Scheduling Challenges for JWST JIM Feb. 19, 2004 Peter Stockman

Major planning constraints Sun avoidance: well known Field of Regard Earth-Moon scattered light: Will constrain some orientations (still not completely understood) Orbit maintenance (11 days between angular momentum dumps for FDF): would constrain roll choice/orientation for long observations Fuel conservation (22 days between angular momentum dumps): would constrain the roll constraints of all observations and potentially the mix of observations in a 22 day period. Red = New and Exciting 2/19/04 JIM

1.) Sun Avoidance The sunshade provides: Simple model based upon 2001 TRW sunshield design by Dennis Skelton The sunshade provides: -5° to 45° pitch from the ecliptic poles ~± 5° of operational roll Required for 10 day fixed-roll NIRSpec observations. 5° safety band in both pitch and roll Sunshield shadows Primary & Secondary Mirrors Primary shadows Secondary Mirror Stayout Zone 2/19/04 JIM 56°

2.) Simple L2, Earth, Moon Geometry in X-Y plane shows how Earth and Moon light can strike OTE ±10° roll shadow band, ±5°in MRD Sun projection The Earth and/or Moon can illuminate the optical surfaces, particularly at L2 orbit (Y and Z) extremes Could be improved by tighter L2 orbit or larger sunshade. Earth Moon 37° 27° L2 JWST 2/19/04 JIM

Earthshine typical example OTE components overhanging sunshield coverage will be illuminated by Earth crescent Northern hemisphere Pitch = 0 Sunshield Roll = 0 Yaw = 45 2/19/04 JIM

Scattered Earthshine can exceed the zodiacal background at l> 3 mm. Worst case assumes: 100% of 1 mirror (SM or PM) 1% dust Nominal BRDF Moonlight is less important (1-3% Zodi) From Larry Petro 2/19/04 JIM

Beckman analyzed one DRM for Earth/Moon Impacts For analysis, he used: Skelton’s stay out zones from 2001 TRW Phase 1 design 15 yr ephemeris and DRM v3.6b Periods exist when either the Earthlight or Moonlight would strike the primary or secondary mirror 15 yr JWST orbit seen from the Sun 2/19/04 JIM

Results 70% of observations were “dark” Earth and Moon each affected 25% of observations: Earth intruded as much as 22° into keep-out zone Moon intruded as much as 30° into keep-out zone Earth seen in L2 XY plane Very little correlation with time, but both Moon and Earth most easily seen at X-Y-Z extremes of the orbit. 1.5Mkm Earth 2/19/04 JIM

The new sunshield (June 2003) is 43% smaller than previous design to reduce angular momentum buildup and mass Design in proposal New design 67% area (based on inner layer) 57% area (based on outer layer) The smaller sunshade will increase the impact of scattered light from the Earth and Moon. 2/19/04 JIM

A rough idea of the constraint and how it changes per year NEP NEP Increased scattered light regions Pattern repeats 90 L2/2 180 L2 ~1 year Can create shorter observing seasons and impact 180 day repeats FOR in JWST frame Z JWST 3 weeks Later JWST L2 L2 Y 2/19/04 JIM

3.& 4.) Angular Momentum and Orbit Maintenance: To determine orbit, FDF is allowing at most 2 momentum dumps per 22 day period (e-folding time for orbit errors). Limited propellant mass for orbit maintenance and momentum dumps has led to concept of 1 dump/22 days (24 hrs before orbit burn) Flexibility for scheduling depends on wheel momentum storage capability 6 wheels = 40 n-m-s 4 wheels = 22 n-m-s 2/19/04 JIM

Schematic Maneuver Sequence Possible Additional Momentum Unload 21-day Tracking Arc Momentum Unloads (~ 1 day prior to SK maneuver) Station-Keeping Maneuvers (8 per rev, ~ 22 days apart) 2/19/04 JIM

Momentum accumulation is dominated by roll offsets in current design 1/5th of 22 day total accumulated in one day! dJ/dt Pitch Roll 2/19/04 JIM

Comparison of momentum accumulation for both new sunshields designs Current design (negative dihedral) Note significant angular momentum due to pitch alone Positive dihedral alternative 2/19/04 JIM

Possible ways to manage angular momentum in the scheduling system Baseline today: Monitor: Check long range plan to see if there is a potential for exceeding the momentum between 22 day dumps. Feasible if problems are rare Restrict average momentum buildup per observation to less than 2 n-m-s average during development of LRP. Constrain roll orientation and start-dates Significantly decreases scheduling flexibility. Failed observations will necessitate replan since all observations would be shifted Actively Manage momentum by balancing angular momentum build-up over each 22 day period (and potentially beyond) in the LRP. Increases science return, but may create a a very brittle schedule. 2/19/04 JIM

DRM shows 30-40% of dump intervals less than 22 days Monitor Study: DRM shows 30-40% of dump intervals less than 22 days Monitor method will not work. Fails in 30-40% of cases even with all reaction wheels working. 2/19/04 JIM

Restricting to an average momentum : 10 day observations need special planning to avoid excessive momentum build-up At high ecliptic latitudes, the visits must be centered within one day: either fixed start times or intervention needed if started early by failure of previous observation 2/19/04 JIM

Restricting to an average momentum : One day visits are also constrained Note drop in available Start-time at high ecliptic latitudes even for a 1-day early visit Restricting average momentum method will be very constraining…worth going to total momentum management 2/19/04 JIM

Restricting average momentum : Loss of reaction wheel leads to drastic constraints Note loss of all flexibility above 45° even for 1 day observations. Restricting average momentum method is not viable. Must go to total momentum management or change vehicle 2/19/04 JIM

A typical 22 day managed schedule Possible 22 day rules Only one 8-10 day obs Only one 4-7 day obs Fill in with 1 day obs 2/19/04 JIM

Summary Overall scheduling of JWST has become more complicated and may significantly impact JWST science: Long observations are almost time critical Full roll (± 5°) is not routinely available Even observations with varying roll but in the same part of the sky will be limited to ≤ 10 day stretches. Thermal radiation from the Earth can produce significant scattered light and preferred observing seasons (potentially impacting NGP & SGP depending on launch date) Angular momentum issue could be mitigated with positive dihedral design, increased momentum wheel capability or added fuel (~ 70 kg). Scattered light issue needs to be confirmed by Ball, STScI, and GSFC (Beckman/Skelton). Larger sunshield makes angular momentum problem worse. 2/19/04 JIM