INTERLUDE 17-1 The Hipparcos Mission
In 1989, shortly before the large, complex, and expensive Hubble Space Telescope was launched by NASA into Earth orbit, a small, simple, and cheap satellite was also deployed by the European Space Agency. Called Hipparcos—a tortured attempt to honor the ancient Greek astronomer Hipparchus of Nicea, who made the first known star map, by creating an acronym out of High Precision Parallax Collecting Satellite—this spacecraft was initially thought to be lost in space. Hipparcos went into the wrong orbit, its onboard engine failed to fire, and its solar panels were heavily pelted with particles in Earth's Van Allen belts. However, before it fell silent in 1993, the engineers managed to control the errant spacecraft, and ESA scientists revamped its mission. Nearly a decade after its launch, its massive database was cataloged and released.

The heart of Hipparcos (shown here before launch) was a small 29-cm mirror designed to determine the positions of stars very accurately. By measuring those positions from two vantage points in orbit (above Earth's turbulent atmosphere), the satellite was able to determine stellar parallaxes with about 10 times greater accuracy than is possible using telescopes on the ground. Since volume scales as the cube of the size, Hipparcos's prime mission was to measure accurate distances for roughly a million stars out to some 200 pc.

In addition to measuring distances to nearby stars with unprecedented accuracy, some other highlights of the Hipparcos mission included the following:

  • In about a million years, the star named Gliese 710 may pose a hazard to planet Earth. Currently this red dwarf star lies 20 pc away in the direction of the Ophiuchus constellation, but its rapid line-of-sight motion will eventually take it within 0.3 pc (less than a light year) of the Sun. As it passes perhaps right through the Oort cloud of comets surrounding our solar system, this dim dwarf star will shine as brilliantly as the red giant Betelgeuse does now. It may also perturb the Oort cloud significantly, sending a shower of comets hurtling in our direction. Astronomers estimate that no star has come this close to the Sun in the past 10 million years or so.
  • Parallax calibrates other "yardsticks" used by astronomers to measure distances in increasingly larger realms and eventually to estimate the size and scale of the entire observable universe. As a result, Hipparcos has affected
    distances far beyond those it was able to measure directly in our stellar neighborhood. In particular, recalculation of distances to a certain class of variable stars (called Cepheids) has put astronomy's entire distance scale on a much firmer foundation.
  • Because of these recalibrations, some distances have changed a little, others need substantial revision. For example, the Large Magellanic Cloud is now thought to be about 10 percent farther away than had previously been estimated. Likewise, the distance to the Andromeda Galaxy has been increased by about a third; its distance is now pegged at 0.9 Mpc (or 2.9 million light years). All distances throughout this text reflect the new values as determined by Hipparcos.
  • The new data have also increased the size, and hence the age, of the universe. The best number for that age, based on Hipparcos measurements, now seems to be about 12—13 billion years. This is the age used in this book.
  • Hipparcos also measured the colors (i.e., temperatures) and apparent brightnesses (and hence luminosities, since the distances are known) of the stars it surveyed. Based on these new data, a major revision of the Hertzsprung—Russell diagram (a key plot of stellar properties used throughout our text) implies that the oldest stars are a little younger than was previously thought. This new analysis of stellar evolutionary tracks seems to reconcile the age of the universe with that of the oldest stars. Prior to the Hipparcos mission, astronomers had faced the embarrassing problem that some stars seemed to be older than the universe itself. No more—this major breakthrough in a heretofore log-jammed subject may well be Hipparcos's greatest contribution to astronomy.

    The Hipparcos mission was not glamorous. This little satellite sent back no pictures for European citizens (who paid for it) and others to marvel at, and it grabbed few headlines in the world's press. Rather, its catalogs, comprising an astonishing 1000 gigabytes of data, resemble huge phonebooks—unexciting to look at but of enormous value for accessing the vital statistics of stars and the entire universe beyond.