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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

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.

notional illustration of Russia's proposed Millimetron as supplied in a NASA presentation

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    $\begingroup$ I'm afraid we should wait for start of development of radio and cooling equipment for Millimetron. I couldn't find any references they already started. Before this we have just a "crystal ball schedule". I expect this development will be the most problematic and expensive part of the program (and so finansial problems of Roscosmos can complicate it further). Sorry for pessimism, I'd like to be wrong :) $\endgroup$
    – Heopps
    Commented Sep 14, 2020 at 13:46

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Provisional launch date 2029 https://millimetron.ru/en/

This project is directly related to the P-2500 (RT-70) radiotelescope at the Suffa radioobservatory. This modification of the RT-70 radio telescope is designed to operate in the millimeter and submillimeter ranges of radio waves. While the radiotelescope is unfinished, there is no point in launching this satellite. enter image description here

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  • $\begingroup$ Thanks! This makes sense; the VLBI will need at least two stations. Space is needed for the very long baseline, not to get above the atmosphere, so the ground station needs to be up and running and the receiver front end proven first. I wonder if the plan is for this dish to be cooled as well? $\endgroup$
    – uhoh
    Commented Sep 13, 2020 at 15:10
  • $\begingroup$ Cooling with helium or nitrogen has been around since at least the 1960s. System noise temperature for this antenna - 2000K. The main feature of the antenna is the special precision of the mirror shape. However, there are spare old RT-70 antennas that are now also in the process of being upgraded to 2030. $\endgroup$
    – A. Rumlin
    Commented Sep 13, 2020 at 16:21
  • $\begingroup$ Cooling the receiver front-end amplifier yes, but the image in my question shows what looks like heat shields around the reflector. I'm thinking that Millimetron will have a cold reflector dish (not just the receiver) and wondering if the ground station will need that too. $\endgroup$
    – uhoh
    Commented Sep 13, 2020 at 17:10
  • $\begingroup$ I think that it is impossible to cool a mirror with an area of 2500 square meters. $\endgroup$
    – A. Rumlin
    Commented Sep 13, 2020 at 17:19
  • $\begingroup$ I see, Millimetron seems to be only 10 meter in diameter and in a vacuum, but if this is much larger, it's not at all possible. :-) $\endgroup$
    – uhoh
    Commented Sep 13, 2020 at 18:02

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