Australia

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Parkes Radio Telescope, Parkes, Australia

32° 59 ′ 59.8 ″ S, 148° 15 ′ 44.3 ″ E

The Dish

If you find yourself in the land down under (or just happen to be lucky enough to live there), then there’s one landmark that shouldn’t be missed, because it’s the star of a movie that is all about science. The film is The Dish , and its star is the Parkes Radio Telescope (Figure 1-1).

Figure 1-1. The Dish; courtesy of Alex Cheal (alexcheal)

Halfway between Melbourne and Brisbane, and 20 kilometers north of the small town of Parkes, stands the gracefully curved dish that since 1961 has been listening to the southern sky for radio transmissions.

It was the Parkes Radio Telescope’s role in picking up transmissions from the Apollo 11 moon landing that made it a movie star. When Neil Armstrong and Buzz Aldrin landed on the moon on July 20, 1969, their transmission of telemetry and television pictures was initially sent to the Goldstone Observatory in the Mojave Desert in California. But problems developed, and NASA switched to getting the signals from the Honeysuckle Creek receiver near Canberra in Australia. Shortly thereafter, they switched to Parkes and found the signal so good that they stuck with the Dish for the rest of the transmission.

The Dish has been involved in a number of other space missions, including Voyager 2 (when it passed close to Uranus and Neptune and returned images of the planets); the Giotto probe that flew close to Halley’s Comet in 1986; and the Galileo probe, which photographed Jupiter in 1997.

The Dish has a small visitor center and two small cinemas showing films explaining radio astronomy and the solar system. While the visitor center is free, there’s a small fee for the cinemas. Since the Dish is located far from civilization, there’s a café serving drinks and meals, and also free picnic facilities and barbeque equipment.

Unfortunately for visitors, the Dish itself is in constant use and is not open for tours. But the observatory occasionally hosts open days when the general public can climb up into the Dish’s rotating mounting, and afterward attend a talk by one of the telescope’s scientists. In the past, the open days have been rounded out by a screening of The Dish under the starlit sky.

As you approach the Dish, switch off anything with a radio in it (such as a mobile phone)—the Dish is listening for very faint radio signals from across the cosmos, so it doesn’t need to listen to you nattering away. Since Parkes is a bit remote, consider making the trip to coincide with a major local event: the Parkes Elvis Festival (held annually in the second week of January) is not to be missed.

But the 64-meter-wide Dish’s main job is radio astronomy, with a special emphasis on pulsars (see sidebar). Over the years, the Dish has been upgraded to make it more and more sensitive to the incredibly faint signals that reach the Earth’s surface.

Practical Information

Information about the Parkes Radio Telescope and other Australian observatories is available from http://outreach.atnf.csiro.au/ .

Pulsars
(The Original Little Green Men)

In July 1967, a team at Cambridge University was working on a radio telescope to listen to signals from quasars (radio sources coming from somewhere in the sky). But they discovered something strange—a pulsating radio signal from the heavens that lasted 40 milliseconds and repeated regularly every 1.337 seconds. At first they thought it must be noise from some earthly source, but quickly realized it was not and named the source LGM-1 (Little Green Men-1).

What they had discovered was a pulsar (or pulsating star), but having never heard a radio source of such regularity coming from the sky, they speculated that it might be a message from a distant civilization, or a navigational beacon for some unknown beings traveling the universe.

Today, LGM-1 is known as the pulsar CP1919, and is one of more than 1,000 known pulsars. And pulsars have been discovered that emit more than just radio waves—they also eject regular bursts of X-rays and gamma rays.

Pulsars are created by rapidly rotating neutron stars. A neutron star is created when a star undergoes a supernova, running out of energy and suddenly collapsing in on itself because of its own gravity. This results in an incredibly dense and compact corpse of a star: a neutron star.

Neutron stars are typically less than 20 kilometers across (the distance from the town of Parkes to the telescope), but contain more mass than the Sun. The interior of a neutron star is made up of neutrons, because the incredible pressure in the star has forced protons and electrons together, eliminating any charge. Deep inside the neutron star, the pressure is so great that there’s probably a soup of even more fundamental particles such as quarks.

Neutron stars are prevented from completely collapsing by the Pauli Exclusion Principle, which says that no two neutrons (in fact, no two identical fermions: see page 118) can occupy the same place at the same time.

It is believed that as the neutron star rotates, its magnetic field interacts with charged particles, leaving its surface to generate electromagnetic radiation. The radiation may be in the radio spectrum or in the form of gamma or X-rays. It leaves the star in a continuous beam emanating from the star’s magnetic north and south poles. This is illustrated in Figure 1-2.

Figure 1-2. A pulsar with its magnetic field and beams of radiation

Since the star’s rotational poles and magnetic poles are offset, the beam isn’t continuously pointed at the Earth, and only sweeps the Earth (appearing to us as a pulse) once per rotation.

In truth, much about pulsars is still not understood, even after 40 years of listening to them. Perhaps the Cambridge University team’s LGM-1 designation was correct after all, and Parkes has been missing extraterrestrial “g’days” since it opened in1961.