Solar System Science for Webb Observers
Frequently Asked Questions (FAQ)
Technical FAQ specifically on Solar System observations. Targeted at the science/technical community.
Solar System Science for Webb Observers
- What's special about Webb for solar system science?
High sensitivity and angular resolution at near-IR and mid-IR wavelengths (see FAQ #16 and #20 below for details). Imaging and spectroscopic coverage from 0.6 to 28.5 μm with low to moderate spectral resolving power (~100 to a few thousand).
- Many important molecules (e.g. H2O, HDO, CO, CO2, S2, CH4), ices, and minerals have strong features in the JWST wavelength range.
- JWST will have the sensitivity to obtain near-IR spectra and mid-IR photometry (albedos) of any Kuiper belt object (KBO) known today.
- Monitoring planetary (and satellite) weather for the duration of mission (minimum 5 years, probably > 10 years), 2 intervals of about 3 months each year.
- What are the top topics for solar system science?
That is up to the scientists who write proposals. But we anticipate and provide support for studies of the outer planets, satellites, comets, asteroids, Kuiper belt objects, and "targets of opportunity". See the science white papers by Lunine et al. and Sonneborn et al. here: http://www.stsci.edu/jwst/doc-archive/white-papers/ . Many of these problems have been included in the JWST Science Operations Design Reference Mission, which describes realistic science programs that could be executed early in the JWST mission. Information about the SODRM is available here: http://www.stsci.edu/jwst/science/sodrm/.
- What does the JWST Science Working Group say about Solar System science?
The JWST Science Working Group (SWG) has worked hard to ensure Webb can observe Solar System objects, and in addition to the white papers above there are a number of ongoing studies and design reference mission programs and the white papers for more information (see links in FAQ #13). The SWG has emphasized these topics: for KBOs and dwarf planets, the composition of surface ices and volatiles in a wide range of bodies in the trans-Neptunian region. For comets, near and mid-infrared spectroscopy of cometary volatiles and organics, and spectroscopic studies of the new class of icy comets in the asteroid belt. For planets and moons, spectroscopy and imaging of the Martian atmosphere, imaging and spectral characterization of the atmospheres of the outer solar system, and monitoring of the Titan methane cycle beyond the end of the Cassini mission.
- What instruments does Webb have?
For a pretty complete description of JWST hardware and anticipated science, see our Space Science Reviews article (pdf). Webb has four science instruments that are described here: http://www.jwst.nasa.gov/instruments.html and here http://www.stsci.edu/jwst/instruments, and summarized here:
- NIRCam covers 0.6 to 5 μm with cameras, broad and narrow-band filters, and a low-resolution grism. NIRCam has a several coronagraphic masks for studying circumstellar disks and for host galaxies around quasars, and for hunting for exoplanets. NIRCam also carries defocussing lenses (for wavefront sensing and also good for exoplanet transits) and a pupil imaging lens (for initial alignment).
- NIRSpec also covers 0.6 to 5 μm with spectral resolving power from ~100 to about 2700. The spectrograph has several fixed slits, an integral field unit with a 3x3 arcsec field of view and 0.1 arcsec slices, and a multiobject spectrograph with ~250,000 addressable slits (4 quadrants of 171x365, each slit about 0.2x0.5 arcsec).
- MIRI goes from 5 to 28.5 μm (including the H2 line at 28.2 μm) with imaging and spectroscopy. The integral field spectrograph has spectral resolving power up to about 3000 in several bands and a field of view of 3.7x3.7 arcsec to 7x7 arcsec (size increasing with wavelength). There is also a low resolution mode with R~100 over 5-14 μm. MIRI has a coronagraph too, with optimized masks at different wavelengths.
- NIRISS is the Near IR Imaging Slitless Spectrometer. It two grism modes. One with R~150 over the 2.2x2.2 arcmin field of view and is good for isolated objects with spectral lines, and it's also good if you can't predict exactly where the object will fall in the field of view. The second grism mode is designed specially for R~700 spectroscopy of bright objects (exoplanet transits). NIRISS also has a sparse aperture synthesis mode with a non-redundant mask to look close to bright stars (or possibly resolve binary KBOs).
- How good is the angular resolution?
The specification is that the telescope is diffraction limited at 2 μm, which means a Strehl ratio of 0.8 and a wavefront error of 150 nm rms. With a 6.5 m telescope, 1.22 λ/D = 0.077 arcsec at 2 μm. The smallest pixels (NIRCam 0.6-2.5 μm) are just 0.034 arcsec. But a lot of the wavefront error is due to imperfect alignment of the parts, and it's possible to do better for a small part of the field of view.
- What can Webb look at?
Anything more than 85° from the Sun as viewed from L2, which includes Mars, Jupiter, Saturn, their satellites, the asteroid belt, and all outer Solar System objects. JWST also can't look farther from the Sun than 135° (i.e. within 45° of the anti-sun position) but outer solar system objects will all be observable some of the time. The field of regard limitations are a fundamental consequence of the observatory thermal design and the sunshield design that keeps the telescope and instruments cold. This means that the Sun, Earth, Moon, Mercury, and Venus, and of course sun-grazing comets and many known NEOs cannot be observed.
- What do I do for really bright objects?
If the desired instrument and detector would saturate during a normal exposures, the observer can specify a subarray of the detector for more rapid readout. The fewer the pixels, the more samples each can get per second. The smallest field for this method for NIRCam is 128x128 pixels, which allows sampling up to 5 per second and a minimum exposure time of 0.18 seconds. We think only Mars will be a real challenge.
- How faint can Webb go?
The combination of large aperture and low temperature makes JWST extremely sensitive across its full wavelength range, one to two orders of magnitude more sensitive than current or recent near- and mid-IR instrumentation. Webb sensitivity will be limited by the zodiacal light background shortward of ~13 microns. Longward of this the background is limited by thermal emission from the telescope optics. There is a comparison of expected JWST sensitivity with other missions here: http://www.stsci.edu/jwst/science/sensitivity.
- How do I determine what exposure my object would need?
Prototype Webb exposure time calculators (ETC) are up and running here: http://jwstetc.stsci.edu. The prototype ETCs cover NIRCam and MIRI full-frame imaging and NIRSpec multi-object spectroscopy. ETCs for all other observing modes, including bright objects, are under development.
- Are there limits on bright object observations?
We are not aware that the near-IR or mid-IR detectors can be damaged by bright light from any celestial source (except the Sun, which will never be in the JWST field of view). However, the detectors have some image persistence after bright sources are observed, so we have to minimize bright target exposures and allow time for recovery before observing fainter objects after bright ones. This subject is still being studied and quantified.
- Can Webb observe near bright planets?
Yes, unless the planet would fall into the Fine Guidance Sensor (FGS) field of view and prevent locating and tracking the guide star. Webb guide stars are generally in the range 17th to 12th magnitude (J), which is not that faint for a big space telescope. The FGS field of view is located several arc minutes from the science instruments.
- Isn't there a lot of glare near bright objects?
Yes, there is expected to be some near-field scattering due to optical imperfections and scattered light. There are predicted JWST point spread functions here:
- What about tracking moving objects?
Webb hardware and flight software can follow an ephemeris for apparent rates of up to 0.030 arcsec/sec with a very small pointing error (spec is 0.017 arcsec rms at 0.003 arcsec/sec). We are working on the software to implement this and it is required to be ready at launch. This rate capability includes Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, their satellites, and comets, asteroids and minor planets at or beyond the orbit of Mars. JWST uses the JPL HORIZONS system for ephemerides of 'standard' objects. The observer needs to supply the ephemeris for objects not in the JPL database. Observing visits are limited to the time the guide star is on the FGS detector (each FGS detector is 2.2x2.2 arcmin). You will need to accurately know the ephemeris of your object in order to put it on the narrow (<1 arcsec) spectrometer slit. The integral field spectrometers (see FAQ #16) provide a larger field of view.
- How do I get observing time?
Write proposals. The JWST proposal process will be similar to HST's. The JWST Science & Operations Center will be located at the Space Telescope Science Institute (STScI) in Baltimore, MD. The HST proposal preparation software already has hooks in it to support JWST proposals, but is not of course ready yet. The first proposals will be solicited about a year before launch. Competition will be fierce! Money will be awarded to successful proposers from U.S. institutions to support their analysis and publication of the observations.
- What is my proprietary time?
The baseline period for exclusive access to your JWST data is one year, as for HST and other missions. Some types of programs will have a shorter or zero exclusive access period. Proposers can also voluntarily reduce or waive their proprietary data rights. After the end of the exclusive access period the observations will be available for archival research.
- What if there's a wonderful surprising event, like a bright comet, a NEO, or a collision of a comet with Jupiter?
Targets of opportunity can be routinely handled in as little as two days. The observatory can slew 90 degrees in under an hour and we will routinely send commands to Webb twice a day.
- How do I put in my ideas and questions?
The Webb project scientists at GSFC and STScI are always interested in good ideas and happy to answer questions. If you are a professional scientist, write to us here: firstname.lastname@example.org or visit our scientific community Wiki page at http://jwstinput.wikidot.com. If you are a member of the public, send your questions to: email@example.com. If you are a member of the Press, please contact Lynn Chandler at (301) 286-2806.
- Is it too late to change the hardware design?
Yes, much too late. The telescope mirrors are finished, the two of four flight instruments are finished and delivered to Goddard, and the other two arrive within a year. It takes until 2018 to be ready for launch because we have to test the instruments, put the telescope together and test it with all the instruments in a giant vacuum tank at an operating temperature of about 40 K.
- How does Webb get community advice?
The JWST Program receives community input or advice through numerous channels. At the highest levels, the NASA Advisory Council's Science Committee provides input to the NASA Administrator on key agency programs such as JWST. The NASA Headquarters Program office and the GSFC Project Office receive advice from the JWST Science Working Group (JWST SWG). Finally, the Space Telescope Science Institute has its own JWST Advisory Committee (JSTAC). Each of these bodies meets several times a year to receive program and project updates and to discuss topics of interest and concern with NASA and its partners. We also have a wiki page for the broader scientific community: http://jwstinput.wikidot.com.
- When do observations start?
Webb will be launched in late 2018 and takes about 2 months to reach its L2 orbit and cool to operating temperature, followed by about 4 months of alignment, checkout, and calibration. The moving target capability will be checked out during this observatory commissioning period and will be available at the start of routine science operations.