American Scientist

To the Editors:

At the Hubble Legacy lecture held at the Griffith Observatory on April 27, 2015, featuring astronomer Laura Danly and three other panelists from NASA and Northrup Grumman, I asked a question about using the NASA spacecraft Orion to service the Hubble Space Telescope or refresh the James Webb Space Telescope with coolant. However, I was informed that the JWST has a cryocooler and thus needs no refreshing—news to me.

I realized my erroneous information came from American Scientist in “The Next Great Exoplanet Hunt ” (May–June 2015), where the article states:

“The infrared detectors onboard the JWST have to be cooled with liquid helium to prevent the thermal fluctuations within the detectors from swamping the astronomical signals. Because the helium will gradually be used up, JWST has a finite and short mission lifetime (5.5 to 10 years).”

Incidentally, after probing further I found that one early consideration was to use solid hydrogen as a coolant—that would be quite a block of ice!

Roy Sykes
Woodland Hills, CA

Drs. Heng and Winn respond:

As pointed out to us by Drs. Jason Kalirai and Jason Tumlinson at the Space Telescope Science Institute (STScI), as well as Mr. Sykes, our article misstated the reason for the finite lifetime of the upcoming James Webb Space Telescope. The mission duration of 5.5 to 10 years is not limited by the supply of liquid helium, as we stated. Rather, it is limited by the supply of hydrazine fuel needed to maintain the spacecraft’s orbit.