Millions of years later, we know better.

"That star was a supernova, one of many that exploded in our part of the galaxy during the past 10 million years," says astronomer Mark Hurwitz of the University of California-Berkeley.

Supernovas near Earth are rare today, but during the Pliocene era of Australopithecus supernovas happened more often. Their source was an interstellar cloud called "Sco-Cen" that was slowly gliding by the solar system. Within it, dense knots coalesced to form short-lived massive stars, which exploded like popcorn.

Researchers estimate (with considerable uncertainty) that a supernova less than 25 light years away would extinguish much of the life on Earth. The blast needn't incinerate our planet.

All it would take is enough cosmic rays to damage the ozone layer and let through lethal doses of ultraviolet (UV) radiation. Our ancestors survived the Pliocene blasts only because the supernovas weren't quite so close.

We know because we can still see the cloud today. It's 450 light years from Earth and receding in the direction of the constellations Scorpius and Centaurus (hence the cloud's name, "Sco-Cen").

Astronomer Jes�s Ma�z-Apell�niz of Johns Hopkins University recently backtracked Sco-Cen's motion and measured its closest approach: 130 light years away about 5 million years ago.

Sco-Cen was still nearby only two million years ago when many plankton, mollusks, and other UV-sensitive marine creatures on Earth mysteriously died. Paleontologists mark it as the transition between the Pliocene and Pleistocene epochs.

Around the same time, according to German scientists who have examined deep-sea sediments from the Pliocene era, Earth was peppered with Fe60, an isotope produced by supernova explosions.

Coincidence?

No one knows. It's a puzzle researchers are still piecing together.

Reconstructing the history of near-Earth supernovas is difficult because old supernovas are elusive. Their glowing shells fade to invisibility in not much more than a million years.

Neutron stars, the collapsed cores of supernova progenitors, last longer, but they are flung across the galaxy by asymmetries in the explosion. Unusual isotopes of iron, like the ones that coincide with the marine extinction, are difficult to find buried under millions of years of sediments.

There is, however, one obvious relic: "All those explosions blew an enormous bubble in the interstellar medium," says Hurwitz, "and we're inside it."

Astronomers call it "the Local Bubble." It's peanut-shaped, about 300 light years long, and filled with almost nothing. Gas inside the bubble is very thin (0.001 atoms per cubic centimeter) and very hot (a million degrees)--that's 1000 times less dense and 100 to 100,000 times hotter than ordinary interstellar material.

The Local Bubble was discovered gradually in the 1970's and 1980's. Optical and radio astronomers looked carefully for interstellar gas in our part of the galaxy, but couldn't find much in Earth's neighborhood. Furthermore, there seemed to be a pileup of gas--like the shell of a bubble--about 150 light years away.

Meanwhile, x-ray astronomers were getting their first look at the sky using orbiting satellites, which revealed a million-degree x-ray glow coming from all directions. "We eventually realized that the solar system was inside a hot, vacuous bubble," says Hurwitz.

Perhaps as soon as this week, NASA plans to launch a satellite--the Cosmic Hot Interstellar Plasma Spectrometer, or "CHIPS"--to study the Local Bubble. "There's a great deal we don't know about it," says Hurwitz, who is the mission's chief scientist. How old is the bubble? What is its internal geography? How fast is it cooling? Data from CHIPS will help answer these questions.

CHIPS will orbit Earth and peer into the bubble using an ultraviolet (UV) telescope. "The gas in the bubble is very bright at extreme UV wavelengths around 170 �," explains Hurwitz.

Other satellites have examined such UV light from the bubble, but CHIPS is better. It has a spectrometer on board with 100 channels ranging from 90 � to 260 �. "The spectrometer is the key," he says.

Like sediments in the Pacific Ocean, gas in the Local Bubble contains supernova-produced iron. "Iron atoms in the bubble have lost many of their electrons--knocked loose by collisions within the hot gas."

CHIPS's spectrometer will be able to detect spectral lines from iron atoms missing 8, 9, 10 and 11 electrons, respectively. By comparing the intensity of those four spectra lines, researchers can map the temperature and density of gas in the bubble.

"If we find a hot spot," says Hurwitz, "that might be the location of the most recent supernova." The spectra will also tell researchers how fast the gas is cooling and thus how old different parts of the bubble might be. A fast-cooling knot of gas which is still hot must be pretty young, for example.

Exploring the internal geography of the bubble is important because what lies inside could affect our planet's future.

During the past few million years, wispy filaments of interstellar gas have drifted into the Local Bubble. Our solar system is immersed in one of those filaments--the "local fluff," a relatively cool (7000 K) cloud containing 0.1 atoms per cubic centimeter.

By galactic standards, the local fluff is not very substantial. It has little effect on Earth because the solar wind and the Sun's magnetic field are able to hold the wispy cloud at bay.

There are, however, denser clouds out there. The Sco-Cen complex, for instance, is sending a stream of interstellar "cloudlets" in our direction. "Some of those cloudlets might be hundreds of times denser than the local fluff," says Priscilla Frisch, an astrophysicist at the University of Chicago who studies the local interstellar medium.

"If we ran into one, it would compress the Sun's magnetic field and allow more cosmic rays to penetrate the inner solar system, with unknown effects on climate and life."

CHIPS will be able to locate dense interstellar clouds by the shadows they cast. Cool clouds are partially opaque to the bubble's UV glow, so they will appear as darker areas in CHIPS maps.

Hurwitz notes that the mission's first sky maps will be rather coarse, with a resolution of 5o x 25o. (The bowl of the Big Dipper, for comparison, is about 10 degrees wide.) Only the largest clouds would appear in those. Later, if the mission is extended beyond its first year, CHIPS will have time to produce sharper maps with 5o x 6o resolution.

Frisch has noted that Homo Sapiens arose only after the local interstellar medium was cleared out. Fewer clouds to run into would promote a stabler climate for our planet, she argues. So perhaps what Australopithecus saw was a good omen, after all....

CHIPS will help us find out.

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