Voyager I and 2 at the edge of the solar system. photo courtesy of NASA The Slightly Enigmatic Message Aboard Voyager. photo courtesy of NASA

by Kim Malville

What’s happening in the sky this month?

Venus continues to be the brightest planet in the sky, hanging close to the southwest horizon. At the start of the month, Venus sets 1 hour and 45 minutes after the sun. By the end of October it hangs around for 2 hours and 30 minutes after sunset. Jupiter rises in the late evening and is the brilliant planet after midnight, where it hangs out in Gemini. Once Venus sets, Jupiter is the brightest planet in the night sky.

October 1: For you early risers, look for the thin waning moon about an hour before sunrise. The moon, Mars, and Regulus form a nice triangle.

October 7-8: The moon has made its voyage from dawn to dusk. A thin waxing crescent appears to the right of Venus in the southwest an hour after sunset.

October 16: You Venus lovers will enjoy seeing the planet close to red Antares, an hour after sunset.

Oct 29: Very wondrously, the triangle reappears in the eastern sky, just ahead of sunrise. The slender moon, Mars, and Regulus form triangle. If you missed it 29 days ago, should try this time.

Voyager passes beyond the solar system

Launched in 1977, Voyager is now more than 11 billion miles from the sun, becoming the first object constructed by humanity to leave our solar realm and enter the vastness of interstellar space. The solar system is dominated by the sun and the wind it produces, which carves out a cavity, protecting us from interstellar cosmic rays. On August 25, 2012 Voyager 1 passed out of that cavity. Its twin, Voyager 2, is slightly behind. Outside, the interstellar wind is produced by the baked debris of thousands of exploded stars in our Milky Way galaxy, memories of countless destroyed planetary systems, including a perhaps a few fried, earth-like planets.

Knowing exactly where interstellar space begins has been a challenge for decades. Most significant of all the data transmitted by Voyager, the number of high energy cosmic rays (fast charged particles) increased on August 25, while the lower energy cosmic rays from the sun decreased.

Given the estimated lifetime of the plutonium battery aboard Voyager 1, its last signals should be heard on Earth around 2025. The spacecraft will boldly continue outward, passing within 1.7 light years of another star in 40,000 years. Perhaps then an advanced civilization will read the message it carries.

Voyager carries a gold-plated copper record with engravings (as well as a stylus) containing information on how to play it. There are images, which will surely puzzle folks in the future as much as they do some of us now. In addition to profiles of naked humans, the image shows the position of the sun relative to the center of our galaxy and pulsars that were known back then, as well as the path of Voyager from Earth to Jupiter and Saturn. It shows nine planets, which is what we believed back in 1977.

The contents of the record were selected by a committee chaired by Carl Sagan and include images of Earth and a variety of natural sounds, including surf, wind, thunder and the songs of birds and whales. There are musical selections from different cultures and spoken greetings from Earth  in fifty-six languages including Esperanto. After NASA had received complaints, the agency chose not to allow Sagan and his colleagues to include a photograph of a nude man and woman on the record, allowing only a silhouette of the couple.

Plutonium doesn’t come from Pluto

It took more than 17 hours for the last message from Voyager to reach Earth. It would have been impossible to send signals over the decades since its launch, without the spacecraft’s three batteries filled with plutonium-238. What strange irony! Most of what humanity knows about the outer planets has come back to Earth on plutonium power. Cassini’s ongoing exploration of Saturn, Galileo’s trip to Jupiter, Curiosity’s exploration of the surface of Mars, and the 2015 flyby of Pluto by the New Horizons spacecraft are all fueled by plutonium. How nice it would be if they could pick up some more of that stuff as they pass by Pluto (just joking). The only natural supplies of plutonium-238 decayed long before Pluto and the other planets formed out of interstellar debris some 4.6 billion years ago. Exploding stars forge the isotope, but its half-life, or time required for 50% to disappear through decay, is just under 88 years, just a tiny human heart-beat in the cosmic time scale. For communication across the solar system, we depend on plutonium-238, a relatively harmless fuel largely acquired as by-product of making nuclear weapons. Original heart pace-makers used plutonium-238 for their batteries, always outlasting their recipients.

The scientific stockpile of the isotope in the US has dwindled to around 36 pounds. That is not much when you consider that the battery powering the Curiosity Rover contains some 10 pounds of plutonium. The implications for space exploration are truly worrying. No more plutonium-238 means no more exploring of 99% of the solar system.

But it doesn’t have to be that way. The required materials, reactors, and infrastructure are all in place to create plutonium-238 already exist. Unlike plutonium-239, plutonium-238 is practically impossible to use for a nuclear bomb. The U.S. government recently approved spending about $10 million a year to reconstitute production capabilities, which the nation closed down almost two decades ago. In March, the DOE produced a tiny amount of fresh plutonium inside a nuclear reactor in Tennessee. But political squabbling and false promises from Russia have stood in the way of a serious effort for producing plutonium-238.

At the end of the Cold War the US ceased production. Russia continued to produce the material by processing nuclear reactor fuel. Russia sold their first batch, weighing 36 pounds, to the U.S. in 1993 for more than $45,000 per ounce. Russia became the planet’s sole supplier. In 2009, they abruptly stopped selling, probably because they also have none left. What extraordinary bedfellows: nuclear bombs, Russia, and the peaceful exploration of outer space.