A Breeze from the Star Signs

NASA spacecraft are monitoring an interstellar wind coming from the constellation Ophiuchus.

Every year in December, something happens that can throw your horoscope out of whack. The sun enters Ophiuchus, the little-known 13th house of the zodiac.

You've probably heard of Capricorn, Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio and Sagittarius. The sun passes through these constellations, one by one, throughout the year. They're the ancient signs of the zodiac.

But Ophiuchus?

Modern astronomers don't divide the sky the same way ancient astronomers did. According to modern star maps, the sun cuts through a 13th constellation, Ophiuchus the Serpent Bearer, between Nov. 30th and Dec. 17th. Astrologically speaking, if you were born between those dates you're no longer a Sagittarian, you're an Ophiuchi! But that's another story....

This story is about what really happens when the sun enters the zodiac's 13th house: An interstellar wind hits our planet.

It's a helium-rich breeze from the stars, flowing into the solar system from the direction of Ophiuchus. The sun's gravity focuses the material into a cone and Earth passes through it during the first weeks of December. We're inside the cone now.

"There's no danger to anyone on Earth," says space physicist George Gloeckler of the University of Maryland. "The helium breeze is a thousand billion billion times (10²¹ times) less dense than Earth's atmosphere. It cannot penetrate to the surface of our planet."

Nevertheless, astronomers are keen to study it.

The breeze is a telltale sign of what lies outside the solar system. Interstellar space, the "void" between the stars, is not empty. It's filled with gigantic clouds of gas and dust. These clouds are the birthplace of stars and planets; they're also the debris left behind when stars explode. The solar system is running into one. Astronomers call it the Local Interstellar Cloud. The sun's magnetic field holds much of the cloud at bay, but some of the cloud's gas does penetrate - hence the breeze.

NASA's ACE (Advanced Composition Explorer) spacecraft, located at the first Lagrange point directly between Earth and the Sun, is perfectly situated to study this breeze. "When Earth moves through the focusing cone (the region of space where the sun's gravity focuses the breeze), so does ACE," explains Gloeckler, who is one of ACE's Lead Co-investigators. "We've been through the cone seven times - once a year since the spacecraft was launched in 1997."

ACE's mission is to study the solar wind, the hot breeze from our own star, so the spacecraft is well equipped to study the interstellar breeze, too. An instrument onboard ACE called SWICS detects helium ions in the breeze, measuring their density, temperature and direction of flow. Using these measurements, along with data from other spacecraft (chiefly SOHO and Ulysses), Gloeckler and colleagues have calculated the properties of the Local Interstellar Cloud.