This is one of my very favorite photos. Earth is tiny – less than a pixel wide – and looks so tentative as if any moment it might disappear in the reflective glare of the sun. It was taken on February 14, 1990 at the end of its primary mission from 3.7 billion miles miles away. NASA mission controllers commanded the probe to spin around and take a family portrait of the solar system.
Carl Sagan, planetary astronomer and creator of the inspirational Cosmos TV series, made the suggestion. Here’s what he had to say about the picture. It recalls Apollo astronaut Neil Armstrong’s vision of Earth:
“From this distant vantage point, the Earth might not seem of any particular interest. But for us, it’s different. Consider again that dot. That’s here. That’s home. That’s us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every “superstar,” every “supreme leader,” every saint and sinner in the history of our species lived there – on a mote of dust suspended in a sunbeam.”
Voyager 1 left the Earth 35 years ago today on September 4, 1977 and took our eyes all the way to Jupiter, Saturn and beyond. Today it’s 11.3 billion miles from home near the edge of the solar system, still within the cocoon of the sun’s influence but just barely. Within the next year or two the probe will sail through the heliopause or boundary of the solar system and into interstellar space.
In May this year Voyager 1 detected an increase in cosmic rays from outside the sun’s realm, a sure sign it’s close to leaving the solar womb. These high-speed particles cut loose during explosive events like supernovae and bound around the galaxy until zipping through the heliopause and heading for Earth. They collide with atoms in the rarefied upper atmosphere and create showers of small, secondary particles that head toward the ground. Only a small number of those ever make it down that far, but they and ones further up are responsible for a portion of our planet’s natural background radiation.
While we’re on the topic of radiation, NASA recently launched the twin Radiation Belt Storm Probes into Earth’s Van Allen radiation belts last Thursday. The belts are two donut-shaped regions filled with high energy particles that surround the Earth. Most of the particles stream in from the sun and get trapped by the planet’s magnetic field. The inner belt, a mix of protons and electrons, can reach down as low as 600 miles; the outer belt is filled with fast-moving electrons and extends outward some 37,000 miles from the surface of Earth.
No one knew about any of this activity before 1958. That’s when the U.S. successfully launched Explorer 1, our country’s first satellite. James Van Allen, a space scientist at the University of Iowa, insisted that a Geiger counter be placed on board to measure cosmic rays in space. He got his way and the counter shot off the scale as it orbited the Earth.
Space scientists came to realize through this and subsequent satellite missions that the “empty” space near our planet was buzzing with activity. The belts were named after their discoverer.
The Radiation Belt Storm probes are identical craft that will fly in separate orbits through the inner and outer Van Allen belts. Since most of the subatomic particles there arrived from the sun, solar activity causes all kinds of interesting fluctuations in the belts. Sometimes they’re chalk full of electrons and can affect satellite electronics; other times the belts are nearly empty and quiet. The probes will sample magnetic and electrical fields in the belts and count how many and what kinds of particles are flying around up there. Scientists hope the data will help better predict space weather which includes things like northern lights. Stay tuned.