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Venus Observations HST Program 13047

Venus Observations HST Program 13047. Objectives. Explain Venus observing strategy. Review areas of special concern with Venus observations and explain how these concerns are being addressed . Determine if implementation is correct and complete. Why Is Venus Special?.

latifah-warren
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Venus Observations HST Program 13047

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  1. Venus ObservationsHST Program 13047

  2. Objectives • Explain Venus observing strategy. • Review areas of special concern with Venus observations and explain how these concerns are being addressed. • Determine if 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 1991, GO 2393 (PI: Bertaux) proposed to observe Mars and Venus with GHRS and was approved by the TAC, but the Venus observations were later removed due to concerns about solar avoidance. • In 1995, HST observed Venus with GHRS and WFPC2 (GO 4518, PI: Esposito and GO 5783, PI: Esposito). • In 1996, two GO Venus proposals (6607, PI: Bertauxand 6771, PI: Na) were approved by the TAC; and there was one GTO/WF2 program (6851, PI: Trauger) to observe Venus. All of these were cancelled by the STScI Director shortly after phase 2 submission.

  5. Venus Observing Program History • In 1997, there was a GTO/STIS program (7581, PI: Moos) to observe Venus. Much implementation work was done, but the program was then withdrawn by the PI. • In 2003, HST observed Venus with STIS (GO 8659, PI: Bullock). • In 2010-2011, HST observed Venus with STIS (NASA 12433, PI: Jessup) • GO program 13047 (PI: Clarke) is now planned for: • October 24, 2013 (2013.297)

  6. Venus Observing Program History • Western Elongation • January 1995 • January 2003 • December 2010 - January 2011 • Eastern Elongation • October 2013

  7. Scientific Goals • Measure Lyman-alpha emissions from deuterium and from hydrogen in order to determine the D/H ratio in the upper atmosphere. • To help distinguish between terrestrial and Venusian Lyman-alpha, observations should happen when the Doppler shift from the combined line of sight orbital motions of Venus and the Earth is maximized. • The Doppler shift requirement results in observing during the 2013 Eastern Elongation.

  8. Observing Strategy • Observe while Venus is near maximum elongation • 4 visits of 1 orbit each • Observe while HST is in Earth’s shadow • A window of 13-16 minutes per orbit • Observe sky background near (~7 degrees) from Venus but within HST’s normal field of regard. • 2 visits of 1 orbit each • The visits may be scheduled in 6 consecutive orbits, but being consecutive is not required.

  9. Observing Strategy Details • Visits currently scheduled on October 24, 2013 • Scheduled in 6 consecutive orbits • Order • Visit 03 – sky background • Visit 01 – Venus • Visit 02 – Venus • Visit 05 – Venus • Visit 06 – sky background • Visit 04 – Venus

  10. Observing Strategy Details Target Visibility Shadow GS Pair Acq at Venus+7D Slew to Venus Observe Venus Slew to Venus+7D Gyro Acqat Venus+7D Observe Venus+7D Single GS Acqat Venus • Initial GS Acq at Venus+7D • A pointing 7 degrees from Venus and 53.6 degrees from the Sun. • Minimizes slew distance to Venus • Precautionary measure to protect against large pointing error after slew • Provides additional sky background data

  11. Observing Strategy Details Target Visibility Shadow GS Pair Acq at Venus+7D Slew to Venus Observe Venus Slew to Venus+7D Gyro Acqat Venus+7D Observe Venus+7D Single GS Acqat Venus • Single GS Acq at Venus • Saves about a minute versus pair acquisition • Venus science observation • One 300s exposure (will increase exposure time, if scheduling allows) • STIS auto wave cal disabled

  12. Venus Appearance at Observation • Apparent V Magnitude: -4.5 +/- 0.3 • Apparent Diameter: 23.0 arcseconds • Phase Angle: 86.3 degrees

  13. STIS Bright Object Screening • Observations will use STIS E140H 1234 with 52X0.5 aperture • Expected count rate from source: • January 2003 Venus observations using 0.2X0.09 aperture saw only 0.25 c/s from geo-coronal + Venusian Ly α • Small area of 8069 aperture may well have missed localized emission • Current program will observe 25”X0.5” area (~ 700X larger) which would give ~ 200 c/s if scaled from 8069 • Martian 52X0.2 aperture observations showed ~ 50 c/s from Martian Ly α • Expect Venusian Ly α to be a few times brighter than Mars and 13047 aperture is 2.5X wider than that used for Mars program • Predict several hundred counts/s from Venusian Ly α (+ geo-coronal line)

  14. STIS Bright Object Screening • Also need to check for bright background objects • O5 star with V=10 would be close to allowed global limit of 200,000 c/s • When comparing with the field, the parallax effects due to HST’s orbital motion need to be included, • Shift in position over HST orbit similar to the angular size of Venus (~23” ) • Procedure: • The program coordinator provided to the instrument scientist with HST-centric ephemerides for both the Venus and Venus-OFFSET-7D target pointings • Current bright object tool was used to check fixed pointings spaced at 10 min intervals along the target path to clear the region swept out by the MAMA observations

  15. 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

  16. 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 7 degree slew to Venus. • Perform guide star acquisition testbefore the Venus observing timeframe. • Tests occurredon September 17 and September 18. • Both were successful.

  17. 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. • Remedy • Try to re-schedule quickly if necessary as in 1995.

  18. 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 stars are • In FGS1 – farthest from STIS. • S8G1000134 • S8G1000147

  19. HST in the Sun • Concern • While slewing away from Venus, HST will be exposed to sunlight and pointed inside the SAZ for up to 1m35s. • HST’s maximum previous exposure to sunlight while pointed inside the SAZ was 3m 12s (Jan 2011). • 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-1067 and EM-MOSES-1268)

  20. Vital Statistics – Prior Venus Observations

  21. Vital Statistics – October 24, 2013 • Total time pointed in SAZ (sunlit & shadow) is 20-21 minutes per orbit.

  22. 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 all prior HST observations of Venus.

  23. 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.

  24. Additional Information

  25. STIS 52X0.5 Slit on Venus in Visit 01

  26. HST’s View of Venus (t=0)

  27. HST’s View of Venus (t=0) Sun Venus Venus+7D Bodies that are relevant to this observing program.

  28. HST’s View of Venus (t=0) Pluto Mercury Saturn & moons Other bodies that happen to be appear in this view.

  29. HST’s View of Venus (t=0) At about this time, HST is beginning its slew from Venus+7D to Venus

  30. HST’s View of Venus (t=5m) terminator Earth Slew to Venus continuing. Sun about to set

  31. HST’s View of Venus (t=10m) Sun has set. Slew to Venus complete. Guide star acquisition in progress.

  32. HST’s View of Venus (t=15m) Science observation beginning

  33. HST’s View of Venus (t=20m) Science observation continuing.

  34. HST’s View of Venus (t=25m) Science observation complete. Slew away from Venus in progress.

  35. HST’s View of Venus (t=30m) Slew away from Venus complete. Pointing at Venus+7D with gyro guiding.

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