Comments below an answer to How does NASA's ASTHROS stratospheric telescope compare to its James Webb space telescope? link to JPL's Paul F. Goldsmith's The Renaissance of High Resolution Astronomical Spectroscopy which describes why submillimeter spectroscopy is important and mentions both current and future facilities.
One of them is ASTHROS as discussed in that question, but one that caught my eye is the one labeled:
Millimetron (Russia) 10m Deployable Antenna cooled to 6 K Earth-Space VLBI Heterodyne spectroscopic instrument TBD
While TDB is a scientific timescale (Barycentric Dynamical Time), TBD means "to be determined". That, together with the notional drawing shown below suggests that this is a future mission perhaps in the feasibility study or planning phases.
Russian radio astronomers have substantial experience in space-based VLBI as well as space-Earth linked VLBI
- Has VLBI been done using any space-based receivers besides Spektr-R?
- What are Spectr-R's major contributions to radio astronomy that could not have been done from Earth?
- How is Spektr-R doing these days?
- What is the largest antenna deployed in space?
- What artificial satellite has the farthest orbit around the Earth?
- Examples of radio correlations over times much longer than interferometric baselines?
The dish is allowed to cool to circa 6 K to minimize the thermal microwave radiation from the dish overwhelming the object being observed. My answer to my question Why doesn't thermal radio emission from a DSN “hot dish” completely swamp the benefits of a cold LNA? explains that since we're operating in the Rayleigh-Jeans regime power per unit frequency scales almost linearly with temperature rather than $T^4$ and that at least for a solid metal dish (but not a wire mesh) the high reflectivity of the reflector means it has an equally low emissivity.
However that was written for the DSN 70 meter dish operating circa 7 to 8 GHz, and Millimetron is said to be sub-millimeter, which is going to be several hundred GHz if not 1 THz or higher. So it makes sense that the drawing shows some nested thermal shields protecting the antenna from radiant heat from the Sun or from Earth or the Moon.
Question: What's the status and timeline for Millimetron? (Russia's 10m Deployable Antenna cooled to 6 K Earth-Space VLBI) How much has been studied, what information is available about its current state of development and technical issues that need to be addressed?
notional illustration of Millimetron as supplied in this NASA presentation.