In a recent study submitted to the Publications of the Astronomical Society of the Pacific, a pair of researchers from the University of California, Los Angeles (UCLA) and the University of California, Berkeley (UC Berkeley) examine the likelihood of extraterrestrial intelligent civilizations intercepting outward transmissions from NASA's Deep Space Network (DSN) that are aimed at five deep space spacecraft: Voyager 1, Voyager 2, Pioneer 10, Pioneer 11, and New Horizons.
Members of the public are free to track such transmissions at DSN Now, which displays real-time data of outgoing and incoming transmissions to all spacecraft at various times.
All five spacecraft completed their primary missions years ago and are traveling farther away from Earth with each passing second.
Voyager 1 officially left our Solar System in August 2012 followed by Voyager 2 in November 2018, and both are currently traveling through interstellar space. The remaining three have yet to cross our Solar System's heliopause, which is the boundary between the solar wind emitted by our Sun and interstellar wind.
So, what led to this recent study examining Earth transmissions being sent to deep space?
Reilly Derrick, who is a first-year Aerospace Engineering undergraduate in UCLA's Samueli School of Engineering, said this recent study was inspired by a 2019 paper examining what stars outside our Solar System four of the five spacecraft (sans New Horizons) will pass within the next few million years.
While this most recent study was just recently published, Derrick started the project during her junior year at St. Ignatius College Preparatory in San Francisco.
"This work [2019 study] identifies the stars that interstellar spacecraft will closely encounter on their paths through the Universe," Derrick recently told Universe Today.
"Our paper expands upon this idea by identifying which stars the terrestrial transmissions to interstellar spacecraft will encounter. These transmissions cover a wider volume of the surrounding Universe, meaning they will encounter more stars.
"Additionally, the transmissions from Earth travel much faster than the spacecraft themselves, so we would expect potential intelligent life to notice and return transmissions much sooner."
For the study, the research duo used a combination of transmission beamwidth between the DSN and the five spacecraft, spacecraft telemetry data from the JPL Horizons System, and data from the Gaia Catalogue of Nearby Stars (GCNS) to ascertain which stars will intercept the outward DSN transmissions from Earth. The GCNS includes 331,312 stars within 100 parsecs of Earth, with one parsec equal to 3.26 light-years.
In the end, the researchers identified the number of stars each spacecraft transmission will encounter as the signals endlessly traverse the cosmos at the speed of light.
From order of least to most stars, the spacecraft transmissions will encounter 142 stars for New Horizons, 241 stars for Pioneer 10, 289 stars for Voyager 1, 325 stars for Voyager 2, and 411 stars for Pioneer 11.
"In addition, we focus on directed transmissions to space, while the majority of the electromagnetic energy that travels away from the Earth goes out in all directions," Dr. Howard Isaacson, who is a researcher in the Astronomy Department at UC Berkeley, recently told Universe Today.
"This focused energy will produce a stronger (although still very weak) signal when it reaches nearby stars. I agree that this catalog of stars is the most important result, providing a window of time that future researchers can use to plan their observations.
"Ideally, we would observe everything in the sky from every telescope in the world all the time, but with limited observing resources, studies like ours can help to optimize those resources."
Along with his duties in the astronomy department, Dr. Isaacson also holds dual research positions with the Search for Extraterrestrial Intelligence with Breakthrough Listen and the California Planet Search.
Since radio signals travel at the speed of light, the researchers then calculated the elapsed time the star would encounter the transmission beam, the year for which potential extraterrestrial intelligent civilizations around these stars would intercept our transmissions, then finally when we might hear a response from such a civilization should they choose to respond immediately.
"Our results include 5 total stars for which we could expect returned transmissions within the 21st century, and 7 total stars for which we could expect returned transmissions within the next 100 years," Derrick tells Universe Today.
"Although this is obviously a very small number compared to all the stars in the surrounding Universe, the fact that there is even a small chance intelligent life surrounding these stars could recognize human transmissions and communicate back to us within our lifetimes is very exciting."
The five stars that Derrick expects Earth to potentially receive a reply by the end of this century are all located within 50 light-years of Earth, and the two additional stars for which Derrick expects a potential reply within the next 100 years are located within 73 light-years of Earth, which are both relatively short distances in terms of cosmic scales.
When asked if they believe we are alone in the Universe, both researchers believe we are not, and Derrick tells Universe Today that the narrow number of stars identified in the study offers an exciting opportunity for future researchers to search for signals.
"Indeed, the pursuit of these types of questions, including our recent discovery that one of five stars like the Sun has a planet like the Earth, have only been possible to answer with modern technology," Dr. Isaacson tells Universe Today.
"We now have the capability of detecting technology from a distant civilization and that is really profound. This study is a contribution to that pursuit. It has not been that long in history, only ~400 years, since Giordano Bruno suggested that other worlds like the Earth may orbit other stars. That claim did not go well for him. Now we have scientific evidence of such worlds, and we are pursuing the next great discovery: Intelligent life beyond the Earth."
Both Voyager spacecraft stand apart from the other three spacecraft used in this study in that they each carry with them a Golden Record that contain a plethora of images and music from Earth, along with instructions on how to play the record should an advanced civilization come across it.
For now, all we can do is wait and continue to scan the heavens for signs of intelligent life in the cosmos other than our own. Will another technological civilization receive our transmissions, and will we get a reply, someday? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!