Thursday, March 13, 2014

Cassini (and other space adventures)

[Approximate Reading Time: 6 minutes]
[Mood: Amused] 

The Cassini-Huygens spacecraft is a marvel of technology. According to NASA, "Cassini-Huygens is one of the most ambitious missions ever launched into space". On NASA's official Cassini web page, we find, "Two elements comprise the spacecraft: The Cassini orbiter and the Huygens probe. In 2004, Cassini-Huygens reached Saturn and its moons. There the spacecraft began orbiting the system in July 2004, beaming home valuable data that will help us understand the vast Saturnian region. Huygens entered the murky atmosphere of Titan, Saturn's biggest moon, and descended via parachute onto its surface." While all that sounds quite clever (well done, humanity!), the saga of how Cassini got to Saturn in the first place speaks even more to humanity's cleverness.

Scientists used a cunning technique to propel Cassini toward Saturn where it now hangs out. Called a "gravity assist" in formal terms, NASA describes the technique: "The rocket that launched Cassini in 1997 was the most powerful available to NASA, but it still wasn't powerful enough to send the nearly 6,000-kilogram (13,200-pound) spacecraft on a direct course to Saturn. Instead, mission designers planned multiple flybys of Venus, Earth and Jupiter, using each planet's gravity to boost Cassini's sun-relative speed and send the spacecraft out to Saturn."

It's humanity's ingenuity on full display. Cassini first goes to Venus, an inner planet closer to Sun, circles it twice, mooching off a small fraction of Venus' orbital kinetic energy in the process, comes back toward Earth, mooches off a fraction of Earth's orbital energy, flies by Jupiter stealing a bit more energy there before finally making it to Saturn. Once there, it uses a similar technique to steal off a little energy from Saturn's moons as it visits Saturn and its moons.

Voyager 1, the farthest human-made object presently, has left the solar system with energy gained from similar gravity assists. According to Wikipedia, "As of Sept 3, 2013, Voyager 1 is over 125.3 AU (18.7 billion km) from the Sun, and is in interstellar space. It gained the energy to escape the Sun's gravity completely by performing slingshot maneuvers around Jupiter and Saturn." The probe is expected to continue its mission until 2025, when it will be no longer supplied with enough power from its generators to operate any of its instruments. At that point, Voyager 1 will become the first object that humanity will have chucked out into interstellar space, a piece of metal and plastic bearing the unique signature of man-made trash found everywhere on Earth.... that is, until Voyager 2 joins it a few years later.

Cassini and Voyager 1 are not the only probes that use gravity assists. And gravity assists are not the only tricks up their sleeves. They're both powered by radioisotope thermoelectric generators (RTGs). These are the electric generators on Voyager 1 that are expected to stop working in 2025. Radioactive materials such as plutonium, curium and strontium are the most often used isotopes in these generators.

RTGs, according to Wikipedia, have been used as power sources in satellites, space probes and such unmanned remote facilities as a series of lighthouses that the former Soviet Union erected inside the Arctic Circle. Safely using RTGs requires containing the radioisotopes long after the productive life of the unit. While the US has safely launched 28 space missions that use RTGs since 1961, "RTGs pose a risk of radioactive contamination: if the container holding the fuel leaks, the radioactive material may contaminate the environment. For spacecraft, the main concern is that if an accident were to occur during launch or a subsequent passage of a spacecraft close to Earth (for example, during one of those gravity assist flybys), harmful material could be released into the atmosphere; therefore their use in spacecraft and elsewhere has attracted controversy." There have been several known accidents involving RTG-powered spacecraft: "In 1969 the launch of the first Lunokhod lunar rover mission failed, spreading polonium 210 over a large area of Russia. Other failed space launches ended with the radioactive material burning up in the atmosphere or falling into the ocean. There were also five failures involving Soviet or Russian spacecraft which were carrying nuclear reactors rather than RTGs between 1973 and 1993. In 1978, Cosmos 954 accidentally reentered Earth's atmosphere, strewing radioactive uranium 235 over 124,000 kilometers in Northern Canada, and exposing several people to harmful radiation. This was the only time the 1972 UN Liability Convention has been invoked." Of course, human beings have been testing nuclear weapons by detonating them on land and in sea for a long time before we invented space technology, so these accidents should not be a surprise. But there's something to be said about a species that blasts nuclear reactors into space.

Notice how the section on Cassini's power system is entitled "Safety", in an Orwellian way!

What do we make of a species, some of the members of which seem to be having so much fun with flybys and plutonium? Imagine a human-like species somewhere far away in the distant galaxies. You notice that their numbers have grown tremendously in the last hundred or so years (from 1.6 billion in 1900 to over 7.2 billion today). During this time, they have also started playing with dangerous substances some of which they launch into space, then bring back perilously close to their planet to do a "flyby". Some of us might say it's perhaps this species' destiny to travel in space. Hence the risky experiments. Some might wonder if that species is behaving in a childish manner. Imagine, further, watching these aliens put some plutonium on a spacecraft and launch it out of their solar system into interstellar space not really caring where it might travel after its power source stops working... essentially hurling a radioactive piece of junk into the universe at a speed of 35,790 mph! Of course, they don't completely rule out the possibility of the probe being found by other intelligent life forms out there. And they kindly include several greetings on a gold-plated copper disc...

Oops... sorry we just poisoned you with radiation, but here are some bird songs from planet Earth! Enjoy!

Children are remarkable for their intelligence and ardor, for their curiosity, their intolerance of shams, the clarity and ruthlessness of their vision. - Aldous Huxley

Hesitation increases in relation to risk in equal proportion to age. - Ernest Hemingway

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