by Prasad Ganti

For forty years, they have been traveling towards the outer solar system and beyond. They are the twin space probes Voyager 1 and Voyager 2. Fueled by nuclear energy in the form of plutonium based thermal generators, Voyager 1 is the only man made object that cleared the boundary of our solar system and is in the interstellar region, the space between stars. Voyager 2 is fast approaching this milestone. They have gone further than any other man made object. Their engines and instruments are running for longer time than any other man made machine.

These space probes embarked on what is known as a Grand Tour. A tour of all the outer gaseous giants, namely Jupiter, Saturn, Uranus and Neptune. It is known as the Grand Tour because all these planets were aligned, an event which happens once in 175 years. It happened in the 1970s and was predicted by Gary Flandro of Jet Propulsion Laboratory earlier in 1964. When this alignment happens, a single space probe can visit each of these planets using the gravity assist of the earlier planet. When it reaches Jupiter, it uses the gravity assist of Jupiter to fling itself towards Saturn and so on. This way, all the four outer planets can be reached faster and with minimum amount of fuel.

The variable distances between the planets and the gravity assists are unique to space travel. We do not find such a concept on Earth. The distance between London and New York is always the same, as far as the human horizon of time can see. The distance between the planets varies because each is moving around the Sun at different speeds, and not in lockstep with each other. The Earth from where the space probes are launched, is itself a moving platform. So are the destination planets. The space probe gains the Earth’s momentum at the time of launch. When it reaches a destination planet, it can be a fly by or get captured by the planet’s gravity. If fly by, it can get a gravity assist like a slingshot, bends its path slightly, and hurl itself at a higher speed towards the next destination. Thus the planet’s momentum gets added to that of the space probe’s. It is like getting a push for free. This concept was amply demonstrated by the earlier probe Mariner 10 which visited Venus and Mercury in 1973-74.

NASA started planning for the twin space probes Voyager 1 and 2 to go on the Grand Tour. The initial plan was to visit Jupiter and Saturn only and later decide on Uranus and Neptune. Both the Voyagers were launched in 1977 within two weeks of each other. They reached Jupiter in 1979. Voyager 1 reached Saturn in 1980 and then went to its largest moon Titan. After that, it moved away from the planetary plane towards the interstellar space. It is important to note that the planets all move around the Sun in almost a single plane, like on the surface of a table. Voyager 1 just moved away from the surface of the table upwards. It could not visit any other planets along this path.

Voyager 2 reached Saturn in 1981. After evaluation it was found that Voyager 2 can proceed to Uranus which it reached in 1986. Voyager 2 next proceeded to Neptune in 1989 after which it left the planetary plane towards the interstellar space. While Voyager 1 went above the surface of the table, Voyager 2 went below the surface of the table. Voyager 1 entered heliopause in 2012. It has since entered the interstellar space, beyond the confines of our solar system.

We learnt a lot about the outer planets and their moons due to the data gathered by the instruments aboard these space probes. Several cameras and spectrographs captured the details and sent them back to Earth. NASA’s deep space network receives such signals from distant space probes. The radio antennas with dishes 200 feet across in California, Australia and Spain catch these feeble signals. Voyager 1 is about 1.3 billion miles away and the radio signal takes about 19 hours to reach us.

The nuclear fuel in the Voyagers is expected to last till 2025. What will happen after it runs out of fuel ? They may loiter around in the interstellar space for some time until captured by another star. Certainly they will not be able to send us any signals back. The golden record, which contains a sample of human music and pictures may be found by some other civilization. The probes will not be able to leave our galaxy as they would need to attain speeds of about a million miles per hour to do so.

The less sophisticated but reliable technology is still working like a charm. Compared to the more complex cell phones and computers we have to throw away every few years.

Below is a picture of the trajectories of the two space probes, courtesy NASA.