Abstract
This article begins by reviewing the necessity and early development of space infrared astronomical observations. It highlights that the two major scientific themes driving further development in the near- and mid-infrared bands are cosmological research and exoplanet detection. The article then introduces the James Webb Space Telescope (JWST) — the most significant current near- and mid-infrared mission — outlining its key technical features and major breakthroughs achieved in these research areas. Furthermore, it discusses the Euclid space telescope and the Nancy Grace Roman Space Telescope (RST). These missions, whose primary scientific objectives are surveying cosmological parameters, feature mosaic near-infrared focal planes with extremely large fields of view. The Atmospheric Remote-sensing Infrared Exoplanet Large survey (ARIEL) mission is also highlighted for its specialized design to acquire near- and mid-infrared spectra of exoplanet atmospheres. Additionally, the article surveys other operating, developing, or planned missions, including SPHEREx, EXCITE, Pandora, JASMINE, GaiaNIR, NEO Surveyor, and the Habitable Worlds Observatory (HWO). Finally, the article notes that while far-infrared technology has progressed more slowly — with missions like SPICA and Origins not yet realized — the Probe Far-Infrared Mission for Astrophysics (PRIMA) holds the promise of filling the "far-infrared gap".
Introduction
The infrared band is one of the most important observation bands in contemporary astronomy . However, due to the severe influence of the Earth's atmosphere, ground-based infrared astronomical observations are limited to a few atmospheric transmission wavelength windows at the short wavelength end (defined as photometric bands such as J, H, K, L, M, N, and Q [ 1-2]) . Most infrared astronomical observations must be carried out in space. It should be noted here that [1] the infrared starting wavelength of modern astronomy is defined as about 1.1 μm (instead of about 0.75 μm for the human eye) , because this is the theoretical cutoff wavelength of the focal plane array of the mainstream charge-coupled device (CCD) used in optical astronomical observations (Si photosensitive material) .
Infrared astronomical observations are further divided into three major bands: near-infrared, mid-infrared, and far-infrared. It is generally accepted that the mid-infrared band starts from 5 μm and ends at about 25－40 μm; the far-infrared band extends to about 200－350 μm and then directly transitions to submillimeter waves. In addition to being strongly absorbed by molecules such as H2O and CO2 (the Earth's atmosphere is completely opaque in the far-infrared band) , the thermal emission of the atmosphere itself also constitutes an extremely strong infrared background for ground-based astronomical observations: the peak falls in the mid-infrared band, and the emission intensity (typical value) increases by 3 orders of magnitude from wavelength 2.2 μm to 3 μm and from 3 μm to 5 μm . In terms of magnitude, for a sky area of the size of a square arcsecond, the atmospheric thermal background [3] is equivalent to magnitude8 in the L band, magnitude2 in the M band, and magnitude ―2 in the N band. Therefore, ground-based astronomical observations in other atmospheric infrared windows other than the J, H, and K bands are extremely difficult in practice.
The Infrared Astronomical Satellite (IRAS) , the first all-sky infrared survey mission to be operational in orbit in 1983, generated a mid-and far-infrared source catalog containing approximately 350, 000 celestial objects through a full-sky survey, marking a milestone in modern infrared astronomy. Following it, three major space-based infrared observatories were established: the Infrared Space Observatory (ISO; 1995－1998) , the Spitzer Space Telescope (SST; 2003－2020) , and the Herschel Space Observatory (HSO; 2009－2013) , which comprehensively conducted photometry, imaging, and spectroscopic observations. Additionally, the Akari satellite (2009－2013) and the Wide-field Infrared Survey Explorer (WISE; 2009－2024) primarily conducted all-sky surveys.
WISE operates only in the near-and mid-infrared bands, while HSO operates in the far-infrared and submillimeter-wave bands. Other missions initially covered the near-, mid-, and far-infrared bands, but once their coolant was depleted, SST, Akari, and WISE were left with only near-infrared observation capabilities (WISE was later renamed the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) , primarily dedicated to searching for near-Earth small objects) . The concepts and development of these space infrared astronomy missions date back much further, and their technical characteristics and scientific achievements will not be elaborated upon in this paper. In addition, the Hubble Space Telescope (HST; 1990 to present) , although operating mainly at room temperature, still possesses highly sensitive near-infrared observation capabilities (wavelength less than 2 μm) ; while the Stratospheric Observatory for Infrared Astronomy (SOFIA) , which began observations in 2010 and has a telescope aperture of 2.4 m, was permanently grounded in 2022 due to excessively high operating costs.
1 Current Development Status
With the development of astronomy, recent and upcoming space infrared astronomy missions place particular emphasis on two major scientific themes: cosmological research and exoplanet detection. Cosmological research in the infrared band can be divided into two categories: one is to observe various celestial objects in the very early universe, including attempts to discover the first generation of galaxies (and even the first generation of stars) ; the other is to use specific celestial object samples within a large spacetime volume to constrain the overall physical parameters of the Big Bang universe. For the former, the JWST, which officially began observational operations in 2022, is highly anticipated by the astronomical community [4-5]; representatives of the latter include the Euclid space telescope launched into orbit by the European Space Agency (ESA) in 2023 and the RST, which is nearing completion of its development by the National Aeronautics and Space Administration (NASA) .
The very early cosmic objects that need to be detected correspond to very large cosmological redshift values, z. It is predicted that the first generation of galaxies may be discovered in the range of z ≈ 10－20. Previously, the HST and SST joint research confirmed the detection of the galaxy GN-z11 with z ≈ 11. The radiation from objects at redshift z, having propagated to this day, would accumulate and be absorbed by Lyα absorption from intergalactic neutral hydrogen gas at different intermediate redshift values (stationary wavelength of 1024 Å) , so detection must be achieved in the infrared band.
In the study of cosmological parameters, the most attention is currently focused on the nature of dark energy, the discovery that drives the accelerated expansion of the universe (a discovery that won the2011 Nobel Prize in Physics) . As the energy density of matter decreases due to cosmic expansion, the influence of dark energy began to emerge around z=2, gradually evolving to its dominant position today. For example, in the galaxy counting sample, the Hα spectral line of hydrogen (stationary wavelength of 6563 Å) is used to measure the redshift of galaxies; z=0.7 corresponds to 1.1 μm, z=2 corresponds to 2 μm, which means that near-infrared spectral observations need to be carried out [6].
One of the key focuses of exoplanet research is to conduct spectral analysis of atmospheric composition to find molecules of interest, especially various molecular combinations that may constitute a habitable atmosphere or be related to life activities [7]. The characteristic vibrational bands of these molecules are mainly in the mid-infrared band, and some are in the near-infrared band (such as the1.4 μm gaseous H2O feature detected by HST) . Considering the masking effect of the host star's brightness, it is currently impossible to directly detect the spectrum of the exoplanet itself in most cases. The main approach is to use the slight changes in the overall spectrum of the star-planet system before and after the transit (and eclipse) phenomenon to subtract the signal of the exoplanet. Even in the case of a hot Jupiter atmosphere dominated by H2, which is conducive to detection, the spectral intensity ratio of the exoplanet to the star is only about 10-4. The spectral signal-to-noise ratio obtained in this way is very limited. Therefore, it is not reliable to directly confirm the individual molecular spectral features. In practice, a large number of parametric atmospheric models are used to fit and solve the entire spectrum, and a sufficiently large wavelength coverage is very important.
To improve the signal-to-noise ratio, multiple transit data need to be superimposed, and larger, more sensitive telescopes need to be used to increase the photon count. A very small number of exoplanets can be resolved from the vicinity of their host stars through imaging (e.g., using coronagraph techniques) , especially distant, newly formed (hotter) Jovian giants, thus enabling direct imaging spectroscopic observations. Because the planet's temperature is much lower than that of its host star, the brightness contrast between the two in the infrared band is smaller, which is most favorable for direct imaging observations.
2 JWST mission
The most important space infrared astronomical facility at present is undoubtedly the JWST [8]. It mainly operates in the near and mid-infrared bands. The telescope adopts a three-mirror astigmatism ablation system and active optics technology. The diameter of the spliced primary mirror is about 6.5 m. Its detection performance is an order of magnitude higher than that of HST and SST in the same band. JWST operates in a weakly stable orbit around the second Lagrange point of the Sun and Earth, located about 1.5×106 km away from the Earth in the direction of the Sun-Earth line. This place is becoming an ideal place to carry out space astronomical observations because it is far away from Earth interference. The huge multi-layered sunshade isolates the solar radiation, and the main body of the telescope is passively radiated to an operating temperature of ≲50 K.
The JWST telescope's focal plane is shared by four large instruments: the Near-Infrared Camera (NIRCam) , the Near-Infrared Sepectrograph (NIRSpec) , the Near-Infrared Imager and Slitless Spectrograph (NIRISS; also used as a precision guide) , and the Mid-Infrared Instrument (MIRI) . The first three operate in the 0.6－5 μm band, while the latter covers the5－28 μm band. JWST offers 17 scientific instrument modes, including direct imaging, slit spectroscopy, seamless spectroscopy, and coronagraph. New technologies employed include a micro-switching array chip capable of simultaneously observing hundreds of targets, and an integrated field-of-view unit that divides a small field of view into dozens of segments to achieve imaging spectroscopy. The near-infrared focal plane detector uses a2k×2k HgCdTe chip, while the mid-infrared band uses a1k×1k Si:As stray band sensor. MIRI also employs a three-stage pulse tube cooler combined with the liquid helium JT effect [9] to reduce the operating temperature of the focal plane detector to below 7 K.
JWST's observational research covers all areas of modern astronomy, and its main scientific objectives are officially summarized as "early universe", "galaxies in the long river of time", "the life and death cycle of stars", and "another world of planets". JWST has confirmed the observation of very early universe galaxies with redshifts as high as 14 [10], and has also found possible signs of the existence of first-generation stars [11-12]. JWST has detected a large number of galaxies with high luminosity in the very early universe [13], suggesting that there may be a very fast galaxy growth rate. This poses a certain challenge to cosmological models and theories of galaxy formation. However, JWST has also newly detected a large class of galaxies called "red dots" in the z≳5 range [14]. This is attributed to the presence of small-mass active galactic nuclei surrounded by dust and gas at the center, which may have made a significant contribution to the high luminosity of very early galaxies. By utilizing the dust penetrability in the infrared band, JWST can also subtract the influence of dust extinction on Cepheid variable stars as distance standard candles, thereby improving the measurement accuracy of the Hubble constant [15].
Regarding exoplanets, JWST first confirmed the detection of CO2 in the atmosphere of an exoplanet in the transit spectrum of WASP-39 b [16], and obtained reliable evidence of the existence of an atmosphere in a rocky planet for the first time in the observation of 55 Cancri e [17], but also basically ruled out the possibility of the existence of an atmosphere in several terrestrial planets among the key observation targets [18]. It should be noted that the reliability of the suspected dimethyl sulfide detection reported in the atmosphere of K2-18 b, which may be an ocean planet [19], has been widely questioned by the astronomical community. In addition, JWST detected a variety of icy molecules in the spectrum at the edge of the molecular cloud [20], including methanol, carboxyl sulfide and possibly acetaldehyde, acetone, ethanol, etc., in addition to H2O, CO2, NH3, CH4; it also detected organic molecules such as ethylene, ethane, and cyanoacetylene in the protoplanetary disk of a newborn low-mass star [21]. At the same time, JWST also obtained many important observation results in the solar system, such as the discovery of an abnormally high CO2/H2O ratio in the comet 3I/ATALAS, an interstellar intrusion object [22].
3 Infrared Sky Survey Mission
Euclid's Visible Camera (VIS) , Near Infrared Spectrometer and Photometer (NISP) , and RST's Wide Field Instrument (WFI) all aim to conduct cosmological parameter surveys using both optical and near-infrared bands [ 23-24 ]. Both have telescope apertures of 1.2 m and 2.4 m. They employ a three-reflector astigmatism correction system. Their field of view is much larger than that of the HST and JWST, and their NISP is close to 0.6. deg2, WFI close to 0.3 deg2; the focal plane is covered by stitching together 36 2k×2k and 18 4k×4k HgCdTe chips respectively, while having high spatial resolution (NISP approximately 0.3 arcseconds/pixel, WFI approximately 0.1 arcseconds/pixel) ; the red end cutoff wavelength is nearly 2 μm [25].
Euclid was launched to the Sun-Earth Second Lagrange Point orbit in 2023 and is currently conducting a galaxy survey in the mid-high galactic latitude region of approximately 15, 000 deg2, aiming to study cosmological parameters using microlensing and galaxy counting (such as baryon acoustic oscillations) . The RST is expected to launch into a similar orbit in 2027, with a galaxy cosmological survey area of approximately 2000 deg2. It will conduct high-redshift Type-Ia supernova cosmological surveys, microlensing exoplanet surveys, and pre-recruited conventional astrophysical surveys. In addition, RST also has a coronagraph that operates only in the optical band and is mainly used for new technology verification.
Currently, Euclid has released a number of early release observations [26] and the scientific results of the approximately 63 deg2 deep field survey [27], covering multiple astronomical research directions, not limited to galaxies and cosmology. Considering the importance of infrared observation, the China Space Station Survey Telescope (CSST) , which will also conduct a cosmological parameter survey, is also equipped with a near-infrared chip developed by the Shanghai Institute of Technical Physics, Chinese Academy of Sciences for optical-near-ultraviolet prime focal plane assistance [28].
In addition to RST, NASA also has a small space-based infrared telescope — SPHEREx—conducting a unique, at least two-year-long galaxy counting survey focused on cosmological parameters . It is scheduled to launch into a sun-synchronous orbit in 2025. The telescope has a20-cm aperture. It is an all-aluminum three-reflector astigmatism ablation system; passively radiated to below 80 K using multi-layer light-shielding plates; the field of view is 3.5 deg×11 deg; a linear variable filter is used to divide the 0.75－5 μm wavelength range into 96 narrow bands, thus innovatively realizing the first full-sky spectral imaging survey; the spatial resolution corresponding to the focal plane HgCdTe detector is about 6 arcseconds/pixel [29]. The SPHEREx survey also has two major scientific objectives, namely various icy molecules in interstellar space (potentially related to life molecules) and predicting the cosmic infrared background fluctuations contributed by the first generation of stars or galaxies.
4 Exoplanet Exploration Missions
JWST cannot guarantee the significant amount of time required for systematic observations of exoplanet transit spectra, thus necessitating the development of a dedicated space infrared mission for exoplanet spectra. A representative of this is ESA's ARIEL telescope [30]. Scheduled for launch in 2029 to the Earth-Sun 2 Lagrange point, it carries a1.1 m×0.7 m all-aluminum off-axis Cassegrain telescope. ARIEL will simultaneously acquire transit (or eclipse) spectra in the bands of approximately 1.2–2 μm (spectral resolution R=λ/Δλ>10) , 2–4 μm (R>100) , and 4–8 μm (R>30) , targeting approximately 1000 exoplanets. The telescope itself will be passively radiated to an operating temperature below 70 K (inherited from the Planck probe's multi-layered V-groove radiator) , with the focal plane detector further cooled by a cooler.
In addition, NASA test-flew a50 cm aperture spherical near-infrared telescope—EXCITE [31], which is used to obtain time-consuming orbital phase time-series spectra of hot Jupiter types; NASA plans to launch the Pandora satellite in 2026, which carries a44 cm aperture Cassegrain telescope. It will obtain transit time-series spectra of a batch of exoplanets in the near-infrared band, and monitor the activity of host stars through synchronous photometry in the optical band, and study the contamination of transit spectra by the latter [32].
NASA’s next-generation flagship ultraviolet/optical/near-infrared space observatory is called the Habitable Worlds Observatory (HWO) , which is currently in the conceptual definition stage. Based on the recommendations of the2020 Decade of Astronomy and Astrophysics by the National Academy of Sciences [33], the primary scientific goal of HWO is to directly image and detect about 25 potentially habitable Earth-like exoplanets using coronagraphs and to reveal atmospheric composition through spectroscopy. It will also study a wide range of astrophysical topics, particularly the evolution of galaxies throughout the history of the universe and the evolution of elemental and molecular abundance in the universe. The proposed launch time is the early 2040s. To improve the exoplanet observation performance of HWO, they are developing a new generation of ultra-low noise near-infrared chips, such as HgCdTe avalanche photodiode arrays [34].
HgCdTe avalanche photodiode arrays are also expected to realize the time delay integration technology required by ESA’s GaiaNIR space astrometry proposal [35], enabling it to obtain high-precision celestial position and parallax information across the entire sky in the near-infrared band, thus compensating for the dust extinction limitation of the Gaia detector in the optical band. The JASMINE telescope [36], which Japan plans to launch in 2028, uses an InGaAs chip to achieve near-infrared astrometry of the central core region of the Milky Way through multiple short-exposure imaging, while using the transit method to search for Earth-like exoplanets in the habitable zone. In addition, NASA will also deploy the NEO Surveyor mission [37] at the Sun-Earth First Lagrange Point in 2027 for the detection of near-Earth small celestial bodies. Its 50 cm aperture telescope will image in two bands (4–5.2 μm and 6–10 μm) , and the data should be usable in other astronomical fields, just like its predecessor NEOWISE.
5 Far-infrared space astronomical observations
Unlike the near-and mid-infrared bands, the development of far-infrared space astronomy has been relatively slow in recent years. In terms of spatial resolution and detection sensitivity, a "far-infrared gap" has emerged, failing to match the JWST and ground-based submillimeter-wave telescope arrays. This situation is due to both scientific and technological reasons. Scientifically, although, as demonstrated by Herschel's numerous observations, the far-infrared band can be used for many cutting-edge astronomical topics (such as planetary system origins and star formation) , its importance cannot be fully realized in the two most prominent and attention-grabbing fields: cosmological research and exoplanet detection. Technologically, due to relatively weak commercial and defense demand, the performance of far-infrared focal plane detectors lags significantly behind that of near-and mid-infrared chips. For example, Herschel's microbolometer array has a maximum size of only 64×32 pixels, with observation efficiency far lower than the large-scale near-and mid-infrared arrays with millions of pixels, and its sensitivity needs to be improved to the background limit. Furthermore, the stringent cooling requirements of far-infrared astronomical observations may significantly increase technical difficulties and development and operating costs.
It is particularly regrettable that Japan, in conjunction with ESA, abandoned the proposal for the mid-and far-infrared space telescope in 2020 after years of demonstration on the SPICA telescope [38]. The SPICA telescope has a2–3 m aperture, which is not as large as Herschel's 3.5 m. The key improvement is that by combining passive radiative cooling and mechanical cooling, the operating temperature is reduced to below 8 K, which is much lower than Herschel's primary mirror temperature of about 80 K. Therefore, it has a detection sensitivity index that is two orders of magnitude higher than Spitzer and Herschel in the wavelength range of 10–100 μm . At the same time, it can avoid the short life problem caused by the use of consumable coolants in focal plane instruments. NASA has demonstrated the Origins mid-and far-infrared space telescope with a6 m aperture (mechanically cooled to 4.5 K) [39], but it failed to obtain final approval in the2020 Decade of Astronomy and Astrophysics [33].
The PRIMA mission [40] has the potential to become the next generation of far-infrared space observatory. NASA plans to choose between the proposal and the X-ray observatory proposal in 2026 and launch it in 2032. PRIMA is an all-aluminum telescope with a diameter of 1.8 m, cooled to 4.5 K by passive radiative cooling combined with spare coolers from the JWST project; it operates in the wavelength range of 24 μm to 235 μm, and its terminal instruments are the spectrometer FIESS (R>85 grating spectrum and R>2000 Fourier spectrum) and the imager PRIMAger (photometry, polarization and R≈10 hyperspectral imaging) , operating in orbit around the second Lagrange point of the Sun. The scientific objectives driving the design of PRIMA were selected by the PI development team, namely the origin of planets and their atmospheres, the co-evolution of galaxies and central supermassive black holes, and the enrichment of dust and metals (astronomically referring to elements other than hydrogen and helium) on cosmic timescales, which will account for 25% of the total observation time. The total number of pixels of the PRIMA focal plane infrared detectors exceeds 12, 000. It uses a superconducting dynamic inductance detector (KID) with ultra-low noise and high sensitivity [41] to make full use of the low thermal background provided by the telescope's deep cooling. The detection sensitivity of this mission to point sources will be10 times higher than that of Herschel, and it will be able to improve the mapping efficiency of the far-infrared spatial-spectral domain by thousands to tens of thousands of times.
Superconducting devices that are sensitive to single photons and have ultra-low readout noise are the research and development hotspots for next-generation mid-and far-infrared array detectors for astronomical use. For example, the superconducting phase transition edge sensor (TES) , which has been developed and improved around SPICA and Origins, is a microbolometer with ultra-high sensitivity, and its array size is striving to move from the10, 000-pixel level to the100, 000-pixel level [42]. However, TES requires the integration of an active superconducting quantum interference device (SQUID) to achieve multiplexed readout, which is an important reason why it was not adopted by PRIMA [40].
6 Conclusion
The high-performance JWST has ushered in a new chapter in space infrared astronomy. Thanks to the significant fuel savings from its successful orbit insertion, which can be used for routine orbit maintenance, its operational lifespan is expected to exceed 10 years. With the ongoing release of data from the Euclid and SPHEREx surveys, the launches of the under-development RST and ARIEL (and likely PRIMA) , and the addition of other small and medium-sized projects, the next decade or so is poised to become a golden age for space infrared astronomy. Meanwhile, domestic institutions such as the Shanghai Institute of Technical Physics of the Chinese Academy of Sciences and the National Astronomical Observatories of the Chinese Academy of Sciences are also developing conventional and superconducting low-noise infrared array detectors for high-sensitivity astronomical observations, striving to overcome this technological bottleneck and lay the foundation for filling the gaps in China's space infrared astronomy field.