Voyager 1’s great escape: The search for interstellar space

by Chris Gebhardt and Jeff Goldader

Like its sister probe Voyager 2, the Voyager 1 spacecraft has been an instrumental force in our continued push to gain a better understanding of our solar system. From its encounters with Jupiter and Saturn, to its ongoing mission to explore the outer boundaries of the solar system, Voyager 1 stands as the farthest man-made object in our solar system and will eventually gain the distinction of being the first man-made object to enter interstellar.

Voyager 1: A spacecraft design overview:

Like its sister probe, Voyager 1 was built to be a robust and sturdy spacecraft, one capable of surviving its encounters with the intense radiation and gravitational fields of the solar system’s giants: Jupiter and Saturn.

But more importantly, Voyager 1 was built to be an instrument of science, not just a survivor in the harsh region of the outer solar system.

Dominated by its large 12ft antenna reflectors, the Voyager spacecraft antennae were the largest graphite epoxy antennae made for a spacecraft at the time of their launch.

The spacecraft were also built with three deployable booms, one of which sported many of the spacecraft’s scientific instruments including the Ultraviolet Spectrometer, the Cosmic Ray experiment, the Plasma experiment, the Photopolarimeter, and the Low-Energy Charged Particle experiment.

A second, longer boom was designed to carry a series of magnetometers and two whip antennae for the plasma radio experiment.

All of the Voyager 1’s experiments, like its sister probe’s, were designed to be compact, lightweight, and draw as little power as possible. Likewise, during periods of interplanetary cruise and high-science data accumulation, accumulated data not immediately transmitted back to Earth was designed to be stored on a digital tape recorder with about 536 megabits capacity (about 67 megabytes), enough to hold roughly 100 images.

But to truly enable the scientific mission tasked by NASA, Voyager 1 and its sister probe needed a powerful source of energy, a source provided by the three Radioisotope Thermoelectric Generators (RTGs) located on the third deployable boom.
An impressive feat of engineering, each of the three cylindrical RTGs on Voyager 1 were built with six layers of four 2-inch diameter plutonium (238) oxide encapsulated in a thin shell of iridium. These shells of iridium are in turn wrapped in graphite yarn and stacked in graphite cylinders.

Each ball produced about 100 watts of thermal energy at launch, for 2400 watts of thermal power from each RTG.

However, like all spacecraft, the power source underwent changes and modifications in the development process.

In fact, during development, it was recognized that the thermocouples (designed to convert the contrast in temperature between the hot plutonium and cool space environment into electricity) were degrading far too quickly in the very warm RTG cylinders

This led to the suggestion that the thermocouple legs be coated with silicon nitride to prevent sublimation of thermocouple material that was causing electrical shorts, which reduced power output.

This careful engineering resulted in robust RTGs on Voyager 1 that have been in continuous operation for exactly 34 years today, though they are only producing about 60% of their original output as of today due to the radiation- and temperature-induced degradation of the thermocouples.

Moreover, just as important as the vehicle’s power source to the scientific mission of Voyager 1 was the ability to precisely aim the spacecraft’s instruments.

This pin-pointing was accomplished through a three-axis-stabilization using hydrazine gas thrusters and gyroscopes. Additionally, certain scientific instruments, like the science scan platform, were designed to be rotated at high enough rates to enable tracking of targets at the high angular rates of motion that would be present during the close approaches to Jupiter and Saturn.

However, none of the instruments would have been able to return any data without the complex control of six interlinked computers on Voyager 1.

To this end, the Computer Command System (CCS) was designed by engineers to control the sequences of activities to be carried out by the spacecraft; the Flight Data Subsystem (FDS) was designed to control the taking and downlink of data; and the Attitude and Articulation Control System (AACS) was designed to control the attitude of the spacecraft and orientation of the science scan platform.

But simply designing these systems to do their jobs was not enough to ensure mission success in the intense radiation fields of Jupiter and, to a lesser extent, Saturn. Specifically, the electronics had to be radiation-hardened to survive the Jupiter encounter.

With all of this forethought and complex engineering, the Voyager 1 spacecraft and its sister Voyager 2 are still functioning to this day and have been essentially remade since their launches as various systems issues forced mission controllers to improvise.

Click here for our two previous Voyager Feature Articles:

In addition, upgrades to the indispensible Deep Space Network have enabled communication with the Voyagers at greater distances than were possible when the spacecraft were first launched.

Voyager 1: Encounters with the outer giants Jupiter and Saturn:

Launched two weeks after its sister spacecraft, Voyager 1 lifted off from the Cape Canaveral Air Force Station’s SLC-41 at 08:56.00 EDT on 5 September 1977 on a Titan III-E Centaur rocket.

Three months and five days later, on December 10, 1977, Voyager 1 entered the Asteroid belt beyond the orbit of Mars.

Despite being the second Voyager probe launched, Voyager 1’s launch propelled the probe to a faster velocity than Voyager 2. As such, on December 19, 1977, Voyager 1 overtook Voyager 2 and sailed out of the asteroid belt on September 8, 1978 – just one year and three days after its launch.

With the Asteroid belt behind it, Voyager 1 began its observations of the Jovian system on January 6, 1979.

Over the next three months, Voyager 1 performed continuous observations of the largest planet in our solar system as it approached the giant.

Because of the greater photographic resolution allowed by Voyager 1’s cameras and the close approach to Jupiter, the greatest observations of the planet and its moons occurred in the 48 hour time period surrounding the time of closest approach to Jupiter.

Among the numerous and shocking discoveries made by Voyager 1 – discoveries followed up by Voyager 2 – were the discovery of rings around Jupiter, the intensity of its radiation belts, and the existence of volcanic activity on Io.

Prior to the passage of Voyager 1, and subsequent pass of Voyager 2, through the Jovian system, volcanic activity had never been observed on a planetary body other than Earth.

In all, Voyager 1 made its closest approach to Jupiter on March 5, 1979 at 12:05.26 UTC at a distance of only 348,890 km from the center of mass of Jupiter.

Additional close flybys on March 5 and 6 included Amalthea at 420,200 km, Io at 20,570 km, Europa at 733,760 km, Ganymede at 114,710 km, and Callisto at 126,400 km. After four months of continuous observations, Voyager 1 ended its tour through the Jovian system on April 13, 1979.

However, the flyby of Jupiter allowed NASA to take advantage of Jupiter’s gravity to alter Voyager 1’s course and fling the craft out toward the arguably most imagination-captivating planet in our solar system: Saturn.

In fact, thanks to the gravity assist, which not only altered Voyager 1’s course, but also increased the speed of craft, Voyager 1 reached Saturn within 1.5 years of its encounter with Jupiter.

Officially, observations of the Saturnian system began on August 22, 1980, and over the course of the next four months, Voyager 1 made several important scientific observations of the planet’s ring system, satellites, atmosphere, and radiation belts.

In all, Voyager 1 made its closest approach to Saturn on November 12, 1980 at 23:46.30 UTC at a distance of only 124,000 km from the planet’s cloud tops.

Additional close flybys in the Saturnian system included Tethys at 415,670 km, Mimas at 88,440 km, Enceladus at 202,040 km, Rhea at 73,980 km, and Hyperion at 880,440 km.

However, the most important moment for Voyager 1 came with its course-specific extreme close approach to Titan, a Saturnian moon discovered the previous year to have a thick atmosphere.

While Voyager 1’s flight controllers had originally hoped to use Saturn’s gravity to alter Voyager 1’s course for a further encounter with Pluto, the discovery of an atmosphere around Titan mandated a change to Voyager 1’s trajectory to study Titan in more detail.

At 05:41.21, Voyager 1 passed just 6,490 km above Titan – gathering invaluable data on this possible life-sustaining body.

Beyond Titan: The family portrait and extended mission:

The encounter with Titan, as worthwhile and necessary as it was,  caused a severe and predicted deflection of Voyager 1’s course that threw the spacecraft out of the elliptic plane of the solar system (toward the “north” of the elliptic).

Thus, the flyby of Titan ended Voyager 1’s grand tour of the solar system and placed the spacecraft at a mind-boggling 17.26 km/s (10.72 mi/s) speed – making it to this day the fastest man-made object in our solar system and ensuring that, barring a collision, Voyager 1 will be the first man-made object to escape the solar system and enter interstellar space. 

But the encounter with Titan was certainly not the end of the Voyager 1’s mission.

Ten years after its Saturnian encounter, Voyager 1 was rotated back toward the solar system, and on Valentine’s Day 1990, the probe was used to take the family portrait of the solar system.

The family portrait of Voyager 1 was the first ever family photo of the solar system and is the source of the famous “Pale Blue Dot” photograph of Earth.

The portrait is, in actuality, a composite image of 60 photographs obtained on February 14, 1990 from a distance of 6 billion kilometers from Earth – the farthest photograph ever taken of the solar system – at approximately 32-degrees above the elliptic plane.

Voyager 1 was specifically chosen for the family portrait because, unlike Voyager 2, it was in a position to view Jupiter without interference from the sun.

The image itself depicts six of the eight planets of the solar system along with the sun. Only Mercury and Mars are not visible due to proximity to the sun and scattered sunlight in the optics, respectively.

Pluto, which is no longer considered a planet, was not included in the photograph due to its distance from the sun and low light reflectivity.

After taking the family portrait, Voyager 1 continued its new mission, to explore the outer reaches of the solar system and send back data to Earth regarding the edge of the solar system.

On November 17, 1998, Voyager 1 officially became the farthest man-made object in the solar system when it over took Pioneer 10 at a distance of 69.4 AU (Astronomical Units – 1 AU equals the average distance between the Earth and the sun).

The Voyager Interstellar Mission: Where no probe has gone before:

As Voyager 1 continues to plow its way toward interstellar space, NASA has repurposed the craft and its sister for the Voyager Interstellar Mission (VIM) – a mission to map the outer boundary of the solar system and explore the interstellar medium outside of our solar system.

As part of the program, NASA tasked Voyager 1 with finding the termination shock (the moment when the sun’s solar wind suddenly drops from hypersonic speeds to subsonic speeds), mapping the heliosheath (a turbulent region beyond the termination shock where the solar wind is disturbed and acted upon by the force of the interstellar medium), and the heliopause (the boundary between the solar system and interstellar space).

While there is some question within the scientific community, it is generally believed that Voyager 1 crossed the termination shock on December 18, 2004 and entered the heliosheath.

The exact date that Voyager 1 crossed the termination shock will never be known because the craft’s solar wind detector failed in 1990. Thus information on the solar wind must be inferred from other functioning instruments on Voyager 1.

This passage through the termination shock marked the beginning of Voyager 1’s passage through the heliosheath – signaling the approach of the interstellar boundary.

Six years later, on December 13, 2010, scientists confirmed that Voyager had reached the area where the sun’s solar wind is turned sideways by the interstellar medium pushing against the heliosphere – the sun’s direct area of influence.

This was a major milestone in the VIM as it further signaled that interstellar space was close.

Currently, it is believed that Voyager 1 entered the heliopause – the area where the solar wind drops to ZERO in relation to the outward push of the sun influence – in June 2010 when consistent ZERO readings began.

However, in an effort to gain more information on this phenomenon, Voyager 1 conducted a test roll on March 8, 2011 to change its orientation in order to better detect the current direction of the solar wind.

For this maneuver, Voyager 1 rotated 70 degrees counterclockwise with respect to Earth. This was the first maneuver of the spacecraft since it took the family portrait in 1990.

Following the roll maneuver, Voyager 1 quickly locked on to Alpha Centauri – its new guide star. This lock on Alpha Centauri will allow Voyager 1 to maintain communication with Earth on a regular basis.

At the time of the roll, Voyager 1 was expected to reach the interstellar boundary between 2015 and 2017. However, observations from Voyager 1, ground based instruments on Earth, and the IBEX (Interstellar Boundary Explorer) satellite have revealed that the distance to the interstellar medium is a lot closer than originally believed.

With refined projections, on June 15, 2011, it was announced that Voyager 1 is now expected to completely traverse the heliopause and exit the solar “at any time”.

When that happens “in the next year or so,” it will represent at fundamental leap forward in humankind’s exploration of space as we begin to actively explore the medium between the stars and form a better understanding of how our solar system interacts with the interstellar medium.

And this exploration of interstellar space will continue from Voyager 1 until at least 2025-2030 as the spacecraft’s RTG power source continues to power its science instruments.

(Images: NASA, NASA APOD, NASA JPL, L2, Caltech).

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