Impetus for Iapetus

PIA11690: Global View of Iapetus’ Dichotomy, NASA/JPL/Space Science Institute

What a strange world Iapetus is! The third largest satellite of Saturn—and the outermost of Saturn’s large satellites—is a moon of many mysteries. We’ll take a look at three of them.

Mystery #1: Iapetus appears to be an original satellite of Saturn, and yet unlike the other regular satellites, its orbit is inclined 15.5˚ relative to Saturn’s equator. The reason for this steep inclination is not well understood.

And, oh, the view! Iapetus is the perfect perch to view Saturn’s rings, as it orbits Saturn every 79.3 days in its steeply inclined orbit.

Saturn from Iapetus at the highest point of its inclined orbit

Mystery #2: Iapetus has the largest albedo dichotomy in the solar system. Why? Iapetus is locked in synchronous rotation as it orbits around Saturn, with the leading hemisphere ten times darker than its trailing hemisphere.

Iapetus has an average visual magnitude of 10.2 west of Saturn and 11.9 east of Saturn. Its albedo ranges from 0.5 to 0.05. (Diagram not to scale)
Bright and dark material on Iapetus. The 500-km-wide crater Engelier is at bottom.

It is thought that the natural state of the Iapetian surface is the bright icy part, with the dark material a thin veneer, less than a meter thick.

Mystery #3: Iapetus has a shape consistent with a body spinning every ~16 hours and yet its rotation period is 79.3 days, and it has a prominent ridge that can be followed 3/4 of the way around the equator.

Walnut-shaped Iapetus with its prominent equatorial ridge
Iapetus’ equator-girdling ridge, up to 20 km high, is heavily cratered and therefore ancient

The surface of Iapetus is heavily cratered, indicating it is very old. Could two comparable-sized objects have collided almost head-on billions of years ago to form Iapetus?

Mountainous terrain along Iapetus’ equatorial ridge imaged by the Cassini spacecraft during its closest flyby on September 10, 2007

As beautiful as spacecraft flyby and orbital images are of Iapetus and the many other interesting moons in our solar system, can you imagine what vistas await us once we start exploring their surfaces with rovers? Anticipation of these images and scientific discoveries surely is an impetus to explore the surface of Iapetus (and other moons) sooner rather than later.

Dark and light material on Iapetus was imaged up close by the Cassini spacecraft during its September 10, 2007 flyby.
Sizes of Iapetus, Earth’s moon, and Earth compared


Bonnefoy, Léa E., Jean-François Lestrade, Emmanuel Lellouch, Alice Le Gall, Cédric Leyrat, Nicolas Ponthieu, and Bilal Ladjelate. “Probing the subsurface of the two faces of Iapetus.” arXiv preprint arXiv:1911.03394 (2019).

Leleu, Adrien, Martin Jutzi, and Martin Rubin. “The peculiar shapes of Saturn’s small inner moons as evidence of mergers of similar-sized moonlets.” Nature astronomy 2, no. 7 (2018): 555-561.

Rivera-Valentin, Edgard G., Amy C. Barr, EJ Lopez Garcia, Michelle R. Kirchoff, and Paul M. Schenk. “Constraints on planetesimal disk mass from the cratering record and equatorial ridge on Iapetus.” The Astrophysical Journal 792, no. 2 (2014): 127.


Climate change is a serious problem requiring immediate attention. We need to reduce greenhouse gas emissions into our atmosphere as fast as possible. Half measures will not do. We are rapidly running out of time before the quality of life for all humans on planet Earth declines, especially for the economically disadvantaged.

A precipitous decline in biological diversity due to habitat loss and extinction of species is of greater concern, and yet it gets very little attention in the mainstream media. While climate change will render large areas of the Earth uninhabitable, biodiversity loss will lead to a partial or complete collapse of the ecosystem humans depend upon for food.

Getting even less attention is the cause of both of these problems: overpopulation. If you were born in 1973, the world’s human population is now twice what it was then. If you were born in 1952, there are three times as many people alive now than there were then. We have a climate emergency and a biodiversity emergency because we have a population emergency. The number of humans on this planet needs to decline, and the only humane way to accomplish that is to have fewer children. It is that simple.

And, yet, we often see this or that news article lamenting the fact that the birth rate in this or that country is too low. That’s crazy! A low birth rate should be a cause for celebration given the current state of the world and its environment. Certainly, a low birth rate does lead to some economic challenges, but these pale in comparison to the challenges we will face if population (and consumption) continue to grow.

As a humanist, I believe that we should do all we can to alleviate and eliminate human suffering. It is our highest moral calling. To be sure, some human suffering is inevitable and necessary when an individual makes poor decisions and suffers the consequences before hopefully making a mid-course correction. But the kind of suffering I am talking about is suffering that is imposed upon a person through no fault of their own, be it the cruelty of other human beings, or the cruelty of nature.

In this light we can see that our economic systems, governments, and most religions are utterly failing us. Nothing short of drastic changes will solve these problems. May wisdom, intelligence, ingenuity, and compassion guide us, rather than fear, ignorance, hatred, and dogma.

There is an organization dedicated to stabilizing human population throughout the world by lowering the birth rate: Population Connection. I encourage you to support their work as I do.

YearPopulationGrowth Factor

World Population Prospects 2019, United Nations.; 17 January, 2020; Dover, Delaware, U.S.A.

Satellite and Meteor Crossings 2019 #2

Edmund Weiss (1837-1917) and many astronomers since have called asteroids “vermin of the sky”, but on October 4, 1957 another “species” of sky vermin made its debut: artificial satellites.  In the process of video recording stars for possible asteroid occultations, I frequently see satellites passing through my 17 × 11 arcminute field of view.

I’ve put together a video montage of satellites I serendipitously recorded between August 9, 2019 and December 22, 2019.  Many of the satellite crossings are moving across the fields as “dashes” because of the longer integration times I need to use for some of my asteroid occultation work. A table of these events is shown below the video. The range is the distance between observer and satellite at the time of observation. North is up and east is to the left.

Satellites in higher orbits take longer to cross the field. In the next video, the originally geosynchronous satellite OPS 1570 (IMEWS-3, “Integrated Missile Early Warning System”) is barely visible until it exhibits an amazing sunglint around 3:41:22 UT.

I caught one meteor on October 6, 2019 at 9:57:43 UT. Field location was UCAC4 515-043597. The meteor was a Daytime Sextantid, as determined using the method I described previously in There’s a Meteor in My Image. The meteor even left a brief afterglow—a meteor train!

Hughes, D. W. & Marsden, B. G. 2007, J. Astron. Hist. Heritage, 10, 21

Bastien and Bastienne…and Beethoven?

Musical mystery, or compositional coincidence?  Wolfgang Amadeus Mozart wrote the music for his one-act opera Bastien and Bastienne in 1768, at the age of 12.  The short Overture to Bastien and Bastienne bears a remarkable resemblance to the opening theme of Beethoven’s Eroica symphony, composed between 1802 and 1804.  Although the keys are different (Mozart’s overture is in G major and Beethoven’s symphony is in E♭ major), could it be that Beethoven had Mozart’s theme in mind while he composed his 3rd symphony?  It is unlikely that Bastien and Bastienne was known to Beethoven, as that music received its first public performance in 1890.  Perhaps, just a coincidence.  Great minds think alike, it appears.

Overture to Bastien and Bastienne, Statatskapelle Dresden, Sir Colin Davis, RCA 74321-56698-2
Symphony No. 3, “Eroica”, Chicago Symphony Orchestra, Sir Georg Solti, London 430 792-2

An Astronomy Retirement Community

Are any of you nearing retirement (as I am) or already retired who might be interested in moving to an astronomy-oriented retirement community? If you are, I encourage you to join the moderated discussion group Dark-Sky Communities at

I am working to establish such a community and would value your input and assistance. That work involves extensive research, networking, writing articles in various publications to reach a wider audience, finding a suitable developer, and seeking benefactors.

Some characteristics of the community I envision include:

  1. Rural location with a dark night sky, but not too far from a city with decent medical facilities, preferably to the northeast or northwest;
  2. Location with an abundance of clear nights and mild winters, probably in Arizona, New Mexico, or West Texas;
  3. Lighting within the community that does not interfere with astronomical activities, strictly enforced;
  4. Community is owned and operated by a benefit corporation or cooperative that will rent a house or apartment to each resident;
  5. Observatories will be available for rental by interested residents who will equip them;
  6. Pro-am collaborative research opportunities will be developed and nurtured;
  7. A community observatory and a public observatory for astronomy outreach will be constructed and maintained;
  8. Lodging will be available for visitors and guests;
  9. There will be opportunities for on-site income operating and maintaining the community or, alternatively, a reduction in monthly rental fees.

Many of us have spent a significant amount of time and energy over the years trying to rein in light pollution in our respective communities and in the wider world, with varying degrees of success. Those efforts should continue, but the grim reality is that light pollution is continuing to get worse almost everywhere.

The opportunity to live in a community of varied interests but with a common appreciation for the night sky and a natural nighttime environment will appeal to many of us. Furthermore, a dark-sky community will afford us opportunities to show the world at large a better way to live.

Traditionally, in the United States at least, if one wants to live under a dark and starry night sky, your only options are to purchase land and build a house on it, or purchase an existing rural home. Not only is buying and maintaining rural real estate unaffordable or impractical for many, many would prefer to live in a rural community, provided that the night sky and nighttime environment are vigorously protected. Rental will also make it easier to move into and out of the community as circumstances change.