Voyage On

Twenty-six years after leaving the Earth, the Voyager spacecraft are still traveling outward on their journey into deep space. We’ll talk about the Voyagers…

In 1977, two spacecraft named Voyager were launched from Earth. These craft revealed Jupiter, Saturn, Uranus and Neptune — and their moons — in ways no one had imagined, In 1989, the Voyager left the planets behind. Voyager 1 is now the most distant man-made object, with Voyager 2 a close second. Even now the Voyagers collecting useful information. Ed Stone is Project Scientist for Voyager.

Ed Stone: The spacecraft were designed back in the early ’70s and at that time no spacecraft had gone as far as Saturn, which was a four-year journey so that was already a major leap. Saturn’s ten times as far from the sun as the earth. But in doing that, we built spacecraft with enough redundancy, that is backup systems, so that they could keep going.

Space scientists have slowly but surely been switching to Voyager’s backup systems …

Ed Stone: Eventually we will not have enough power to keep all of the instruments going, and we will have to shut them off probably what we will do is not shut them off permanently, but cycle between the instruments, so that we will continue to sample the full range of what’s measured out there …

The data collected at the termination shock will tell scientists just how far the sun’s magnetic field and the solar wind reach.

According to Dr. Stone, Voyager 1 is now 87 times farther from the sun than the Earth is.

From JPL’s Voyager Web site: “Both Voyagers are headed towards the outer boundary of the solar system in search of the heliopause, the region where the Sun’s influence wanes and the beginning of interstellar space can be sensed. The heliopause has never been reached by any spacecraft; the Voyagers may be the first to pass through this region, which is thought to exist somewhere from 5 to 14 billion miles from the Sun. Sometime in the next 10 years, the two spacecraft should cross an area known as the termination shock. This is where the million-mile-per-hour solar winds slows to about 250,000 miles per hour Ñ the first indication that the wind is nearing the heliopause. The Voyagers should cross the heliopause 10 to 20 years after reaching the termination shock. The Voyagers have enough electrical power and thruster fuel to operate at least until 2020.”

Nearly thirty years old today, scientists estimate that the Voyagers’ power supply will only last twenty to thirty more. In the meantime, the team is taking steps to make the machinery and instruments last as long as possible, while maintaining the stream of data being reported back to earth. Here, Stone explains the functions of some of the instruments, and plans for dealing with the decreasing power supply:

Ed Stone: “Well, we have enough electrical power to last about another 20 years — that will take us to a distance of around 150 astronomical units for Voyager 1 — and there’s a reasonable chance that that will be outside the heliosphere and in interstellar space.”

Ed Stone:”We have over the years switched to our backup systems on the spacecraft. These spacecraft were designed back in the ’70s and at that time no spacecraft had gone as far as Saturn, which was a four year journeyÉ But in doing that, we built spacecraft with enough redundancy, that is backup systems, so that they could keep going. And we have slowly but surely been switching to our backup systems as we prepare for essentially the last half of the journey now.”

Ed Stone: “The instruments themselves do not have backup, except for within the instruments. For instance, we have right now active on each spacecraft a magnetometer, which measures the magnetic field being carried out by the solar winds. We have an instrument which measures the solar wind itself on Voyager 2. We have another instrument that measures energetic particles – these are particles which are moving a few percent the speed of light. And then we have another instrument which measures cosmic rays, which are particles moving with 10% the speed of light or more. And then the fifth instrument we have running is an instrument which measures waves within the solar wind, in the plasma itself, and radio waves coming from the edge of the heliosphere.”

Ed Stone: “Actually, we had 11 investigations when we began. Many of them were focused just on the planets themselves and those we have turned off. We had an imaging system; we had an infrared spectrometer to look at the infrared emissions from the planets. We had an ultraviolet spectrometer which we are still using to look outward. And we had a photo-polarimeter which calibrated the amount of reflected sunlight off of the planets. In addition to that we used the radio system itself to explore the atmospheres of the planets as we flew behind the planets.”

Ed Stone “Eventually we will not have enough power to keep all of the instruments going and we will have to shut them off or probably what we will do is cycle between the instruments, so that we will continue to sample the full range of what’s measured out there, and we will do this cycling between the instruments until we don’t have enough power to do that. On the order of 10 years from now, we may have to do some power cycling among the instruments.”

During their initial planetary mission, the Voyagers made unprecedented discoveries about the planets and their moons. Here, Stone talks about some of the highlight discoveries of those years:

Ed Stone “At Jupiter, we [not only] found that the great red spot is a huge hurricane-like storm system, but we found that it was only the largest of literally dozens of such storm systems that riddle its atmosphere. And of course, the biggest surprise was Io. Here is this moon, about the size of our moon, but it has 100 times more volcanic activity than earth, with volcanic eruptions literally hundreds of miles high and volcanic craters which are as big as the island of Hawaii. Another satellite of Jupiter nearby is Europa; it has the smoothest icy surface – the surface is water ice – in the solar system. The Galileo mission has flown close up and has shown strong evidence that the reason it’s so smooth is some ice pack on a liquid water ocean, which of course is very exciting from the standpoint of the possibility of life and the origin of life. Then Saturn of course – there we found that the rings are much more complex than we had known from earth. And there are many moons, some of which are shepherding narrow rings around Saturn. We flew close to Titan and found it has a very deep thick atmosphere, mainly nitrogen like that here on earth, but no oxygen. Instead of that, there is natural gas or methane, which is being converted into complex organic molecules, which are raining down on the surface of Titan.”

Notes on the Termination Shock

The solar winds from our sun blow outward until the pressure of the surrounding space slows them and turns them back on themselves. This creates a huge bubble of ionized gas around the sun and planets called the heliosphere. The edge of the heliosphere is the edge of our solar system.

Scientists predict Voyager 1 and 2 will leave the solar system in around 10 to 20 years. That’s when they’ll cross the point where the solar wind streaming out from our sun is stopped by interstellar winds. But before that, the solar winds are slowed down from supersonic to subsonic speeds. That point is called the Termination Shock.

Ed Stone: “You see, because the solar wind is supersonic, it has to go through a sonic shock and slow down abruptly so it can turn around before it hits the interstellar wind. We are getting close to that shock.”

Researchers predict that the Voyagers will reach the Termination Shock in the next one to two years. The encounter would provide the first real data on the size of the heliosphere and how far the craft have left to travel before leaving it. But there’s a wild card: impending changes in the solar winds.

Ed Stone: “Over the solar cycle the pressure of the wind changes. It’s at a minimum right now. In about 2 years the winds will settle down and the heliosphere will expand. And in a certain sense we’re in a bit of a race here to see if we can get to the shock before the shock starts moving away from us as the heliosphere expands again.”

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