'Spooky action at a distance' is still spooky no matter how you explain it. Einstein famously used the phrase to describe quantum entanglement, where two entangled particles appear to interact instantaneously even though separated in space. Now we're talking about using the effect for communications, following the news that European scientists have proven that entanglement persists over a distance of 144 kilometers. Fortunately for would be communicators, a pair of entangled photons can be created in a process called Spontaneous Parametric Down Conversion. Once entangled, the photons stay entangled until one of them interacts with a third particle. When that happens, the other photon changes its quantum state instantaneously. The beauty of entanglement for communications is that anyone trying to listen in on a message invariably disrupts the entangled system, a result that would be easily detectable. The security potential is obvious in a world where so much banking information takes...

Given how tricky it is to pick up accidental radio signals -- "leakage" -- from extraterrestrial civilizations, how hard would it be to communicate with our own probes once they've reached a system like Alpha Centauri? A front-runner for interstellar communications is the laser. JPL's James Lesh analyzed the problem in a 1996 paper, concluding that a 20-watt laser system with a 3-meter telescope as the transmitting aperture could beam back all necessary data to Earth. It's a system feasible right now. Right now, that is, if we had some way to get the telescope, just a bit larger than the Hubble instrument, into Centauri space. But even though propulsion lags well behind laser technology for such a mission, we're continuing to study how lasers can help closer to home. Their high frequencies allow far more data to be packed into the signal, but the highly focused beam also uses a fraction of the power of radio. Data return becomes less of a trickle and more of a flood (imagine...

As we move up the frequency ladder toward optical communications, each step takes us closer to the kind of data traffic we'll need for deep space missions into the Kuiper Belt and beyond. The idea is to pack as much information as possible into the signal. A stream of data transmitted from an antenna spreads at a diffraction rate that is determined by the wavelength of the signal divided by the diameter of the antenna. Higher frequencies, then, give us a much narrower signal, alleviating bandwidth crowding. And a laser communications system makes fewer demands upon a spacecraft's power sources than radio. So watch developments like the recent experiment performed by the Japan Aerospace Exploration Agency (JAXA) with interest. The agency carried out a successful optical test using laser beams between its 'Kirari' satellite (also known as the Optical Inter-orbit Communication Engineering Test Satellite) and a mobile ground station in Germany. The downlink occurred with the satellite at...

Centauri Dreams is saddened to learn of the death of Eberhardt Rechtin, a pioneer of deep space exploration. The list of his accomplishments is long: Rechtin served as CEO of Aerospace Corporation (El Segundo, CA), as chief engineer of Hewlett-Packard, and as director of the Defense Advanced Research Projects Agency. But his career is best remembered for his work on the Deep Space Network that today allows us to track probes at the edge of the heliosphere. Building that network was a towering achievement, and one that seemed unlikely in the 1950s, when Rechtin developed Microlock, a system of receivers that, when deployed at different ground stations, could compensate for the shifting frequencies produced by rockets in flight. Those of us who remember the days right after Sputnik may recall that there was an intense effort to expand America's ground tracking capabilities thereafter. Microlock was at the heart of it, expanding to the system that tracked the first US satellites. The...

Why is it so tricky to deliver large amounts of data from space? One key issue is frequency -- because the amount of data that can be transmitted varies with the square of the frequency, higher frequencies give you more bang for the buck. Moving the Deep Space Network from today's X-Band (between 8.40 and 8.45 GHz for deep space work and between 8.45 and 8.50 GHz for near-Earth operations) to the Ka-Band (31.80 to 32.30 GHz) will increase the network's capabilities by a factor of four or five. But the real goal is optical communications, where the far narrower signal carries a vastly increased amount of information. We need that kind of data-packing not only to get around spectrum-crowding as more and more spacecraft need to talk, but also to handle the high resolution imagery and video we'll want to see from future deep space missions. "It can take hours with the existing wireless radio frequency technology to get useful scientific information back from Mars to Earth. But an optical...

All but lost in the recent news of the Stardust sample return and New Horizons launch, the Messenger spacecraft continues on its journey to Mercury. And significant science has already occurred, particularly the laser link set up across a record 24 million kilometers (15 million miles) between the spacecraft and Earth. Laser communications with spacecraft are still in their infancy, but this test showed the potential of moving past microwaves into far more effective laser channels. The beauty of a laser signal is that it spreads much more slowly than conventional radio signals, a huge factor given the need to return significant data at maximum speed. Consider this: the Mars Pathfinder mission returned a radio signal that spread to hundreds of times the diameter of the Earth by the time it reached us. The 23-watt signal of the distant Voyagers broadcasts a beam now a thousand times the Earth's diameter. These numbers play havoc with signal strength. Voyager's signal to Earth during...

One reason we need to re-think our communications strategies is that our resources are so limited. The Interplanetary Internet Project, for example, points out as a major justification for its work that if we can network spacecraft in distant planetary environments, we can sharply cut back on the amount of antenna time needed. After all, having a trio of spacecraft (including an orbiter, perhaps, and two rovers) linking their data to a single relay would mean a unified data download to Earth. The IPN idea would create new networking protocols that could make these things happen. But research continues on other fronts, including the technology we use to transmit signals. An upcoming paper discusses phased arrays, wherein a large number of mini-transmitters could send a combined beam into the sky. Systems like these are familiar to those working with military radar but have been hitherto unavailable for cost reasons for civilian uses. The paper, by Louis Scheffer at Cadence Design...

Celestial mechanics seems a long way from atomic physics, but new work by scientists and engineers suggests some remarkable parallels. In fact, the mathematics describing both have provided new designs for space missions, as witness the Genesis spacecraft, which returned particles from the solar wind to Earth. Genesis' highly unstable orbit was controlled by the L1 Lagrange equilibrium point, a point between Earth and the Sun where the gravity of both bodies is balanced. The orbit is an example of a chaotic trajectory identical to those traversed on the atomic level by highly excited electrons. Image: The extraordinary orbit of the Genesis spacecraft, a lesson in controlling chaos. Credit: Jet Propulsion Laboratory. The linkage between orbits and atoms is discussed in an article running in an article called "Ground Control to Niels Bohr: Exploring Outer Space with Atomic Physics," running in the October 2005 issue of Notices of the American Mathematical Society. The article is also...

Getting an interstellar probe to its target involves navigation of a high order. 'Marker' stars -- stars that are both bright and distant enough to have relatively fixed positions for the duration of the journey -- often show up in the scientific literature. Thus Rigel and Antares, both of which are far larger than the Sun, are attractive markers. Rigel (Beta Orionis) is some 800 light years distant, while Antares (Alpha Scorpii) is over 500 light years away. Are there other, better kinds of markers? Perhaps so, according to the European Space agency. ESA, through its Ariadna initiative, is homing in on using pulsars for navigation. Ariadna operates under ESA's Advanced Concepts Team to study new space technologies through linkages with the European academic community; it's a way to strengthen the agency's ties with independent researchers. As in the US, creating such connections is tricky business, but Ariadna is already doing interesting work, as its new study on pulsars suggests....

Building the first interplanetary laser link, a project known as the Mars Laser Communications Demonstration, will be one of the topics discussed at an upcoming conference in Anaheim. The gathering targets the latest developments in optical technologies from key players in the industry, but it's the use of lasers for space communications that catches Centauri Dreams' eye. As we go deeper and deeper into the Solar System and beyond, we'll need these technologies to allow for effective data return. Ponder this: because radio signals fall off in intensity with the square of their distance, a spacecraft twice as far from Earth sends a signal with four times less strength. The 23-watt signal of Voyager has spread to a beam width 1000 times the diameter of Earth by the time it reaches us. The Jet Propulsion Laboratory's James Lesh says that makes the Voyager signal twenty billion times less powerful than what it takes to run a digital wristwatch. Move to Alpha Centauri and the dropoff...

Here's an interesting observation from Joss Bland-Hawthorn, who is head of instrument science at the Anglo-Australian Observatory in Sydney: "Astronomers are losing vast amounts of data from recent satellite missions to Mars. We collect a hundred times more than we can transmit back." The comment appears in the current issue of New Scientist, in an article by Maggie McKee called "Mars Laser Will Beam Super-fast Data." And the problem identified is one that will plague us more and more the farther we get from Earth. Radio signals are inherently less efficient than lasers, and not only because shorter wavelengths can carry more information in the same unit of time. A laser signal transmitted from a Mars orbiter, says New Scientist, will only spread to a width of a few hundred kilometers by the time it reaches the Earth. A radio signal, by contrast, diffuses rapidly with distance. How rapidly? Well, JPL's James Lesh told me in a telephone interview last year that the Mars Pathfinder...

Although using older protocols, NASA and ESA demonstrated how interplanetary networking will change space exploration by linking one of the Mars rovers (Opportunity) to ESA's Mars Express orbiter. Both spacecraft were using the Proximity-1 protocol developed by the Consultative Committee for Space Data Systems, which has worked for years on standardizing methods of transmitting data from spacecraft. On the horizon are the protocols being developed by the InterPlanetary Internet project, which aim at eventually providing a communcations infrastructure throughout the Solar System.