Abstract
As humanity explores the Solar System, the further our spacecraft get from Earth the further their data signals have to travel. We look at some of the biggest obstacles that come up when attempting to transfer data billions of kilometers across space using a power- and weight-limited spacecraft.
As humanity explores the Solar System, the further our spacecraft get from Earth the further their data signals have to travel. We look at some of the biggest obstacles that come up when attempting to transfer data billions of kilometers across space using a power- and weight-limited spacecraft.
Main Text
Exploring the planets beyond Earth has long been a dream of humanity, one that finally started to come true on October 4, 1957, when Sputnik first launched into orbit. The early spacecraft was little more than a metal sphere with a radio stuck inside, its bleeps and bloops informing the world of its journey around the planet. While this was enough for it to take hold of the public imagination and top headlines around the world, it only served to fuel humanity’s desire to see the distant planets of our Solar System. But it also became apparent that while it might be possible to send a robotic ambassador to the other planets, just sending back bleeps and bloops would never satisfy the public. The next generation of spacecraft were equipped with more sensors, such as magnetometers and particle sensors, but while streams of numbers might be interesting to scientists, the public wanted more, especially as the spacecraft started venturing away from Earth and toward the Moon. It wasn’t enough to go there—people wanted to see it. They wanted pictures.
This created something of a problem for early spacecraft designers. While it was fairly straightforward to put a camera into a spacecraft, getting the pictures back was not. This was in an age not just before digital cameras but when pretty much all photography was recorded onto film that had to be chemically developed. It could be possible to return the film canisters to Earth for processing, which the US military had been doing for high-altitude spy photography since 1946. But this meant that not only could they not look at the photography while in flight, they had to return the spacecraft to Earth in good enough condition to use the photographs.
Instead, both the Soviets and US began working on alternatives. The first of these was a simple light sensor that scanned in the view a few pixels at a time, transferring the image into a stream of brightness measurements that could be easily transmitted as a radio signal. The first of these images was sent back in 1959 by NASA’s Explorer 6 spacecraft, showing the cloud cover over the Pacific Ocean (Figure 1). It was a blurry image, but one that proved the concept could work.
Figure 1.
The First Photo of Earth from a Satellite in Orbit
This is the first crude picture obtained from the Explorer 6 satellite launched August 7, 1959. It shows a sun-lighted area of the Central Pacific Ocean and its cloud cover. The picture was made when the satellite was about 17,000 miles above the surface of the earth on August 14, 1959. At the time, the satellite was crossing Mexico. The signals were received at the South Point, Hawaii, tracking station. Image credit: NASA, Explorer 6 satellite.
Meanwhile, the Soviets took a different tack. Rather than creating the image in a transmittable format, they instead chose to take the photographs traditionally, develop them in space, and then use a scanner similar to those used to transmit television images at the time to send them back to Earth. In October 1959, they put the system to dramatic use to take the first ever images of the far side of the Moon—images that quickly graced front pages around the world.
This method, however, was never meant to be a long-term solution. Developing images took time, meaning you couldn’t have a real-time image stream. Ultimately, NASA began using TV cameras that could take and transmit images almost instantly. This allowed them to build the Ranger impactor probes—spacecraft that were sent on a collision course with the Moon, sending back a constant stream of data as they got ever closer to the surface. As the spacecraft were utterly obliterated during impact, the live feed was vital as there was no chance to develop images and transmit them after the encounter.
Around this time, human exploration of the Moon in the form of the Apollo program began to come to the fore, and in turn robotic exploration began to look toward more distant targets: the planets. However, while the Moon is a “mere” 385,000 km away, our nearest planet, Venus, never comes closer than 40 million km.
This means two things. For one, the time a signal takes to make the return journey from spacecraft to Earth goes up. While at worst it can take just 2.7 s for a signal to get from Earth to the Moon and back, for Venus it can take anywhere between 4 min when it’s close to Earth and almost half an hour when it’s on the other side of its orbit, making real-time communications impossible. While this can add a lot of complications when it comes to operating spacecraft, in terms of data downloads it just means you have to wait that little bit longer.
The bigger issue instead is caused by the fact that signal strength drops by an inverse square law, and so increasing the distance by a factor of 100 decreases the signal by 10,000 times. To solve half of this problem, NASA built the Deep Space Network, a series of radio receivers dedicated to picking up signals from interplanetary spacecraft. As well as being very sensitive, the receivers were placed around the world, allowing them to detect the signals from spacecraft no matter where they were in the sky.
The other half of the issue was a tougher solve—the spacecraft themselves were going to need more powerful radios. For spacecraft that intended to remain in space, this mostly meant increasing the size of the solar panels that powered them. The real problem was the landers. As Venus was covered with a thick layer of clouds, solar panels weren’t an option, meaning landers had to be battery powered, and bigger batteries start getting heavy very quickly—a big problem when your launch rocket has a strict weight limit.
The first generation of Soviet landers to Venus communicated directly with Earth using a high-gain, directional antenna with a low-gain back up (a fortunate addition when the first successful Venus lander, Venera 7, landed on its side and could only communicate with the low-gain antenna). However, Venus’s hellish environment meant the landers could only survive at most an hour on the surface. Combined with the radio’s tiny bandwidth, there was no way they could send images all the way back to Earth on their own.
Luckily, the Soviets came up with the idea of sending the lander along with another spacecraft that would remain above the planet to act as a relay. This meant that the lander only needed to be able to communicate the short distance to the relay spacecraft, which could then use its bigger, more powerful radio to communicate the information back home at a much more leisurely pace. Using this method, the Venera 9 mission sent back the first ever image from the surface of another planet on October 22, 1975. It’s a method that has stood the test of time, as almost all modern planetary landing missions use this method to get their data home.
Of course, using a second spacecraft adds in another point of failure. The European Space Agency felt the pain of this in 2004, when their Huygens lander began its descent toward Saturn’s moon, Titan. The lander had hitched a ride to the moon on board NASA’s Cassini orbiter and was using the larger spacecraft as a relay to transmit all its data back to Earth. During the landing itself, some of the world’s largest radio telescopes managed to pick up a direct signal from Huygens, though it was far too faint to extract any data. It did, however, confirm the lander was on and transmitting on both channels.
It was an unpleasant shock, then, when the Cassini data stream started coming in and only one of the channels was carrying any data. It later turned out that Cassini had never turned on its receiver to listen out for Huygens as it made its descent—a single command missed out of thousands had robbed the world of 350 images from the only time a spacecraft has visited the surface of another planet’s moon.
As spacecraft move farther out into the Solar System, there is another problem that starts creeping in. Traveling the billions of kilometers to the outer planets takes not just years but decades, and the spacecraft are so far away from the Sun that solar panels would have to be the size of football fields to soak up enough Sun to power them. Instead, most long-distance spacecraft are powered by radio thermal generators, which are extremely efficient but have a low power output. To compensate for both this and the huge distances, the bandwidth of its radio is extremely low—in the case of the New Horizons spacecraft, just a few kilobits per second. Though it flew past the dwarf planet Pluto on July 14, 2015, it took the spacecraft another 15 months to upload the 6.25 Gb of data. With multiple science groups all wanting data from different instruments first and the risk the spacecraft could suddenly stop working without warning, the New Horizons team had to carefully prioritize the data, ensuring each team got their data in a fair order, with the most important information coming first.
For most of the problems discussed so far, the issues naturally improve as technology develops. Lighter batteries, better transmitters, and more sensitive receivers back on Earth have all helped spacecraft transmit more data farther and more clearly. But this constant improvement only applies to new spacecraft—you can’t upgrade something once it’s already been launched. The two longest-serving spacecraft, Voyager 1 and 2, are currently giving humanity its first ever glimpse of interstellar space, having sailed out beyond the Sun’s protective magnetic bubble. But, as they were built in the 1970s, they are recording this unique view with 1970s technology—an 8-track recording tape with just 69.63 kb of memory.
Fortunately, the signal is sent back to Earth in a format that can be interpreted digitally, otherwise NASA would frantically be buying up all the 8-track tape recorders still left in existence to decode the signal. Instead, they just call on their software developers to dust off some of their more archaic programming languages.
As we look toward sending out missions not to just our planets but perhaps to other stars as well, future-proofing a spacecraft’s data transmission is vital. It could take hundreds of years to reach another star. Imagine the frustration of sending a spacecraft all that way, laying out hundreds of booster receiver stations to relay the signal back Earth, only to receive a century-old data stream no one knows how to decode.
Every year, humanity pushes farther and farther out into the void that surrounds our planet, and fortunately, thanks to the work put in by those on Earth planning those missions, we’re also able to take in the view.
Biography
About the Author
Ezzy Pearson is a space journalist and author of Robots in Space: The Secret Lives of Our Planetary Explorers, chronicling the adventures of robotic spacecraft that have reached out and touched another planet, available now from the History Press. She is the news editor of BBC Sky at Night Magazine.