Date: July 15, 2011

Title: Discovery of Neptune’s Thirteen Moons

Podcaster: Diane Turnshek

Organization: Carnegie Mellon University

Description: Happy birthday to Neptune! This week marks the completion of one orbit around the Sun since Neptune was discovered 165 years ago. I claim that the pattern of discovery of Neptune’s moons illustrates how the process of science itself has changed, from one man, working alone, to hundreds of researchers working together on satellite mission teams. In Part I, we look at the past and how each of the thirteen moons of Neptune was discovered and in Part II (July 18), we look to the future at how the fourteenth and beyond will be found.

Bio: Diane Turnshek teaches astronomy at the University of Pittsburgh, St. Vincent College and Carnegie Mellon University, where she also coordinates physics outreach. She hosts a monthly public lecture series at Allegheny Observatory. In her outreach role, she has visited schools, libraries, camps, Scouts and Congress and podcasted with (“The Science of StarGate 1 and 2”) and 365Daysof Diane consults with people who wish to use accurate science, from SF authors to opera companies. Find her short fiction published in Analog Magazine and elsewhere.

Sponsor: This episode of 365 Days of Astronomy is sponsored by 3D apps for the iPhone, iPad and iPod Touch.


Hi, this is Diane Turnshek and welcome to 365 Days of Astronomy. I’m vacationing this week from my job as outreach coordinator in the Department of Physics at Carnegie Mellon University. I’m talking to you from Alpha, my science fiction and fantasy writing workshop for teens.

Happy birthday to Neptune! This week marks the completion of one orbit around the Sun since Neptune was discovered 165 years ago. Let’s celebrate this occasion with some virtual champagne, good company and speculation. I researched and wrote a shorter article on this same topic for inclusion in the special Neptune edition of Argentus, produced by Stephen H. Silver. If you find Neptune fascinating, as I do, I recommend this issue, which includes ten articles by astronomers, science fiction authors and some people who were onsite as these discoveries were being made. You can download the Neptune Argentus issue for free on the web. The link is included in our transcript notes on the 365 Days of Astronomy website. My favorite of the Argentus articles contains passages from Chris McKitterick’s novel “Transcendence” published in 2010 from Hadley Rille Books. Set in the Neptune system, the book follows a scientist who is exploring an alien artifact. Here’s an excerpt from the novel:

“Clarisse Chang’s security station hung stationary in Neptune’s upper atmosphere, nearly in orbit around the pale blue gas giant—high enough from the planet’s center of mass that onboard gravity barely exceeded Earth’s. Even at this altitude, winds could be potentially destructive. Roiling clouds swept past like mountains, as if the station were hurtling across smooth white and blue terrain, flying over hundred-kilometer valleys and then crashing through liquid cliffs. The tiny disk of the sun shown at a steep angle, tingeing the sky’s horizons a faint purple while overhead, stars pricked pure blackness. Triton, Neptune’s largest moon, was rising. For a moment, its disk partially eclipsed the sun’s.”

Just beautifully written.

Lately, I’ve been interested in how discoveries are made. Accidental discoveries are made all the time in science. Being in the right place at the right time with the know-how to recognize a discovery isn’t that rare. Teflon, penicillin, Velcro and rayon are a few instances. Serendipity also played a large role in the vulcanization process of rubber, the discovery of iodine and helium and the development of saccharin. It’s skill, mixed with the proper tools and varying doses of luck. For example, Galileo may have actually noticed Neptune, but failed to recognize it as a planet. “Expect the Unexpected” was the motto for the Space Telescope Science Institute in the early days. We can’t know all the possible discoveries from our research efforts before we start; that’s what makes it exciting!

I claim that the pattern of discovery of Neptune’s moons illustrates how the process of science itself has changed. Only seventeen days after Neptune was found in 1846, Triton, the largest moon, was discovered by one man, British astronomer William Lassell. He had made his fortune in the beer industry allowing him the free time and funds necessary to pursue his interest in astronomy. Like the great scientists of his era, Lassell furthered the field by the lone pursuit of knowledge.

Triton, with a diameter of 2700 kilometers and a thin atmosphere of nitrogen and small amounts of methane and carbon dioxide, revolves around Neptune in a retrograde orbit. Nearly every body in our solar system revolves around the Sun and rotates on its axis counterclockwise as seen from the North. When we find a moon orbiting in the opposite direction that the planet rotates, like the two moons of Mars, it indicates that the satellite wasn’t formed with the planet, but was captured at a later date. Several moons share that honor in the flock orbiting Neptune. They’re considered to be captured Kuiper Belt Objects (KBOs).

Triton is a dense, geologically active moon with polar caps and geysers. Smooth, flat plains that are not heavily cratered indicate past icy lava flows. Triton is as large as Pluto and tidally locked to Neptune. Triton’s retrograde orbit is slowly decaying and that will eventually bring the moon so close to the gas giant that the planet’s differential gravitational forces will rip the moon into another ring.

Gerard P. Kuiper discovered Nereid in 1949. He was regarded by many as the father of modern planetary astronomy. He postulated that planetesimals, formed at the birth of the solar system, may someday be found orbiting in the icy reaches just beyond the orbit of Neptune, but he died before they were discovered. He wasn’t the only one who speculated that there might be icy bodies that far out in the solar system. Kenneth Edgeworth is also credited with this idea and the belt is often named for him as well. These KBOs have been found to orbit 30 to 55 AU from the Sun in a disk aligned with the plane of the solar system. Gerard Kuiper also introduced the idea that Pluto and short-period comets came from this region. Besides the Kuiper Belt, astronomical objects bearing his name include the Kuiper Airborne Observatory (named to honor the work he did with infrared spectroscopy), a minor planet, several binary stars and craters on the Moon, Mercury and Mars. His discoveries are credited to his extraordinary eyesight and his complete dedication and devotion, the long hours of work and the seriousness with which he approached his studies, demanding near perfection of himself and the researchers around him.

Neried has the most eccentric orbit of any moon that we’ve found in the solar system. It is almost seven times further at its greatest distance from the planet than the closest point. Triton’s capture (millions of years ago) is likely the cause the Neried’s disrupted orbit.

Proteus, Thalassa, Despina, Galatea, Larissa and Naiad were seen when Voyager 2 passed the system in 1989. From Earth, they are so close to the planet that they are hidden in the glare.

Proteus is the second largest moon of Neptune, but not completely spherical and extremely dark with only 6% of sunlight reflecting off the surface–a sharp contrast to Triton’s extremely bright surface that reflects 70%. At 5 x 109 kg, Proteus is thought to be just under the mass limit for a moon to form a spherical shape.
Larissa had originally been spotted by a team of scientists using ground-based telescopes in 1981. Harold Reitsema, William Hubbard, Larry Lebofsky and David Tholen were looking for a ring around the planet Neptune by monitoring the luminosity of a star as the planet eclipsed it. A quick fluctuation in the brightness, a decrease of only a few seconds, indicated not a ring, but a small moon. Larissa wasn’t seen again until Voyager 2 passed by.

Stephen Synott and the Voyager 2 Team share the credit for Despina, Galatea and Proteus, and Richard Terrile and the Voyager Imaging Team share discovery for Thalassa. One of the satellite discovery papers from Voyager 2 has fourteen authors.

Halimede, Laomedeia and Sao were discovered in 2002 (Matthew Holman, et al.) using Earth-based telescopes. Distant and tiny, they were missed by Voyager 2 and are most likely captured satellites, orbiting Neptune up to 125 times the distance from the Earth to the Moon. Halimede has a retrograde orbit. The 4-m Blanco telescope at CTIO in Chile and the 3.6-m Canada France Hawaii telescope on Mauna Kea were used. Again, the discovery papers have numerous authors.

Matthew Holman and another team found distant Neso, then lost it again and the International Astronomical Union refused to acknowledge their find. Persistence paid off and the team reestablished Neso’s position a year later in 2003. Psamathe has a similar orbit to Neso. These two moons are thought to originally have been one moon that broke apart. The 8.3-m Subaru telescope on Mauna Kea was used in this search-and-find mission by astronomer Scott S. Shepard and his co-workers.

So here’s where I leave you to ponder for a couple days. How will science in the future be different? Will the trend of larger and larger collaborations continue? Will the researchers not even know each other? Will we have papers with a thousand authors on them? Are bigger telescopes the only way to go?
Tune in on the 18th July for the conclusion of “The Discovery of the Neptune’s Thirteen Moons.” Thanks for listening to 365 Days of Astronomy. This is Diane Turnshek, signing off.

Special Neptune edition of Argentus

Chris McKitterick’s novel Transcendence (available free)

End of podcast:

365 Days of Astronomy
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