Micrometeoroid Avoidance Zone: Policies and Procedures

Micrometeoroid impacts on JWST optical components have the potential to degrade the image quality, leading to a reduction in the effective lifetime. NASA commissioned a Micrometeroid Working Group (MmWG) to examine this issue and identify potential mitigations.

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The MmWG identified a Micrometeoroid Avoidance Zone (MAZ), a cone of diameter 75 degrees, centred on JWST’s orbital direction of motion (the ram direction). This direction is expected to see the largest flux of higher-velocity micrometeoroids. The MmWG recommended minimizing observations within the MAZ, setting an implementation goal of <15% (by time) of the Cycle 2 program, with <10% as an aspirational goal. Meeting this goal is estimated to reduce major impacts by a factor of 2, with a corresponding reduction in the growth of the wavefront error. 

The MmWG also recommended that JWST should be scheduled to avoid pointing in the direction of major meteor showers, notably the Geminids and Quadrantids. These concerns will likely affect scheduling on 2-4 days during the year.

This article describes the steps being taken to help achieve the MAZ goals in Cycle 2.


STScI has developed JDox material that provide a description of the issue (see JWST Micrometeoroid Avoidance Zone (MAZ)) and a guide for proposers on how to reduce the MAZ exposure of their observations (see APT Micrometeoroid Avoidance).

Proposal preparation

Community members submit proposals for JWST observations using the Astronomer’s Proposal Tool (APT). As part of the process, APT uses a Visit Planner to determine when observations can schedule; the proposer can set Special Requirements (eg orientations, timing constraints) that affect the scheduling windows. APT (as of version 2022.7, release date 17 Nov. 2022) has been modified to give proposers a warning when a visit has scheduling windows that are more than 70% within the MAZ. Under those circumstances:

  • The APT Visit Planner provides a visual representation of the non-MAZ scheduling windows (the Micrometeoroid Safe Zone).
  • Proposers are given links to the JDox documentation providing guidance on how to reduce the MAZ exposure.
  • Adjusting the scheduling Special Requirements to reducing the MAZ overlap below 70% will remove the warning from the proposal.
  • If it is not possible to reduce the MAZ overlap below 70%, the proposer must provide an explanation in a text box on APT. The warning changes to an informational flag. The text explanation is available to STScI staff, but will not be included in the Telescope Allocation Committee (TAC) version of the proposal.
  • The TAC will review proposals based on the propose science, without regard to scheduling constraints.

Post-TAC procedures

The Cycle 2 TAC will meet during the last two weeks in April 2023. The TAC recommendations will be presented to the STScI Director on May 8, 2023 at the Cycle 2 Director’s Review.

  • Processing of accepted proposals starts once the Director has formally approved the Cycle 2 program.
  • The construction of the Long Range Plan (LRP) starts when the proposals are ready to be incorporated into the schedule.
  • Accepted proposals are subject to science review by members of the Instruments Division (INS). INS will inspect each program for overly restrictive special requirements and will ask observers to loosen those requirements where possible. Science reviews occur through the cycle.
  • The LRP will be constructed initially without NIRSpec MSA observations, with an aim to minimizing the placement of observations in the MAZ. The initial LRP will be completed in ~1.5 months.
  • NIRSpec MSA observations will be added once the initial LRP is in place, generally choosing orientations that place the observations outside the MAZ;
  • It may be necessary to place observations in the MAZ to maintain efficient use of telescope time.
  • The LRP will be constructed to avoid pointing in the direction of major meteor showers.
  • The full LRP (save Target of Opportunity and Director’s Discretionary Time observations) will be available in late June.
  • Once the full LRP is available, the MAZ usage will be calculated.
    • If MAZ usage is below 10%, then proceed with LRP implementation;
    • If MAZ usage is between 10 and 15%, then high usage programs will be identified and program coordinators will work with science policy and the PIs to use standard procedures (orientation changes, fewer scheduling constraints) adjust observations to reduce MAZ usage;
    • If MAZ usage exceeds 15%, a Working Group will be constituted for additional review of programs with extensive (>40%) observations scheduled in the MAZ:
      • The Working Group includes representatives from JWSTMO, science policy, user support & scheduling.
      • The Working Group is charged with reducing MAZ usage with minimal impact on the science goals of each program.
      • Observations may be deferred to future cycles.
      • The Working Group will assess whether alternative strategies might be available for some observations that would still enable the proposers to reach their science goals. These options might include adding observing time to accommodate looser scheduling of mosaics, overlapping instrument fields-of-view without requiring a 180-degree flip, and/or obtaining observations at less optimal orientations. The final decision on any program will balance the risk of leaving observations in the MAZ against extending the observing period unduly by pushing observations to later epochs, potentially reducing overall observing efficiency.
    • If, after applying mitigations, MAZ usage remains above 20%, the Working Group will identify observations that could be removed from the current cycle to reduce usage below 20%:
      • Observations will be prioritized for down-selection based on the relative ranking of programs from the TAC review.
      • Observations may be cancelled if they cannot be deferred to future cycles.
      • Grant funding for an affected program will generally be adjusted only if the cancelled observations represent a major part of the program.

Protocols for scheduling in-cycle programs

The Long Range Plan is a dynamic entity, subject to change in-cycle as programs are updated, Target-of-Opportunity programs are triggered, new Director’s Discretionary programs are approved, and failed observations are re-scheduled. The following criteria will be adopted to schedule in-cycle observations

  • Time critical observations will be scheduled based on the requirements to achieve the science goals, without regard to MAZ compliance. There is no requirement for review by the Telescope Time Review Board.
  • Observations that are not time critical, including repeats of failed observations, will generally be scheduled when they are MAZ-compliant. Principal Investigators of those programs should provide a scientific justification to the TTRB if they believe that there is a compelling case for non-compliant scheduling. The TTRB will review the request and rule accordingly.

The level of overall MAZ-compliance will be reviewed midway through each cycle, and appropriate adjustments made to scheduling specific observations if such is deemed necessary.

These policies are subject to revision in Cycle 2 and future cycles should circumstances demand operational changes.

Notable updates

Originally published