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?
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 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: Bertaux and
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.
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but the program was then withdrawn by the PI.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)
Venus Observing Program History

Western Elongation




January 1995
January 2003
December 2010 - January 2011
Eastern Elongation

October 2013
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.

Observing Strategy

Observe while Venus is near maximum elongation


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.


4 visits of 1 orbit each
2 visits of 1 orbit each
The visits may be scheduled in 6 consecutive
orbits, but being consecutive is not required.
Observing Strategy Details
Visits currently scheduled on October 24, 2013
 Scheduled in 6 consecutive orbits
 Order

1.
2.
3.
4.
5.
6.
Visit 03 – sky background
Visit 01 – Venus
Visit 02 – Venus
Visit 05 – Venus
Visit 06 – sky background
Visit 04 – Venus
Observing Strategy Details
Target Visibility
Shadow
GS Pair Acq at
Venus+7D
Slew to
Venus
Observe
Venus+7D

Observe Slew to
Venus
Venus+7D
Single GS
Acq at Venus
Gyro
Acq at
Venus+7D
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
Observing Strategy Details
Target Visibility
Shadow
GS Pair Acq at
Venus+7D
Slew to
Venus
Observe
Venus+7D

Single GS
Acq at Venus
Gyro
Acq at
Venus+7D
Single GS Acq at Venus


Observe Slew to
Venus
Venus+7D
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
Venus Appearance at Observation
Apparent V Magnitude: -4.5 +/- 0.3
 Apparent Diameter: 23.0 arcseconds
 Phase Angle: 86.3 degrees

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)
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
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 7 degree slew to
Venus.
Perform guide star acquisition test before the Venus observing
timeframe.
Tests occurred on September 17 and September 18.
 Both were successful.

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.
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

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)
Vital Statistics – Prior Venus
Observations
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
Vital Statistics – October 24, 2013
Observation
Science
Angle to
Exposure Sun Limb
Time
(degrees)
Sunlit Time inside SAZ
While Slewing Toward
Venus
Visit 01
5m
46.6
0m 30s
S8G1000134 in FGS1
<= ~70” from centerline
Visit 02
5m
46.6
0m 52s
S8G1000134 in FGS1
<= ~60” from centerline
Visit 04
5m
46.6
1m 10s
S8G1000147 in FGS1
<= ~45” from centerline
Visit 05
5m
46.6
0m 43s
S8G1000134 in FGS1
<= ~60” from centerline

Guide Star
Total time pointed in SAZ (sunlit & shadow) is 20-21 minutes per orbit.
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.
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
STIS 52X0.5 Slit on Venus in Visit 01
HST’s View of Venus (t=0)
HST’s View of Venus (t=0)
Sun
Venus+7D
Venus
Bodies that are relevant to this observing program.
HST’s View of Venus (t=0)
Pluto
Mercur
y
Saturn &
moons
Other bodies that happen to be appear in this view.
HST’s View of Venus (t=0)
At about this time, HST is beginning its slew from Venus+7D to Venus
HST’s View of Venus (t=5m)
terminator
Earth
Slew to Venus continuing. Sun about to set
HST’s View of Venus (t=10m)
Sun has set. Slew to Venus complete. Guide star acquisition in progress.
HST’s View of Venus (t=15m)
Science observation beginning
HST’s View of Venus (t=20m)
Science observation continuing.
HST’s View of Venus (t=25m)
Science observation complete. Slew away from Venus in progress.
HST’s View of Venus (t=30m)
Slew away from Venus complete. Pointing at Venus+7D with gyro guiding.