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Comment by nextaccountic | Hacker News Reader

> In addition, according to Carl Hansen, a flight systems engineer at the Space Telescope Science Institute (the science operations center for JWST), a comparable X-band antenna would be so large that the spacecraft would have trouble remaining steady for imaging.

Why would a large antenna make the spacecraft less steady? What's the mechanism behind it?

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At a guess: solar pressure.

The antenna is steered to point towards the DSN antenna on the earth. A larger moving mass would make it harder to maintain telescope pointing while the antenna is moving.

In reality, the antenna pointing is 'paused' during each science observation, unless pointing is needed due to the length of the observation.

Oh, the antenna doesn't need to just point in the general direction of Earth, it needs to point to somewhere in the surface. That makes sense, having a narrower beam would save power and achieve higher bitrates

Does this mean it has only a 12-hour window to transmit? Or there's multiple antennas on Earth?

It uses NASA’s Deep Space Network. There are three stations around the globe (California, Australia, and Spain), spaced so that there is near continuous coverage for deep space missions. JWST points it’s antenna at the station that is currently in view.

However, the ground stations are shared between many missions, so they are not available for JWST all the time. Expectation is that JWST gets 8-12 hours/day of DSN time.

Could also be more resonant modes in the structure that would need to be damped out.

It's because large, directive antennas are still 'dish' style and have to be mechanically pointed at the target (Earth based DSN receiving antennas). That pointing causes vibrations and potentially a shifting center of mass.

Phased arrays allow beam pointing without mechanical movement, but are very expensive for large high gain antennas.

I believe it is both due to solar wind and the fact that it would act as a tiny light sail.

Solar wind exposure perhaps?

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