Interstellar Ambassadors
Published on Jul 12, 2006 at 12:56 pm.
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Filed under space exploration.
In the evening of March 2, 1972, an Atlas-Centaur rocket flamed into life and lifted off from Cape Canaveral (If you look at international launch data, they will say that the rocket launched on March 3, 1972, since it was after midnight Greenwich Mean Time, but it was still March 2 in Florida). Atop the rocket was a small hexagonal piece of equipment, 36cm tall, with sides 71cm long. This was the heart of the Pioneer 10 spacecraft. Affixed to the central body were antennae, radiothermal generators, magnetometers, and cameras. Within moments the craft was hurling through space at over 51,000 kph. After only about 11 hours, it was farther from Earth than our Moon. Pioneer 10 continued onward. Unlike previous spacecraft, it was not headed for anything as nearby as the Moon, nor to Venus, our nearest planetary neighbor, nor even the red planet Mars. Pioneer 10 was the first spacecraft aimed at the outer Solar System. It passed through the asteroid belt and entered the Jupiter system in November 1973. It didn’t stop there, though. The little spacecraft passed closest to Jupiter December 3, 1973. Jupiter’s gravity pulled the craft inward and faster. As it slung past Jupiter, it left faster than it had approached. This gravitational boost, called a slingshot effect, hurled the little vessel outwards towards interstellar space.
Pioneer 10 is remarkable because it was the very first interstellar spacecraft ever launched from Earth. It continued onwards, and in fact continues to this day, though it no longer operates. It is now nearly 91 astronomical units (AU) from the Sun. An astronomical unit is the average distance that Earth is from the Sun. Pioneer 10 is so far away that it takes nearly 13 hours for light to reach it from the Sun, or for radio signals to reach it from Earth. It is currently moving at a bit over 12 km/s outward in the general direction of the constellation Taurus. But it isn’t alone. One month after Pioneer 10 launched, Pioneer 11 launched to follow it. Though launched only a month behind Pioneer 10, Pioneer 11 did not arrive at Jupiter until one year later. Now, Jupiter was in position to hurl Pioneer 11 outward with a slingshot effect, as it did to Pioneer 10, but this time towards the great ringed planet Saturn. Pioneer 11 passed closest to Saturn on September 1, 1987. Saturn’s gravity hurled Pioneer 11 onward out of the Solar System. Currently, Pioneer 11 is a little less than 72 AU from the Sun, moving at about 11.5 km/s. Being closer, it takes light only a bit under 10 hours to reach Pioneer 11 from the Sun. Pioneer 11, like its sibling Pioneer 10, is no longer functioning.  From Earth, Pioneer 11 would appear to be in the general direction of the constellation Scutum.The Pioneer 10 and 11 spacecraft were the first man-made objects that were sent out of the Solar System. As a sort of public relations gimmick, NASA affixed a plaque to the side of each vessel bearing a figure of a human man and woman, a schematic of the Solar System, and a map showing the Sun’s location relative to several pulsars. The Pioneers are not heading towards any other star system, and if they were it would take tens of thousands of years for them to arrive. Then, they would be a tiny piece of metal flotsam flying through the system. It would be an incredible miracle if it were to be found, even if it passed through a star system filled with space faring aliens. And, if it were found, the plaque would likely be completely illegible, having been eroded by micrometeorites, interstellar dust collisions, etching from interstellar medium, and general chemical degradation. And, even if it were still able to be read, the map would be useless. Pulsars are rotating neutron stars. As they rotate, “hot spots†beaming radiation into space sweep across space light the beams of a lighthouse. When these beams pass by our line of sight, they give a burst of radio static — the pulses. If the beam doesn’t sweep past, you don’t see the pulsar. So, it would be pretty unlikely that anyone finding the plaque would be in a location to see the pulsars so described, unless they were fairly near the Solar System anyway.But, the Pioneers are not the only craft leaving the Solar System. In late summer, 1977, two larger, much more costly, and substantially more sophisticated spacecraft were launched on similar missions: Voyager 1 and Voyager 2. Interestingly, Voyager 2 was launched first, on August 20, 1977, and Voyager 1 followed on September 5, 1977. Though Voyager 2 launched first, Earth and Jupiter were better placed just two weeks later, so Voyager 1 actually arrived at Jupiter first, on March 3, 1979. Voyager 2 followed, arriving August 9, 1979. Jupiter hurled both craft onwards towards Saturn. Voyager 1 passed Saturn November 13, 1980, and Voyager 2 passed August 27, 1981. I was in college when these events occurred. Amazing amounts of data began flowing back. So much of what we knew about Saturn changed from when I began college until when I left. (Hmm. OK, so I never actually left. Now, I am just at the front of the classroom instead of being a student.)   Voyager 1 passed by Saturn with a trajectory that allowed the best study of the rings, but this meant that Saturn’s gravity hurled it away from the plane of the planets’ orbits, so it would never again pass by another of the Sun’s planets. It is currently the most distant man-made object, at a distance of just under 100 AU from the Sun. It is so far that it takes nearly 14 hours for signals to reach Earth from the spacecraft, which is still functioning, and heading in the general direction of the constellation Ophiuchus 17.1 km/s. However, using Saturn to slingshot itself onward, Voyager 2 continued on to Uranus, passing that planet January 30, 1986. I was a graduate student then. Uranus then propelled Voyager 2 on to Neptune, which it passed August 15, 1989. By that point, I was teaching astronomy. The first textbook that I used teaching had very little to say about Neptune, since it was written before the first spacecraft had even been there! Currently, Voyager 2 is about 80 AU from the Sun, and it takes about 11 hours for signals to go between Earth and the spacecraft. Voyager 2 is heading outwards at 15.6 km/s in the general direction of the constellation Telescopium. Though the idea of the plaque on the Pioneers seemed cute, and it was a major public relations thing for NASA, it drew serious criticism. The problem was that the artists, not knowing if aliens would recognize the concept of clothing, had drawn the people on the plaque naked, and anatomically correct. This turned out to be a serious public relations blunder for NASA. So, the Voyagers had a different public relations gimmick. A gold plated long playing record was affixed to the sides of the craft. That way, any aliens finding it would simply take the record and put it on their turntable in their flying saucer, spinning it at 33 1/3 rpm and listen to the sounds of Earth. The president will greet them, there will be representative music, the sound of rain, birds chirping, and whales singing. Yeah, right. Better hope the aliens have turntables with the right size stylus. All four of these craft are our interstellar ambassadors, leaving the Solar System. But they now have a new cousin. Earlier this year, the New Horizons spacecraft blasted off on a mission to Pluto. Eventually, it will pass Pluto and continue onward through the Kuiper Belt, becoming the fifth spacecraft to leave the Solar System. It has a way to go, though, as it is currently less than 2.75 AU from the Sun (as of this writing), but it is moving fast. It is moving at nearly 26 km/s, meaning that it is moving at the incredible clip of almost 5.5 AU per year! Gravity is pulling on it, slowing it down, but it will remain the fastest craft ever launched from Earth.-Astroprof
