Ministerio de Economía y Competitividad Gobierno de Canarias Universidad de La Laguna CSIC Centro de Excelencia Severo Ochoa

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Nov. 8, 2016

JOHN STANSBERRY: “JWST may finally tell us much more about where the different classes of KBOs came from”

Since he was a PhD student dealing with the volcanism of Io, one of Jupiter's satellites, he has been constantly trying to get to know the coldest and distant objects in the Solar System by space missions Spitzer, Herschel, and the future James Webb Space Telescope. The confines of our planetary system and the instruments on board space satellites are the specialty of John Stansberry, researcher of the Space Telescope Science Institute (STScI), in the United States, and lecturer of the XXVIII Canary Islands Winter School of Astrophysics , organized by the Instituto de Astrofísica de Canarias (IAC).

 

By Elena Mora (IAC)

 

“Space telescopes are not subject to the blurring of images caused by atmospheric turbulence”.

“Telescopic exploration is much more limited in the detail it can provide for individual objects, but allows us to explore many more objects than spacecraft missions”.

“JWST is making great progress towards on on-schedule launch at the end of 2018.

 

Question: How do we explore the outer parts of the Solar System?

Answer: Our knowledge of the solar system beyond Jupiter is based on both robotic exploration using spacecraft (the Voyager 1 & 2 missions, Cassini-Huygens, New Horizons), and on extensive telescopic observations. Voyager 1 and Cassini explored the Saturn system, while Voyager 2 went on to encounter the Uranus and Neptune systems. These missions returned rich data sets that offered us our first close-up looks at the moons of these planets, and of the rings of Saturn. Cassini explored the Saturn system in incredible detail, for example revealing fluvial features on the surface of Titan, and hydrothermal eruptions from Enceladus.

Telescopic exploration is much more limited in the detail it can provide for individual objects, but allows us to explore many more objects than spacecraft missions. Telescopic data have allowed us to discover the existence of the Kuiper belt and map its structure, revealed that binary Kuiper belt objects (KBOs) are quite common, allowed us to determine the sizes and albedos of over 100 KBOs be detecting the heat they emit, and determine the composition of the surfaces of tens of KBOs, and the visible and near-IR colors of over 100 of them.

Q: What are the advantages of space telescopes with respect to terrestrial telescopes?

A: Space telescopes offer four (at least) distinct advantages over ground-based telescopes. First is that they are not subject to the blurring of images (‘seeing’) caused by atmospheric turbulence. At visible and near-IR wavelengths, seeing typically limits the spatial resolution that can be achieved, rather than the diameter of the telescope primary mirror (although more and more large ground-based telescopes now use adaptive optics to deliver near diffraction-limited imaging, particularly in the near-IR).

Second is the ability to observe at wavelengths where the atmosphere is strongly absorbing or opaque. For example, many molecules have fundamental absorption bands in the 2.5 – 5 micron region, but ground-based instruments have only limited access to those wavelengths due to atmospheric absorption.

Third is that space-based observatories are in a very stable environment, and so their performance is very stable regardless of where they are pointed. This greatly simplifies absolute calibration compared to ground-based data, since no correction is needed for observations taken at different elevation angles or ‘airmass’. Also, gravity does not distort the telescope optics, so the image quality is very stable over the entire sky, and as a function of time. This helps in applications such as detecting faint sources near much brighter ones, and in making very precise astrometric measurements.

Fourth is that space-based telescopes can be operated at very low temperatures, and don’t have to look through a warm atmosphere. This provides huge improvements in sensitivity to thermal emission from solar system (or other) targets. This is because emission from the telescope itself, and the photon-noise associated with that emission, can be orders of magnitude lower than for ground-based telescopes.

Q: The launch of the James Webb Space Telescope (JWST) is planned for the end of 2018. In what phase of the process is it now?

A: JWST is making great progress towards on on-schedule launch. All 4 science instruments have been delivered and undergone two rounds of cryogenic testing. The telescope (18 segments in the 6.5m primary, secondary, tertiary and fine-steering) mirrors have been integrated with the mirror support structure, as has the instrument module. The telescope and instruments system will undergo cryogenic testing in late 2017, in a vacuum chamber built for the Apollo lunar missions. After that it will be integrated with the spacecraft and sunshade, undergo warm testing, and then be shipped to Kourou, French Guiana, for launch on an Ariane 5.

Observation planning tools are reaching an advanced state of development, and will be used first by JWST ‘guaranteed time observers’ (instrument PI’s and their science teams) to submit their GTO observing plans in April 2017. General observers will submit their science proposals by February 2018. Pipeline data processing software is also in development, and will fully support observations of moving targets. The planning and scheduling system is also nearly complete.

All of these software systems, and handling of science proposals, are being developed at the Space Telescope Science Institute (which also operates the Hubble space telescope).

Q: Apart from its size (6,5 m), what other advantages has the JWST in comparison to  the Hubble Telescope? What options the JWST offers for doing science?

A: JWST gathers photons ~6 times more quickly than Hubble, because of the larger mirror. JWST is optimized to operate at wavelengths from 2 – 28 microns, and also offers good performance at wavelengths from 0.6 – 2 microns. At 2 microns the angular resolution provided by JWST is comparable to that of Hubble, and at the shortest wavelengths it should do better.

JWST offers imaging and spectroscopic modes. Imaging can be done through numerous filters at different wavelengths. Observations in all instrument modes are supported for moving targets.

Q: Recently, the images captured by the spacecraft New Horizons have allowed to discovery in the Cthulhu region of Pluto some mountains covered of material that could be condensed frozen methane from its atmosphere. What does this discovery mean? Could this frozen material behave as the water does in the atmosphere of our planet?

A: Many of Pluto’s mountain peaks are coated in methane (CH4) ice, and the hypothesis is that CH4 in Pluto’s atmosphere is behaving like H2O in Earth’s atmosphere. Pluto is so cold that CH4 can only be in its solid or gas phases, so CH4 rain can’t occur, but CH4 snow potentially can. Alternatively, the CH4 on the mountains may be condensing directly onto the surface from the gas as the atmosphere flows over the tops of the peaks and cools.  N2 is the dominant gas in Pluto’s atmosphere, as it is on Earth. However, N2 on Pluto is also present in the solid phase on the surface. This leads to a distinctly un-Earthly situation where the pressure of the atmosphere depends on the surface temperature, so seasons on Pluto will not only result in CH4 and N2 condensing an subliming, but the entire atmosphere may largely freeze out when Pluto travels to the outer portions of its elliptical orbit.

Q: In spite of all scientific advances achieved about the evolution and formation of the Solar System, what answer about it has not been found yet?

A: We currently don’t understand the link between dynamical classes of KBOs and their compositions. By allowing us to measure compositions of many smaller (more typical) KBOs in the 2-5 micron region, where absorption features are stronger, JWST may finally tell us much more about where the different classes of KBOs came from, what the composition of the proto-solar disk was at those locations, and significantly enhance our understanding of physical and chemical conditions in that disk.

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