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Guest Editorial

How Far Can We Hear?
by Michael Busch
Originally posted to, used by permission

The current and recent past radio leakage from the Earth is dominated by two things:

  • Narrow beams from high-power radars, both scientific and military, which are used to monitor objects throughout the inner solar system and in Earth orbit respectively.
  • The constant-frequency carrier waves of ground-based television broadcasts.

The radar beams are potentially detectable by current radio facilities such as the Arecibo Observatory and FAST and the planned future Square Kilometer Array at distances of tens to hundreds of thousands of lightyears. However, they are transient and only very rarely aimed at any star because they’re tracking objects moving across the sky in the foreground. So unless an astronomer on an Earth duplicate at Tau Ceti were aiming their equivalent of the Deep Space Network directly at the solar system, we would be very unlikely to pick up those radar beams.

Television broadcasts are aimed towards the local horizon, because that’s where most of the customers are. This means that a given transmitter sweeps across much of the sky twice a day - once when that part of the sky is rising, and once when it is setting. The exact region of the sky that is being covered by a given transmitter depends on its latitude, but there are enough TV broadcasters around the world to sweep across the sky in all directions many times a day. While the modulation on the television broadcasts that carries the images and sound would be very hard to detect across interstellar distances, the carrier waves are relatively easy to see because each occupies only a narrow frequency range.

We can measure the pattern of TV carrier waves coming off the Earth in multiple ways: by plotting out the known transmitter power and locations of different TV broadcast antennas, or by monitoring the very small fraction of the radio leakage that is reflected back at Earth by the Moon. The latter was first done in the late 1970s. Given that information, we can estimate how far away different radio telescopes could detect the current TV carrier wave radio leakage from Earth. The Arecibo Observatory and FAST could each detect the TV carrier wave leakage out to a distance of about sixteen lightyears. That’s a little bit further away than Tau Ceti is. However, both telescopes are too far north to observe Tau Ceti. Arecibo and FAST are both single dishes built into roughly spherically-shaped holes in the ground and are constrained to look within a certain number of degrees of straight overhead. This prevents them from viewing objects that are too far north or south on the sky. FAST can look at things within 2º of Tau Ceti, but that’s not quite close enough.

Current radio telescopes in the southern hemisphere, such as Parkes, could only pick up the current TV carrier wave radio leakage at a distance of 3.4 lightyears or so—which is not as far as Proxima Centauri (the nearest star) and less than 30% of the distance to Tau Ceti. However, once the Square Kilometer Array is completed in Australia and Africa, it would be able to detect the current TV carrier wave radio leakage at a distance of about fifty lightyears for objects in the southern hemisphere sky.

Over the course of a year; the SKA would be able to detect the carrier wave leakage at the distance of Tau Ceti, establish that the emission came from radio transmitters on a planet the size of Earth with a rotation period of one day on an orbit in the star’s habitable zone (by tracking the Doppler shift of the signals), and map out the transmitter locations across the portion of the planet’s surface visible to them. With more extensive monitoring, it would be able to detect different transmitters turn on or off or change operating frequency.

Radio SETI searches have been ongoing to varying degrees since Frank Drake’s first project in 1960. SETI astronomers have not found any alien radio signals, but they’ve considered in detail what they’d be able to learn from any that they might find.

Disclaimer: The opinions expressed in editorials are those of the individual authors, and do not necessarily reflect the position of The SETI League, Inc., its Trustees, officers, Advisory Board, members, donors, or commercial sponsors.

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