Summary of the recent Report of the 2010 Senior Review of the Astrophysics Division Operating Missions April 6 – 9, 2010 from space.com...
A NASA advisory panel is recommending that the Wide-field Infrared Survey Explorer (WISE) mission end in October as originally planned instead of continuing to search for comets, asteroids and stars during a three month extended phase.
While the WISE mission is expected to produce significant results, NASA's 2010 Astrophysics Senior Review Committee said there was not adequate scientific justification to continue the mission once the spacecraft depletes its supply of hydrogen used to cool the onboard telescope and detectors.
The WISE spacecraft built by Ball Aerospace & Technologies Corp. of Boulder, Colo., carries an infrared telescope built by the Space Dynamics Laboratory of Logan, Utah, that is designed to detect the faint glow of distant objects with instruments chilled to the point where they produce no detectable infrared light.
The original plan for the 10-month WISE mission, which was launched in December and managed by the Jet Propulsion Laboratory in Pasadena, Calif., includes one month for on-orbit checkout, followed by a six-month survey of the entire sky in four wavelengths of infrared light.
During the last three months of the original mission, the WISE team plans to conduct a second survey covering half the sky in those four infrared wavelengths.
Because the spacecraft and telescope remain in excellent condition, Ned Wright, WISE principal investigator and a professor at the University of California, Los Angeles, proposed a three-month extension of the mission to complete the second half of the second sky survey in two of the four infrared wavelengths because those images could be captured even when the hydrogen supply is exhausted and the instruments can no longer be chilled.
That additional three-month project, known as Warm WISE, would have added $6.5 million to the program's $320 million price tag, according to NASA spokesman J.D. Harrington. WISE mission officials also proposed spending $8 million on an Extended Source Catalog, a detailed database of large objects, such as galaxies and interstellar clouds of gas and dust, revealed in WISE imagery but not of primary concern to astronomers looking at specific stars or comets.
Currently, WISE is producing approximately 7,500 images a day in each of four infrared wavelengths.
That original mission "should produce a catalog and image atlas of great utility to the entire astronomical community," according to the report of the NASA review panel. "Although it is impressed with the promise of the cryogenic mission, the Senior Review Committee did not find adequate scientific justification in the proposal for the cost of either the Warm Extension or the Enhanced Data Products."
Link: Space.com Article
From the report of NASA's 2010 Astrophysics Senior Review Committee:
The WISE mission will produce a survey of the entire sky at wavelengths 3.4, 4.6, 12, and 22 μm. WISE was launched into a Sun-synchronous orbit on 2009 December 14, and began its prime survey mission on 2010 January 14. WISE has a cryogenic primary mission utilizing a solid hydrogen cryostat, a 40 cm telescope, and four infrared detector arrays. The focal plane has four 1024x1024 infrared detector arrays, HgCdTe for the two shorter wavelengths and Si:As for the two longer wavelengths. The former will remain available after cryogen exhaustion. WISE maps the sky in a highly redundant manner by scanning the sky continuously, taking out the orbital smearing by using synchronized motions of a scanning mirror. WISE produces a complete sky map in 6 months. Data products for the primary mission include an Image Atlas and a point source catalog.
Cryogenic WISE will also identify and characterize a number of Near Earth Objects(NEO) using the 12 and 22 μm windows. These bands will not be available after exhaustion of the cryogen, thus no NEO work is anticipated in the warm mission phase.
The proposal to the SRC included two main elements: a three-month extension of WISE flight operations after cryogen depletion to complete the second-pass sky coverage in the 3.4 and 4.6 μm bands, and enhanced data products and enhanced data analysis tools.
Spacecraft/instrument health & status:
WISE is currently conducting its prime cryogenic mission. All systems on the instrument payload and spacecraft are performing well. The first pass on the sky should be complete in July 2010, and the solid hydrogen cryogens are expected to last until October 2010. The 9-month cryogen lifetime should be sufficient to cover half the sky for a second pass.
After the depletion of the cryogens, the focal planes and optics are predicted to radiatively equilibrate at 72 ± 4 K. This temperature should be low enough to continue operation of the two shorter wavelength channels with little change in performance.
The WISE cryogenic survey will be orders of magnitude more sensitive than previous all sky surveys at these wavelengths. The WISE survey is expected be an important astronomical database for decades to come.
The team is highly experienced and includes experts in both the instrumentation and the processing of very large astronomical data sets.
Relevance to NASA priorities:
WISE is relevant to NASA priorities since the WISE catalog will contain positions and infrared photometry for all types of astronomical objects ranging from solar system objects to sources in the distant universe. In particular, Warm WISE would contribute to the NASA strategic goal of understanding the formation of low mass stars and the properties of massive exoplanets by increasing the sample of known objects.
The final release of the Warm WISE data is scheduled for the fourth quarter of CY2012.
Synergy with other missions and ground-based work:
It is expected that the WISE data will be important as a general source of infrared information for other missions as well as for ground-based work. In particular, WISE will provide targets for Spitzer verification and further characterization.
The proposal did not convincingly demonstrate to the SRC the need for the three-month extension for the warm mission. The primary science example, the detection of ultra-cool brown dwarfs, would certainly benefit from the warm mission, but the transformational benefit of increasing the sample size from, for example, 85 to 170 objects, was not shown. Most of the other examples were statistical in nature and could reasonably be performed over the half of the sky covered twice in the cryogenic mission. In terms of the general survey sensitivity, only the 4.6 μm band is expected to show a significant improvement in sensitivity from the extension. While the enhanced reliability in the repeated regions is desirable, the requested funding seems high for the anticipated improvement.
The science return for the investment in the Absolute Brightness Calibrated Atlas was not well demonstrated. For many studies of extended structures, manual background corrections of the individual 1.56ºx1.56º images should be sufficient. The WISE Variability and Proper Motion Database will likely have limited utility given the several-year delay between the observations and the availability of the tool. Moreover, the cadence of the WISE observations is far from optimal for synoptic studies. The requested funding for the WISE Extended Source Catalog appears to be very high for the perceived effort required to produce the catalog.
The SRC was unconvinced about the need for the Custom Image Co-adder Tool and the Custom Source Extractor given their costs.
Overall assessment and recommendations:
The WISE prime mission should produce a catalog and image atlas of great utility to the entire astronomical community. Although it is impressed with the promise of the cryogenic mission, the SRC did not find adequate scientific justification in the proposalfor the cost of either the Warm Extension or the Enhanced Data Products.
The SRC recommends no funding for the extended Warm WISE mission nor for theenhanced tool development.
Link: Report of the 2010 Senior Review of the Astrophysics Division Operating Missions April 6-9, 2010
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16 May 2010
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