YM COLUMN ARCHIVE

July 16, 2008

Voyager 2 escaping from the solar system

Lifetime of artificial satellites and planetary spacecrafts does not last so long but only for several years. Ongoing scientific satellites of Japan are AKEBONO (launched in 1989), GEOTAIL (1992), HAYABUSA (2003), SUZAKU (2005), REIMEI (2005), AKARI (2006), HINODE (2006) and KAGUYA (2007), among which the longest active is AKEBONO becoming 20 years next year.

http://www.isas.jaxa.jp/j/enterp/missions/index.shtml

In this respect, two Voyagers launched in 1977 and still transmitting data of outer solar system are rare cases of longest duration. According to the science magazine “Nature” issued on July 3, Voyagers are still engaged in activities and keep sending valuable scientific data to earth.

http://www.astroarts.co.jp/news/2007/09/05voyager/index-j.shtml

It was August 31, 2007 when it was reported that Voyager 2, which is the only spacecraft that ever visited Uranus and Neptune passed “termination shock” often referred to as the farthest reaches of the solar system. In summer of 2007, Voyager 2 was said to have been still staying in the magnetic region called “gigantic bubble” in the heliosphere ten billion kilometers away from the sun. The “bubble” is caused by the solar wind emitting from sun at extremely high rate, which pushed out interstellar medium coming from beyond the solar system. Nevertheless, solar wind has to inevitably collide head on with the flow of the interstellar medium somewhere in the region beyond Neptune. The collision reduces solar wind velocity of one million miles per hour to slower speed than sonic velocity. The region of the collision is called “termination shock”.

Incidentally, it was 94.1 AU (astronomical unit: 1 AU is 150 million kilometers; average distance between sun and earth) when Voyager 1 passed termination shock in December 2004. This time Voyager 2 is reported to have passed the termination shock at 83.7 AU. What does the difference mean?

Solar activities were at its maximum in 2004. At this time the power of solar wind was so much intensified that it strengthened pressure of heliopause to push out termination shock. Despite Voyager 1 was going further away from sun at a violent speed, so was termination shock so that Voyager 1 could not pass over termination shock. When solar activity eventually slowed down and heliosphere turned to contraction from expansion, and then Voyager 1 could take hold of termination shock.

On the contrary, it was declining phase of solar activities when Voyager 2 approached the farthest reaches of heliosphere. Voyager 2 was at 83.4 AU when plasma device onboard Voyager 2 alarmed closing in to termination shock, which means that Voyager 2 passed termination shock at 83.7 AU only 30 days later. However, movement of termination shock was so complicated that there were counted five times when the spacecraft deemed to have passed termination shock. Each of five times indicated sharp spikes on plasma device and magnetometer. And finally Voyager 2 escaped from the turbulent region.

There is an interesting find in the said “Nature”. Scientists thought so far that temperature of solar wind plasma would rise to one million degrees due to rapid slowdown of kinetic energy caused by solar wind velocity reducing to subsonic speed from one million miles per hour. What actually happened was that temperature rose by only 100 thousand degrees. Seventy percent of total energy looks like having disappeared somewhere.

The answer was brought from an unexpected source. The NASA’s two spacecrafts “STEREO” were launched in 2006 into Earth’s orbit around the sun to obtain stereo pictures of sun’s surface and measure magnetic fields and ion fluxes associated with solar explosions. Contrary to their mission objectives, STEREO’ sensors detected neutral atoms of high energy seemingly coming from outer planets of solar system during an observation period from June to October of 2007.

The article of Nature explains what the neutral atoms are. The super high-energy particles of solar wind reaching termination shock collide with cold flow of interstellar medium. The rest of seventy percent of total solar wind energy warms up cold interstellar medium and creates the region called “heliosheath” beyond termination shock.

Thus Voyager 2 safely passed through termination shock and entered heliosheath. The next destination is heliopause, the last fort of solar system, from where Voyager 2 will sail out into interstellar space.

http://planetary.org/news/2008/0710_30_Years_into_its_Journey_Voyager_2.html

I welcome your opinions on this column to the following E-mail address.
matogawa@planetary.or.jp

(Translated by The Planetary Society of Japan)