Thirty-four years after launch, Voyager 2 continues to explore

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On August 20, 1977, the intrepid spacecraft Voyager 2 launched from the Cape Canaveral Air Force Station, FL on what was supposed to be only a four year mission to Jupiter and Saturn. But exactly 34 years later, Voyager 2 has cemented itself into the upper echelons of unmanned space exploration, continuing to beam back data as it searches for the barrier between our solar system and interstellar space.

Launch and the cruise to Jupiter:

Despite being designated as the second probe in the Voyager family, Voyager 2 received the honor of being the first Voyager probe launched into space on what was – at the time – supposed to be an extremely truncated “grand tour” mission (only visiting   Jupiter and Saturn and none of the remaining outer planets).

Fitted atop a Titan III-E/Centaur rocket, Voyager 2 was launched on August 20, 1977 at 10:29.00 EDT from Space Launch Complex 41 at Cape Canaveral after a difficult birthing period that on more than one occasion threatened to completely derail the Voyager Program.

However, all was not smooth sailing for Voyager 2. Shortly after the spacecraft’s escape from Earth orbit, Voyager 2’s science instrument boom did not fully deploy, though it was very close to the fully deployed position and nonetheless functioned properly.

After recovering from this early-mission issue, Voyager 2 continued its outward trek toward Jupiter and entered the asteroid belt on December 10, 1977. Nine days later, Voyager 2 was overtaken by sister probe Voyager 1 which, despite being launched two weeks after Voyager 2, was launched at a higher velocity.

In April 1978, due to a ground error resulting in the failure to send expected commands to Voyager 2, the spacecraft’s fault-protection software assumed the primary radio receiver had failed and switched to the backup receiver.

When this happened, the backup receiver suffered a short in a key component: the tracking loop capacitor. This left the backup receiver unable to adjust for frequency drifts of the signal from Earth.

When controllers commanded Voyager 2 to resume using the primary receiver, the primary receiver failed completely, resulting in a week of waiting for the fault-protection software to again command use of the degraded backup receiver.

Controllers ultimately learned to compensate for the limitations of the backup receiver.

After 10 months in the asteroid belt, Voyager 2 exited the belt on October 21, 1978.

A complete pre-launch history and development of the Voyager Program can be found here:
http://www.nasaspaceflight.com/2011/08/voyagers-unprecedented-on-going-mission-exploration/

An encounter with Jupiter:

On April 25, 1979, less than two years after its launch, Voyager 2 began observations of the Jovian system.

During the four months of scientific investigations, Voyager 2 made several significant discoveries, not the least of which was the discovery of three previously-unknown moons.

Two of these moons, Adrastea and Metis were discovered to have orbits just outside the rings of Jupiter, while the third moon, Thebe, was identified as having an orbit between Amalthea and Io.

Furthermore, Voyager 2 collected data on Europa’s intersecting linear features. During the Voyager 1 flyby of Europa early in 1979, scientists discovered the linear markings and hypothesized that they were caused by tectonic forces or “crustal rifting.”

A plan was devised to collect higher resolution images of the features with Voyager 2. When these images were transmitted back to Earth, scientists were shocked to discover that the features lacked almost any topographic relief – furthering the mystery surrounding Europa.

In all, Voyager 2’s data – correlated with data from Voyager 1 – allowed scientists to understand that Europa is in fact internally active because of tidal heating in its gravitational tug of war with Jupiter.

Based on Voyager 2 data, Europa was also believed to have a thin layer of water ice floating on a subterranean ocean about 30 miles below the moon’s surface.

On July 9, 1979, Voyager 2 made its closest approach to Jupiter at 18:29.00 EDT, coming within 570,000 km of the largest planet in our solar system.

During the flyby, additional rings around Jupiter were discovered by Voyager 2, and the tell-tale Great Red Spot was shown to be an anticyclone storm complex.

Voyager 2 also discovered several smaller storms in Jupiter’s upper atmosphere as well as eddies in the planet’s bands.

However, the most-significant discovery came not at Jupiter but at its well-known moon Io.

During its pass through the Jovian system, Voyager 2 helped with the discovery of active volcanism on Io. The discovery marked the first time that volcanism was observed on a celestial body other than the Earth and the first time the evidence was gathered about recent (within months) volcanic activity on a planetary body other than Earth.

Moreover, Voyager 2’s encounter with Jupiter was precisely planned to use Jupiter for a gravity/velocity assist maneuver to fling Voyager 2 onto the proper course heading for an encounter with Saturn.

The gravity assist boosted Voyager 2’s outward velocity to “solar system escape velocity” levels, forever placing Voyager 2 on a course out of the solar system.

Voyager 2 made its last observation of Jupiter on August 5, 1979.

A short hop to Saturn:

Two years after completing operations at Jupiter, Voyager 2 arrived at the Saturnian system, with continuous observations of the system beginning on June 5, 1981.

During its pass through the Saturnian system, Voyager 2 conducted extensive evaluations of Saturn’s primary moons Titan and Enceladus, as well as the moons Iapetus, Hyperion, Helene, Dione, Calypso, Mimas, Pandora, Atlas, Janus, Epimetheus, Telesto, Tethys, Rhea, and Pheobe.

The closest approach to any object in the Saturnian system by Voyager 2 was its flyby of the moon Enceladus at just 87,010 km.

During the probe’s flyby of Saturn, Voyager 2 was guided on a specific course to allow it to probe Saturn’s upper atmosphere via radio link.

These observations allowed scientists to determine that Saturn’s upper atmospheric temperature was -203 degrees C at a mean pressure level of seven kilopascals and -130 degrees C at a mean pressure level of 120 kilopascals.

Saturn’s northern polar region was also observed to have a season temperature variation on the order of 263 degrees C cooler than the other temperature regions sampled.

Voyager 2 made its closest approach to Saturn on August 25, 1981 (exactly 4 years five days after launch) at 23:24.05 EDT (0324.05 UTC 26 August 1981) at a distance of 161,000 km.

This flyby was specifically designed to do something no other space probe had attempted before, use Saturn for a gravity assist maneuver to alter Voyager 2’s course for a close encounter with yet another planet: Uranus.

While initially not at all a part of the Voyager missions up the crafts’ launches, NASA decided to take advantage of the health of the spacecraft and the rare planetary alignment of the gas and ice giant planets to send Voyager 2 off to visit Uranus.

This was made possible solely because of the earlier successful close flyby of Saturn’s moon Titan by the Voyager 1 probe. Titan was a crucial mission objective of the Voyager Program, and the success of the Voyager 1 flyby allowed NASA to alter Voyager 2’s trajectory to ensure the proper encounter with Saturn’s gravitational field for a course alteration to Uranus.

The final observation of the Saturnian system was made on September 25, 1981.  

However, shortly after concluding operations at Saturn, Voyager 2’s science scan platform seized, and its gear and shaft were damaged because of the apparent migration of lubricant away from the gear-shaft interface due to overuse in a short amount of time.

Between Voyager 2’s encounter with Saturn in 1981 and Uranus in 1986, controllers developed a technique called “image motion compensation.” This involved moving the scan platform at slow rates, which was found to be possible despite the damage, in conjunction with thruster firings to rotate the entire spacecraft at a rate that would allow a target to be tracked long enough for imaging.

 This technique was designed to reduce smearing of the lubricant during the long photographic and sensor exposures required at Uranus, where the sunlight would be nearly 400 times dimmer than at Earth.

The sharp images that would later be obtained of Uranus’ moon Miranda and Neptune’s moon Triton proved the success of the Voyager team’s efforts.

Encounter with Uranus’ chaotic system:

Truly going for the first of two times where no man-made object had (or today, has) gone before, Voyager 2 began its encounter with the Uranian system on November 4, 1985.

The first time any probe had visited the Uranus and its moon, and the event upon which we base vast amount of our knowledge of Uranus on, Voyager 2 returned a wealth of data to Earth about this true oddity of the solar system family.

With an axial tilt of 97.77 degrees, Uranus has, by far, the largest axial tilt of any planet in the solar system; additionally, Uranus’ odd axial tilt was measured by Voyager 2 to have an odd effect on the planet’s magnetic field – a field that was discovered by Voyager 2.

Measurements from Voyager 2 found that Uranus’ magnetic field was comparable in intensity to that of Earth’s. Moreover, there were found to be large variations in the field given its significant offset of 60 degrees from the planet’s axis of rotation.

Basically, Uranus’ magnetic field does not emanate from the planet’s poles like Earth’s magnetic field does; it emanates from a location 60 degrees from the poles and 30 degrees from the planet’s equator.

This offset, coupled with Uranus’ axial tilt, creates a magnetotail trailing Uranus that is twisted into a long corkscrew by the planet’s rotation around its axis.

This has led to the hypothesis that Uranus’ magnetic field is generated at some depth within Uranus where water molecules, under specific high pressure, become electrically conducting.

Moreover, a surprising find was that, despite the axial tilt throwing one pole into total sunlight for half a Uranian year and the other pole into complete darkness for half an orbit, the temperature’s throughout the planet’s cloud tops were relatively equal.

Voyager 2 was also able to determine that Uranus’ “day” was comparable to that of Saturn and Jupiter at 17 hours 14minutes.

Voyager 2 also took specific measurements of Uranus’ composition, confirming that it differed greatly in chemical composition from Jupiter and Saturn.

This confirmation of Uranus’ composition (the gases hydrogen, helium, methane, hydrogen deuteride, and the ices ammonia, water, ammonium hydrosulfide, and methane) has led a growing number of scientists to refer to Uranus as the first of two Ice Giants (Neptune being the second).

Voyager 2 further measured Uranus’ radiation belts, which were found to have a similar intensity as those of Saturn.

Voyager 2 also discovered 10 new moons of Uranus, and Miranda was revealed to be one of the oddest moons ever identified – with huge oval structures surrounded by faults of 12 miles deep and a combination of older and younger surfaces.

These observations have led scientists to hypothesize the Miranda was actually torn apart from either gravitational forces or a planetary collision at one point in time and then reaggregated into its present form.

A further detailed studied of remaining moons showed a wide difference in apparent surface feature age.

Likewise, Uranus’ rings were found to be extremely young and unlike those of Jupiter and Saturn.  A study of Uranus’ rings by Voyager 2 revealed that the rings are most likely the remnants of moons that collided with one another in the chaotic orbital environment around Uranus and were not formed at the same time as Uranus.

During its observations of Uranus, Voyager 2 also discovered two previous-unknown rings.

Voyager 2’s closest encounter with Uranus occurred on January 24, 1986 at 12:59.47 EST at a distance of 81,500 km.

The closest approach of Voyager 2 to any object in the Uranian system occurred on 24 January 1986 at 10:50.00 EST when the probe passed within 29,000 km of Miranda.

With the amazing continued good health of Voyager 2, NASA decided that it could not pass up the rare planetary alignment to send Voyager 2 to yet another planet: Neptune.

On to Neptune – disproving “Planet X”:

Approaching the Neptunian system, Voyager 2 began observations of the farthest planet of our solar system on June 5, 1989 – exactly eight years after it began observations of the Saturnian system.

Voyager 2 made its closest approach to Neptune – and its closest approach to any object planet other than Earth – on August 24, 1989 at 23:56.39 EDT.

During its flyby of Neptune, Voyager 2 discovered an anticyclone, dubbed the Great Dark Spot (which has since disappeared). This has led to the theory that the Great Dark Spot was actually a hole in the cloud deck of Neptune and not a storm like Jupiter’s Great Red Spot.

Voyager 2 also confirmed that Neptune is comprised primarily of hydrogen, helium, and methane – like Uranus – and is thus sometimes (and increasingly) classified as an Ice Giant instead of a gas giant like Jupiter and Saturn.

But most importantly, Voyager 2’s pass through the Neptunian system allowed for the first accurate measurement of the planet’s mass, which was found to be 0.5 percent less than previously calculated.

This error in mass, equivalent to the mass of Mars, was enough to disprove the theory of another planet somewhere in the vicinity acting upon Neptune and Uranus’ orbits.

Previously, the inaccurate measurements of Neptune’s mass had resulted in skewed orbital predictions for Uranus and Neptune. Thus, scientists – assuming the mass measurements were correct – had concluded that another planet, Planet X, had to exist as its gravity would explain the oddities of Uranus’ and Neptune’s observed v. predicted orbits.

The accidental discovery of Pluto during the search for Planet X ultimately proved more frustrating than helpful as Pluto was found to have a significantly low mass that did not account for the observed orbital perturbations of Uranus and Neptune.

However, using Voyager 2’s information, the mass of Neptune was recalculated downward and new orbital predictions were run. The lower mass accounted for the skewed orbital predictions of Uranus and Neptune, and the Planet X theory was largely discarded.

But Voyager 2 still had one more task to perform: a flyby of Neptune’s moon Triton.

This flyby involved altering Voyager 2’s course to swing it over the northern pole of Neptune and use Neptune’s gravity to fling Voyager 2 down toward Triton.

The flyby was extremely successful and sent Voyager 2 sailing away from any other planetary body in the solar system.

Years later, with the reclassification of Pluto in 2006 out of the planet category, Neptune officially became the last planet in our solar system. Thus, in a de facto state, Voyager 2 became the only probe to complete the “Grand Tour” proposal to visit all of the outer planets in one mission.

Further, the reclassification of Pluto meant that all of the planets in the solar system had now been visited and studied by at least one probe – with Voyager 2 completing the overall one visit per planet study with the flyby of Neptune in 1989.

With the conclusion of Neptunian observations on October 2, 1989, Voyager 2’s primary mission ended officially on December 31, 1989.

From Neptune to the outer edges of the solar system:

Following its encounter with Neptune, Voyager 2 was flung out of the ecliptic plane of the solar system at a -55 degree declination. Thus, Voyager 2 was placed on a permanent course out of the solar system in the “southern” portion of the heliosphere – the sphere of direct influence of the sun’s solar wind over the interstellar medium (the hydrogen and helium that permeates the galaxy).

Continuing its outward trek from the solar system, NASA left Voyager 2 operational to take advantage of a robust spacecraft as it plowed toward the edge of the solar system.

Specifically, for both Voyager 2 and its sister spacecraft Voyager 1, NASA developed a new mission of the Voyagers: find and explore the barrier between the solar system and interstellar space.

To this end, Voyager 2 reached the first part of this barrier on August 30, 2007 when it crossed the Termination Shock – a standing shock wave and the immediate area where the solar wind dramatically and suddenly drops from supersonic to subsonic speed in relation to the sun.

Crossing the Termination Shock at a distance of 76 AU (Astronomical Unit – 1 AU equals the average distance between the Earth and the sun) revealed that the heliosphere is not a perfect bubble and behaves quite different in the “southern” region than it does in the “northern” region where Voyager 1 is escaping the solar system.

Passing through the Termination Shock brought Voyager 2 into the heliosheath – the area within the heliosphere where the solar wind is slowed and made turbulent by the interstellar medium.

Evidence from Voyager 2 and Voyager 1 has revealed that the heliosheath is filled with 100 million-mile-wide bubbles created by the impact of the solar wind and the interstellar medium.

Voyager 2 is currently in the heliosheath travelling outward at a velocity of 15.464km/s and is 14,285,000,000 km away from Earth (of 95.49 AU from Earth).

It is the second farthest man-made object from Earth, behind only its sister Voyager 1. It is twice as far from the sun as Pluto but not quite beyond the orbital distance of the dwarf planet Eris.

Voyager 2 is currently expected to reach the heliopause – the area in the heliosphere where the solar wind is stopped completely by the force of the interstellar medium in 2015.

However, recent observations of the interstellar boundary by the IBEX spacecraft have resulted in a recalculation downward of the length of the heliosheath and distance to the heliopause.

Thus, Voyager 2 could reach the heliopause before 2015.

Beyond the Heliopause – the Voyager Interstellar Mission:

Once Voyager 2 passes through the heliopause, the only remaining event will be to actually cross the barrier and officially exit the solar system – a continuation of Voyager 2’s current Voyager Interstellar Mission (VIM).

Provided there are no major systems malfunctions and no collisions with other space bodies, Voyager 2 is expected to continue transmitting data back to Earth until at least 2025 – over 48 years after its launch and well after it exits the solar system and enters interstellar space.

To this very day, all Voyager 2 data beamed back to Earth is done in real-time at a rate of 160 bits per second due to efforts to conserve power on the spacecraft.

To manage the dwindling power reserves and communications so as to maintain science data return as long as possible, with as limited support staff as possible, the Voyager team has developed a “thirty year plan” to operate the spacecraft and gradually power down non-essential systems.

The control strategy for the VIM is to store a sequence of repeating commands managing the health and safety of the spacecraft, as well as basic data acquisition and transmission.

The “baseline sequence” is stored in the CCS memory and manages the science instruments and communications functions using 11 pre-loaded “block routines” that accomplish tasks such as the recording and playing back of data and gyro and antenna operations.

Non-repetitive science or engineering events are now managed using “overlay sequences,” which cover spans of several months as well as “mini-sequences” uplinked for specific events.

A “Backup Mission Load,” a sequence of commands to allow the spacecraft to operate autonomously should the ability to receive data from Earth be lost, has also been sent to the spacecraft. The spacecraft now have information onboard to automatically keep the antennae pointed at Earth through the 2017-2020 timeframe.

Most importantly, though, a key part of the thirty year plan is to manage the gradual shutdown of systems that are the least essential to the continued gathering of particle and field data called for by the VIM.

The exact order and times of the shutdowns of the remaining instruments have not yet been determined, but a rough schedule is has been developed.

If commanding from Earth is still possible beyond gyro shutdown, the remaining science instruments will be powered down in a to-be-determined order beginning around 2020.

At that time, communications will still be possible, and sufficient hydrazine is available for almost 50 years at the current rate of consumption.

Should command ability be lost, the Backup Mission Load will activate and take the following steps to reduce power consumption and configure the spacecraft for the VIM: the scan platforms and all the instruments on them will be powered down; initialize the Low Energy Charged Particle instrument for data gathering; initialize the Cosmic Ray experiment for data gathering;

Furthermore, the Backup Mission Load will configure the communications transmitter system power, configure the thrusters to a redundant configuration, and terminate gyroscope and tape recorder use on predetermined dates.

(Images via NASA JPL, NASA, NASA APOD, L2 Historical)

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