Venus Observations

HST Program 8659

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 cycle 4, HST observed Venus with GHRS and WFPC2 (GO programs 4518 and 5783).

v In cycle 6, 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 Director shortly after phase 2 submission.

v In cycle 7, 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 Much implementation work was done, but the program was then withdrawn by the PI.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 cycle 9, the TAC approved GO program 8659 to observe Venus. This program was planned for June 2001 but did not execute due to the failure of STIS side 1 electronics.

v Program 8659 is now re-planned for January 26, 2003.

Scientific Goals

v High resolution STIS spectrum of Lyman-av Assess the abundance of deuterium in the Venus

atmospherev Constrain models of Venus atmosphere and

climate evolution

Observing Strategy

v Observe while Venus is near maximum elongationä In cycle 11, there are two 10 day long windows

separated by 6 months.v Observe while HST is in Earth’s shadow

ä A window of 11-13 minutes per orbitv One HST orbit is sufficient to conduct

observation.v Generic TDRSS scheduling will be employed.

Which Elongation?

v Eastern: August 2002, Western: January 2003v Both are possible, but Western is easier.

Target Visibility

Shadow

Target Visibility

ShadowEastern Elongation:

Western Elongation:

Eastern vs. Western Elongation

Target Visibility

GS Pair Acq at Venus+5D

Slew to Venus

Target Visibility

Shadow

Single GS Acqat Venus

Observe Venus

Slew toVenus+5D

ObserveVenus+5D

GS Pair Acq atVenus+5D

Observing Strategy Details

v Initial GS Acq at Venus+5Dä A pointing 5 degrees from Venus and 51.01

degrees from the Sun.ä Minimizes slew distance to Venusä Precautionary measure to protect against large

pointing error after slew

Observing Strategy DetailsTarget Visibility

GS Pair Acq at Venus+5D

Slew to Venus

Target Visibility

Shadow

Single GS Acqat Venus

Observe Venus

Slew toVenus+5D

ObserveVenus+5D

GS Pair Acq atVenus+5D

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

v Venus science observationä 4.25m exposureä STIS auto wave cal disabled

Observing Strategy DetailsTarget Visibility

GS Pair Acq at Venus+5D

Slew to Venus

Target Visibility

Shadow

Single GS Acqat Venus

Observe Venus

Slew toVenus+5D

ObserveVenus+5D

GS Pair Acq atVenus+5D

Venus+5D Observationä Geocoronal Lyman-a foreground and

interplanetary Lyman-a background.ä 16m exposureä For subtraction from Venus Lyman-alpha line

Venus Appearance at Observation

v Sun angle: 46.00 degreesv Apparent magnitude: -4.4v Surface brightness: +1.5 (average for 1 square

arcsecond of illuminated portion of disk)v Apparent diameter: 21.12 arcsecondsv Phase angle: 81.24 degrees

Areas of Concern

v Guide star acquisition failurev Spacecraft of 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 Precautionsä The chances are minimized by the short 5 degree slew

to Venus.ä In cycle 4, additional FHST maps were also inserted by

GSFC; but now there are automatic FHST maps, so this is not an issue.

Guide Star Acquisition Failure

v Precautionsä A guide star acquisition test was planned in advance.

F Guide star ID is 00624100797.F Test successfully completed on July 15, 2002.

Problem Shortly Before Observation

v Concernä An unrelated problem may threaten the Venus

observing window.ä In cycle 4, the Venus observation was originally lost

due to an unrelated safing; but it was quickly replanned and executed successfully.

ä In May 2001, the current program was delayed due to the failure of the STIS side 1 electronics.

Problem Shortly Before Observation

v Remedyä The current Venus observation could probably be

rescheduled within a couple of weeks after January 26.ä A reduction of exposure time or an additional orbit

would be required.ä The Sun angle would be reduced to 44-45 degrees.ä There might be conflicts with other time critical

observations.

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 magnitude 11.1 and will be

about 16.2 arcminutes from Venus.

HST FOV

HST in the Sun

v Concernä While slewing away from Venus, HST will be exposed

to sunlight and pointed inside the SAZ for ~3m 0s.ä In cycle 4, the maximum time that HST was exposed to

sunlight while pointed inside the SAZ was ~2m 20s.ä Pointing inside the SAZ while HST is in sunlight can

be tolerated for a maximum of about 15 minutes before the temperature of the paint inside the OTA would become a contamination issue.

HST in the Sun

v Precautionsä Guide star search radius could be reduced in order to save

~30 seconds of acquisition time.ä This would reduced HST’s time in sunlight to ~2m 30s.ä In cycle 4, the slew away from Venus was started sooner

by editing the SMS to reduce the time pad between PCPTERM and slew from 60s to 20s.F This pad has since been reduced to 15s, so there is no longer

any time to be saved here.

HST Safing

v Concernä If HST were to enter inertial hold safe mode while

pointed within the SAZ, that would be bad.v Precautions

ä 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 cycle 4.

HST Safing

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

software sun point safe mode might not be achieved.v Precaution

ä Be ready to command HST into hardware sun point safe mode.

Proposed Schedule for Remaining Preparation

v Observation will be part of SMS 2003.025ä Calendar boundary moved to 2003.025:23 from 2003.027 due to

tight constraints of GOODS and KBO large programs which are executing in the same time period.

v Test SMS to GSFC: week of December 16, 2002.v 2003.025 calendar starts building on January 16, 2003.v Pre-FRR: January 17, 2003 10:00-11:00 LMB 229 (video

conference with STScI in room 112)v FRR: January 21, 2003 9:30-11:00 NASA Building 3 room

S107A (video conference with STScI in room 112)

Additional Information

Detailed Timeline

between Begin guide star acquisition at VENUS+5D. (VENUS+5D is a pointing 5 degrees2003.026:08:11:01 from Venus in the direction away from the Sun. Sun angle for this pointing is 51.01 and degrees) Exactly when this falls in this orbit depends on what other unrelated2003.026:09:07:14 observations precede it in the schedule. The important thing is that this will take place in the orbit before the Venus observation.2003.026:09:07:14 VENUS-CRESCENT enters Earth avoidance. (VENUS-CRESCENT is the science target. It is a pointing a few arcseconds offset from the center of Venus. Sun angle for this pointing is 46.00 degrees.)2003.026:09:32:28 HST enters Earth’s shadow2003.026:09:41:24 begin slew to VENUS-CRESCENT2003.026:09:47:02 end slew2003.026:09:47:02 begin guide star acquisition2003.026:09:47:08 VENUS-CRESCENT exits Earth avoidance2003.026:09:53:34 end guide star acquisition2003.026:09:53:42 begin science observation of VENUS-CRESCENT2003.026:10:01:18 end science observation

Detailed Timeline

2003.026:10:01:23 begin slew to VENUS+5D2003.026:10:02:56 HST exits Earth's shadow2003.026:10:07:00 end slew2003.026:10:07:00 begin guide star acquisition at VENUS+5D2003.026:10:13:32 end guide star acquisition2003.026:10:13:40 begin science observation at VENUS+5D2003.026:10:33:01 end science observation2003.026:10:33:01 begin STIS readout2003.026:10:36:01 end readout2003.026:10:36:01 begin STIS wavelength calibration2003.026:10:37:27 end wavelength calibration2003.026:10:37:27 begin STIS readout 2003.026:10:38:26 end readout2003.026:10:38:26 begin end-of-visit overhead2003.026:10:42:48 end visit 01 of program 8659

RA, DEC, Sun Angle, Roll

v HST will be at nominal roll (99 degrees V3) during Venus Observation

Time R.A. Dec. Sun Sep. (utc) (degrees) (degrees) (degrees) =============================================================2003-JAN-26 09:15:00.0 259.1056 -20.0229 46.01 2003-JAN-26 09:20:00.0 259.1096 -20.0234 46.01 2003-JAN-26 09:25:00.0 259.1136 -20.0238 46.01 2003-JAN-26 09:30:00.0 259.1177 -20.0243 46.01 2003-JAN-26 09:35:00.0 259.1217 -20.0247 46.01 2003-JAN-26 09:40:00.0 259.1257 -20.0252 46.01 2003-JAN-26 09:45:00.0 259.1297 -20.0256 46.01 2003-JAN-26 09:50:00.0 259.1338 -20.0261 46.01 2003-JAN-26 09:55:00.0 259.1378 -20.0265 46.00 2003-JAN-26 10:00:00.0 259.1418 -20.0270 46.00 2003-JAN-26 10:05:00.0 259.1458 -20.0274 46.00

RA, Dec, Sun Angle, Roll

Time R.A. Dec. Sun Sep. (utc) (degrees) (degrees) (degrees) ============================================================= 2003-JAN-26 10:10:00.0 259.1499 -20.0278 46.00 2003-JAN-26 10:15:00.0 259.1539 -20.0283 46.00 2003-JAN-26 10:20:00.0 259.1579 -20.0287 46.00 2003-JAN-26 10:25:00.0 259.1619 -20.0292 46.00 2003-JAN-26 10:30:00.0 259.1660 -20.0296 46.00 2003-JAN-26 10:35:00.0 259.1700 -20.0301 46.00 2003-JAN-26 10:40:00.0 259.1740 -20.0305 46.00 2003-JAN-26 10:45:00.0 259.1780 -20.0310 46.00 2003-JAN-26 10:50:00.0 259.1821 -20.0314 46.00 2003-JAN-26 10:55:00.0 259.1861 -20.0319 46.00 2003-JAN-26 11:00:00.0 259.1901 -20.0323 46.00 2003-JAN-26 11:05:00.0 259.1941 -20.0327 46.00 2003-JAN-26 11:10:00.0 259.1982 -20.0332 46.00 2003-JAN-26 11:15:00.0 259.2022 -20.0336 46.00

Guide Star IDs

v The guide star system gave the following suitable pairs to use for the setup pointing prior to the Venus observation:

0622702670F10622701453F2 0622702670F10622700796F20622600804F20622701480F1 0622600506F20622601372F3 0622600624F20622701753F1 0622702670F10622700972F20622601774F20622701175F3 0622701753F10622700591F30622601058F20622600428F3 0622600020F20622601581F1 0622601619F10622600428F3 0622600624F20622601581F10622701753F10622600138F2 0622701480F10622700829F30622601619F10622601058F2 0622600612F20622601372F30622600800F20622601581F1 0622601372F30622600548F20622601530F10622600428F3 0622601204F20622601581F10622600376F20622601581F1 0622600290F30622601530F10622601619F10622600290F3 0622601204F30622601581F10622600708F20622600772F3 0622601058F20622601530F1 0622600820F20622601372F3 0622601372F30622601577F2 0622600138F20622700591F3

Guide Star IDs

v The guide star for the Venus observation is 0624100797F1.v For the pointing to which HST will slew after Venus, the

guide star system gave the following suitable pairs:0622600624F2 0622601629F1 0622601018F2 0622600826F20622600376F2 0622600652F20622701175F30622700806F30622700454F3 0622601701F1 0622600800F2 0622701445F3 0622601879F2 0622601628F1

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.