Venus Observations HST Program 12433

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 1995, HST observed Venus with GHRS and WFPC2 (GO 4518 and GO 5783). v In 1996, 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 STScI Director shortly after phase 2 submission. v In 1997, 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 Much implementation work was done, but the program was then withdrawn by the PI.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 2003, HST observed Venus with STIS (GO 8659). v NASA program is now planned for: ä December 27-28, 2010 ( ) ä January 22, 2011 ä January 27, 2011

Scientific Goals  Coordinated observing with ESA’s Venus Express mission. v Map variations in SO 2, SO, S 2, and other UV absorbers. v Constrain models of Venus atmosphere and climate evolution

Observing Strategy v Observe while Venus is near maximum elongation v Observe while HST is in Earth’s shadow ä A window of 9-14 minutes per orbit v Three visits of 2 orbits each.

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 GS Pair Acq at Venus+5D Observing Strategy Details Target Visibility Shadow Slew to Venus Single GS Acq at Venus Observe Venus Slew to Venus+5D GS Pair Acq at Venus+5D v Initial GS Acq at Venus+5D ä A pointing 5.5 degrees from Venus and 51.6 degrees from the Sun. ä Minimizes slew distance to Venus ä Precautionary measure to protect against large pointing error after slew

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 GS Pair Acq at Venus+5D Observing Strategy Details Target Visibility Shadow Slew to Venus Single GS Acq at Venus Observe Venus Slew to 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 Visit 01: two 153s exposures, Visit 02: two 58s exposures, Visit 03: 116s and 2.5s exposures ä STIS auto wave cal disabled

Venus Appearance at Observation v Apparent magnitude: /- 0.2 v Surface brightness: +1.5 (average for 1 square arcsecond of illuminated portion of disk) v Apparent diameter ä Visit 01: 28.5 arcseconds ä Visit 02: 21.4 arcseconds ä Visit 03: 20.4 arcseconds v Phase angle ä Visit 01: 97.3 degrees ä Visit 02: 89.9 degrees ä Visit 03: 79.4 degrees

Vital Statistics Observation Science Exposure Time Angle to Sun Limb (degrees) Sunlit Time inside SAZ While Slewing Away from Venus Guide Star January s per orbit46.32m 20s January s46.02m 30s Dec 28, 2010 orbit 1 153s46.22m 14sS in FGS1 on centerline Dec 28, 2010 orbit 2 153s46.23m 1sS in FGS1 ~50” from centerline Jan 22, 2011 orbit 1 58s46.11m 57sS8OF in FGS1 12” from centerline Jan 22, 2011 orbit 2 58s46.11m 50sS8OF in FGS1 on centerline Jan 27, 2011 orbit 1 120s45.73m 12sS8L in FGS1 36” from centerline Jan 27, 2011 orbit 2 120s45.73m 2sS8LA in FGS2 29” from centerline

Areas of Concern v Guide star acquisition failure v Spacecraft or 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 Too late to perform guide star acquisition test. Stars are in solar avoidance zone. ä Science exposures will proceed even if GS acquisition fails. F Special commanding specified to ignore “take data” flag. F AQEXPIRE timer adjusted via SMS edit.

Problem Shortly Before Observation v Concern ä An unrelated problem may threaten the Venus observing window. ä In 1995, the Venus observation was originally lost due to an unrelated safing; but it was quickly replanned and executed successfully. ä The 2003 Venus observations were delayed from 2001 due to the failure of the STIS side 1 electronics.

Problem Shortly Before Observation v Remedy ä Visit 01: Second choice coordination opportunity with Venus Express on January 23, 2011 ä Visit 02: ??? ä Visit 03: ???

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: F Visit 01: magnitude 12.4 and 15.5 arcminutes from Venus. F Visit 02: magnitude 11.7 and 16.8 arcminutes from Venus. F Visit 03 ; Orbit 1: magnitude 11.2 and 13.8 arcminutes from Venus ; Orbit 2: magnitude 11.5 and 12.5 arcminutes from Venus

HST FOV – Visit 01

HST FOV – Visit 02

HST FOV – Visit 03 Orbit 1

HST FOV – Visit 03 Orbit 2

HST in the Sun v Concern ä While slewing away from Venus, HST will be exposed to sunlight and pointed inside the SAZ for up to 3m 32s. ä HST’s maximum previous exposure to sunlight while pointed inside the SAZ was 2m 30s (2003 observation). ä Pointing inside the SAZ while HST is in sunlight can be tolerated for at least 15 minutes before the temperature of the paint inside the OTA would become a contamination issue. (EM-MOSES-1267 and EM-MOSES-1268)

HST in the Sun v Precautions ä Guide star search radius was reduced in order to save seconds of acquisition time.

HST Safing v Concern ä HST must not enter inertial hold safe mode while pointed within the SAZ. 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 both prior HST observations of Venus.

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.

Additional Information

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 Target Visibility Shadow Slew to Venus Single GS Acq at Venus Observe Venus Slew to Venus+5D Observe Venus+5D GS Pair Acq at Venus+5D Display as seen from HST

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.