deep space communication
PERSPECTIVES

Solving the Data Transmission Challenge in Deep Space

By Jason Batt, Creative and Editorial Manager, 100 Year Starship

Humanity has crossed a historically important threshold in the last decade and most of our population didn’t notice. With the constant rotating shift of global personnel on the space station in Earth’s low orbit, there are and likely always will be humans living off-planet. We have become not just a space-faring civilization but a space-inhabiting one.

With each successive push, as we move beyond orbit to the moon and then to the red planet of Mars, we are faced with communication challenges. In the near future, our greatest gap in human to human communication might be an approximate forty-minute turnaround time for data to go to Mars and receive a reply back1. But as we move beyond Mars into deep space and then venture out beyond the solar system grasping for the stars in the interstellar medium, the gap becomes monumental.

Data is not just storage and processing. Sending and receiving are critical points. In deep space and interstellar missions, sending and receiving of data might just be the hurdles we’re unable to leap. The laws of the universe and the physical composition of interstellar space make communicating at deep space and interstellar distances challenging, although not (entirely) impossible.

The Vast Distances across Space

Let’s frame the first (and largest) problem: the distance. When we’re moving around our Earth in planes that can get us from one coast to another in hours, we struggle to grasp the enormity of space.

The distance between the Sun and Proxima Centauri, our nearest star, is 4.35 light years. Light travels 9.46 trillion kilometers in a year. So Proxima Centauri is 40,208,000,000,000 kilometers (or 268,770 Astronomical Units—the distance between the Earth to the Sun) away.

The distance between LA and New York is 3,941 kilometers.

The space probe that would become the most-distant human-made object launched from Earth on September 5, 1977, and finished its “grand tour” of the planets around 19892. It is heading out into interstellar space at nearly 61,000 km/h3. It’s now over 20 billion kilometers from Earth, and counting4.

If we shrunk the space between the Sun and Proxima Centauri down and placed the Sun where Los Angeles is and Proxima Centauri where New York is, Voyager would’ve only traveled 1.84 kilometers in 42 years and wouldn’t even have left Southern California.

deep space communication

Big Challenges to Data Transmission in Space

So why is the distance so challenging?

First is the degradation of the signal. In transmitting data digitally, the Bit Error Rate (BER) is the amount of data that is altered or lost due to “noise, interference, [or] distortion.” BER is a risk in all information sent digitally. When that data is sent across the stars, through the interstellar medium, the likelihood of errors increases exponentially. The errors occur as the transmissions encounter clouds of ionized gas: invisible pockets of distortion that are challenging to anticipate precisely.

Second is the ever-increasing energy demands of sending a signal over such massive distances. According to the inverse-square law, “The power of any signal falls with the square of the distance traveled.” Energy becomes the critical factor in data transmission. Any alteration to the message creates energy use: length of data transmission, sending to multiple sources at once, rate of data transmission. The delicate balance of maintaining enough energy to reach the destination versus keeping the exponentially rising energy needed to a manageable limit is a difficult process5,6.

Oh, and one other thing: this is difficult if we’re talking stationary targets. If it’s planet to planet or star to star, we might be able to pull it off. But trying to communicate with a moving object is far more difficult. The challenge of communicating to a distant interstellar receiver is only compounded if that target is a ship traveling at the speeds necessary to cross that great gap.

Communication at Interstellar Distances

How do we send data? How do we receive data? Using current methods, a simple approach is with a large ground-based or orbiting antenna dish hooked up to a nuclear power supply (remember the energy requirement is the square of the distance traveled). Assuming one bit per second, we can send roughly 40 megabytes (MB) of data every decade.

One idea held by the space research community posits that there are large portions of the microwave spectrum through which most of the material encountered in interstellar space would be transparent for. If we’re sending signals, we know the frequency. But this only addresses the BER challenge and not the energy requirement.

Using the Sun as a Gravitational Lens

There is a unique idea that has come out of Italy that might address both of these issues: turn the Sun into an antenna of sorts. One highly-regarded astronomer, space scientist and mathematician – in multiple presentations, papers, and books – has explored the use of the Sun’s gravitational lensing effect as a way to communicate at interstellar distances7. Gravitational lensing is a discovery that is derived from the widely-accepted general theory of relativity. We’ve even observed them—hundreds so far. Their use is of obvious benefit in the detection of extrasolar planets.

This researcher from Italy hopes that the Sun could be used to search for current extraterrestrial radio signals and then used to provide for sending and receiving data from other stars.

deep space communication

A gravitation lens works much like an optical lens except that its focal point is actually a focal line that is constructed of several focal points.

The same space scientist concludes (and proves) that the Bit Error Rate of transferring data will be unacceptable without gravitational lensing using any other method even to the short distance of Proxima Centauri. The ultimate dream and the end is the creation of a “radio bridge” between two stars. After digging through the various calculations, he determined that placing a probe behind Alpha Centauri A (the larger of the trinary star system) might require just one-tenth of a milliwatt to have perfect communication between the Sun and Alpha Centauri A through two 12-meter FOCAL spacecraft antennas8.

However, all of this assumes technology that has yet to be invented. True, nothing exists yet that can actually make the gravitational lens real.

Probing New Ideas for Interstellar Data Transmission

Another approach is an interstellar version of Sneakernet. “What’s Sneakernet,” you might ask? It’s how the largest search engine in the world transmits massive amounts of data9. In fact, if you want to transfer even a few hundred gigabytes of data, it’s generally faster to do overnight delivery of a hard drive than to send the files over the internet8. Until recent years, Cuba, largely cut off from the developing world, still had a massive proliferation of internet content through the physical distribution of media on USB drives hand-distributed one person to another10.

120 terabytes of data, on a 100 megabit connection, would take nearly four months. Or it can be duplicated on a few hard drives and shipped overnight via ground transportation companies.

deep space communication

One space-based program working on interstellar space missions is proposing sending a micro-computer with a solar sail in the next decade or two to Alpha Centauri and the intent is to get that up to .20 lightspeed (which will still take it nearly 20 years to reach our nearest star)11.

40 years sounds like a long time and it’s one of the reasons that physical data packages are overlooked. Long ago, a well-known nanotechnology scientist put the work into debunking all of the reasons why probes wouldn’t work. The conclusion was probes aren’t more expensive at the travel times we’re talking about in the interstellar medium12. In the author’s eyes, what does an extra decade or century mean when the data is guaranteed to arrive whole and all at once?

In one interesting approach, the founder of a global space-advocacy non-profit has suggested that rather than data drives, the physical delivery system should be microbes. He concluded, “So, bacteria can be projected across interstellar space at essentially no power cost to the transmitting party, beyond that required to launch them to planetary escape velocity13.” How do you load those packages up with important data? The genetic material of common bacteria has potentially millions of bases – bits of data – encoded14. This nanotechnology scientist concluded that bacteria can store data at a density of 900TB per gram15.

Photos from the Grandkids

All of this seems theoretical now, but that threshold of becoming a space-inhabiting civilization snuck up on us. Future generations might be exploring deep space and, back here on Earth, getting summer vacation pictures from them won’t be as easy as hopping on social media. Communication has always been critical for our connection and growth as a species. A famous saying concludes that when barriers to communication are removed, nothing would be too great to us as a species. Our greatest, worldwide effort to reach the planets and ultimately the stars might truly slow down our ever-increasing communication. Each minute and each bit of data will become even more valuable than it is today.

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Sources:

  1. Time Delay Between Mars And Earth – Mars Express. http://blogs.esa.int/mex/2012/08/05/time-delay-between-mars-and-earth/
  2. Voyager – Mission Timeline. https://voyager.jpl.nasa.gov/mission/timeline/#event-voyager-1-launches
  3. NASA Is About to Launch the Fastest Spacecraft in History. Target: The Sun! https://www.space.com/41447-parker-solar-probe-fastest-spacecraft-ever.html
  4. https://voyager.jpl.nasa.gov/
  5. Design for minimum energy in starship and interstellar communication. https://arxiv.org/abs/1402.1215
  6. End-to-end interstellar communication system design for power efficiency. https://arxiv.org/abs/1305.4684
  7. Maccone FOCAL mission. http://kiss.caltech.edu/workshops/ism/presentations/Maccone%20FOCAL%20mission.pdf
  8. Mathematical SETI: Statistics, Signal Processing, Space Missions. https://books.google.com/books?id=cuJMzvdIWMcC&pg=PA385#v=onepage&q&f=false
  9. FedEx still faster than the internet – Pingdom Royal. https://royal.pingdom.com/fedex-still-faster-than-the-internet/
  10. Inside Cuba’s massive, weekly, human-curated sneakernet. https://boingboing.net/2018/05/03/inside-cubas-massive-weekly.html
  11. Breakthrough Initiatives. https://breakthroughinitiatives.org/news/12
  12. Debunking the Myths of Interstellar Probes. http://www.rfreitas.com/Astro/ProbeMyths1983.htm
  13. Interstellar Communication Using Microbes: Implications for SETI. https://www.centauri-dreams.org/2017/12/21/interstellar-communication-using-microbes-implications-for-seti/
  14. Bacterial Genomes | Department of Microbiology. https://micro.cornell.edu/research/epulopiscium/bacterial-genomes/
  15. Scientists Discover How to Store Data in Bacteria | HuffPost. https://www.huffpost.com/entry/bacteria-that-can-store-data_n_813894?guccounter=1

3 Comments

  1. Most sources for this article are technically correct, but the author lacks the scientific background to competently understand and work with the information.

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