Bad Astronomy. Misconceptions and Misuses Revealed, from Astrology to the Moon Landing 'Hoax'

The Hubble Space Telescope floats freely over the Earth, prepared to take another observation of an astronomical object. Despite its clear views of the universe, the telescope is never more than a few hundred kilometers above the surface of Earth.

(Image courtesy NASA and the Space Telescope Science Institute.)

Lenses are good for smaller telescopes but become unwieldy when they are bigger than about a half-meter (20 inches) across. They have to be supported from the edges, lest you block their view. Large lenses are extremely heavy, which makes them difficult to use. They also need to be placed at the aperture of the telescope, at one end of a long tube. That placement makes the telescope unwieldy and very temperamental to balance.

Since only the front side of a mirror is needed, it can be supported all along its backside, making mirrors easier to use. A mirror reflects light, but a lens has to have light pass through it, which can dim that light. Even better, when you’re making a mirror, you only need to grind and polish one side and not two. That’s a pretty good savings over a lens.

Incidentally, the CNN web site made the same mistake less than a year later. I don’t blame them, though. The Space Telescope Science Institute, which is charged with running the scientific aspects of Hubble, sponsored a PBS program about the telescope. In one episode, I heard an announcer introduce a segment as, “Through Hubble’s Lens.” If even PBS can get it wrong, what chance does everyone else have?

Size Does Matter

Many people are surprised at the large size of the Hubble satellite. It’s roughly as big as a school bus. However, they are usually further surprised when told that the telescope is rather small as such things go. The primary mirror is 2.4 meters (8 feet) across. That may sound big compared to you or me, but there are many telescopes in the world more than four times that size. Even when Hubble was built it was not the biggest telescope. The legendary Hale Telescope at Pasadena’s Palomar Observatory has a 5-meter (nearly 17-foot) mirror, and that one was built in 1936.

Not that Hubble is all that tiny. A full-scale mockup of it stands in a building at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. It’s about five stories high and looms impressively over people as they walk by. Covered in shiny foil to reflect the Sun’s light and keep the observatory cool, it looks like the world’s largest TV dinner.

The reason Hubble isn’t as big as some ground-based observatories is because it’s hard to get something big into space. Hubble was designed to fit inside the Space Shuttle, and that put an upper limit on its size. The Next Generation Space Telescope, designed to observe infrared light and planned for launch in 2009, will be at least six meters (19 feet) across. One design calls for the mirror to be folded, and when it’s out in space the mirror will unfold like a flower. Hubble’s mirror, on the other hand, is basically one giant piece of glass, making it very heavy. If the mirror were any bigger, the spacecraft itself would have to be substantially larger to support it, making it impossible for the Space Shuttle to lift it.

A Drop in the Bucket

A related misconception is that a telescope’s most important function is to magnify an object, or make it look “closer.” That’s only partly true. It helps to make a small object look bigger, of course, but the real reason we make telescopes bigger is to collect more light. A telescope is like a rain bucket for light. If you are thirsty and want to collect rainwater, it’s best to use a wide bucket. The wider the bucket you use, the more rain you collect. It’s the same for telescopes: the bigger the mirror, the more light you collect from an object. The more light you gather, the fainter an object you can see. The unaided eye can pick out perhaps 10,000 stars without help, but with the use of even a modest telescope you can see millions. With a truly big telescope billions of stars become detectable.

The biggest telescopes on Earth have mirrors about 10 meters (33 feet) across, about the width of a small house. There are currently plans to build much larger telescopes. One design calls for a mirror 100 meters (109 yards) across! It’s called the OWL, for Overwhelmingly Large Telescope. It’ll cost a lot, but probably still less than Hubble did. A lot of that cost will probably go into simply finding a place to put it.

So Hubble may be small, but remember, it’s above the atmosphere. The air glows, which washes out faint objects when viewed from the ground (see chapter 11, “Well, Well: The Difficulty of Daylight Star Sighting”). Hubble has darker skies and can see much fainter objects. The atmosphere also moves, so stars seen from the ground wiggle and dance (see chapter 9, “Twinkle, Twinkle, Little Star”). This spreads out the light from stars, making faint ones even more difficult to detect, especially if they are near brighter stars, which overwhelm them. With Hubble above the atmosphere, it avoids this effect and can more easily spot fainter stars. Between the much darker sky and ability to see faint objects, it holds the record for detecting the faintest objects ever seen: in a patch of sky called the Southern Deep Field, one of Hubble’s cameras spotted objects ten billion times fainter than you can see with your unaided eye. That’s a pretty good reason to loft a telescope a few hundred kilometers off the ground.

The Case for Space

Still, it’s not easy getting something that size into space. For a long time, Hubble was the largest single package delivered to orbit from the Space Shuttle. The Shuttle can only get a few hundred kilometers above the Earth’s surface, and schlepping the 12-ton Hubble up made it even harder to get there. Using the Shuttle’s robot arm, in April 1990 astronaut Steve Hawley gently released Hubble into the Earth’s orbit, where it still resides, about 600 kilometers (375 miles) above the Earth’s surface. It’s another common misconception that Hubble is like the starship Enterprise, boldly going across the universe to snap photos of objects no one has snapped before. In reality, the distance from Hubble to the surface of the Earth is about the same as that between Washington, D.C., and New York City. Hubble is only marginally closer to the objects it observes than you are! Sometimes it’s actually farther from them; it may be observing an object when it’s on the far side of its orbit, adding a few hundred kilometers to the distance the light travels from the object to Hubble’s mirror.

Film at 11:00

Which brings us to yet another common misthought about Hubble. Despite what many newspapers and television programs may say, Hubble has never taken a single photograph of an object. Hubble isn’t a giant camera loaded with ISO 1,000,000 film. Hubble uses electronic detectors to take images of objects. These detectors are called charge-coupled devices, or CCDs. You’ve probably seen or used one of these yourself: handheld video cameras have been using CCDs for years, and digital cameras use them as well. They are much better than film for astronomy because they are far more sensitive to light, making it easier to detect faint objects. They are stable, which means that an image taken with one can be compared to another image taken years later. That comes in handy when astronomers want to look for changes in an object’s shape or position over time. CCDs store data electronically, which means the data can be converted to radio signals and beamed back to Earth for processing. That’s their single biggest advantage over film for space telescopes. Who wants to go all the way into orbit just to change a roll of film?

Psst! Can You Keep a Secret?

When Hubble points at an object, it’s pretty likely to show us something we cannot see from the ground. That makes the data highly desirable, and of course that means a lot of competition to get time on the telescope. There aren’t all that many astronomers around, but time on Hubble is an even rarer commodity. Once a year or so an announcement is made asking for proposals to use Hubble. Typically, NASA gets six times as many proposals as Hubble can physically observe during the upcoming year. Six-to-one odds are bit longer than most people like, but there is only so much observing time in a year. This creates a funny situation; a public telescope must, for a short time, have its data kept secret.

This time is called the proprietary period, and it is designed to give the astronomer a chance to look at the data. It may sound odd to keep Hubble data secret. After all, everyone’s tax dollars paid for it, so shouldn’t everyone have the right to see the data right away?

This may sound like a fair question, but really it’s flawed reasoning. Your tax dollars pay for the IRS; then why not get access to your neighbors’ tax returns? Ask the military for the blueprints for their latest secret fighter jet and see how far that gets you.

Now, to be fair, these really are secrets and there are good arguments for them to be. Hubble data are not really secret. But there is still good reason to let an astronomer have them for a year before they are released.

To see why, imagine for a moment you are an astronomer (if you are an