Current and Future Space and Airborne Observatories for ISM Studies

With its five exchangeable scientific instruments (SIs), SOFIA filled nicely the large spectral gap in the FIR between JWST and ALMA and provided further complementary capabilities like high spectral resolution at JWST wavelengths with EXES and the ability to observe very bright sources with FORCAST, that filled in the overexposed areas in MIR maps of the Galactic Center region, made by Spitzer (Hankins et al., 2020). The recent discovery of water on the sunlit surface of the Moon by Honniball et al. (2021) falls into that category as well. The heterodyne instrument GREAT covers such important atomic fine structure lines as [C ii] and [O i] at the highest spectral resolutions of up to R $\approx 10^{6}$ and fills in the spectral gaps that are inaccessible for ALMA due to atmospheric extinction. This enabled not only the discovery of new molecules in the ISM like Helium Hydride (Güsten et al., 2019), but also very detailed kinematic studies e.g. feedback processes in Orion by Pabst et al. (2019) that triggered a very successful SOFIA legacy program by Schneider et al. (2020). When sensitivity became an issue and could be gained by sacrificing spectral resolution, in particular for extragalactic work, the FIFI-LS spectrometer provided a good alternative for observations of fine structure lines as shown by Fadda et al. (2021), Spinoglio et al. (2022) or Pineda et al. (2018). Last but not least, where very high sensitivity was required to reveal the peak of the cold dust emission of high redshift objects, HAWC+ provided the FIR imaging capability. HAWC+, however, also provided an entire new dimension for ISM studies, that had before only briefly been available with ISO in the FIR. Polarization mapping revealed the vectors of magnetic fields in the ISM thanks to the FIR emission of aligned elongated dust particles. Many publications sparked a lot of new observational as also theoretical interest in this previously rather dormant field (Pillai et al., 2020; Lopez-Rodriguez et al., 2021; Zielinski et al., 2021).

In the face of the tremendous scientific successes of this true ISM-Machine, the decision by NASA and DLR to end the SOFIA mission after only 9 observing cycles, is certainly very hard to understand. Following the recommendations from the Flagship Mission Review from 2019, the project has transformed since then with a tremendous growth in science productivity as demonstrated in the SOFIA Status and Future Prospects Report (Rangwala et al., 2022)¹¹1This report was already prepared for NASA’s Senior Review Process.. Annual publication rates for SOFIA have doubled over the past three years on topics ranging from the Earth to high-z galaxies (Schmelz et al., 2021).

The Decadal Survey Astro 2020 recommended to NASA to terminate the SOFIA mission, which unfortunately was based on outdated ($>2$ years) and incorrect information²²2SOFIA science addresses 50% of Astro 2020 key science questions, not 10%.. NASA holds Astro 2020 recommendations as superior to Senior Review process results and hence removed SOFIA from the Senior Review.³³3This avoided potentially ending up with two contradicting recommendations. Arguments that SOFIA’s science productivity was insufficient can be easily refuted by comparing the observing time that is spent on average per refereed publication to that of Herschel. Eight years after launch Herschel had provided about 23,500 hours of observing time and produced 2,145 publications, resulting in $\approx$11 h$/$paper. SOFIA with 3458 hours and 330 publications after 8 years since achieving full science operational capability in 2014 results in very similar 10.5 h$/$paper.

Fortunately the last year was particularly productive in terms of observations, so there is a considerable amount of science data in the IRSA archive. As there is only a minimal post-operational phase of one year planned by NASA at this point, we hope DLR will provide the means to conduct data reprocessing also for the time before Cycle 5, advanced water vapor and pointing analysis and more comprehensive corrections, which are currently not included in the plans. In the next section we’ll lay out that the time to the next FIR mission might be rather long. Already collected FIR photons might thus be even more valuable for astronomy and funds for maximizing their scientific usability will be well spent.

Fig. 2 illustrates the history of FIR astronomy by showing the operational phases of all major observatories as green boxes, starting in the sixties until today and the current outlook towards 2045. Up to today, there was an almost continuous capability to supply astronomers with current FIR observations except for the few years between ISO and Spitzer. With the sudden cancellation of SOFIA, which was originally scheduled to continue until 2034, and the cancellation of SPICA by ESA in 2021, the opportunities for FIR data collection have become sparse. In Rangwala et al. (2022) Page 4, a traceability matrix can be found, that links Astro 2020 science questions to key measurements in the MIR and FIR, that could have been performed with SOFIA. This list should still be useful as a collection of science requirements for the design of future stratospheric- and space-observatories.

Even though Astro 2020 recommended the cancellation of SOFIA, it acknowledged the importance of the FIR spectral region for astrophysics and recommended the launch of a Probe space mission for 2030 that will specialize either in FIR- or X-ray- astronomy. NASA followed this up by issuing an announcement of opportunity and a proposal deadline of October 2023, a downselection end of 2025, a cost cap of 1B$\$$ excluding the launcher and a launch date not later than 2032 (NASA-SMD, 2022). If history is a guide, such a schedule is highly optimistic. In reality a launch might rather be expected in the mid 2030s, not to mention that continuing the SOFIA mission until its planned end would have cost substantially less, especially when taking into account the launcher as well.

Given that the X-ray community is also competing for another opportunity, it is everything but a done deal that NASA’s probe mission will be dedicated to the FIR. If that doesn’t happen, then also the dream of a more ambitious true observatory for the FIR such as ORIGINS in the 2040s (Meixner et al., 2019) may become unrealistic with observational FIR astronomy having lost a lot of its expertise by then.

Therefore at this point it is quite important for the FIR community to look towards the future which at least in space will be the Probe mission. There are four mission proposals for the FIR named PRIMA (PI, Jason Glenn)⁴⁴4https://prima.ipac.caltech.edu, SPICE (PI Lee Mundy)⁵⁵5https://asd.gsfc.nasa.gov/spice/index.html, FIRSST (PI Asantha Cooray) and SALTUS (PI Chris Walker), which were presented at the IR Astrophysics Workshop 2022 in Colorado. The concepts comprise more traditional space observatories with cold telescopes like PRIMA and FIRSST, and more unusual ones like the interferometer SPICE or the large inflatable telescope concept SALTUS. Details as presented at the workshop are available at the workshop website (IRSTIG, 2022).

In the meantime the FIR community should also investigate other opportunities to reclaim a permanent capability in that part of the spectrum. This will in particular enable more time dependent FIR astronomy, that we consider being still in its infancy. The fairly short life spans of FIR missions so far have been a hindrance while time-domain astronomy has really taken off in other parts of the electromagnetic spectrum. The astrophysical community should investigate the available potential in the FIR.

SOFIA was likely the last airplane observatory, and future stratospheric platforms will probably be of the lighter-than-air category. Current balloon experiments are, however, rather short lived, extremely weather dependent with very few launch opportunities, can’t stay in a particular region for long and have only a 50 % survival rate upon landing. Such missions are still seen rather as serving technology maturation and the training of instrumentalists than being able to support serious general observatory type projects for the astronomical community. This school of thought needs to change as better technologies become available that could address many of the shortcomings mentioned above. Longer lived robotic stratospheric platforms with propulsion may also be interesting to a wider community including UV- and FIR-astronomy but also climate research and general Earth observation (Miller et al., 2014).