ESA's Planck Satellite: The magnetic field along the galactic plane

Credit: ESA/Planck Collaboration. 

Acknowledgment: M.-A. Miville-Deschênes, CNRS – Institut d’Astrophysique Spatiale, Université Paris-XI, Orsay, France

While the pastel tones and fine texture of this image may bring to mind brush strokes on an artist's canvas, they are in fact a visualisation of data from ESA's Planck satellite.

The image portrays the interaction between interstellar dust in the Milky Way and the structure of our Galaxy's magnetic field.

Between 2009 and 2013, Planck scanned the sky to detect the most ancient light in the history of the Universe, the cosmic microwave background.

It also detected significant foreground emission from diffuse material in our Galaxy which, although a nuisance for cosmological studies, is extremely important for studying the birth of stars and other phenomena in the Milky Way.

Among the foreground sources at the wavelengths probed by Planck is cosmic dust, a minor but crucial component of the interstellar medium that pervades the Galaxy. Mainly gas, it is the raw material for stars to form.

Interstellar clouds of gas and dust are also threaded by the Galaxy's magnetic field, and dust grains tend to align their longest axis at right angles to the direction of the field.

As a result, the light emitted by dust grains is partly 'polarised', it vibrates in a preferred direction, and, as such, could be caught by the polarisation-sensitive detectors on Planck.

Scientists in the Planck collaboration are using the polarised emission of interstellar dust to reconstruct the Galaxy's magnetic field and study its role in the build-up of structure in the Milky Way, leading to star formation.

In this image, the colour scale represents the total intensity of dust emission, revealing the structure of interstellar clouds in the Milky Way.

The texture is based on measurements of the direction of the polarised light emitted by the dust, which in turn indicates the orientation of the magnetic field.

This image shows the intricate link between the magnetic field and the structure of the interstellar medium along the plane of the Milky Way.

In particular, the arrangement of the magnetic field is more ordered along the Galactic plane, where it follows the spiral structure of the Milky Way. Small clouds are seen just above and below the plane, where the magnetic field structure becomes less regular.

From these and other similar observations, Planck scientists found that filamentary interstellar clouds are preferentially aligned with the direction of the ambient magnetic field, highlighting the strong role played by magnetism in galaxy evolution.

The emission from dust is computed from a combination of Planck observations at 353, 545 and 857 GHz, whereas the direction of the magnetic field is based on Planck polarisation data at 353 GHz.

Two dynamos drive Jupiter's magnetic field

Jupiter cut open: The magnetic field lines illustrate the high complexity of the magnetic field inside the planet, which, however, quickly decreases beyond the metallic layer (black line). 

On the surface, a dipolar part that is inclined by ten degrees with respect to the axis of rotation dominates. 

The thickness of the field lines is a measure of the local magnetic field strength. 

In the equatorial region, a jet produces bundles of field lines with a pronounced east-west orientation at the transition to the metallic layer. 

The coloured contours represent the radial surface field. Red indicates field lines directed outwards, blue inwards; green denotes a weak field. 

The colour coding of the sections represents the field in the east-west direction – red indicates eastwards, blue westwards. 

Credit: J. Wicht, MPS

Superlatives are the trademark of the planet Jupiter.

The magnetic field at the top edge of the cloud surrounding the largest member of the solar system is around ten times stronger than Earth's, and is by far the largest magnetosphere around a planet.

Just why this field has a similar structure to that of our own planet although the interiors of the two celestial objects have a completely different structure, has mystified researchers for a long time.

With the aid of the most detailed computer simulations to date, a team headed by the Max Planck Institute for Solar System Research in Göttingen has now succeeded in explaining the origin of the magnetic field deep inside the gaseous giant.

Magnetic fields are always generated when electric currents flow. The Earth is surrounded by a magnetic field because, deep in its interior, there is a circulating molten mass of iron and nickel.

This motion gives rise to electric currents that generate Earth's familiar dipolar magnetic field, in much the same way as a bicycle dynamo operates. Physicists call it the geo-dynamo, but how does the dynamo inside of Jupiter work?

Jupiter consists predominantly of hydrogen and helium.

Photos of the planet show coloured bands of cloud and gigantic tornados such as the Great Red Spot.

The temperature at the upper cloud boundary is minus 100 degrees Celsius, but temperature, pressure and electrical conductivity increase enormously with increasing depth.

At a depth of just under 10,000 kilometres and a pressure of several million atmospheres, the hydrogen even becomes conductive like a metal, an exotic state of matter which does not exist on Earth.

It is still unclear whether there is a rocky core at the centre of the planet; it could possibly amount to around 20 percent of the Jupiter radius, corresponding to 14,000 kilometres.

Previous computer simulations on the formation of the magnetic field had to greatly simplify this complex structure.

The upper gaseous region and the lower metallic region were treated separately, for example.

Thus, no computation correctly reproduced the strength and the form of the magnetic field as determined by space probes.

"Several colleagues assumed that certain physical quantities changed suddenly at the transition to the region of the metal-like conducting hydrogen," says project leader Johannes Wicht from the Max Planck Institute for Solar System Research in Göttingen, but new models from colleagues at the University of Rostock seem to prove that this is probably not the case.

The properties change gradually over the whole gas layer so that the separate treatment of the outer and inner region is hardly justified.

The important step forward here was the fact that, for the first time, the Göttingen-based physicists dealt with all regions of the planet in the same simulation.

To this effect, the Max Planck Society's huge Hydra supercomputer in Garching had to spend around six months on the computation.

The result was impressive: it portrayed Jupiter's magnetic field more or less as space probes had determined it in nature.

"The main part of the magnetic field, which looks so similar to Earth's magnetic field, is generated deep inside the planet, where the properties no longer change so strongly," says Wicht.

NASA's Messenger: Mercury's magnetic field reveals its interior is different from Earth's

Earth and Mercury are both rocky planets with iron cores, but Mercury's interior differs from Earth's in a way that explains why the planet has such a bizarre magnetic field, UCLA planetary physicists and colleagues report.

Measurements from NASA's Messenger spacecraft have revealed that Mercury's magnetic field is approximately three times stronger at its northern hemisphere than its southern one.

In the current research, scientists led by Hao Cao, a UCLA postdoctoral scholar working in the laboratory of Christopher T. Russell, created a model to show how the dynamics of Mercury's core contribute to this unusual phenomenon.

The magnetic fields that surround and shield many planets from the sun's energy-charged particles differ widely in strength.

While Earth's is powerful, Jupiter's is more than 12 times stronger, and Mercury has a rather weak magnetic field.

Venus likely has none at all. The magnetic fields of Earth, Jupiter and Saturn show very little difference between the planets' two hemispheres.

Within Earth's core, iron turns from a liquid to a solid at the inner boundary of the planet's liquid outer core; this results in a solid inner part and liquid outer part.

The solid inner core is growing, and this growth provides the energy that generates Earth's magnetic field. Many assumed, incorrectly, that Mercury would be similar.

"Hao's breakthrough is in understanding how Mercury is different from the Earth so we could understand Mercury's strongly hemispherical magnetic field," said Russell, a co-author of the research and a professor in the UCLA College's department of Earth, planetary and space sciences.

"We had figured out how the Earth works, and Mercury is another terrestrial, rocky planet with an iron core, so we thought it would work the same way but it's not working the same way."

Mercury's peculiar magnetic field provides evidence that iron turns from a liquid to a solid at the core's outer boundary, say the scientists, whose research currently appears online in the journal Geophysical Research Letters and will be published in an upcoming print edition.

"It's like a snow storm in which the snow formed at the top of the cloud and middle of the cloud and the bottom of the cloud too," said Russell.

"Our study of Mercury's magnetic field indicates iron is snowing throughout this fluid that is powering Mercury's magnetic field."

The research implies that planets have multiple ways of generating a magnetic field.

Hao and his colleagues conducted mathematical modeling of the processes that generate Mercury's magnetic field.

In creating the model, Hao considered many factors, including how fast Mercury rotates and the chemistry and complex motion of fluid inside the planet.

The cores of both Mercury and Earth contain light elements such as sulfur, in addition to iron; the presence of these light elements keeps the cores from being completely solid and "powers the active magnetic field–generation processes," Hao said.

Hao's model is consistent with data from Messenger and other research on Mercury and explains Mercury's asymmetric magnetic field in its hemispheres.

He said the first important step was to "abandon assumptions" that other scientists make.

"Planets are different from one another," said Hao, whose research is funded by a NASA fellowship. "They all have their individual character."

More Information: 'A dynamo explanation for Mercury's anomalous magnetic field.' Authors: Hao Cao, Christopher Russell, et al. - Article first published online: 19 JUN 2014 DOI: 10.1002/2014GL060196

Satellite X-ray observations: Neutron star with doughnut-shaped magnetic field and axial wobble

An artist's impression of a magnetar with an intense torroidal magnetic field in its core. 

Credit: NASA /CXC /M.Weiss

When a massive star dies, it can collapse under its own gravity with such force that it produces a supernova, leaving behind an extremely dense remnant consisting almost entirely of neutrons, a neutron star.

Some neutron stars, known as magnetar, possess powerful magnetic fields, which are stronger than any other known magnetism in the Universe.

These intense magnetic fields somehow produce high-energy x-ray pulses, but this process is not well understood.

Kazuo Makishima from RIKEN's MAXI Team and Teruaki Enoto from the RIKEN Nishina Center for Accelerator-Based Science in collaboration with the University of Tokyo and NASA have now found evidence that the magnetar 4U 0142+61 'wobbles' about its rotational axis, implying that the sphericity of the star is distorted due to an intense donut-shaped magnetic field at its core.

"Magnetars emit high-energy 'hard' x-rays, but the origins of these emissions are unknown," explains Makishima.

"We observed 4U 0142+61 using the Suzaku x-ray astronomy satellite (formerly known as Astro-E2) to find out whether the magnetar's emissions change over time."

The magnetar had previously been measured to spin at a rate of one revolution in about 8 seconds and to produce x-ray pulses of the same period, but Makishima and his co-workers noticed slow fluctuations in the arrival times of the x-ray pulses.

They attributed these fluctuations to axial wobble, known as free precession.

The star's axis precesses with a period that differs very slightly from the star's rotation period, and the slow beat between the two periods changes the observed emissions.

"The idea of free precession was not in my mind when we started the data analysis," says Makishima, "but I was familiar with it through my long experience with spinning satellites."

"The precession is most likely caused by a slight deformation of the magnetar, and the deformation is possibly due in turn to internal magnetic fields that are even stronger than the external visible fields."

The findings suggest that the magnetar is deformed from a perfect sphere due to an extremely strong, tightly wound toroidal magnetic field buried deep in the star's core.

The results therefore support the hypothesis that the hard-x-ray pulses are produced by consuming magnetic energy.

Makishima's team plans to analyze a third dataset from 4U 0142+61 and search the Suzaku data for other magnetars that might show similar effects.

"We will also propose observations of these objects with ASTRO-H, the powerful successor to Suzaku, which will be launched in 2015," he says.

More information: Makishima, K., Enoto, T., Hiraga, J. S., Nakano, T., Nakazawa, K., Sakurai, S., Sasano, M. & Murakami, H. Possible evidence for free precession of a strongly magnetized neutron star in the magnetar 4U 0142+61. Physical Review Letters 112, 171102 (2014). DOI: 10.1103/PhysRevLett.112.171102

ESA SWARM Launch

The European Space Agency (ESA) on Friday launched a trio of hi-tech satellites on an unprecedented mission to map anomalies in Earth's magnetic field.

The 230-million-euro ($276-million) Swarm mission blasted off in fog aboard a Rokot launcher from Plesetsk in northwestern Russia at 1602 GMT, ESA showed in a live feed.

The launch, postponed from November 14, was the third by the Russian-made Rokot from Plesetsk this year, the Russian defence ministry said.

ESA said the satellites reached a near-polar orbit 91 minutes after launch and all sent signals back home.

The sophisticated monitors are identical, each weighing 470 kilod (1,034 pounds) and carrying instruments on an extendable boom.

They are due to operate at extremely low altitudes close to the edge of the atmosphere to measure the strength, orientation and fluctuations of the Earth's geomagnetic field.

• Two will fly initially at 460 kilometres (287 miles), reduced after four years to just 300 kilometres (180 miles).
• The third will start at 530 kilometres (331 miles), to offer a different angle of view.

The project aims at providing the most accurate measurements ever of Earth's magnetic field, in a mission designed to last at least five years.

The magnetism derives mainly from superheated liquid iron and nickel, which swirl in the outer core about 3,000 kilometres (1,800 miles) beneath the planet's surface.

Like a spinning dynamo, this subterranean metal ocean generates electrical currents and thus a magnetic field.

But the field is not constant.

The gap between the magnetic north pole and the geographical north pole has been widening since 2001 at the rate of 65 kilometres (40.6 miles) per year, compared with just 10 kilometres (six miles) per year in estimates in the early 1990s.

In addition, the magnetic field has been weakening. Since the mid-19th century it has lost around 15 percent of its strength.

Some experts wonder if this is a prelude to a reversal of magnetic polarity, something which usually occurs around every 200,000 to 300,000 years but is now considered long overdue.

ESA SWARM: Explaining the earth's Magnetic Field - Animation video

An introduction to Earth's magnetic field: what it is, where it comes from and what are the benefits and advantages to life on Earth.

Physicists monitoring huge solar event - Magnetic Field Reversal - Video

The sun's magnetic field is poised to reverse its polarity. The effects of the event will be closely monitored by Stanford solar physicists. Credit: Kurt Hickman

The sun's magnetic field is poised to reverse its polarity. The effects of the event, which occurs every 11 years, will ripple throughout the solar system and be closely monitored by Stanford solar physicists.

Every 11 years, the sun undergoes a complete makeover when the polarity of its magnetic field – its magnetic north and south – flips. The effects of this large-scale event ripple throughout the solar system.

Although the exact internal mechanism that drives the shift is not entirely understood, researchers at Stanford's Wilcox Solar Observatory have monitored the sun's magnetic field on a daily basis since 1975 and can identify the process as it occurs on the sun's surface. This will be the fourth shift the observatory has monitored.

New polarity builds up throughout the 11-year solar cycle as sunspots – areas of intense magnetic activity – appear as dark blotches near the equator of the sun's surface.

Over the course of a month, a sunspot spreads out, and gradually that magnetic field migrates from the equator to one of the sun's poles.


As the polarity moves toward the pole, it erodes the existing, opposite polarity, said Todd Hoeksema, a solar physicist at Stanford since 1978 and director of the Wilcox Solar Observatory.

The magnetic field gradually reduces toward zero, and then rebounds with the opposite polarity.

"It's kind of like a tide coming in or going out," Hoeksema said. "Each little wave brings a little more water in, and eventually you get to the full reversal."

The effects of this event are widespread: The area of space where the sun's magnetic field exerts its influence – called the heliosphere – stretches well beyond Pluto, past NASA's Voyager probes near the edge of interstellar space.

The sun is also typically at the peak of its activity during a magnetic field reversal, which, in addition to an increased number of sunspots, is marked by a surge in solar flares and mass ejections.

The sun's changing magnetic field and the bursts of charged particles can interact with Earth's own magnetic field, one manifestation of which is a noticeable uptick in the occurrence and range of auroras.

Earth's magnetic field can also affect major electronic systems, Hoeksema said, such as power distribution grids and GPS satellites, so scientists are keen to monitor the heliosphere.

"We also see the effects of this on other planets," Hoeksema said. "Jupiter has storms, Saturn has auroras, and this is all driven by activity of the sun."

The Sun's Heliosphere is about to flip

An artist's concept of the heliospheric current sheet, which becomes more wavy when the sun's magnetic field flips.

Something big is about to happen on the sun.

According to measurements from NASA-supported observatories, the sun's vast magnetic field is about to flip.

Todd Hoeksema

"It looks like we're no more than 3 to 4 months away from a complete field reversal," says solar physicist Todd Hoeksema of Stanford University. "This change will have ripple effects throughout the solar system."

The sun's magnetic field changes polarity approximately every 11 years. It happens at the peak of each solar cycle as the sun's inner magnetic dynamo re-organizes itself.

The coming reversal will mark the midpoint of Solar Cycle 24. Half of 'Solar Max' will be behind us, with half yet to come.

Hoeksema is the director of Stanford's Wilcox Solar Observatory, one of the few observatories in the world that monitor the sun's polar magnetic fields.

The poles are a herald of change. Just as Earth scientists watch our planet's polar regions for signs of climate change, solar physicists do the same thing for the sun.

Magnetograms at Wilcox have been tracking the sun's polar magnetism since 1976, and they have recorded three grand reversals—with a fourth in the offing.

Solar physicist Phil Scherrer, also at Stanford, describes what happens: "The sun's polar magnetic fields weaken, go to zero, and then emerge again with the opposite polarity. This is a regular part of the solar cycle."

A reversal of the sun's magnetic field is, literally, a big event. The domain of the sun's magnetic influence (also known as the "heliosphere") extends billions of kilometers beyond Pluto.

Changes to the field's polarity ripple all the way out to the Voyager probes, on the doorstep of interstellar space.

When solar physicists talk about solar field reversals, their conversation often centers on the "current sheet."

The current sheet is a sprawling surface jutting outward from the sun's equator where the sun's slowly-rotating magnetic field induces an electrical current.

The current itself is small, only one ten-billionth of an amp per square meter (0.0000000001 amps/m2), but there's a lot of it: the amperage flows through a region 10,000 km thick and billions of kilometers wide.

Electrically speaking, the entire heliosphere is organized around this enormous sheet.

During field reversals, the current sheet becomes very wavy. Scherrer likens the undulations to the seams on a baseball.

As Earth orbits the sun, we dip in and out of the current sheet. Transitions from one side to another can stir up stormy space weather around our planet.

Cosmic rays are also affected. These are high-energy particles accelerated to nearly light speed by supernova explosions and other violent events in the galaxy.

Cosmic rays are a danger to astronauts and space probes, and some researchers say they might affect the cloudiness and climate of Earth.

The current sheet acts as a barrier to cosmic rays, deflecting them as they attempt to penetrate the inner solar system. A wavy, crinkly sheet acts as a better shield against these energetic particles from deep space.

As the field reversal approaches, data from Wilcox show that the sun's two hemispheres are out of synch.

"The sun's north pole has already changed sign, while the south pole is racing to catch up," says Scherrer.

"Soon, however, both poles will be reversed, and the second half of Solar Max will be underway."

When that happens, Hoeksema and Scherrer will share the news with their colleagues and the public.

Star Tau Boo's baffling magnetic flips

Artist's impression of the giant exoplanet orbiting Tau Boötis, viewed through the star's magnetic arcs. 

Credit: David Aguilar, CfA

The first observations of the complete magnetic cycle of a star other than the Sun are proving a puzzle to astronomers.

Tau Boötis, known as Tau Boo (τ Boo), is a yellowish star that is a little brighter than our Sun. It is located 51 light years away in the constellation of Boötes.

It is host to a giant exoplanet about six times the mass of Jupiter, which orbits Tau Boo every 3.3 days.

In 2007, the magnetic field of Tau Boo was seen to flip: the first time this was observed to happen in a star other than the Sun.

Since then the team has observed four reversals in polarity and is now able to confirm that the star has a rapid magnetic cycle of no more than two years – compared to 22 years for the Sun.

This cycle will subject the orbiting hot-Jupiter to very fast changes in its surrounding environment. Dr Rim Fares will present findings at the National Astronomy Meeting in St Andrews on Thursday 4 July.

"The Sun's magnetic field is a bit like a giant bar magnet, with a north pole and south pole. Every 11 years, during solar maximum (the peak of sunspot activity), the Sun's poles swap over. It takes two flips to restore the magnetic field to its original orientation, so the Sun's magnetic cycle lasts 22 years," explained Dr Fares.

"Tau Boo has the same magnetic behaviour as the Sun, but its cycle is very fast compared to the solar one.

We've seen changes at regular intervals of about a year that are clearly not chaotic, so we can now be sure that we are looking at the star's magnetic cycle lasting at most two years."

Artist’s impression of the magnetic field of Tau Boötis. 

Credit: Karen Teramura, University of Hawaii Institute for Astronomy

The reasons for Tau Boo's fast cycle are still unclear. As well as having the only proper cycle yet observed, Tau Boo is also unique in being the only star where magnetic reversals have been seen that is orbited by detected planets.

Dr Fares and her colleagues made the discovery whilst undertaking a mini-survey of 10 stars orbited by hot-Jupiters, massive Jupiter-like planets that orbit very close to their star and experience scorching temperatures.

Observations of the stars' magnetic fields were compared to observations of stars without hot-Jupiters.

The team aimed to understand how the magnetic environment of stars affects the planets embedded within them and whether the planets themselves have an influence on the magnetic behaviour of the star.

"There are still some big questions about what's causing Tau Boo's rapid magnetic cycle. From our survey, we can say that each planetary system is particular, that interactions affect stars and planets differently, and that they depend on the masses, distance and other properties of the system," explained Dr Fares.

Read the full story here

New Research Re-Calculates date of Moon's Magnetic Dynamo by 160 million years

Mosaic of the near side of the moon as taken by the Clementine star trackers. 

The images were taken on March 15, 1994. 

Credit: NASA

A multi-disciplinary team of international researchers has found evidence to suggest the moon's dynamo persisted until at least 3.6 billion years ago.

In their paper published in the Proceedings of the National Academy of Sciences, the team says this pushes back the date for the dynamo approximately 160 million years.

Currently, the moon has no global magnetic field, but analysis of rocks brought back by Apollo astronauts showed that it did at one time. To create such a field, the moon would necessarily have had some churning in its interior—a dynamo.

Evidence of a dynamo inside the moon has led scientists to propose different theories as to how it might have come about.

Some scientists suggest it might have been due to an impact that knocked the internals loose and set them moving for a period of time.

Others theorise it might have been more likely due to differences in heat distribution during radioactive decay, prompting liquid shifting.

Clément Suavet

The lead author, Clément Suavet is currently assigned to the MIT Department of Earth, Atmospheric and Planetary Sciences (EAPS).

To gain a better understanding of the moon's dynamo and how it might have occurred, researchers have been working to more clearly define when it came about, how strong it was and how long it lasted.

To that end, researchers with this latest effort went back to the moon rocks that started the whole debate.

Using newer technology to analyse the rocks, they found that they had, on average, fields of 13–70 microtesla—the higher readings are on a par with that of Earth's magnetic field.

More importantly, they found that the rocks showed that a dynamo existed as far back as 3.6 billion years ago.

 Suavet says, 'This new finding shoots down the idea of the dynamo forming due to a large impact.'

That's because other research has shown that no impacts large enough to cause a dynamo have occurred since approximately 3.72 billion years ago—well before the age of the samples found but that still doesn't reveal the actual cause.

Though they can't prove it, the group suggests the dynamo mostly likely occurred due to interaction with Earth's gravity—likening it to a tug-of-war between the solid mantel and the liquid core, resulting in a constant internal churning.

More information: Persistence and origin of the lunar core dynamo, PNAS, Published online before print May 6, 2013, doi: 10.1073/pnas.1300341110

Signs Changing Fast: Voyager at Solar System Edge

Voyager 1, which launched on Sept. 5, 1977, is 11 billion miles (18 billion kilometers) from the sun. Voyager 2, which launched on Aug. 20, 1977, is close behind, at 9.3 billion miles (15 billion kilometers) from the sun.

Two of three key signs of changes expected to occur at the boundary of interstellar space have changed faster than at any other time in the last seven years, according to new data from NASA's Voyager 1 spacecraft.

For the last seven years, Voyager 1 has been exploring the outer layer of the bubble of charged particles the sun blows around itself. In one day, on July 28, data from Voyager 1's cosmic ray instrument showed the level of high-energy cosmic rays originating from outside our solar system jumped by five percent.

During the last half of that same day, the level of lower-energy particles originating from inside our solar system dropped by half. However, in three days, the levels had recovered to near their previous levels.

A third key sign is the direction of the magnetic field, and scientists are eagerly analyzing the data to see whether that has, indeed, changed direction. Scientists expect that all three of these signs will have changed when Voyager 1 has crossed into interstellar space. A preliminary analysis of the latest magnetic field data is expected to be available in the next month.

"These are thrilling times for the Voyager team as we try to understand the quickening pace of changes as Voyager 1 approaches the edge of interstellar space," said Edward Stone, the Voyager project scientist based at the California Institute of Technology, Pasadena, Calif.

"We are certainly in a new region at the edge of the solar system where things are changing rapidly. But we are not yet able to say that Voyager 1 has entered interstellar space."

The levels of high-energy cosmic ray particles have been increasing for years, but more slowly than they are now. The last jump - of five percent - took one week in May. The levels of lower-energy particles from inside our solar system have been slowly decreasing for the last two years.

Scientists expect that the lower-energy particles will drop close to zero when Voyager 1 finally crosses into interstellar space.

"The increase and the decrease are sharper than we've seen before, but that's also what we said about the May data," Stone said. "The data are changing in ways that we didn't expect, but Voyager has always surprised us with new discoveries."

Voyager 1, which launched on Sept. 5, 1977, is 11 billion miles (18 billion kilometers) from the sun. Voyager 2, which launched on Aug. 20, 1977, is close behind, at 9.3 billion miles (15 billion kilometers) from the sun.

"Our two veteran Voyager spacecraft are hale and healthy as they near the 35th anniversary of their launch," said Suzanne Dodd, Voyager project manager based at NASA's Jet Propulsion Laboratory, Pasadena. "We know they will cross into interstellar space. It's just a question of when."