打ち上げから４２年　ボイジャー２号が迫る星間空間の謎

【パリＡＦＰ＝時事】米航空宇宙局（NASA）の探査機ボイジャー2号が、太陽系を包む泡構造「太陽圏」を離脱した際の観測データを解析した5件の論文が4日、英科学誌ネイチャー・アストロノミーに発表された。ボイジャー2号は1977年8月20日に打ち上げられ、今も太陽圏を越えた領域からのデータを地球に送り続けている。（写真は米航空宇宙局が提供したボイジャーのイメージ画像）
　科学者らはボイジャー２号と双子の探査機ボイジャー１号から送られてくる観測データを比較することで、数々の疑問に答えが出ると期待していた。だが、ボイジャー２号が太陽風や磁場、太陽の影響範囲である太陽圏と星間空間との境界を飛び交う宇宙線などに関する謎を解決するごとに、また新たな謎が出てくるという。
　太陽圏は太陽系を包む荷電粒子の泡で、宇宙線に対する保護膜のような役割を果たしている。激しい風にさらされた吹き流しのような形状をしており、太陽磁場と、太陽から放出される電離したプラズマ粒子の太陽風で形成されている。この太陽風は時速３００万キロの速度に達することもある。
　ボイジャー２号は今回、理論で予測され、ボイジャー１号によって観測されていた太陽圏外縁部の「磁気バリア」の存在を確認した。この磁気バリアは、「ヘリオポーズ（太陽圏界面）」と呼ばれ、太陽風の荷電粒子と恒星間風（星間物質の流れ）が衝突する、比較的薄い接触境界領域となっている。
　さらに、粒子の漏出という謎も出てきた。
　ボイジャー１号が太陽圏の境界を通過した時は、反対方向に高速で進んでいる太陽系外空間からの粒子、特に宇宙線が検出された。だが、「ボイジャー２号の場合は、真逆だった」と、論文執筆者の一人で米カリフォルニア工科大学のエドワード・ストーン教授は指摘する。「太陽圏を離れると、内部から外へ漏出している粒子が観測され続けた」
　この漏出の謎について、荷電粒子の測定に関する今回の論文の執筆者であるジョンズ・ホプキンス大学応用物理学研究所のトム・クリミギス氏は、１１年周期で増減を繰り返す太陽活動との関連を示唆している。
　打ち上げから４２年を過ぎたボイジャー１号と２号は５年以内に電力が尽き、データの送信が途絶える見通しだ。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2019/11/06-08:55）

A probe launched by NASA four days after Elvis died has delivered a treasure trove of data from beyond the solar bubble that envelops Earth and our neighbouring planets, scientists reported Monday.
But for every mystery Voyager 2 has solved about the solar winds, magnetic fields and cosmic rays that buffet the boundary between interstellar space and the Sun's sphere of influence, a new one has cropped up.
Voyager 2 left Earth's orbit in 1977 a month before its twin Voyager 1, but took seven years longer to reach the heliosphere's outer limit some 18 billion kilometres (more than 11 billion miles) away.
Shaped something like a windsock in a stiff breeze, the heliosphere is formed by the Sun's magnetic field and solar winds that can reach speeds of three million kilometres per hour.
It can be compared to a cosmic supertanker ploughing through space, said Edward Stone, a professor at the California Institute of Technology and lead author of one of five articles published in Nature Astronomy.
As it moves through the interstellar medium -- the vast expanses of space between stellar fiefdoms -- there's a wave in front, just as with the bow of a ship, Stone told journalists by phone.
Scientists hoped to answer a number of questions by comparing data sent back by the two probes, which pierced the Sun's protective bubble at different angles and locations.
- Particle 'leakage' -
We didn't have any good quantitative data of how big this bubble is that the Sun creates around itself with supersonic solar wind and ionised plasma speeding away in all directions, Stone said.
Voyager 2 confirmed, for example, the existence of a magnetic barrier at the outer edge of the heliosphere that had been predicted by theory and observed by Voyager 1.
But contrary to all expectations and predictions, the magnetic field direction did not change when Voyager 2 crossed the heliopause, Leonard Burlaga, a scientist at the NASA Goddard Space Flight Center and lead researcher for one of the studies, told AFP.
The so-called heliopause is the relatively thin contact boundary where solar wind of charged particles and interstellar wind collide.
Scientists were also surprised that it took 80 days for Voyager 2 to cross this magnetic barrier, while its sister probe did so in less than a day.
And then there's the leakage enigma.
As Voyager 1 crossed the heliosphere threshold, it detected particles from outer space -- notably cosmic rays -- racing the other way.
On Voyager 2, it was just the opposite, said Stone. Once we left the heliosphere, we continued to see particles leaking from the inside out.
In at least one case, it was a similarity between the two missions that was perplexing.
- Johnny B. Goode -
This is very strange, said Tom Krimigis, a scientist in the Applied Physics Laboratory at Johns Hopkins University and senior author of a study reporting on measurements of charged particles.
One crossing (of the heliopause) occurred at the solar minimum, when solar activity is the least, and the other at the solar maximum, he told journalists.
If we take our models at face value, we expect that there would be a bigger difference.
The Sun's activity waxes and wanes in 11-year cycles.
The missions also measured incoming cosmic rays that grew stronger as the probes approached heliopause, with direct implications for the health of manned space missions into deep space.
If an astronaut moves closer to the source [of cosmic rays], it is going to be important to understand how much intensity there is, Stone said.
A factor of three is big when we're talking about the affect of radiation on life.
Built to last five years, Voyager 1 and Voyager 2 set out to explore the solar system's outer planets.
After 42 years in action, they are still going strong, although both will run out of power and fall silent within five years.
But that does not mean they will disappear, said Bill Kurth, a researcher at the University of Iowa and co-author of the study focusing on plasma waves.
They will outlast Earth, he said. They are in their own orbits around the galaxy for five billion years or longer, and the probability of them running into anything is almost zero.
If intelligent life in a far corner of the Milky Way finds either probe one day, a golden record including drawing of a naked man and woman, bird and whale songs, and Johnny B. Goode by Chuck Berry will be on board.