Venus Observations HST Program 12433

1. Venus ObservationsHST Program 12433

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

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

4. Venus Observing Program History • In 1995, HST observed Venus with GHRS and WFPC2 (GO 4518 and GO 5783). • 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. • 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 Much implementation work was done, but the program was then withdrawn by the PI.

5. Venus Observing Program History • In 2003, HST observed Venus with STIS (GO 8659). • NASA program 12433 is now planned for: • December 27-28, 2010 (2010.361-362) • January 22, 2011 • January 27, 2011

6. Scientific Goals • Coordinated observing with ESA’s Venus Express mission. • Map variations in SO2, SO, S2, and other UV absorbers. • Constrain models of Venus atmosphere and climate evolution

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

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

9. Observing Strategy Details Target Visibility Target Visibility Target Visibility Shadow Shadow GS Pair Acq at Venus+5D Slew to Venus Slew to Venus Observe Venus Observe Venus Slew to Venus+5D Slew to Venus+5D GS Pair Acqat Venus+5D Single GS Acqat Venus Single GS Acq at Venus GS Pair Acqat Venus+5D • Single GS Acq at Venus • Saves about a minute versus pair acquisition • Venus science observation • Visit 01: two 153s exposures, Visit 02: two 58s exposures, Visit 03:116sand 2.5s exposures • STIS auto wave cal disabled

10. Venus Appearance at Observation • Apparent magnitude: -4.1 +/- 0.2 • Surface brightness: +1.5 (average for 1 square arcsecond of illuminated portion of disk) • Apparent diameter • Visit 01: 28.5 arcseconds • Visit 02: 21.4 arcseconds • Visit 03: 20.4 arcseconds • Phase angle • Visit 01: 97.3 degrees • Visit 02: 89.9 degrees • Visit 03: 79.4 degrees

11. Vital Statistics

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

13. Guide Star Acquisition Failure • Concern • If attitude error is greater than the search radius, the guide star acquisition could fail resulting in the loss of the science. • Precautions • The chances are minimized by the short 5 degree slew to Venus. • Too late to perform guide star acquisition test. Stars are in solar avoidance zone. • Science exposures will proceed even if GS acquisition fails. • Special commanding specified to ignore “take data” flag. • AQEXPIRE timer adjusted via SMS edit.

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

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

16. FGS Exposure To Venus • Concern • 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. • Precautions • Choose a guide star that is as far as possible from Venus. • The chosen guide star is: • Visit 01: magnitude 12.4 and 15.5 arcminutesfrom Venus. • Visit 02: magnitude 11.7 and 16.8 arcminutes from Venus. • Visit 03 • Orbit 1: magnitude 11.2 and 13.8 arcminutes from Venus • Orbit 2: magnitude 11.5 and 12.5 arcminutes from Venus

17. HST FOV – Visit 01

18. HST FOV – Visit 02

19. HST FOV – Visit 03 Orbit 1

20. HST FOV – Visit 03 Orbit 2

21. HST in the Sun • Concern • While slewing away from Venus, HST will be exposed to sunlight and pointed inside the SAZ for up to 3m 32s. • HST’s maximum previous exposure to sunlight while pointed inside the SAZ was 2m 30s (2003 observation). • 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)

22. HST in the Sun • Precautions • Guide star search radius was reduced in order to save 10-30 seconds of acquisition time.

23. HST Safing • Concern • HST must not enter inertial hold safe mode while pointed within the SAZ. • Precautions • Disable inertial hold safe mode such that any condition that would normally lead to inertial hold leads to software sun point safe mode instead. • This was done in both prior HST observations of Venus.

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

25. Additional Information

26. Observing Strategy Details Observe Venus+5D Target Visibility Target Visibility Target Visibility Display as seen from HST Shadow Shadow GS Pair Acq at Venus+5D Slew to Venus Slew to Venus Observe Venus Observe Venus Slew to Venus+5D Slew to Venus+5D Observe Venus+5D Single GS Acq at Venus Single GS Acq at Venus GS Pair Acqat Venus+5D GS Pair Acqat Venus+5D

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

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

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

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

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

32. 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

33. 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.

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

35. 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.

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

37. 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.