves about a minute versus pair acquisition v Venus science observation ä 4.25m exposure ä STIS auto wave cal disabled

Observing Strategy Details Target Visibility GS Pair Acq at Venus+5D Slew to Venus Target Visibility Shadow Single GS Acq at Venus Observe Venus Slew to Venus+5D Observe Venus+5D GS Pair Acq at Venus+5D  Venus+5D Observation  Geocoronal Lyman-  foreground and interplanetary Lyman-  background. ä 16m exposure ä For subtraction from Venus Lyman-alpha line

Venus Appearance at Observation v Sun angle: degrees v Apparent magnitude: -4.4 v Surface brightness: +1.5 (average for 1 square arcsecond of illuminated portion of disk) v Apparent diameter: arcseconds v Phase angle: degrees

Areas of Concern v Guide star acquisition failure v Spacecraft of instrument problem shortly before scheduled observing time v FGS exposure to Venus v HST being exposed to sunlight while pointed within the Solar Avoidance Zone (SAZ) v HST safing while pointed within the SAZ

Guide Star Acquisition Failure v Concern ä If attitude error is greater than the search radius, the guide star acquisition could fail resulting in the loss of the science. v Precautions ä The chances are minimized by the short 5 degree slew to Venus. ä In cycle 4, additional FHST maps were also inserted by GSFC; but now there are automatic FHST maps, so this is not an issue.

Guide Star Acquisition Failure v Precautions ä A guide star acquisition test was planned in advance. F Guide star ID is F Test successfully completed on July 15, 2002.

Problem Shortly Before Observation v Concern ä An unrelated problem may threaten the Venus observing window. ä In cycle 4, the Venus observation was originally lost due to an unrelated safing; but it was quickly replanned and executed successfully. ä In May 2001, the current program was delayed due to the failure of the STIS side 1 electronics.

Problem Shortly Before Observation v Remedy ä The current Venus observation could probably be rescheduled within a couple of weeks after January 26. ä A reduction of exposure time or an additional orbit would be required. ä The Sun angle would be reduced to degrees. ä There might be conflicts with other time critical observations.

FGS Exposure To Venus v Concern ä If the attitude error is large enough (> 300 arcseconds), an FGS could possibly see Venus and cause potential bright light exposure to the FGS photo-multiplier tubes. v Precautions ä Choose a guide star that is as far as possible from Venus. ä The chosen guide star is magnitude 11.1 and will be about 16.2 arcminutes from Venus.

HST FOV

HST in the Sun v Concern ä While slewing away from Venus, HST will be exposed to sunlight and pointed inside the SAZ for ~3m 0s. ä In cycle 4, the maximum time that HST was exposed to sunlight while pointed inside the SAZ was ~2m 20s. ä Pointing inside the SAZ while HST is in sunlight can be tolerated for a maximum of about 15 minutes before the temperature of the paint inside the OTA would become a contamination issue.

HST in the Sun v Precautions ä Guide star search radius could be reduced in order to save ~30 seconds of acquisition time. ä This would reduced HST’s time in sunlight to ~2m 30s. ä In cycle 4, the slew away from Venus was started sooner by editing the SMS to reduce the time pad between PCPTERM and slew from 60s to 20s. F This pad has since been reduced to 15s, so there is no longer any time to be saved here.

HST Safing v Concern ä If HST were to enter inertial hold safe mode while pointed within the SAZ, that would be bad. v Precautions ä Disable inertial hold safe mode such that any condition that would normally lead to inertial hold leads to software sun point safe mode instead. ä This was done in cycle 4.

HST Safing v Concern ä If there is an anomaly with the 486 flight computer, software sun point safe mode might not be achieved. v Precaution ä Be ready to command HST into hardware sun point safe mode.

Proposed Schedule for Remaining Preparation v Observation will be part of SMS ä Calendar boundary moved to :23 from due to tight constraints of GOODS and KBO large programs which are executing in the same time period. v Test SMS to GSFC: week of December 16, v calendar starts building on January 16, v Pre-FRR: January 17, :00-11:00 LMB 229 (video conference with STScI in room 112) v FRR: January 21, :30-11:00 NASA Building 3 room S107A (video conference with STScI in room 112)

Additional Information

Detailed Timeline between Begin guide star acquisition at VENUS+5D. (VENUS+5D is a pointing 5 degrees between Begin guide star acquisition at VENUS+5D. (VENUS+5D is a pointing 5 degrees :08:11:01 from Venus in the direction away from the Sun. Sun angle for this pointing is and degrees) Exactly when this falls in this orbit depends on what other unrelated and degrees) Exactly when this falls in this orbit depends on what other unrelated :09:07:14 observations precede it in the schedule. The important thing is that this will take place in the orbit before the Venus observation. in the orbit before the Venus observation :09:07:14 VENUS-CRESCENT enters Earth avoidance. (VENUS-CRESCENT is the science target. It is a pointing a few arcseconds offset from the center of Venus. Sun angle target. It is a pointing a few arcseconds offset from the center of Venus. Sun angle for this pointing is degrees.) for this pointing is degrees.) :09:32:28 HST enters Earth’s shadow :09:41:24 begin slew to VENUS-CRESCENT :09:47:02 end slew :09:47:02 begin guide star acquisition :09:47:08 VENUS-CRESCENT exits Earth avoidance :09:53:34 end guide star acquisition :09:53:42 begin science observation of VENUS-CRESCENT :10:01:18 end science observation

Detailed Timeline :10:01:23 begin slew to VENUS+5D :10:02:56 HST exits Earth's shadow :10:07:00 end slew :10:07:00 begin guide star acquisition at VENUS+5D :10:13:32 end guide star acquisition :10:13:40 begin science observation at VENUS+5D :10:33:01 end science observation :10:33:01 begin STIS readout :10:36:01 end readout :10:36:01 begin STIS wavelength calibration :10:37:27 end wavelength calibration :10:37:27 begin STIS readout :10:38:26 end readout :10:38:26 begin end-of-visit overhead :10:42:48 end visit 01 of program 8659

RA, DEC, Sun Angle, Roll v HST will be at nominal roll (99 degrees V3) during Venus Observation Time R.A. Dec. Sun Sep. (utc) (degrees) (degrees) (degrees) ============================================================= 2003-JAN-26 09:15: JAN-26 09:20: JAN-26 09:25: JAN-26 09:30: JAN-26 09:35: JAN-26 09:40: JAN-26 09:45: JAN-26 09:50: JAN-26 09:55: JAN-26 10:00: JAN-26 10:05:

RA, Dec, Sun Angle, Roll Time R.A. Dec. Sun Sep. (utc) (degrees) (degrees) (degrees) ============================================================= 2003-JAN-26 10:10: JAN-26 10:15: JAN-26 10:20: JAN-26 10:25: JAN-26 10:30: JAN-26 10:35: JAN-26 10:40: JAN-26 10:45: JAN-26 10:50: JAN-26 10:55: JAN-26 11:00: JAN-26 11:05: JAN-26 11:10: JAN-26 11:15:

Guide Star IDs v The guide star system gave the following suitable pairs to use for the setup pointing prior to the Venus observation: F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F3

Guide Star IDs v The guide star for the Venus observation is F1. v For the pointing to which HST will slew after Venus, the guide star system gave the following suitable pairs: F F F F F F F F F F F F F F1

HST’s View of Venus (t = 0) Sun Venus Venus+5D Earth Venus is in Earth occultation. The Sun is not. HST would be pointed at Venus+5D at this time.

HST’s View of Venus (t = 5 min) The Sun is moving toward setting.

HST’s View of Venus (t = 10 min) The Sun is moving toward setting. The terminator becomes visible on the Earth.

HST’s View of Venus (t = 15 min) The Sun is very close to setting.

HST’s View of Venus (t = 20 min) The Sun has set. HST is now in the Earth’s shadow.

HST’s View of Venus (t = 25 min) Venus is very close to rising. At about this time, HST will begin its slew from Venus+5D to Venus

HST’s view of Venus (t = 30 min) Venus has risen. At about this time, HST will complete its slew to Venus; and the guide star acquisition will begin.

HST’s View of Venus (t = 35 min) At about this time, the guide star acquisition will finish; and the science observation will begin.

HST’s View of Venus (t = 40 min) At about this time, the science observation will finish; and the slew to Venus+5D will begin.

HST’s View of Venus (t = 45 min) The Sun is very close to rising.

HST’s View of Venus (t = 50 min) The Sun has risen. HST is now out of the Earth’s shadow. The slew to Venus+5D finishes a couple of minutes after sunrise.