ESA Science & Technology - NG-CryoIrTel: Next Generation-Cryogenic cooled Infrared Telescope

Mission Justification

To reveal the origin and evolution of galaxies, stars and planets is one of the ultimate goals of astronomy. To achieve the goal, observations in the mid- and far-infrared are essential, since it is in this range that astronomical objects emit most of their radiation as they form and evolve in regions where obscuration by dust prevents observations in the visible and near infrared.

Over the past quarter of a century successive space infrared observatories (IRAS, IRTS, ISO, Spitzer, AKARI) have revolutionised our understanding of the evolution of stars and galaxies. Mid- to far-infrared observations have led to stunning discoveries such as the Ultra Luminous Infrared Galaxies (ULIRGS), the basic processes of star formation from "class 0" pre-stellar cores through to the clearing of the gaseous proto-planetary discs, and the presence of dust excesses around main sequence stars.

The Herschel Space Observatory launched in 2009 extends this work in the far-infrared and sub-mm, and JWST, due for launch in 2018, will provide a major boost in observing capability in the 2 – 28 μm range. Although Herschel and JWST are powerful missions, they have some constraints for the observations in the mid- and far-infrared.

Comparison of natural backgrounds (zodiacal emission, Galactic cirrus, and cosmic microwave background radiation) with those of thermal radiation from telescopes as a function of temperature. Credit: SPICA consortium.

The thermal radiation of telescopes onboard Herschel and JWST (if it is used for the far-infrared) is brighter than natural background radiation by a factor of one million in the far-infrared. Hence their sensitivity in the far-infrared could be degraded by a factor of one thousand from that of the natural background limited observations. On the other hand, previous cryogenically cooled missions (e.g. IRAS, IRTS, ISO, Spitzer, and AKARI) had a telescope smaller than 1m. Hence, if a 3-m-class space telescope is to be cooled below 6K, huge sensitivity improvement can be expected from that of Herschel.

SPICA, which is required to be cooled below 6K, can achieve superior sensitivity by reducing the background radiation by a factor of a million from previous missions.

Hence SPICA was proposed in this context and is optimised for mid- and far-infrared astronomy with a cryogenically cooled, large telescope. SPICA was propose as an observatory that would provide imaging and spectroscopic capabilities in the 5 to 210 μm wavelength range with a 3.2 m telescope cooled to a temperature less than 6 K.

From SPICA to NG-CryoIRTel

Discussion between ISAS/JAXA and ESA in 2013 concluded that the scheme for SPICA was not compatible with a timely and robust implementation of the mission. Both JAXA and ESA believe that a more balanced sharing of responsibilities, with an enhanced ESA participation to the mission, would lead to a lower risk and to a more robust mission implementation. Any significant extension of the ESA-contributed elements would bring the mission into the medium mission range, however, with the implication that the mission would need to be proposed by the interested scientific community to an ESA call for missions where it would be peer-reviewed together with other proposals submitted to the same opportunity.

To support the European and Japanese science community in defining a post-SPICA mission for a medium (M-class) mission call, a smaller Next Generation-Cryogenic cooled IR Telescope (NG-CryoIRTel) has been assessed in a CDF study, trying to meet a set of science achievements similar to the ones proposed for the previous SPICA mission.

NG-CryoIRTel will cover the full 20 to 210 μm wavelength range, including the missing 28 μm to 55 μm octave which is out of the Herschel and JWST domains, with unprecedented sensitivity and spatial resolution. Hence, NG-CryoIRTel will be the only observatory of its era to bridge the wavelength gap between JWST and ALMA, and carry out unique science programmes.

Science Objectives

The main science objectives, derived from the previous SPICA study and which NG-CryoIRTel tries to enable are:

• Investigate how stars and galaxies form and evolve over cosmic ages
• Observe thousands of obscured, distant galaxies and determine the processes that govern their evolution
• Investigate how our Solar System relates to other planetary systems and how life could evolve elsewhere
• Characterise oxygen, water, ice and rocks in young planet-forming systems and study their relation to the rocks and ice in our own Solar System

Besides these objectives, the NG-CryoIRTel might also be suitable for other science cases requiring a large cryogenically cooled telescope.

Mission Requirements

The main mission requirement for this study is to accommodate a 2m class telescope, operating below 6K, on a spacecraft maximising the re-use of Planck V-Groove technology and Gaia bipods that can be separated in-orbit for the passive cooling, using active coolers provided by JAXA and re-use the service module architecture from Herschel or Euclid.

As an option, the CDF study also investigated the growth potential of the selected architecture assessing the maximum telescope size that can be accommodated on the selected V-Groove configuration.

Study Results: Baseline

Study Results: Option

Full details can be found in the CDF study report: NG-CryoIRTel, Assessment of Next Generation Cryogenic InfraRed Telescope.