BOSTON — As the refurbished Hubble Space Telescope returns to work, let's hear it for Europe's retired astronomical satellite, Hipparcos. Ongoing studies of its data are giving Hubble scientists fundamental knowledge that helps them understand what they see.
Earlier this month, astronomer Michael Feast of the University of Cape Town in South Africa reported that his Hipparcos team believes it knows why the universe as seen by Hubble seems younger than the oldest stars. It turns out that Hubble scientists - and other astronomers - have been using a faulty "ruler" to measure distances.
Dr. Feast says his analysis shows the universe to be 10 percent larger - and hence a billion years older - than Hubble scientists have recently estimated. Likewise, his study cuts some 4 billion years off the age assigned to the oldest stars in our galaxy, an estimate that also depends on the stars' distances. Feast and his colleagues tentatively assign the universe an age of about 12 billion years and the old stars an age of around 11 billion years. If their conclusions hold up to further analysis, they will have cured what Feast calls "a nonsensical contradiction that was a headache for cosmologists."
The Hubble and Hipparcos orbiting telescopes have needed each other. One of the Hubble's key tasks is to refine our knowledge of the size and age of the universe. One of Hipparcos's main tasks was to improve the accuracy of the basic distance-measuring tools a hundredfold. It was designed for what astronomers call astrometry - precise measurement of the relative positions and motions of stars.
Astrometry bears the same relation to the cosmic "landscape" that surveying does to geography. By determining what is where on a relatively small scale, it lays the basis for larger-scale mapping. It goes back thousands of years.
A little more than 2,100 years ago, for example, the Greek philosopher Hipparchus used astrometry to measure the distance to the moon. It's no accident that the satellite had a similar-sounding name. The difference in spelling comes from the nerdish practice of cobbling together initial letters from technical terms to make a name - in this case, HIgh Precision PARallax COllecting Satellite.
The technique works like this. Observers note a star's position on the sky as seen from two opposite points along Earth's orbit. They call the angle between the two positions the star's parallax. Then, like a surveyor sighting a landmark from opposite ends of a baseline, they calculate the star's distance by triangulation. This works for stars within a couple of thousand light years.
Beyond that, astronomers estimate distance by noting how dim a "standard" light source appears to be. A certain type of supernova-star explosion or variable stars whose pulsating rhythms reflect their intrinsic brightness are typical "standard candles." To build confidence in such distance estimates, astronomers need to test them against astrometric measurements whenever a "standard candle" is within range.
That's what the Hipparcos data now enables analysts to do with a precision never possible before. Just as the Hubble has a clearer view by orbiting above the atmosphere, so too did Hipparcos see things more sharply. It could resolve angular differences of only a couple of thousandths of a second. That's sharp enough to see an astronaut on the moon. Launched in August 1989 and retired in August 1993, Hipparcos poured down data at rates equivalent to 50 volumes of Shakespeare's complete works per day.
It's taken analysts years to work up those data to the point where crucial findings such as Feast's improved cosmic "ruler" are beginning to emerge.
Like the Hubble with its flawed mirror, Hipparcos broke astronomers' hearts when a booster rocket failure marooned it in the wrong orbit.
And as with the Hubble, the mission team's ingenuity turned that potential tragedy into an outstanding success. Now the Hubble mission itself is benefiting from the European Space Agency's triumph.