Philosophical Society of Washington

Minutes of the 2373rd Meeting

At the Cosmos Club, Washington, DC
February 10, 2017

President Larry Millstein called the 2373rd meeting of the Society to order at 8:04 p.m.  He announced the order of business and welcomed new members.  President Millstein presented a summary of the 35th meeting of the Society, held in 1872.  The minutes of the previous meeting were read and approved.  President Millstein then introduced the speaker for the evening, John M. Grunsfeld, Astronaut and Former Associate Administrator, Science Mission Directorate, NASA.  His lecture was titled “After the James Webb Space Telescope: In-Space Assembly of Very Large Space-Based Observatories”.


Dr. Grunsfeld began by explaining that we do not yet know exactly what kind of observatories will follow the Hubble and the James Webb Space Telescope, but the answers provided by those groundbreaking telescopes have raised ever more intriguing and detailed questions to ask with their successors. Ultimately, Dr. Grunsfeld explained, the goal of space-based observatories is to collect the data needed to answer the big questions: where did we come from, where are we going, and are we alone in the universe.

Dr. Grunsfeld reiterated that these are scientific questions, not philosophical ones, and presented a graph charting the improvements in telescope capabilities since Galileo’s telescope in the 1600s.  The first telescopes were crude, but still 100 times more sensitive than the human eye. Subsequent telescopes over the next four centuries increased in resolving power by orders of magnitude until the Hubble was launched into orbit and made humanity’s first observations from above the atmosphere, achieving a 10 trillion-fold improvement over the human eye with only a 2.4 meter telescope. Despite the much larger size of Earth-based observatories such the 200 inch telescope at Mount Palomar and the Soviet-era BTA 6-meter telescope in Russia, Dr. Grunsfeld described how the exquisite clarity of the images Hubble sent back “blew them all away.”

Showing a selection of famous images taken by the Hubble, Dr. Grunsfeld affirmed the importance of wonder and awe in driving scientific inquiry.  The Hubble images are striking in part because they are the same quality as our eyes, if we could be in space close enough to see the nebulae and solar systems with our naked eye. Even so, the Hubble is only so large and thus its range is limited, and it can only see visible light, so it cannot see past the outer edges of the gas and dust clouds that within which stars and planets form. Size does matter, Dr. Grunsfeld repeated.

The James Webb Space Telescope will solve these shortcomings by being larger and having infrared capabilities, letting it look through gas and dust to study the formation of stars and planets.  JWST will be placed at Lagrange Point L2, one million miles from Earth, and shaded from the sun so that the primary mirror cools to 40 degrees above absolute zero, making it incredibly sensitive to infrared radiation. Furthermore, JWST’s multipart mirror of interlocking gold-coated hexagons includes the ability to correct its optics in flight, further improving its sensitivity.

Dr. Grunsfeld invited the audience to imagine the moment when we definitively locate a small, wet, rocky planet in the habitable zone around a star, a second Earth.  Such an “Earth 2.0,” if we can see it, will be fainter than the faintest galaxy in the Hubble Deep field survey, which broke the cosmic current distance record for the furthest imaged object, a galaxy with a redshift of 11.1 z, suggesting that we are seeing light from when it was only 400 million years old—an instant compared to the 13.4-billion-year age of our Milky Way.

Dr. Grunsfeld explained that the search for life hinges on the detectability of biosignatures, which can be spotted by analyzing planetary images for absorption spectra consistent with oxygen, carbon dioxide, methane, water vapor, and even chlorophyll.

Dr. Grunsfeld summarized the case for larger space-based observatories by paraphrasing Carl Sagan, explaining that “extraordinary claims require extraordinary apertures.”  200 hours of observation of the Earth at 10 parsecs—about 30 light years away—with a 4-meter telescope would produce inconclusive data as to whether there were biosignatures on the planet.  A 16-meter telescope, on the other hand, would resolve these spectral lines and provide a definitive answer about life on Earth.

Dr. Grunsfeld explained that aperture size is only about 15 percent of the overall project cost for a telescope, meaning that a small additional investment can provide orders of magnitudes more value by not only letting us discern distant planets more accurately, but could allow us to image them fast enough to detect the changes in their color that might be associated with clouds, oceans, and continents.  One proposal implementing these principles is the Large Segmented Space Telescope, a plan for an 8 to 16-meter successor to the JWST. Dr. Grunsfeld explained that the LSST is based on the same design techniques as the JSWT and could be built and launched with today’s technology.

Dr. Grunsfeld concluded by reiterating that for the first time, we have the technology needed to seek answers to our grandest scientific questions, and he urged ambition in pursuing them.

After the conclusion of the talk, President Millstein invited questions from the audience.

One questioner asked about the value of moon-based observatories. Dr. Grunsfeld noted that the moon would block terrestrial radiation, but otherwise would not be particularly useful for astronomical observations because it still has a gravity gradient, plus dust and thermal variation. And, as Hubble has proven, we can fix telescopes in space.

After the question and answer period, President Millstein thanked the speaker, made the usual housekeeping announcements, and invited guests to join the Society.  At 9:35 p.m., President Millstein adjourned the 2373th meeting of the Society to the social hour.


Attendance: 119
The weather: Overcast (and clear for billions of miles after that)
The temperature: 3°C

Respectfully submitted,

Preston Thomas
External Communications Director


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