On November 26, 2018 at 2:52:59 ET, NASA did it again—the agency’s InSight probe successfully landed on Mars after an entry, descent, and landing maneuver later dubbed “six and a half minutes of terror.” The moniker fits because NASA engineers couldn’t know right away whether the spacecraft had made it safely down to the surface because of the current time delay (roughly 8.1 minutes) for communications between Earth and Mars. During that window of time, InSight couldn’t rely on its more modern, high-powered antennas—instead, everything depended on old-fashioned UHF communications (the same method long utilized in everything from TV antennas and walkie-talkies to Bluetooth devices).
Eventually, critical data concerning InSight’s condition was transmitted in 401.586Mhz radio waves to two CubeSats called WALL-E and EVE, which in turn relayed the data at 8Kbps back to huge 70 meter antennas on Earth. The CubeSats had been launched on the same rocket as InSight, and they followed along on the trip to Mars in order to observe the landing event and send back data immediately. Other Mars orbiters like Mars Reconnaissance Orbiter (MRO) were out of position and couldn’t initially provide real-time communications with the lander. That’s not to say that the entire landing coverage hinged on two experimental CubeSats (each the size of a briefcase), but the MRO would have relayed InSight’s landing data only after further delay.
InSight’s entire landing truly put all of NASA’s communications architecture—called the Mars Network—through its paces. The signal the InSight lander beamed back at relay orbiters was sure to reach Earth even if one or more of the orbiters failed. WALL-E and EVE were there to pass information through immediately, and they did just that. If those CubeSats didn’t work for some reason, the MRO was ready to step in. Each piece worked as a node in an Internet-like network making it possible to route packages of data through multiple terminals made with different kinds of hardware. Right now, the most efficient tool is the MRO spacecraft, which can relay data at a maximum rate of 6Mbps (a current record for planetary missions). But NASA had to work with much less communications muscle in the past—it’s also going to need much more in the future.
The Deep Space Network
As NASA has increased its footprint in space, better space communications systems have been steadily appearing to extend coverage: first the goal was low-Earth orbit, then geosynchronous orbit and the Moon second, and soon farther into deep space. It started with crude portable radio tracking stations deployed by the US Army in Nigeria, Singapore, and California to receive telemetry data from Explorer 1, the first artificial satellite the US successfully launched into orbit back in 1958. And slowly but surely, that basis evolved into the advanced communications systems of today.
Douglas Abraham, a Strategic and Systems Forecasting Lead at NASA’s Interplanetary Network Directorate, highlights three independently developed space communication networks today. The Near Earth Network supports spacecraft in low-Earth orbit. “It’s a collection of antennas, mostly between 9 and 12 meters. There are a few larger ones that are 15 and 18 meters,” says Abraham. Next, slightly above geosynchronous Earth orbit, there are several telecommunications Tracking and Data Relay Satellites (TDRS). “These can look down at low-Earth orbiters and communicate with them, and this information then gets relayed from the TDRS satellites to the ground,” Abraham explains. “That’s the trafficking and data relay satellite system generally known as NASA’s Space Network.”
But even the TDRS was not enough to communicate with spacecraft flying way beyond the Moon to other planets. “So we had to build a network covering the entire Solar System. This is the Deep Space Network,” says Abraham. The Mars Network is an extension of the DSN.
Given its reach and ambitions, the DSN is the most complicated of these systems. At its core, the DSN is a collection of very large antennas measuring from 34 to 70 meters in diameter. Multiple 34-meter antennas and one 70-meter antenna operate in each of the three DSN sites. One site is located at Goldstone, California, another sits outside of Madrid, Spain, and the third resides outside of Canberra, Australia. These facilities are placed approximately 120 degrees apart around the globe to ensure 24/7 coverage for all spacecraft beyond the geosynchronous orbit.
The 34-meter antennas are DSN’s daily drivers, and they come in two variants: older high-efficiency antennas and relatively modern beam waveguide antennas. The difference is that the beam waveguide version has five precision radio frequency mirrors that reflect signals along a tube to the control room below the ground, where electronics analyzing those signals are better shielded from all sources of interference. The 34-meter antennas working separately, or in arrays of two or three dishes, can close most links NASA needs to be closed. But for special occasions when the distance is too large even for a few 34-meter antennas working together, people running the DSN use their 70-meter behemoths.
“They are important in several situations,” Abraham says of the larger antennas. The first is when a spacecraft is so far from Earth that it would be impossible to close the link with a smaller dish. “The New Horizons mission, which currently is way past Pluto, or the Voyager spacecraft, which is beyond the Solar System, are good examples. Only 70-meter antennas can get through to them and get their data back to Earth,” Abraham explains.
The 70-meter dishes are also used when a spacecraft can’t communicate with its high-gain antenna, either because of a planned critical event like an orbit insertion or because something has just gone terribly wrong. A 70-meter antenna was used to safely bring Apollo 13 back to Earth, for instance. It also received Neil Armstrong’s famous, “That’s one small step for a man. One giant leap for mankind” message. Even today, the DSN is the most advanced and sensitive telecommunications system in the world. “But for a number of reasons, it is close to its limits,” Abraham warns. “There’s not much room to improve the radio frequency technology the DSN relies on. We’re running out of low-hanging fruit to go for.”
Listing image by NASA Ames