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The James Webb Space Telescope's Plan for Operations and Instrument Capabilities for Observations in the Solar System [1]

['Stefanie N. Milam', 'Nasa Goddard Space Flight Center', 'Greenbelt Road', 'Greenbelt', 'Md', 'Usa', 'Stefanie.N.Milam Nasa.Gov', 'Nasa.Gov', 'Cristina.A.Thomas Nasa.Gov', 'John A. Stansberry']

Date: 2023-04

The James Webb Space Telescope (JWST) is an infrared-optimized observatory with a 6.5 m diameter segmented primary mirror and instrumentation that provides wavelength coverage of 0.6–28.5 μm, sensitivity 10× to 100× greater than previous or current facilities, and high angular resolution (0 07 at 2 μm). Details on the science goals, mission implementation, and project overview are summarized in Gardner et al. (2006). The science instruments have imaging, coronagraphic, and spectroscopic modes that provide spectral resolving power of ∼100 < RP < ∼3000, integral field units, and near-infrared multi-object spectroscopy. The capabilities of JWST will enable important studies throughout the Solar System beyond Earth's orbit, many of which are discussed in this special issue. JWST will provide access to the important 3 μm region free of the strong atmospheric absorptions that restrict observations from Earth. The target of opportunity response time can be as short as 48 hr, enabling timely observations of important events.

The JWST mission is led by NASA's Goddard Space Flight Center, with major mission participation, including science instrumentation, by the European Space Agency (ESA) and Canadian Space Agency (CSA). The JWST observatory will be launched by an Ariane 5 rocket provided by ESA. The observatory is designed for a five-year prime science mission, with consumables for 10 years of science operations. JWST will be operated for NASA by the Space Telescope Science Institute in Baltimore, MD. The first call for proposals for JWST observations will be released in late 2017.

After launch in 2018 October, JWST will enter a Lissajous orbit, free of Earth and lunar eclipses, around the Sun–Earth L2 point. This orbit simplifies planning and scheduling and minimizes thermal and scattered-light influences from the Earth and Moon. The telescope and science instruments are passively cooled to 40 K by remaining in the shadow of a 160 m2 five-layer sunshield. The mid-IR detector is further cooled to 6.5 K by a cryocooler.

The optics provide diffraction-limited performance at wavelengths ≥2 μm (where the point-spread function (PSF) FWHM is 64 mas). Scattered light is a concern for observing near bright targets such as the giant planets and brighter asteroids. Details of mirror-segment edges are being incorporated into JWST PSF models and will improve their fidelity. Unfortunately, accurate scattered-light performance will not be known until on-orbit testing is completed. The commissioning plan for the observatory will include such testing, and the scope of the tests is under discussion as of this writing.

The four science instruments on JWST (NIRCam, NIRISS, NIRSpec, and MIRI) cover the wavelength range from 0.6 to 28.5 μm and offer superb imaging and spectroscopic sensitivity (see some additional detail in Tables 1 and 2). Full details on the MIRI instrument can be found in the special issue of PASP recently published (e.g., Rieke et al. 2015). Sub-array readouts will enable observations of the giant planets and many bright primitive bodies in a variety of instrument modes. Saturation limits and special modes are presented in Norwood et al. (2016a). The science instruments share the telescope focal plane with a Fine Guidance Sensor (FGS—a part of the CSA contribution to the mission), as illustrated in Figure 1.

Figure 1. JWST science instrument and guider fields of view as they project onto the sky. The Sun and ecliptic north directions are shown for a line of sight in the ecliptic plane, at a solar elongation of 90°, and in the direction of observatory orbital motion. The line of sight is restricted to elongations of 85°–135°.(A color version of this figure is available in the online journal.) Download figure: Standard image High-resolution image

Table 1. JWST Imaging Modes Mode Instrument Wavelength Pixel Scale Field of Viewa (μm) (arcseconds) Imaging NIRCama 0.6–2.3 0.032 2 2 × 2 2 NIRCama 2.4–5.0 0.065 2 2 × 2 2 NIRISS 0.9–5.0 0.065 2 2 × 2 2 MIRIa 5.0–28 0.11 1 23 × 1 88 Aperture Mask Interferometry NIRISS 3.8–4.8 0.065 5 1 × 5 1 Coronagraphy NIRCam 0.6–2.3 0.032 20'' × 20'' NIRCam 2.4–5.0 0.065 20'' × 20'' MIRI 10.65 0.11 24'' × 24'' MIRI 11.4 0.11 24'' × 24'' MIRI 15.5 0.11 24'' × 24'' MIRI 23 0.11 30'' × 30'' Note. aMIRI and NIRCam provide sub-array imaging to facilitate observations of bright objects and coronagraphic imaging for the study of extra-solar planetary systems. NIRCam has two modules, giving a total field of view of 2 2 × 4 4 when both are used. NIRISS AMI employs a ∼5'' × 5'' sub-array. MIRI and NIRCam provide sub-array imaging to facilitate observations of bright objects and coronagraphic imaging for the study of extra-solar planetary systems. NIRCam has two modules, giving a total field of view of 24 when both are used. NIRISS AMI employs a ∼5''5'' sub-array. Download table as: ASCIITypeset image

Table 2. JWST Spectroscopy Modes Mode Instrument Wavelength Resolving Power Field of View (μm) (λ/Δλ) Slitless Spectroscopy NIRISS 1.0–2.5 150 2 2 × 2 2 NIRISS 0.6–2.5 700 Special Modea NIRCam 2.4–5.0 2000 2 2 × 2 2 Multi-Object Spectroscopy NIRSpec 0.6–5.0 100, 1000, 2700 3 4 × 3 4 0 2 × 0 5b Single Slit Spectroscopy NIRSpec 0.6–5.0 100, 1000, 2700 slit widths 0 4 × 3 8 0 2 × 3 3 1 6 × 1 6 MIRI 5.0–∼14.0 ∼100 at 7.5 μm 0 6× 5 5 slit Integral Field Spectroscopy NIRSpec 0.6–5.0 100, 1000, 2700 3 0× 3 0 MIRI 5.0–7.7 3500 3 0 × 3 9 MIRI 7.7–11.9 2800 3 5 × 4 4 MIRI 11.9–18.3 2700 5 2 × 6 2 MIRI 18.3–28.8 2200 6 7 × 7 7 Notes. aThis mode is specific to bright point-source targets (e.g., exoplanets). See the NIRISS instrument page for details ( bAny configuration of 0 2 × 0 5 micro-shutters (365 (dispersion) × 171 (spatial) shutters per quadrant). This mode is specific to bright point-source targets (e.g., exoplanets). See the NIRISS instrument page for details ( http://www.stsci.edu/jwst/instruments/niriss/science-with-niriss ). Any configuration of 05 micro-shutters (365 (dispersion)171 (spatial) shutters per quadrant). Download table as: ASCIITypeset image

Moving target observations are executed by controlling the position of a guide star in the FGS such that the science target remains stationary in the specified science instrument. The JWST attitude control system will track objects moving at rates of at least 30 mas s−1. The target ephemeris is represented in the observatory attitude control system as a fifth-order polynomial, enabling tracking of objects (such as Io) that have large apparent accelerations. Pointing stability (and therefore image quality) for moving targets is predicted to be better than 10 mas over 1000 s, comparable to that for fixed targets.

Figure 2 illustrates how the JWST "speed limit" of 30 mas s−1 affects the kinds of targets that can be observed, and at what heliocentric distances. Ephemerides for 11,000 near-Earth objects (NEOs), 170 known comets, 300 main belt asteroids (MBAs), and 130 Centaurs and trans-Neptunian objects (TNOs) were retrieved from the Jet Propulsion Laboratory's Horizons system for the years 2019–2020 at one-day spacing. JWST was chosen as the observer location (specified by entering "@JWST" for the observatory), and the analysis was restricted to times when the target fell within the JWST field of regard (see below).

Figure 2. Apparent rates of motion of 11,467 NEOs (observable in 2019), 170 Comets, and 305 main belt asteroids as seen from JWST during 2019–2020. Only the rates while objects are in the 85°–135° elongation range are included. Top left: points each give the apparent rate for one object on one date, plotted vs. the heliocentric distance of the object on that date. Top right: differential (black line) and cumulative (gray line) histograms of the data shown in the top left panel. Vertical lines give the mean apparent rate for the sample (dash–triple-dot) and the 30 mas s−1 JWST speed limit (dashed). The cumulative curve is re-normalized to be on the same scale as the differential curve. On any given date when an NEO is within the JWST elongation limits, there is a 91% probability that it can be tracked at or below the 30 mas s−1 limit. Bottom left: similar histograms for known comets. Only one comet can't be observed on any date in the two-year period, while six can only be observed for a limited time. Bottom right: similar histograms, but for the MBA sample. While these results are generally encouraging, it is important to note that signal to noise in a given exposure time and spatial resolution on a target both peak at epochs when the target is closest to the observatory and that the apparent rates of motion are typically highest at that time. If JWST can be made to track targets at rates higher than 30 mas s−1, there will be a significant benefit in reducing necessary integration times and enhanced resolution. Download figure: Standard image High-resolution image

The biggest impact of the 30 mas s−1 tracking-rate limit is for NEOs, where for any randomly chosen day there is a 9% chance that the target would be moving too fast to be tracked (see also Thomas et al. 2016). Panel (a) of Figure 2 shows that NEOs are preferentially moving too fast for JWST when they are nearest to the Sun (this is primarily because that is when they are closest to JWST as well). For the known comets, there is about a 1% probability that a target can't be tracked on any given day. For MBAs, Centaurs, and TNOs, all targets can be tracked on any day. The JWST ephemeris currently available in Horizons is nominal, but representative of the class of orbits the observatory may ultimately follow. Plans are in place to update the observatory ephemeris in Horizons about every two weeks after launch, so uncertainties in the JWST position will be a small component in the uncertainty on moving target location in the science data. Absolute pointing accuracy is limited by the accuracy of the guide-star catalog and on the moving-target ephemeris itself. Accuracy of the guide-star catalog is expected to improve dramatically as a result of the GAIA mission, so JWST pointing accuracy relative to guide stars should be well under 1''. Ephemeris uncertainty depends on the particular science target and is not particular to whether a target is observed from JWST or elsewhere.

The JWST sunshield design requires the orientation of the telescope line of sight to always be in the solar elongation range of 85°–135° (see Figure 1). The observatory boresight can be pointed to any position on the celestial sphere within this elongation range. This fundamental pointing limitation for JWST is similar to that for other passively cooled infrared missions such as Spitzer and Herschel (Wilson & Scott 2006; Pilbratt et al. 2010). Objects close to the ecliptic are typically observable during two ∼50 day intervals per year. However, the motion of some Solar System objects may further limit this observability. For example, Mars is only observable in 2018, 2020, and 2022 (see Villanueva et al. 2016). When the observatory pointing lies near the ecliptic plane the instrument fields of view are highly restricted in their position angle regardless of date or solar elongation, as shown in Figure 1. The roll angle about the boresight has a +/−5° range.

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[1] Url: https://iopscience.iop.org/article/10.1088/1538-3873/128/959/018001

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