Venus Observations
HST Program 12433
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.

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.

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program was then withdrawn by the PI.
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
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

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.
Observing Strategy Details
Target Visibility
Target Visibility
Shadow
Target Visibility
Shadow
GS Pair Acq
at Venus+5D
Slew to
Venus
Observe Slew to
Venus Venus+5D
GS Pair
Single GS
Acq at
Acq at Venus
Venus+5D

Slew to
Venus
Observe Slew to
Venus Venus+5D
Single GS
Acq
at Venus
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
GS Pair
Acq at
Venus+5D
Observing Strategy Details
Target Visibility
Target Visibility
Shadow
Target Visibility
Shadow
GS Pair Acq
at Venus+5D
Slew to
Venus

Observe Slew to
Venus Venus+5D
GS Pair
Single GS
Acq at
Acq at Venus
Venus+5D
Single GS Acq at Venus


Slew to
Venus
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: 116s and 2.5s exposures
STIS auto wave cal disabled
Observe Slew to
Venus Venus+5D
GS Pair
Acq at
Venus+5D
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
Vital Statistics
Observation
Science
Exposure Time
Angle to
Sun Limb
(degrees)
Sunlit Time inside SAZ
While Slewing Away
from Venus
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 FGS1
on 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 FGS1
12” from centerline
Jan 22, 2011
orbit 2
58s
46.1
1m 50s
S8OF000650 in FGS1
on 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
Guide Star
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

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.

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.
Problem Shortly Before Observation

Remedy



Visit 01: Second choice coordination opportunity with
Venus Express on January 23, 2011
Visit 02: ???
Visit 03: ???
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 arcminutes from 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

HST FOV – Visit 01
HST FOV – Visit 02
HST FOV – Visit 03 Orbit 1
HST FOV – Visit 03 Orbit 2
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)
HST in the Sun

Precautions

Guide star search radius was reduced in order to save 1030 seconds of acquisition time.
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.
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.
Additional Information
Observe
Venus+5D
Observing Strategy Details
Target
Target
Target
Visibility
Visibility
Visibility
Shadow
Display as seen
from HST
Shadow
GS Pair Acq
at Venus+5D
Slew to
Venus
Single GS
Acq
at Venus
Observe Slew to
Venus Venus+5D
Slew to
Venus
Observe
Venus+5D
Single GS
GS Pair
Acq
Acq at
at Venus
Venus+5D
Observe Slew to
Venus Venus+5D
GS Pair
Acq at
Venus+5D
HST’s View of Venus (t = 0)
Earth
Venus+5D
Sun
Venus
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.