Skip to main content
Telescopes and space missions

Telescopes and space missions

Voyager heads for interstellar space

25 Aug 2009

Despite completing its grand tour of the solar system 20 years ago, NASA’s Voyager mission is still set to provide new insights about interstellar space, as Richard Corfield explains

Voyager 2 – still going strong

Two decades ago today — 25 August 1989 — an extraordinary spacecraft took its final, stunning photos of the distant planet Neptune, before heading towards the edge of the solar system and the interstellar void beyond. Voyager 2 was launched by NASA in 1977 to take advantage of a rare alignment of the planets that occurs only once every 176 years. The alignment allowed the craft — and its sister ship Voyager 1 — to take a “grand tour” of the outer planets of the solar system, picking up speed at each planetary rendezvous via so-called gravity-assist manoeuvres.

Voyager 2, launched on 20 August 1977, crossed the ring plane of Saturn, then headed on to Uranus and Neptune, returning the most spectacular pictures ever taken of those distant, giant planets. Voyager 1, meanwhile, blasted off on 5 September 1977 on a faster orbit that would allow it to do a fly-by of Titan — one of Saturn’s four moons. After its cloud-skimming encounter with that moon, Voyager 1 headed upwards at an angle of 35° to the plane of the ecliptic, leaving the solar system behind forever.

The two sister craft now form the Voyager Interstellar Mission (VIM). Although it was never certain when the Voyager craft blasted off in 1977 that they would survive for so long, VIM is still an official NASA mission. It receives some $4.7m each year — with funding planned through until at least 2012 — and employs 40 staff at NASA’s Jet Propulsion Laboratory.

Both craft are currently investigating the physics of the outer solar system and will continue to do so until they cross over into interstellar space. Five instruments are running on the spacecraft, transmitting data about the strength of the local magnetic field as well as the speed, temperature and density of the solar wind — the stream of particles from the Sun — and the intensity of energetic ions and cosmic rays.

Asymmetric bubble
VIM is led by veteran California Institute of Technology physicist Edward Stone, who has been involved with the Voyager programme since 1972. Having originally co-ordinated the 11 teams of scientists that were involved in the planetary phase of the mission, the 73 year old remains as excited as ever about the science that the Voyager craft are doing, particularly now that they are in the most distant part of the “heliosphere” — the bubble of space blown by the solar wind into the interstellar medium.

“It was our message saying ‘We, as a civilization, now can do this!'” Edward Stone, California Institute of Technology, leader of the Voyager Interstellar Mission (VIM)

Voyager 1 entered this region, known as the “heliosheath”, in December 2004 after crossing the “termination shock”, which is where the solar wind — normally travelling at about 400 km s-1 — is abruptly slowed down to subsonic speeds as it presses against the hydrogen and helium ions of interstellar space. Voyager 2 followed its sister craft into the heliosheath in August 2007. What Stone and his colleagues are now trying to do is understand how the solar wind gets deflected as it interacts with the interstellar wind, which is proving more complex than models suggest.

One of the first surprises, as Stone and colleagues reported in Nature last year (454 71), was that the heliosphere is not spherical but asymmetric. Indeed, the southern boundary was found to be some 1500 million kilometres closer to the Sun than the northern boundary. “[This] tells us that there is something outside that is pressing more strongly on the south than on the north — most likely a magnetic field,” says Stone. “So we’re very interested in working with the models of the heliosphere to interpret and predict the direction of the local interstellar magnetic field and its strength.”

Stone claims to already have some information about the direction and strength of the magnetic field of interstellar space based on the fact that the heliosphere is distorted. But to measure the field directly, the Voyager spacecraft will have to cross the final frontier of the solar system — the heliopause — where the pressure of the interstellar wind and of the solar wind balance. Nobody knows for sure when Voyager 1 — the more distant of the two spacecraft — will enter interstellar space because no-one knows exactly where it begins, but Stone estimates that it will be within the next five or six years.

Beyond the final frontier
Telltale signs that Voyager 1 has left the heliosphere will, Stone thinks, be a change in the magnetic field and ion flow, and an increase in energetic particles from the Milky Way hitting the craft, which will no longer be protected by the heliosheath. “The radiation environment will be much more intense from supernovae that have occurred in the Sun’s region over the last 10 million years or so,” he says.

“Voyager represents the benchmark for outer solar-system exploration” John Zarnecki, The Open University, UK

Stone and his colleagues have also been surprised by the physics of the termination shock. Models assumed that the energy of the solar wind, which has to go somewhere when it slows down, would heat the wind from, say, 104 K before the shock to 106 K afterwards. But instead Voyager found that the temperature of the wind after the shock was only 105 K.

“We now believe that that energy went into heating ions that actually came in as atoms from interstellar space,” Stone says. “So once again we are trying to improve the models to understand this interaction.”

To Stone, the Voyager legacy is alive and well — a human adventure as much as a scientific one — and he applauds the original decision of a committee led by Carl Sagan to place two golden discs on the sides of the craft, containing sound and images about life on Earth, that may one day be our calling card to another civilization. “It was our message saying ‘We, as a civilization, now can do this!’,” he says.

But the craft have also inspired others too, such as space scientist John Zarnecki from the Open University in the UK, who was a principal investigator of the Huygens probe that landed on the surface of Titan on 14 January 2005. “To my generation, [Voyager] represents the benchmark for outer solar-system exploration,” he says. “Everything that we have done since then is built on it. There is no question that the success of the Voyager 1 fly-by of Titan in 1980 inspired me to believe that it was possible to send a probe there.”

Copyright © 2024 by IOP Publishing Ltd and individual contributors