Bakersfield College

September 21, 2013

Bakersfield Night Sky – September 21, 2013
By Nick Strobel

Last week a major milestone (or 12 billion milestone) was reached in our exploration of the universe when Voyager 1 became the first human-built thing to leave our solar system. We now have a craft that is truly in interstellar space! The Sun produces a magnetic bubble around itself that keeps out the high-energy charged particles (plasma) produced by other stars as they live and die (including supernovae). Inside of that bubble lies all of the planets and dwarf planets (even troublemaker Eris). The Sun's magnetic field is carried outward by the solar wind, charged particles from the Sun flying outward, away from the Sun at a million or so miles per hour. Eventually, the pressure outward by the solar wind is matched by the interstellar pressure. The boundary where the number of solar wind particles drops off is 11.3 billion miles out from the Sun.

Hold on a minute! Didn't I give a distance of 12 billion miles in the first sentence? So, what's the 11.3 billion mile distance I give a few sentences later? Well, since we haven't been in interstellar space before, there's a bit of uncertainty of what it's like and we have to figure out what's going on. The Voyager team was getting some confusing, even contradictory information from the instruments that measure the charged particles, magnetic field strenth, and magnetic field direction back in the late summer of 2012. Some of the observations were what you would expect to find in interstellar space and other observations, particularly of the magnetic field direction, indicated that Voyager was still inside the Sun's bubble. It wasn't until a particularly powerful burst of plasma from the Sun caused the plasma around Voyager 1 in mid-spring 2013 to oscillate in a way that only would happen for interstellar plasma and different from what its sister craft, Voyager 2, was measuring at its closer distance to the Sun, that the Voyager team was finally convinced that Voyager 1 was in interstellar space. It took several months of figuring out what the data were telling the team to bring us to this September's announcement. When the Voyager team went back to observations of late summer and fall 2012, they found a faint set of plasma oscillations in mid-fall 2012 that said Voyager 1 was in interstellar space then and the confusing plasma/magnetic field data from a bit earlier in late August 2012 was when Voyager 1 actually crossed the boundary into interstellar space. Well, hindsight certainly is 20-20! When they figured out what the boundary was really like, they could pinpoint to the day when Voyager 1 entered interstellar space: August 25, 2012 at a distance of 11.3 billion miles from the Sun. Last week Voyager 1 was 11.7 billion miles (round up to 12 billion miles) out from the Sun. In a further departure from the perfect bubble boundary, it looks like the fuzzy bubble is a bit squashed in the direction that Voyager 2 is travelling, so it will probably reach interstellar space at a closer distance from the Sun than Voyager 1 did. You can view the current distances of Voyager 1 and 2 and the solar wind to interstellar plasma (cosmic rays) ratio at the Voyager homepage http://voyager.jpl.nasa.gov .

The Voyager spacecraft were launched in 1977 and are still transmitting data 36 years later with a power of just 23 watts. By the time their weak signals reach the Earth, the signal strength has been diluted to a billion-billionth of a watt (no, I'm not stuttering, that's a billion-billionth = 10^{-18} or 0.000000000000000001). The weak radio signals are picked up by the large 34-meter and 70-meter radio dishes at the Deep Space Network stations in Goldstone, CA (30 miles north of Barstow); Canberra, Australia; and Madrid, Spain. It takes the radio waves traveling at the speed of light about 17 hours to travel the distance between the Voyagers and Earth. They really are way out there!

Another boundary for the solar system is where the gravity influence from the Sun becomes as weak as that from the rest of the stars. That happens at the edge of the very large cloud of iceberg chunks or comet nuclei called the Oort Cloud. The Oort Cloud is from where comets with very long orbit times ("long orbital periods") such as Comet ISON come. The outer edge of the Oort Cloud is about 9.3 trillion miles out from the Sun and the Voyagers won't get there for another 30,000 years. In about 40,000 years Voyager 1 will be closer to the star AC +79 3888 than to our Sun. The star AC +79 3888, not Alpha Centauri, will be the closest star to the Sun by that time. The Voyagers will both have fallen silent well before then. They generate electricity using radioisotope thermoelectric generators that convert heat from the radioactive decay of plutonium into electricity. There should be enough plutonium left to power all of the instruments on the spacecraft until at least 2020. After that the instruments will be switched off one-by-one until 2025 to conserve electricity. For a few years after that we'll still get some radio signals from the spacecraft until they finally fall silent.

Since we knew that the Voyagers were going to be the first craft to leave the solar system, the Voyager team wanted our first steps into interstellar space to be a positive greeting from planet Earth. They attached a gold-plated copper disk ("the Golden Record") to each of the Voyagers that carried the sights and sounds of Earth. Each record is encased in a jacket of aluminum and they are to be played on the record player that is also onboard the Voyagers. Hopefully, the extra-terrestrials are intelligent enough to decipher the symbols used to explain how to play the record. You can view and hear the contents of the Golden Record at http://goldenrecord.org .

Our autumn season officially begins tomorrow afternoon (at 1:44 PM Pacific time to be more precise). The season begins when the Sun's midpoint crosses a point in space called the "equinox". This is where the Sun's annual path among the stars, called the "ecliptic", intercepts the projection of the Earth's equator onto the sky, called the "celestial equator". The Sun is moving southward at the September equinox, so this marks autumn for the Northern Hemisphere when the nights become longer than the daylight, getting ever longer until the winter solstice in December. Since the Sun is moving southward, the Southern Hemisphere will experiencing their spring season and lengthening daylight "down under".

Five days later, Comet C/2012 S1 ISON will pass 2.0 degrees (knuckle-to-knuckle distance at arm's length) north of Mars at seen from Earth on the morning of September 27th as shown in the first star chart below. You'll need a telescope to spot Comet ISON north of Mars. On October 1st, Comet ISON will pass within just 6.5 million miles of Mars. You can join in the Comet ISON observing campaign or just keep track of what Comet ISON is doing at the NASA Comet ISON Observing Campaign website at www.isoncampaign.org . This NASA-coordinated campaign is composed of hundreds, growing to thousands, of amateur astronomers and professional astronomers using equipment as simple as digital SLR cameras on up to large telescopes working with NASA scientists and NASA's resources to find out as much as possible about this new new visitor to the inner solar system. Our best views of Comet ISON will happen around Thanksgiving time to almost Christmas time. As of now, the comet is not looking like it will be as bright as hoped but comets that are first-time visitors to the inner solar system are especially unpredictable as far as what their brightness will be. All we can do is wait and see.

In the early morning sky Jupiter becomes visible low in the east at about 1:20 AM. It is at the eastern edge of Gemini. When Mars becomes visible at 3:45 AM, Jupiter and Gemini will be about a third of the way up in the sky. Jupiter will be the brightest star-like object in the pre-dawn sky. The first star chart below shows the view for 5:30 AM when Jupiter will be over halfway up in the sky. Mars has moved to the space between Cancer and Leo since my early September column. To the right (southward) of Jupiter will be the brightest true star in the night sky, Sirius at the nose or neck of Canis Major (depending on how you connect the dots). Above Canis Major will be Orion with his three famous belt stars. Above Jupiter and Gemini will be the stars of Auriga high overhead at the zenith with the bright whitish-yellow star Capella at the north end of it. Further north you'll be able to pick out the Big Dipper part of Ursa Major. A Waning Crescent Moon will pass under Jupiter on the morning of September 28th and then under Mars between the mornings of September 30th and October 1st.

In the evening sky Venus will shine in the west brighter than any star. It will be next to the stars of Libra but you'll be hard pressed to find any of the dim stars of that constellation in light-polluted skies. In about three weeks, Venus will have moved to the stars of Scorpius. Slightly to the right of Venus is Saturn at the edge of Virgo. Mercury is barely above a flat horizon a half hour after sunset but see if you can spot it with binoculars on September 24th when it will be right next to Spica, the bright star at the bottom of Virgo. The second chart below shows this view.

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.

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Nick Strobel
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com

Kern Community College District