Voyager 2 is not lost in interstellar space, NASA announced Tuesday. The U.S. space agency’s Jet Propulsion Laboratory (JPL) confirmed on Twitter that it has detected faint radio signals from the venerable probe — “something like listening to the heartbeat of the spacecraft” — and that this confirms it remains in good condition and is emitting information from beyond the Solar System. The problem is that this information is no longer reaching Earth correctly, due to a human error that occurred two weeks ago.
On July 21, a series of routine commands sent to Voyager 2 inadvertently caused a two-degree deviation in its main antenna. As the probe is nearly 20 billion kilometers (12.4 billion miles) from Earth, a small error is enough to cause the signal to become lost in the void: communication between the probe and the antennas of the Deep Space Network (DSN), which receive its signals at various points on the planet, has therefore been interrupted. “Data being sent by the spacecraft is no longer reaching the DSN, and the spacecraft is not receiving commands from ground controllers,” NASA explained in a statement last Friday.
Eventually, late Tuesday, the DSN antenna in Canberra, Australia, picked up a faint signal from Voyager 2′s beacon, confirming that the disconnection was not due to a malfunction and that the onboard equipment is still transmitting. Although the situation remains unchanged, as the detected waves are so weak that they do not allow for the transmission of information, this news provides hope that contact will be recovered in the coming days.
The Canberra antenna has continued to transmit commands in the general direction of Voyager 2, which was launched in 1977. After all, the probe’s route has not changed: one of the fastest human spacecraft ever built, it continues to move away from Earth at a speed of almost 1.5 million kilometers per day. The idea is that if the spacecraft hears any of these signals, it could perhaps regain its orientation. But it is still too early to tell whether that emergency maneuver will work. Although they are very short command sequences, traveling at the speed of light they take 18 hours to reach Voyager 2; and if the reorientation is successful, it would take another 18 hours for the signals from the spacecraft to travel back to Earth.
If contact is not recovered in the next few days, it will be necessary to wait until October 15. An automatic maneuver, which is performed several times a year, was already scheduled for that date, so that the probe itself can autonomously re-establish its position facing the Sun.
Into the interstellar void
At the distance from Earth where Voyager 2 is there is not much to see. Only darkness, barely mitigated by the twinkling of thousands of stars and the diffuse luminosity of the Milky Way dividing the celestial vault in two. The Sun is just another star, albeit a little brighter. The Earth, the Moon and the inner planets are difficult to distinguish, hidden by their own radiance. The only tangible things are waves of plasma, subatomic particles; some arrive from the distant Sun, guided by invisible magnetic lines of force; others, from the interstellar void.
Voyager 2 was launched on August 20, 1977, with the objective of investigating Jupiter and Saturn on a mission that was originally scheduled to last about 12 years. It has already quadrupled its lifespan and its discoveries include not only details of its two original targets, but also of the other two giant planets, Uranus and Neptune, and their respective families of satellites: it is the only spacecraft to have visited them. Curiously, Voyager 2 was launched a few weeks before its twin, Voyager 1. This was done because the latter would follow a faster trajectory that would allow it to reach Jupiter sooner. Saturn would be its next port of call, with special interest in studying its satellite, Titan, which made it necessary to adopt a course that made it impossible to reach Uranus, whose exploration and that of Neptune would be reserved for Voyager 2.
Since then, the three antennas of the DSN, located in Australia, the U.S. and Spain, have been following the journeys of the two Voyager probes. They no longer do so continuously, but only from time to time. Sometimes orders are sent to adjust orientation, check low hydrazine reserves for the position control motors, or to test techniques to reduce power consumption.
On November 5, 2018, after 41 years of travel, Voyager 2 officially left the Solar System. On that day, when it was approximately 18 billion kilometers (11.2 billion miles) from Earth, its sensors registered a sort of jump. The probe had gone from being enveloped by the hotter, fainter plasma generated by the solar wind to a cooler, denser plasma that bathes the interstellar space beyond the borders of our immediate celestial neighborhood.
The technicians at the JPL know that the Voyager spacecraft have little life left. In fact, they keep tracking them almost as a challenge, to see how long they are able to transmit and how long the antennas can hear them. The two probes are powered by small plutonium nuclear reactors. At those distances, solar panels would have been useless. But 46 years of operation have used up almost all the nuclear fuel. That is why most of the onboard instruments were switched off at the end of their original missions, as a measure to reduce consumption and maximize the life of the generators. Today, only half a dozen devices continue to send data, all of them referring to the abundance of plasma and cosmic rays in the interstellar medium.
With these energy-saving measures, it is possible that the two probes will remain active until 2030. After that, the isotope generators will cool down and will no longer produce enough power to activate the radio. Voyager will go silent and continue its journey to infinity. Their long-term destiny is to orbit the galaxy among the other stars. When our Sun is extinguished, those small pieces of metal may be the only reminder of our existence.
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