Voyager sees solar wind run out of puff

Artist's impression of the heliosphere

Artist's impression of the bubble—the heliosphere—blown in interstellar space by the solar wind. The Voyager 1 spacecraft is soon to cross the boundary into interstellar space.

  • Voyager 1 launched on September 5, 1977
  • The most distant spacecraft from Earth – 17.4 billion km
  • Now exploring the boundary between Solar System and interstellar space

The 33-year-long odyssey of NASA’s Voyager 1 spacecraft has reached a new milestone…a distant point at the edge of our Solar System where there is no outward motion of solar wind.

The solar wind is a stream of hot ionised gas, or plasma, emanating directly outward from the Sun. It forms a bubble—known as the heliosphere—around our Solar System. The solar wind travels at supersonic speed until it crosses a shockwave called the termination shock. At this point, the wind dramatically slows down and heats up in the heliosheath.

Outside the bubble lies true interstellar space, through which blows a gentle “interstellar wind”.

Now hurtling toward that interstellar space some 17.4 billion kilometres from the Sun, Voyager 1 has crossed into an area where the solar wind speed has slowed to zero.

Scientists suspect the solar wind has been turned sideways by the pressure from the interstellar wind.

The event is a major landmark in Voyager 1’s passage through the heliosheath, the turbulent outer shell of the Sun’s sphere of influence, and the spacecraft’s upcoming departure from our Solar System.

Voyager spacecraft

The twin Voyager spacecraft were launched in 1977 to investigate the outer planets of the Solar System.

“The solar wind has turned the corner,” said Ed Stone, Voyager project scientist based at the California Institute of Technology. “Voyager 1 is getting close to interstellar space.”

Voyager shows us something new

Launched on September 5, 1977, Voyager 1 crossed a region called the termination shock in December 2004 into the heliosheath. Scientists have used data from Voyager 1’s Low-Energy Charged Particle Instrument to deduce the solar wind’s velocity.

When the speed of the charged particles hitting the outward face of Voyager 1 matched the spacecraft’s speed, researchers knew that the net outward speed of the solar wind was zero. This occurred in June, when Voyager 1 was about 17 billion kilometres from the Sun.

Because the velocities can fluctuate, scientists gathered four more monthly readings before they were convinced the solar wind’s outward speed actually had slowed to zero.

Analysis of the data shows the velocity of the solar wind has steadily slowed at a rate of about 72,400 kph each year since August 2007, when the solar wind was speeding outward at about 209,000 kph. The outward speed has remained at zero since June.

The results were presented at the American Geophysical Union meeting in San Francisco.

“When I realised that we were getting solid zeroes, I was amazed,” said Rob Decker, a Voyager Low-Energy Charged Particle Instrument co-investigator and senior staff scientist at the Johns Hopkins University Applied Physics Laboratory.

“Here was Voyager, a spacecraft that has been a workhorse for 33 years, showing us something completely new again.”

Entering a new frontier

Scientists think Voyager 1 has not crossed the heliosheath into interstellar space. Crossing into interstellar space would mean a sudden drop in the density of hot particles and an increase in the density of cold particles.

Researchers are putting the data into their models of the heliosphere’s structure and should be able to better estimate when Voyager 1 will reach interstellar space.

They currently estimate Voyager 1 will cross that frontier in about four years.

Diagram of Voyager and Pioneer spacecraft positions

Where are they now? Positions of the two Voyager spacecraft, and Pioneers 10 and 11. Voyager 1 (bottom right corner) is the most distant.

“In science, there is nothing like a reality check to shake things up, and Voyager 1 provided that with hard facts,” said Tom Krimigis, principal investigator on the Low-Energy Charged Particle Instrument, who is based at the Applied Physics Laboratory and the Academy of Athens, Greece.

“Once again, we face the predicament of redoing our models.”

A sister spacecraft, Voyager 2, was launched in August 20, 1977 and has reached a position 14.2 billion kilometres from the Sun.

Both spacecraft have been travelling along different trajectories and at different speeds. Voyager 1 is travelling faster, at a speed of about 61,100 kph, compared to Voyager 2’s velocity of 56,300 kph.

In the next few years, scientists expect Voyager 2 to encounter the same kind of phenomenon as Voyager 1.

The Voyagers were built by NASA’s Jet Propulsion Laboratory, which continues to operate both spacecraft.

Adapted from information issued by NASA / JPL.

Get SpaceInfo.com.au daily updates by RSS or email! Click the RSS Feed link at the top right-hand corner of this page, and then save the RSS Feed page to your bookmarks. Or, enter your email address (privacy assured) and we’ll send you daily updates. Or follow us on Twitter, @spaceinfo_oz

Filed Under: AstronomyFeatured storiesNews ArchiveSpaceflight

Tags:

RSSComments (1)

Leave a Reply | Trackback URL

  1. […] This post was mentioned on Twitter by Alice Gorman, Jonathan Nally. Jonathan Nally said: Voyager sees solar wind run out of puff http://goo.gl/fb/0I15L […]