MIT OpenCourseWare: watch MIT lectures for free

For example, you can watch here a fascinating lecture entitled "Some Funny Things Happened on the Way to the Moon: A History of MIT's Participation in the Guidance, Navigation and Control of the Apollo Spacecraft".

1967: astronomers discover alien beacons!

Listen to this sound, which has been recorded pointing a radiotelescope towards the constellation of Vela. 

Now imagine that you are the first to ever record such a signal, and have the confirmation that it can not be some kind of man-made radio frequency interference. The idea of an alien origin would occur to you. This is the fascinating story of the discovery of pulsars.

The first pulsar was observed on November 28, 1967, by Jocelyn Bell Burnell and Antony Hewish.The observed emission from the pulsar was pulses separated by 1.33 seconds, originated from the same location on the sky, and kept to sidereal time. In looking for explanations for the pulses, the short period of the pulses eliminated most astrophysical sources of radiation, such as stars, and since the pulses followed sidereal time, it could not be man-made radio frequency interference. When observations with another telescope confirmed the emission, it eliminated any sort of instrumental effects.

At this point, Burnell notes of herself and Hewish that "we did not really believe that we had picked up signals from another civilization, but obviously the idea had crossed our minds and we had no proof that it was an entirely natural radio emission. It is an interesting problem—if one thinks one may have detected life elsewhere in the universe, how does one announce the results responsibly?" Even so, they nicknamed the signal LGM-1, for "little green men" (a playful name for intelligent beings of extraterrestrial origin).

It was not until a second pulsating source was discovered in a different part of the sky that the "LGM hypothesis" was entirely abandoned.

A pulsar (portmanteau of pulsating star) is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. This radiation can only be observed when the beam of emission is pointing toward the Earth, much the way a lighthouse can only be seen when the light is pointed in the direction of an observer, and is responsible for the pulsed appearance of emission. 

In 1974, Antony Hewish became the first astronomer to be awarded the Nobel Prize in physics. 

Pulsar maps have been included on the two Pioneer Plaques as well as the Voyager Golden Record. They show the position of the Sun, relative to 14 pulsars, which are identified by the unique timing of their electromagnetic pulses, so that our position both in space and in time can be calculated by potential extraterrestrial intelligences.

[the plaque attached to Pioneer 10]

[source]

Space History: Pale Blue Dot & Voyager 1

I do enjoy contemplating this famous picture, "Pale Blue Dot", farthest picture of the Earth ever taken. It was shot by Voyager 1 in 1990, while travelling outside our solar system at a velocity of 64000 km/h and a record distance of 6 billion kilometers.

Astronomer Carl Sagan related his thoughts on a deeper meaning of the photograph:

From this distant vantage point, the Earth might not seem of any particular interest. But for us, it's different. Consider again that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every "superstar," every "supreme leader," every saint and sinner in the history of our species lived there – on a mote of dust suspended in a sunbeam. 

The Earth is a very small stage in a vast cosmic arena. Think of the rivers of blood spilled by all those generals and emperors so that in glory and triumph they could become the momentary masters of a fraction of a dot. Think of the endless cruelties visited by the inhabitants of one corner of this pixel on the scarcely distinguishable inhabitants of some other corner. How frequent their misunderstandings, how eager they are to kill one another, how fervent their hatreds. Our posturings, our imagined self-importance, the delusion that we have some privileged position in the universe, are challenged by this point of pale light. Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity – in all this vastness – there is no hint that help will come from elsewhere to save us from ourselves.

The Earth is the only world known, so far, to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment, the Earth is where we make our stand. It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another and to preserve and cherish the pale blue dot, the only home we've ever known.

—Carl Sagan, Pale Blue Dot: A Vision of the Human Future in Space, 1997 reprint, pp. xv–xvi [source]

Regarding Voyager 1, it is still operational and responding to commands broadcast from Earth.

On February 17, 1998, it became the farthest man-made object from Earth, passing Pioneer 10 at 69 AU from the Sun. Since then, Voyager 1 has been the farthest manmade object from Earth, and there are no probes predicted to be launched in the next 20 years that will pass the probe.

On December 3, 2012, NASA scientists announced that Voyager 1 had discovered a previously unknown region of the heliosphere. Described as a "magnetic highway," here the pressure of the interstellar medium sweeps back the Sun’s magnetic field and with it many of the slower moving particles emerging from within the solar system. These are mixed with faster moving particles entering the solar system from the interstellar medium. The magnetic field in this newly discovered region is 10 times more intense than Voyager 1 encountered before the termination shock. It is expected to be the last barrier before the spacecraft exits the solar system completely and enters interstellar space. Voyager 1 is predicted to enter the interstellar medium between 2012–15 [source].