Bakersfield Night Sky – November 7, 2015
By Nick Strobel
Last weekend was a busy one for solar system news. On October 28th the Cassini spacecraft took its deepest dive into the geysers at the south pole of Saturn's moon, Enceladus, and a dead comet whizzed by the Earth on the morning of Halloween.
Last Friday the Cassini team released the first set of pictures from Cassini's plunge through the geysers at Enceladus' south pole on October 28th. Cassini got as close as 30 miles above the surface of Enceladus, zipping by at about 19,000 miles per hour. On this close flyby, the Cassini team wanted to see if there is molecular hydrogen in the geysers that would be independent evidence for hydrothermal activity on the ocean floor. Also, although organic compounds have already been discovered in the geyser material, the Cassini team will be looking for additional, more complex organic molecules than what has already been detected. Cassini's instruments don't have the resolution to determine if the organic molecules are from biological reactions. There are geophysical ways of making organic molecules. Finally, the Cassini team wanted to see if the geysers are more like jets of squirted material or are a curtain going along the full extent of the fractures at the south pole. That will tell us how long Enceladus could have been active.
Another amazing fact is that the amount of material Cassini flew through to make these measurements is equivalent to just a tiny drop of water. Although Cassini was not designed to sample active cryovolcanic plumes, the Cassini team designed the spacecraft so well and they have the ingenuity to take advantage of opportunities and surprises like this.
Cassini will make one more flyby of Enceladus in December but from a much higher altitude of 3100 miles to get global measurements of the amount of heat coming from the moon's interior. Starting next year, Cassini will be adjusting its orbit around Saturn to fly between Saturn's innermost ring and its upper atmosphere. Near the end of 2017, Cassini will execute a controlled descent (and death) into Saturn's atmosphere as its propellant tanks run dry. This will ensure that Cassini won't accidentally contaminate any of Saturn's moons, especially Enceladus or Titan, by crashing into them far into the future. By 2017, the northern hemisphere will be in summer and Cassini will have witnessed a full switch of the seasons in Saturn's hemispheres (winter to summer in the north and summer to winter in the south). Saturn's orbit is about 29 years long, so each season is slightly more than 7 Earth years long.
On Halloween morning, the "Great Pumpkin" Halloween asteroid (2015 TB145) flew by Earth at 22 miles per second (over 79,000 miles per hour) at a distance about 30% farther away than the Moon. Radar imaging of the object shows it to be fairly round with a diameter of about 2000 feet. Infrared data shows the object to be very dark, reflecting just 6% of the sunlight. That's slightly brighter than most comet nuclei that reflect just 3 to 5% of the sunlight but significantly darker than asteroids that reflect about 15 to 20% of the sunlight reaching them. Couple that with its very elongated 3.1-year orbit and it is more likely that 2015 TB145 is a dead comet---one that has run out of volatiles from repeated heating of multiple passages near the Sun.
Keeping to the Halloween theme and acknowledging our predisposition to seeing faces in any inanimate object, NASA noted that the two side-by-side craters with low radar reflectivity made the radar image look like a human skull. Also, acknowledging the internet firestorms that can get started from a near-Earth object encounter like this, NASA made sure to note in the first few press releases that the gravity from this dead comet is way, way too small to affect the Earth. No effects on tides or tectonic plates.
The Great Pumpkin Halloween asteroid was discovered on October 10th as part of NASA's Near-Earth Object Observation program, nicknamed "Spaceguard". So far, this program has discovered 98% of the NEOs. The next time an object this large is known to get this close to the Earth is in August 2027 when 1999 AN10 flies by at about the distance of the Moon. That asteroid is about 2600 feet in diameter. For more details, see http://neo.jpl.nasa.govand main JPL website at www.jpl.nasa.gov .
Saturn continues to be the lone planet in our evening sky and it's getting harder to see it since it is so low in the southwest after sunset. By the middle of the month, it will be lost from view as it heads for conjuction with the Sun (going behind it) on November 30th. In the early morning sky, the planets Venus, Mars, and Jupiter were doing a little dance with each other at the end of October. Now Venus and Mars have pulled away from Jupiter. Two nights ago, Venus and Mars were close enough to each other that you could cover both with your thumb at arm's length. Tonight (tomorrow pre-dawn) they will be about two thumbs apart. They will fit within the field of view of your binoculars until Friday morning.
Want to see more of the stars at night and save energy? Shield your lights so that the light only goes down toward the ground. See www.darksky.org for how.
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com