Wednesday, August 30, 2017

The Rings of Saturn Viewed Planet-Side and the Final Days of Cassini

On August 20, NASA's Cassini spacecraft completed another pass between Saturn and its rings. The below animation shows the ring system from above (sunlit side) and then below (shadow side). All of the ring system is seen. But due to the angle, the rings and the ring divisions appear foreshortened (squeezed together). Also, the inner C ring looks larger in the foreground.

Saturn's ring system as seen during the Cassini pass between the rings and Saturn on August 20, 2017. Image Credit: NASA/JPL-Caltech/Space Science Institute

The Grand Finale is Nearly Complete

The spacecraft is quickly approaching its  mission-ending dive into the atmosphere of Saturn on September 15. An April 22 gravitational assist from Saturn's moon Titan put the craft on its final path. But several mission milestones remain before then.

The spacecraft is expected to lose radio contact with Earth within about one to two minutes after beginning its descent into Saturn's upper atmosphere. But on the way down, eight of Cassini's 12 science instruments will be operating. In particular, the spacecraft's ion and neutral mass spectrometer (INMS), which will be directly sampling the atmosphere's composition, potentially returning insights into the giant planet's formation and evolution. On the day before, September 14, other Cassini instruments will make detailed, high-resolution observations of Saturn's auroras, temperature, and the vortices at the planet's poles. Cassini's imaging camera will take it's last views on September 14 and then be shut down. Below are some highlights from the final days of Cassini.

September 9 -- Cassini will make the last of 22 passes between Saturn itself and its rings -- closest approach is 1,044 miles (1,680 kilometers) above the clouds tops.

September 11 -- Cassini will make a distant flyby of Saturn's largest moon, Titan. Even though the spacecraft will be at 73,974 miles (119,049 kilometers) away, the gravitational influence of the moon will slow down the spacecraft slightly as it speeds past. A few days later, instead of passing through the outermost fringes of Saturn's atmosphere, Cassini will dive in too deep to survive the friction and heating.

September 14 -- Cassini's imaging cameras take their last look around the Saturn system, sending back pictures of moons Titan and Enceladus, the hexagon-shaped jet stream around the planet's north pole, and features in the rings.

September 14 (5:45 p.m. EDT / 2:45 p.m. PDT) -- Cassini turns its antenna to point at Earth, begins a communications link that will continue until end of mission, and sends back its final images and other data collected along the way.

September 15 (4:37 a.m. EDT / 1:37 a.m. PDT) -- The "final plunge" begins. The spacecraft starts a 5-minute roll to position INMS for optimal sampling of the atmosphere, transmitting data in near real time from this point to end of mission.

September 15 (7:53 a.m. EDT / 4:53 a.m. PDT) -- Cassini enters Saturn's atmosphere. Its thrusters fire at 10 percent of their capacity to maintain directional stability, enabling the spacecraft's high-gain antenna to remain pointed at Earth and allowing continued transmission of data.

September 15 (7:54 a.m. EDT / 4:54 a.m. PDT) -- Cassini's thrusters are at 100 percent of capacity. Atmospheric forces overwhelm the thrusters' capacity to maintain control of the spacecraft's orientation, and the high-gain antenna loses its lock on Earth. At this moment, expected to occur about 940 miles (1,510 kilometers) above Saturn's cloud tops, communication from the spacecraft will cease, and Cassini's mission of exploration will have concluded. The spacecraft will break up like a meteor moments later.

A Very Long Mission at Saturn

As Cassini completes its 13-year tour of Saturn, its Grand Finale -- which began in April -- and final plunge are just the last beat. Following a four-year primary mission and a two-year extension, NASA approved an ambitious plan to extend Cassini's service by an additional seven years. Called the Cassini Solstice Mission, the extension saw Cassini perform dozens more flybys of Saturn's moons as the spacecraft observed seasonal changes in the atmospheres of Saturn and Titan. From the outset, the planned endgame for the Solstice Mission was to expend all of Cassini's maneuvering propellant exploring, then eventually arriving in the ultra-close Grand Finale orbits, ending with safe disposal of the spacecraft in Saturn's atmosphere.

Mountains of New Data on the Saturn System

Since its launch in 1997, the findings of the Cassini mission have revolutionized our understanding of Saturn, its complex rings, the amazing assortment of moons and the planet's dynamic magnetic environment. The most distant planetary orbiter ever launched, Cassini started making astonishing discoveries immediately upon arrival and continues today. Icy jets shoot from the tiny moon Enceladus, providing samples of an underground ocean with evidence of hydrothermal activity. Titan's hydrocarbon lakes and seas are dominated by liquid ethane and methane, and complex pre-biotic chemicals form in the atmosphere and rain to the surface. Three-dimensional structures tower above Saturn's rings, and a giant Saturn storm circled the entire planet for most of a year. Cassini's findings at Saturn have also buttressed scientists' understanding of processes involved in the formation of planets.

Why End the Mission?

The spacecraft is running low on the rocket fuel used for adjusting its course. If left unchecked, this situation would eventually prevent mission operators from controlling the course of the spacecraft.

Two moons of Saturn, Enceladus and Titan, have captured news headlines over the past decade as Cassini data revealed their potential to contain habitable – or at least "prebiotic” – environments.

In order to avoid the unlikely possibility of Cassini someday colliding with one of these moons, NASA chose to safely dispose of the spacecraft in the atmosphere of Saturn. This will ensure that Cassini cannot contaminate any future studies of habitability and potential life on Enceladus and Titan.

For more information about the Saturn system and the Cassini mission, click on the links below.

NASA Cassini Mission

NASA Saturn/Cassini Mission Coverage


Sunday, August 27, 2017

Asteroid 3122 Florence Close Approach September 1

On September 1, asteroid 3122 Florence will become the largest asteroid to fly by Earth since near-Earth asteroids were discovered a century ago.

Asteroid 3122 Florence will pass by Earth on September 1, 2017, at a distance of about 4.4 million miles. Image Credit: NASA/JPL-Caltech

Measurements made by the Spitzer Space Telescope and NEOWISE asteroid-hunting instrument suggest that Florence is around 2.7 miles (5 km) in diameter. The asteroid will pass 4.4 million miles from Earth, about 18 times the distance from the Earth to the moon.

Many known asteroids have passed closer to Earth than Florence will, but those were estimated to be smaller. NASA has tracked near-Earth objects since 1998.

Background on Florence

The body called Florence was first detected March 2, 1981 by American astronomer Schelte “Bobby” Bus from Australia’s Siding Spring Observatory. The discovery was provisionally labeled 1981 ET3. In 1993, it was acknowledged as asteroid discovery number 3122 and was named 3122 Florence in honor of Florence Nightingale (1820-1910), the founder of modern nursing.

3122 Florence is a stony asteroid of the Amor group, classified as near-Earth object and potentially hazardous asteroid (PAH). It orbits the sun at a distance of 1.0–2.5 AU once every 2 years and 4 months (859 days). Its orbit has an eccentricity of 0.42 and an inclination of 22° with respect to the ecliptic. The PHA classification is due to both the body’s absolute magnitude (H ≤ 22) and its minimum orbit intersection distance (MOID ≤ 0.05 AU).

Observing Opportunity

For visual astronomers, 3122 Florence will be clearly visible in the night sky beginning August 27. On September 1, 3122 Florence will pass 0.04723 AU (7,066,000 km; 4,390,000 mi) from Earth, brightening to apparent magnitude 8.5, when it will be visible in small telescopes for several nights as it moves through the constellations Piscis Austrinus, Capricornus, Aquarius and Delphinus.

NEO Close Approaches in 2017

Florence is just one of a few bodies passing Earth this year. In January, asteroid 2017 AG13 snuck up on astronomers. The body was between 36 and 111 feet wide and passed Earth at half the distance to the moon. Another asteroid in the same size range, 2012 TC4, is scheduled to pass roughly one-fourth the distance to the moon—between 4,200 miles and 170,000 miles—on October 12, 2017.

Earth-Based Radar Observing

The size and proximity of Florence make it a perfect target for ground-based radio telescope observations. Radar imaging is planned at NASA's Goldstone Solar System Radar in California and at the National Science Foundation's Arecibo Observatory in Puerto Rico. The resulting radar images will show the real size of Florence and could reveal surface details as small as about 30 feet (10 meters).

NEO Tracking Continues

Currently, NASA is tracking 1,826 near-Earth objects classified as Potentially Hazardous Asteroids, which have some risk of striking Earth in the future. Among those are several larger than Florence, including 1999 JM8 at 4.3 miles across, 4183 Cuno at 3.5 miles across and 3200 Phaeton at 3.2 miles across. None have come as close as Florence. Florence won’t make a closer pass until around the year 2500.

More information about asteroids and near-Earth objects can be found at:

For more information about NASA's Planetary Defense Coordination Office, visit:


Friday, August 25, 2017

Journey to the Shadow

New friend Scott Wargo made this video of totality during the August 21 eclipse, as seen from our Greenwood, South Carolina location. I've already shared this link on some sites, so apologies if you have already seen this. Just trying to be thorough. Enjoy.

Journey to the Shadow

Friend Craig MacDougal made this 13-minute YouTube video to capture the sights and sounds of the August 21 eclipse. Watch the Walmart as the sky grows erily darker and folks get more excited, see the darkness and hear the shouting during totality, and then watch how the world seems to quickly return to normal afterward. Did it really happen? I'm pretty sure it did!

Monday, August 21, 2017

Journey to the Shadow

I have no photos from totality. But I have memories. Wow! Amazing. It was so otherworldly. The corona had the look of a Star Fleet delta shield with a solar prominence thrown in. The experience was so worth the trip. I will be scanning the recordings for the eclipse as it appeared when I saw it with my own eyes. We broke camp about 30 minutes after the end of totality. We are making our way back home with the rest of humanity. ETA, sometime after midnight Eastern Time.

Journey to the Shadow

More observers collected as we got closer to totality. One family had some guests, a missionary family back from Niger. We had some fun with a pinhole projector.

Journey to the Shadow

As we waited through the partial phase, Craig played a little Frisbee to pass the time.

Journey to the Shadow

We met up with Craig's friends in Greenwood. We are camped in a shaded area by the parking lot of a Super Walmart. Partly cloudy, but we still consider it a great spot. Also, we aren't relocating again. And so we wait.

Journey to the Shadow

It is the morning of Eclipse Day and we have a new plan: Go West! Craig's careful study of the weather models and satellite imagery gave him a headache. But suggested that around eclipse time in South Carolina, the chances of cloud cover will be lower the farther west you go. Therefore, we've decided to drive to Greenwood, west of Columbia. There we will meet an old friend of Craig's who will be observing there with friends. Being Eclipse Day, we expect traffic to be a challenge. So, for our dash across the state, we will take a few roads we hope will be less traveled. And by doing so, we hope that will make all the difference. The optimistic travel-time estimate is about 2 hours 30 minutes. If there are significant delays on the road, we may get to Greenwood after first contact (C1, the start of the partial phase). But as long as we are in place before totality (the time from C2 to C3), then no worries! The rough eclipse contact times for Greenwood, SC (Eastern Time) are: C1: 1:10:25 pm; C2: 2:39:22 pm; C3: 2:41:51 pm; C4: 4:04 pm. Queue the John Williams music!...Trivia Item 1: Experiencing a total solar eclipse where you live happens on average about once in 375 years. Trivia Item 2: 12.2 million Americans live in the path of this total eclipse. Of course, with visitors, that number will be much higher! About 200 million people (a little less than 2⁄3 the nation's population) live within one day's drive of the path of this total eclipse. In addition, millions of Americans will be able to view a partial eclipse, weather permitting.

Sunday, August 20, 2017

Journey to the Shadow

We found near our motel this concrete elephant and giraffe. Since golf is big in this area, we think they are remnants of a miniature golf course for the kids. We had dinner at the local Shoney's, watched the local news and weather, and chatted about the eclipse with the staff. After dinner, we returned to the restaurant with a couple pairs of eclipse glasses and a copy of the eclipse times for local viewing. We gave the glasses and times to the staff and wished them well.

Journey to the Shadow

We have arrived at the Best Western in Santee, South Carolina. The agenda for the evening includes: (1) Get the wi-fi going, (2) forage for dinner at a local establishment, (3) construct some pinhole projectors to have on hand for the eclipse, and (4) study the latest weather models for tomorrow. Oh, and (5) get a good night's rest. Trivia: The last time a total solar eclipse occurred exclusively in the U.S. was in 1778.

Journey to the Shadow

We reached South Carolina and topped off the tank. We're back on the road to Santee. Trivia: The last total eclipse in the United States occurred on Feb. 26, 1979. The last total eclipse that crossed the entire continent occurred on June 8, 1918.

Journey to the Shadow

Now in Georgia. Behold the image of a sign partially eclipsed (ha!) by another car taking a pic of the same sign! Trivia: The umbra (or dark inner shadow) of the moon will be traveling from west to east from almost 3,000 miles per hour (in western Oregon) to 1,500 miles per hour in South Carolina.

Journey to the Shadow

On our way! Note the car signage. If others can advertise their fanaticisms, so can we. Trivia: The total eclipse will be viewable throughout a 70-mile-wide path that crosses 14 of the United States from Oregon to South Carolina.

Journey to the Shadow

via Instagram

Tuesday, July 04, 2017

I Can Name That Eclipse in Five Notes

Total solar eclipses have always captivated our attention. They have even made their way into our music. See how many of these songs you recognize.

March 7, 1970 total solar eclipse. Image Credit: NSO/AURA/NSF

You’re So Vain, composed and performed by Carly Simon

. . . you flew your Lear jet up to Nova Scotia
To see a total eclipse of the sun

This is the only known recorded song with lyrics that mention a specific eclipse. But which one? The recording was released November 1972. And Nova Scotia had recently experienced two total solar eclipses, one on March 7, 1970 and another on July 10, 1972. Simon has reported that she wrote the song in 1971. If Simon was reflecting on recent events, then she probably meant the total solar eclipse of March 7, 1970.

Eclipse, by Pink Floyd from ‘Dark side of the Moon’

… and everything under the sun is in tune
but the sun is eclipsed by the moon.

Total Eclipse of the Heart, sung by  Bonnie Tyler.

…Once upon a time there was light in my life
But now there's only love in the dark
Nothing I can say
A total eclipse of the heart…

If we go farther back in time, we come across a smattering of sheet music published in the 1800s and early-1900s.

The Total Eclipse Gallop, composed by E. Mack and published in 1919 by Lee & Walker. This song commemorates the August 7, 1869 total solar eclipse, which was visible across the continental United States.

Eclipse, composed by Herman Darewski (1883-1947) was published in 1919.

Other memorable dance tunes that are now long forgotten include:

Eclipse March, 1899 published by Troedel & Co., National Library of Australia.

Eclipse Polka,  by Giuseppe Bistolfi. Published by Kansas City, MO: J.W. Jenkins Son, 1889. University of Missouri, Kansas City Sheet Music Collection

Eclipse Polka, 1853, Published by Wm. Vanderbeek and Son,

Eclipse Waltz, 1854, W. C. Peters and Sons

Eclipse Polka, 1874, Lee & Walker

Eclipse Quickstep, 1885, Richards, J. G

Eclipse Gallop, 1885, Spear & Dehnhoff

Eclipse Schottische, 1884, Stewart, S. S.

For more information on the August 21 total eclipse, and eclipses in general, visit:


Monday, July 03, 2017

Solar Viewing Safety

When it comes to looking at the sun, safety comes first! Here are some important questions and answers about solar viewing safety.

Anatomy of the Human Eye. Image Credit: NASA

Why is it not safe to look at the sun even when only a small part of it is visible?

The rods and cones in the human retina are very sensitive to light. Even a thin sliver of the sun’s disk covers thousands of these light-sensitive cells. Normally during daylight conditions, the iris contracts so that only a small amount of light passes through the lens and then reaches the retina. This level of indirect sunlight is perfectly OK and the eye has evolved over millions of years to safely see the daylight world under most circumstances. The problem is that the sun’s surface is so bright that if you stare at any portion of it, no matter how small, it produces enough light to damage individual retinal cells.  It takes a few seconds for this to happen, but afterwards you will see a spot as big as the solar surface you glimpsed when you look away from the sun at some other scenery. Depending on how long you gazed at the sun and how badly the retinal cells were damaged,  this spot will either fade away in time or remain permanent.  You should never assume that you can look away quickly enough to avoid eye damage because every person is different in terms of their retinal sensitivity, and you do not want to risk being the one who damages their eyes just to try to look at the sun. If you want to see what the sun looks like, use a properly-equipped telescope. Or just go online and view thousands of pictures taken of the sun by telescopes and NASA spacecraft.

Is it true that you should not look at the sun even during a total solar eclipse?

There is a misunderstanding being circulated that during a total solar eclipse when the moon has fully blocked the light from the sun, that there are still harmful ‘rays’ that can injure your eyes.  This is completely false. When the bright photosphere (the visible surface) of the sun is completely covered, only the faint light from the corona is visible, and this radiation is too weak to have any harmful effects on the human retina.

The misunderstanding comes about because of using the general term ‘solar eclipse’ to describe both the total phase when the sun disk is completely blocked, and the minutes before and after totality when there is still some of the sun’s disk visible. It is harmful to view even a sliver of the sun disk because of its intensity, and so to simply say that you should not view a solar eclipse is rather inaccurate.

Do lunar and solar eclipses have any noticeable effect on humans?

There is no evidence that eclipses have any physical effect on humans. However, eclipses have always been capable of producing profound psychological effects. For millennia, solar eclipses have been interpreted as portents of doom by virtually every known civilization. These have stimulated responses that run the gamut from human sacrifices to feelings of awe and bewilderment. Although there are no direct physical effects involving known forces, the consequences of the induced human psychological states have indeed led to physical effects.

How are eyes damaged by staring at the sun?

Solar retinopathy is a result of too much ultraviolet light flooding the retina. In extreme cases this can cause blindness, but is so painful that it is rare for someone to be able to stare at the sun for that long. Typically, eye damage from staring at the sun results in blurred vision, dark or yellow spots, pain in bright light or loss of vision in the center of the eye (the fovea). Permanent damage to the retina has been shown to occur in approximately 100 seconds, but the exact time before damage occurs will vary with the intensity of the sun on a particular day  and with how much the viewer's pupil is dilated  from decongestants and other drugs they may be taking.  Even when 99% of the Sun's surface (the photosphere) is obscured during the partial phases of a solar eclipse, the remaining crescent Sun is still intense enough to cause a retinal burn. Note, there are no pain receptors in the retina so your retina can be damaged even before you realize it, and by then it is too late to save your vision!

Where can I get the right kind of solar filter to view the eclipse?

Many people will obtain eclipse viewing glasses. To date, three manufacturers have certified that their eclipse glasses and hand-held solar viewers meet the ISO 12312-2 international standard for such products: Rainbow Symphony, American Paper Optics, and Thousand Oaks Optical. These companies may be found online and the glasses ordered, but you really need to order your glasses many months in advance because of the anticipated huge audience that could number in the hundreds of millions. If you are a photographer or amateur astronomer, you will want professional-grade solar filters to cover your binoculars, telescope or camera. Companies like Thousand Oaks Optical and others you can find by using the keyword ‘Solar filters’ have these filters for sale, but again due to the large number of likely customers along the path of totality, you need to order your filter many months in advance. You will also need some time to learn how to use the filter with your optical system, and if you are photographing the eclipse, take lots of test shots to get the right solar disk size and sharpness.

Is it only the bright light that is dangerous when viewing the sun?

Actually, although filters and glasses do safely block the intense sunlight that is known to damage retinas, the infrared ‘heat’ from the sun can also make viewing uncomfortable as it literally warms the eye.  This is why staring at the sun for minutes at a time even with proper filters can still over-heat the tissues and fluids in the eye, and the consequences of this heating can be dangerous as well. To avoid this problem before totality takes place, try not to use your filters without frequently looking-away to cool your eyes. During totality, there is no adverse heating of the eyeball since the solar disk is not visible.

Isn’t this ‘safety’ issue about eclipse viewing, a bit overblown?

Absolutely not!  You cannot look at the sun without suffering severe damage. We have many built-in reflexes to prevent this. The ONLY exception is in viewing solar eclipses. It is an inherently dangerous activity that you have to do very carefully in order not to suffer eye damage. There are specific steps you can take, based on the experience of  thousands of professionals, not only in astronomy but in medicine. So, bottom line: read the safety warnings and make sure you understand how to view the eclipse before  August 21, so that the only lasting impression you have is a wonderful memory of the event, not a damaged retina!

For more information on the eclipse, and how to safely view it, visit: