venus observations hst program 12433. objectives v explain venus observing strategy. v review areas...

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 Much implementation work was done, but the program was then withdrawn by the PI.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 12433 is now planned for:ä December 27-28, 2010 (2010.361-362)ä January 22, 2011ä January 27, 2011

Scientific Goals

v Coordinated observing with ESA’s Venus Express mission.

v Map variations in SO2, SO, S2, and other UV absorbers.

v Constrain models of Venus atmosphere and climate evolution

Observing Strategy

v Observe while Venus is near maximum elongationv 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 Acqat Venus+5D

Slew toVenus

Target Visibility

Shadow

Single GSAcq at Venus

ObserveVenus

Slew toVenus+5D GS Pair

Acq atVenus+5D

Observing Strategy DetailsTarget Visibility

Shadow

Slew toVenus

Single GSAcqat Venus

ObserveVenus

Slew toVenus+5D

GS PairAcq atVenus+5Dv 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 Venusä Precautionary measure to protect against large pointing error after slew

Target Visibility

GS Pair Acqat Venus+5D

Slew toVenus

Target Visibility

Shadow

Single GSAcq at Venus

ObserveVenus

Slew toVenus+5D GS Pair

Acq atVenus+5D

Observing Strategy DetailsTarget Visibility

Shadow

Slew toVenus

Single GSAcqat Venus

ObserveVenus

Slew toVenus+5D

GS PairAcq atVenus+5D

v Single GS Acq at Venusä Saves about a minute versus pair acquisition

v Venus science observationä Visit 01: two 153s exposures, Visit 02: two 58s exposures, Visit

03: 116s and 2.5s exposuresä STIS auto wave cal disabled

Venus Appearance at Observationv Apparent magnitude: -4.1 +/- 0.2v Surface brightness: +1.5 (average for 1 square

arcsecond of illuminated portion of disk)v Apparent diameter

ä Visit 01: 28.5 arcsecondsä Visit 02: 21.4 arcsecondsä Visit 03: 20.4 arcseconds

v Phase angleä Visit 01: 97.3 degreesä Visit 02: 89.9 degreesä Visit 03: 79.4 degrees

Vital Statistics

ObservationScience Exposure Time

Angle to Sun Limb (degrees)

Sunlit Time inside SAZ While Slewing Away from Venus Guide Star

January 1995 22-109s per orbit 46.3 2m 20s

January 2003 255s 46.0 2m 30s

Dec 28, 2010 orbit 1

153s 46.2 2m 14s S924000129 in FGS1on centerline

Dec 28, 2010 orbit 2

153s 46.2 3m 1s S924000129 in FGS1~50” from centerline

Jan 22, 2011 orbit 1

58s 46.1 1m 57s S8OF000650 in FGS112” from centerline

Jan 22, 2011 orbit 2

58s 46.1 1m 50s S8OF000650 in FGS1on centerline

Jan 27, 2011 orbit 1

120s 45.7 3m 12s S8L2000017 in FGS1 36” from centerline

Jan 27, 2011 orbit 2

120s 45.7 3m 2s S8LA045471 in FGS2 29” from centerline

Areas of Concern

v Guide star acquisition failurev Spacecraft or instrument problem shortly before

scheduled observing timev FGS exposure to Venusv 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 Concernä 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 Concernä 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 Remedyä Visit 01: Second choice coordination opportunity with

Venus Express on January 23, 2011ä Visit 02: ???ä Visit 03: ???

FGS Exposure To Venus

v Concernä 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 Precautionsä 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 Concernä 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 Precautionsä Guide star search radius was reduced in order to save 10-

30 seconds of acquisition time.

HST Safing

v Concernä HST must not enter inertial hold safe mode while

pointed within the SAZ.

v Precautionsä 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 Concernä If there is an anomaly with the 486 flight computer,

software sun point safe mode might not be achieved.

v Precautionä Be ready to command HST into hardware sun point

safe mode.

Additional Information

Target

Visibility

GS Pair Acqat Venus+5D

Slew toVenus

Target

Visibility

Shadow

Single GSAcqat Venus

ObserveVenus

Slew toVenus+5D

ObserveVenus+5D

GS PairAcq atVenus+5D

Observing Strategy Details

Target

Visibility

Shadow

Slew toVenus

Single GSAcqat Venus

ObserveVenus

Slew toVenus+5D

ObserveVenus+5D

GS PairAcq atVenus+5D

Display as seen from HST

HST’s View of Venus (t = 0)

Sun

Venus

Venus+5DEarth

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.