Voyager 1 is the spacecraft that holds the record for distance reached from Earth, 21.337 billion kilometers, and time spent in space remaining in operation: 45 years. For some time, however, it has presented a bizarre and paradoxical problem.
The Attitude and Articulation Control Subsystem (AACS) is sending data to the ground stations that at the moment seems absolutely senseless. The AACS has been successfully controlling the orientation of spacecraft in space for 45 years; that is, it ensures that all instruments are properly aimed. In particular the antenna, which must always be arranged so that the main lobe of the electromagnetic power emission is towards the Earth in order to communicate.
In other words, if the data currently received and generated by the AACS were truly the description of the probe’s position, then we would not have been able to receive it or we would have received at least a weaker signal. Furthermore, no system seems to have entered “safe-mode”, a kind of hibernation in case of emergency.
The functioning of the AACS
Voyager’s AACS works according to the criterion of stabilization on three axes: the orientation of the probe is in fact fixed in space with respect to some external references. For Voyager the references are the Sun and a set of “fixed” stars, so defined because they are far enough away to make the relative motion with respect to the solar system small or negligible.
Knowing therefore the position of the Sun and stars, the AACS computer is able to obtain the position of the Voyager and the angle of each of its instruments, including the antenna to communicate. In the event of an error, therefore, the hydrazine thrusters (nominal 16 plus 8 backup) are commanded to fire to realign the instruments.
JPL has been committed to the development of three-axis control technology since 1959; however the real development, after the experiments of the Mariner and Viking spacecraft sets, took place with the two Voyager missions. In 1975, with NASA’s huge budget cuts, cost-effective and efficient solutions had to be found to problems that were already more complex than those of the past. In fact, compared to previous probes, Voyager has a much lower structural stiffness.
The delicate shape of the Voyager
The RTGs powered by Polonium for the production of electricity are mounted on arms outside the main structure to avoid radioactive losses near the scientific instruments. Furthermore, the magnetometer is also positioned away from the main body, to avoid interference with the magnetic fields artificially generated by the probe instruments.
Finally, the scanning and image acquisition platform was also fixed like this, to have a better field of view. During maneuvers, these extended arms flex slightly, causing a return stress that makes the AACS requirements much more stringent when maneuvering than when cruising. In particular, the AACS must simultaneously manage the maneuver and the disturbances caused by the maneuver itself.
It was for this reason that backup thrusters were added for the sole purpose of resisting maneuvers during the encounter with the planets (Jupiter, Saturn and Titan). It can really be said that the JPL worked to perfection as the backup thrusters were put back into use in 2017 due to the deterioration of the nominal ones and they worked without problems after 37 years of inactivity and exposure to the space environment.
The on-board computer
The AACS continued to function flawlessly for 45 years thanks to its 45-year-old HYPACE control computer. HYPACE stands for Hybrid Programmable Attitude Control Electronics and is made up of one of the first digital circuits ever built, associated with a classic analog circuit; It consists of the same 4K-28bit board as the Viking Orbiter combined with a transistor-transistor (TTL) logic of integrated circuits to create a processor with cycles of 28 microseconds (10 MHz), capable of thus performing a million cycles (operations / calculations) per second.
Voyager was the first probe to achieve this computational capacity and its prolonged operation is further proof of the validity of this system. Another reason for the long life of this probe is the high redundancy, both at the hardware and at the software level.
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A first reason why this is so difficult to find an answer to the recent mysterious anomalies of the AACS it is precisely our ignorance about the environment in which the Voyager is flying: every second that passes, extends our knowledge of space by 16 km. At this moment, in fact, Voyager is in interstellar space beyond the limit called “Termination shock”, exceeded in 2004, that is, where the particles of the solar wind reach a subsonic speed, and beyond the limit of the heliopause, exceeded in 2012, that is the area where the solar wind is completely stopped by the interstellar medium, balancing it in terms of pressure. The interstellar medium is the term by which we mean the rarefied material consisting of gas and dust that is found between the stars.
In fact, as well as being unknown, interstellar space is an extremely radioactive environment that clearly puts a strain on hardware (with the demolition of thermo-optical properties) and software. In particular, a very energetic radiation, when it strikes an electronic component, inside the satellite, can cause a so-called bitflip: the change, in a binary code of a 0 in 1 or vice versa, creating unpredictable problems.
A second reason that makes it difficult to find out the reason for the error is the communication delay of 20 and a half hours. That is, it takes about two days to send a message and receive a reply. Considering all this however, Suzanne Dodd, project manager for Voyager 1 and 2 at JPL, remains positive:
The probes are both 45 years old, far beyond their original plans. […]. There are huge challenges for the engineering team, but I think if there is a way to solve the problem with the AACS, they will find it.
In fact, it is absolutely remarkable to think that there are still extremely specialized engineers in the operation of software and hardware conceived almost 50 years ago and who allow these extreme missions to continue.
Voyager 1’s RTGs will provide sufficient power until 2025, when it reaches 25 billion kilometers away, hopefully sending useful data until that date when it should reach the hydrogen wall before the bow shock. Then, around 2042, when it will certainly no longer work, it will reach bow shock.
This is the area where the interstellar medium becomes subsonic, in an effect similar to the solar wind crashing into the Earth’s magnetosphere. 30,000 years from now, Voyager 1 will completely exit the Oort Cloud and enter the gravitational pull field of another star. Finally, in 38,000 years it will bring its gold record to about 1.7 light years from the star Gliese 445 in the constellation Giraffe.
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