Magazine article American Scientist

Letters

Magazine article American Scientist

Letters

Article excerpt

JWST's Limiting Factor

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), 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 Hüls, 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.

Botanical Details

To the Editors:

The phytolith story presented by Thomas Hart ("Phytoliths: The Storytelling Stones Inside Plants," March-April) was quite exciting. The new discoveries reveal so much detail about the diets . and surroundings of extinct animals, as well as the flora of ancient environments. I anticipate that my paleo- and ethnobotanical students will enjoy reading this article. I did find, however, that some botanical details, which were admittedly peripheral to Hart's main story, were erroneous or misleading.

For example, lignin is not a tissue, as claimed. Tissues are aggregates of cells that work in concert to undertake their characteristic useful functions. Lignin, on the other hand, is a type of macromolecule that particular cell types infuse into their cell walls in order to provide specific functionality for the several kinds of tissues that contain them. The author described lignin as providing "a defensive function." While this is doubtless true when lignin is produced by phloem fiber or certain periderm cells (for instance, cork cells), the primary role of lignin is to increase the physical strength of cell types found mainly in xylem tissue. Lignified waUs resist collapse due to transpiration tension occurring in tracheary elements and increase the plant's weight-bearing ability. Lignin is the molecular feature that makes wood woody; its main function is increasing structural strength.

The claim that "the same genes that regulate the production of lignin... also regulate phytolith development" is misleading. None of the entries in the article's bibliography support this statement. Although my survey of the literature was not exhaustive, I was only able to find one study that correlates one gene with the presence of both lignified cells and phytolith-bearing tissue. This research studied one genus in the periderm (rind) of its fruits, meaning one specific kind of tissue in one kind of organ. Phytoliths are usually not found accompanying lignified cells, and they are often found associated with cells that do not contain lignin. This gene locus (Hr) appears to control the distribution of specialized cell types, rather than the molecular synthesis (production) of the materials, one an organic phenylpropanoid macromolecule and the other a mineral inclusion. These are compositionally very different from each other, so their actual "production" is unlikely to involve the same genes. …

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