Space Sunday: Voyager at 45

August and September 2022 mark the 45th anniversaries of the launches of Voyager 1 and Voyager 2, NASA’s twin interplanetary – and now interstellar – explorers.

Designed to take advantages of a planetary alignment which occurs once every 176 years, allowing the use the gravities of one of the outer planets to “slingshot” a vehicle on to the next, the two Voyager mission vehicles remain in operation today, and continue to stand at the forefront of our understanding of the local space surrounding our solar system.

Voyager 1 continues to set records as the furthest man-made object from Earth – it is now over 23.3 billion kilometres away – whilst Voyager 2 remains famous for giving us our first detailed views of Uranus and Neptune during its 20-year voyage through the outer solar system.

Products of the 1970s, the Voyager craft stand as museum pieces by today’s standards. Each has around 23 million times less memory than a modern cellphone, their communications systems can only transmit and receive data some 38,000 times slower than a modern cellular network, and they record the data they gather on an 8-track tape recorder prior to transmitting it back to Earth. Nevertheless, the amount of knowledge they have gathered and returned to us about the outer reaches of the solar system, the heliosphere (the bubble of space around the Sun in which the solar system resides), the heliopause (the boundary between that Sun-dominated “bubble” and the galaxy at large) and the realm of interstellar space beyond that bubble.

Operated by NASA’s Jet Propulsion Laboratory (JPL), the Voyager craft were launched in reverse order, with Voyager 2 lifting-off on August 20th, 1977 and Voyager 1 following on September 5th, 1977. The reason for this ordering was simple: during the development of the mission, Saturn’s moon Titan, known to have an atmosphere, was identified as a primary target for fly-by investigation, and so was assigned to Voyager 1.

However, in order to reach the moon, the vehicle would have to follow a course that would carry it over Saturn’s northern reaches, and throw it “down” and out of the plane of the ecliptic and away from any chance of reaching the outer planets. Instead, Voyager 2 was tasked with completing the “grand tour” of the major planets – Jupiter, Saturn, Uranus and Neptune, and in order to achieve this, it would have to be launched first.

Even so, thanks to the nature of orbital mechanics requiring Voyager 2 to be thrown out on a more circular, “indirect” path towards Jupiter whilst Voyager 1 could be launched more directly towards Jupiter meant it could reach the gas giant first, arriving in January 1979, having “overtaken” Voyager 2 in December 1977. . Its passage through the Jovian system revolutionised our appreciation of the Galilean moons of the system, after which it travelled on to its November 1980 encounter with Saturn and then Titan.

Voyager 2’s more circular trajectory meant it did not reach Jupiter until July 1979, six months behind Voyager 1, but its route allowed it to make a much closer fly-by of Europa, the ice-covered Galilean moon, giving scientists the first hint of the nature of the mechanisms at work deep within the moon.

From here the vehicle journeyed on to an August 1981 encounter with Saturn and then Uranus in 1986 and then Neptune in August 1989, whilst Voyager 1 continued onwards toward the heliopause, all of which I covered in  Space Sunday: Voyager at 40.

In 2010, Voyager 1 commenced a two-year transition from the space dominated by the Sun and its outward flow of radiation, and the realm of interstellar space. The first indications that it was beyond the influence of the Sun’s radiation came in later 2012 – although it was not until March 2013 that this was empirically confirmed through analysis of multiple data returned by the vehicle.

Voyager 2 commenced its voyage through the heliopause in 2013; however, as it was still travelling within the plane of the ecliptic, it was effectively travelling through a “thicker” part of the “bubble wall” of the heliosphere, so it did not enter interstellar space until November 2018.

Even so, and possibly confusingly, neither craft have actually departed the solar system per se. This is because the “size” of the solar system is measured in two ways: the influence of the Sun’s outward flow of radiation and by the influence of its. Despite having passed through the former, both craft are sill within space affected by the latter, and neither will reach the Oort Cloud – the source region of long-period comets and seen as marking the outer limits of the Sun’s gravitational influence – for another 300 years.

As such, both of the nuclear-powered vehicles are now engaged in a multi-vehicle mission (having been joined in it by the likes of the New Horizons spacecraft, the Parker Solar Probe and others) referred to as the Heliophysics Mission.

The Heliophysics Mission fleet provides invaluable insights into our Sun, from understanding the corona or the outermost part of the sun’s atmosphere, to examining the sun’s impacts throughout the solar system, including here on Earth, in our atmosphere, and on into interstellar space. Over the last 45 years, the Voyager missions have been integral in providing this knowledge and have helped change our understanding of the sun and its influence in ways no other spacecraft can.

– Nicola Fox, director of the NASA’s Heliophysics Division

Today, as both Voyagers explore interstellar space, they are providing humanity with observations of uncharted territory. This is the first time we’ve been able to directly study how a star, our sun, interacts with the particles and magnetic fields outside our heliosphere, helping scientists understand the local neighbourhood between the stars, upending some of the theories about this region, and providing key information for future missions.

– Linda Spilker, Voyager’s deputy project scientist at JPL

Given their age, both vehicles – and the people who have overseen their operations over the decades (the mission team spans two generations of scientists) – have faced some significant challenges. Within a year of its launch, for example, Voyager 2 suffered a complete failure of its primary radio receiver, and has been using its back-up receiver ever since.

More recently, Voyager 1 has experienced an issue that caused status information about one of its onboard systems to become garbled – a problem which is still being investigated. Between these two events are a multitude of others that have been faced and resolved, help the missions forward in their successes.

Most notably, however, both vehicles have been at the mercy of the diminishing output from their radioisotope thermoelectric generators (RTGs), the nuclear “batteries” that generate heat through the radioactive decay of plutonium pellets which can be converted into electrical power.

At launch, these plutonium pellets could generate around 470 watts of usable power. To date, around 30% of the total available plutonium available across all three RTGs used by each vehicle has been lost, reducing overall usable electrical power by over 40%. As a result, five of the major science instruments on each Voyager craft have been shut down over the past several years, and the heaters of five more on each vehicle have also been disabled – although the instruments themselves have remained functional, despite being at temperatures well below their operational minimum.

It is believed that at the current rate of decay, the remaining plutonium will reach a point where it can no longer supply sufficient heat to generate the power needed to keep the remaining instruments running in around 2025, when further shutdowns will be required

A further increasing challenged face by both Voyager vehicles is that of distance. It takes signals from Voyager 1 around a day to send a signal to Earth, with the inverse-square law, meaning that as the distance increases, so communications are at increasingly lower data rates. Even so, it is anticipated Voyager 1 has around another 5-6 years of meaningful communications capability left, and Voyager 2 around 7 years – although these figures are again estimates.

The Voyagers have continued to make amazing discoveries, inspiring a new generation of scientists and engineers. We don’t know precisely how long the mission will continue, but we can be sure that the spacecraft will provide even more scientific surprises as they travel farther away from the Earth.

– Suzanne Dodd, current  Voyager Project Manager, JPL.

Artemis Update

The first of NASA’s Space Launch System (SLS) rockets has completed its third – and hopefully final – trip to the launch facilities at Pad 39B of NASA’s Kennedy Space Centre ahead of its upcoming launch as a part of the Artemis 1 mission to cislunar space.

The mobile launch platform carrying the SLS and its Orion spacecraft left the Vehicle Assembly Building at the Kennedy Space Centre at approximately 02:00 UTC on August 17th, 2022, arriving at the pad facilities some 10 hours later. The roll-out follows the final round of checks-outs and inspections of the rocket and its payload following the recent wet dress rehearsal (WDR) launch fuelling and countdown simulation in June.

This return to the pad is to allow time for the vehicles (rocket and Orion) to be put through their final pre-launch preparations and eventual fuelling of the rocket ahead of the first launch window opening on August 29th, 2022. The work includes checking connections for data, power and other commodities at the pad, as well as servicing the solid rocket boosters, whose hydraulic power units will be fuelled with hydrazine to operate their thrust vector control systems. There will also be “program-specific engineering tests” of the vehicles at the pad.

The fist launch window opens at 12:33 on August 29th, and runs for 2 hours. Formal operations in readiness for this launch attempt will commence on August 27th, with actual propellant loading for the rocket starting about eight hours ahead of the window opening.

If all goes as planned, the launch will signal the start of a 42-day uncrewed mission which will send the Orion spacecraft into a distant retrograde orbit around the Moon to test its systems before it returns to Earth to splash down off the coast of San Diego, California. Should this particular window be missed, further launch windows will be available on September 2 (2 hours), and September 5th (90 minutes).

I’ll have more on the mission specifics in the next Space Sunday update.

In the meantime, and on August 19th, NASA released details of the shortlist of 13 locations within the Moon’s South Polar regions which have been selected for the Artemis 3 mission – the first crewed landing on the Moon by America since 1972’s Apollo 17.

These locations each include multiple sites that could host landings the Human Landing System (HLS) vehicle SpaceX is contracted to provide to NASA for the mission, and which is based on their yet-to-fly Starship vehicle. SpaceX is said to have “worked very closely with NASA scientists and technologists to identify these 13 regions.”  All of them have been selected as a mix of their suitability for landing the 50-metre tall HLS and their inherent scientific value.

All of the locations cover an area some 15 km on a side, and lie within 6-degrees of the lunar South Pole. Thirteen areas have been selected to allow for the broadest number of options for continuous daylight within multiple sites within one more more of the locations, to ensure any given site has the necessary levels of sunlight by which to achieve a safe landing. Narrowing of the list to a defined set of “primary” and “back-up” locations will commence around 18 months prior to any date set for an Artemis 3 launch, with a final selection being made in “sufficient time” to allow for crew training.