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, No. 14

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.

 Eclipse Video


Journey to the Shadow, No. 13

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!

 Eclipse Video


Monday, August 21, 2017

Journey to the Shadow, No. 12

August 21, 2017

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, No. 11

August 21, 2017

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, No. 10

August 21, 2017

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


Journey to the Shadow, No. 9

August 21, 2017

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, No. 8

August 21, 2017

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, No. 7

August 20, 2017

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, No. 6

August 20, 2017

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, No. 5

August 20, 2017

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, No. 4

August 20,2017

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, No. 3

August 20, 2017 

All packed and waiting for my cohort, Craig MacDougal. The black bag has pieces of foam core board for pinhole projectors. Trivia: On Aug. 21, 2017, all of North America will view – weather permitting -- a partial eclipse, when the moon obscures part of the sun.

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, No. 2

August 19, 2017

The Journey to the Shadow gets serious tomorrow as we leave Tampa, Florida for Santee, South Carolina.