Surfers in space: on an interstellar internet

In the future, when human activity will expand towards the Moon and Mars, and the number of spacecraft around these bodies will increase immeasurably compared to the situation existing today, there will be a need for a long-term satellite communication system that will cover the expanses of the solar system. How would such a system work?

Tal Inbar, "Galileo"

Daily communication that we have been used to for almost a thousand years does not bother us, except when a malfunction occurs that causes us to suddenly not receive satellite TV broadcasts, or the quality of the satellite internet somewhere is slightly impaired; A short time usually passes until the fault is repaired, and life returns to its normal course.

However, communication failures with research spacecraft located at the edges of our solar system can wreak havoc on space missions that last years and cost hundreds of millions of dollars, and even more. In the future, when human activity will expand towards the Moon and Mars, and the number of spacecraft around these bodies will increase immeasurably compared to the situation that exists today, a long-term satellite communication system will be needed. In this article we will review some of the aspects related to construction Space communication network which will spread over the vastness of our solar system.

Deep Space Network

Communication between research spacecraft and Earth is point-to-point communication, meaning the spacecraft is in line of sight to Earth and transmits to one known and pre-calculated location. At the time of writing these lines, for example, a spacecraft is on its way to Pluto, and daily communication is carried out with spacecraft located around Saturn, Mars, and with several other research spacecraft, some of which have left the solar system, such as Voyager.

Due to the great distance at which unmanned research spacecrafts that study the planets operate, and due to the problems of the power required for radio transmissions from the spacecrafts (which are always limited by the size of their solar cells or the power of the nuclear unit that supplies electricity to some of them), a system of very large antennas has been established on Earth, allowing Communication with the various spacecraft.

This system is called the Deep Space Network (DSN) and is operated for the American space agency NASA by the JPL Jet Propulsion Laboratory. The array is based on huge adjustable antennas installed in California, Spain and Australia, with the angular distance between each station allowing continuous contact with the spacecraft despite the Earth's rotation around its axis.

Communication satellites around planets

At this stage of exploring the solar system using spacecraft there are still no satellites orbiting planets and serving as dedicated communication satellites whose main role is as communication satellites to Mars, the Moon and other planets. However, even today spacecraft from different countries know how to "talk" to each other, and thus, for example, robots operated by the United States, which roam the surface of Mars, can transmit information through a spacecraft of the European Space Agency orbiting Mars.

The compatibility between the communication protocols is the result of cooperation and long discussions between the space agencies. With the increase in the volume of communications that will be transmitted from Mars to Earth, there will be a need for dedicated spacecraft relay stations, such as communications satellites in Marsstationary orbit - that is, around Mars - or several communications satellites that will float in the asteroid belt between the orbit of Mars and that of Jupiter.

From the earth to space: the internet model

The Internet is essentially decentralized - there is no single center where all the information is stored and from which the communication is managed. In recent years, this distributed model has been at the center of the planning of various space agencies, led by NASA, when it comes to characterizing the communication architecture in the solar system for the years to come.

Planetary Internet must be resistant to various disturbances (such as storms on the surface of the Sun), transmission interruptions (for example when a spacecraft is behind a planet) and allow the mission to continue despite these conditions. Of course, the delay in communication caused by the distance must also be taken into account - for example, the delay in communication between the Earth and a spacecraft located around Mars can range from 3.5 minutes to 20 minutes.

A fundamental difference in the planetary Internet compared to the Internet on Earth lies in the fact that there will not be a continuous connection between two end users - this should be taken into account, and so, for example, information packets passing from a spacecraft to a relay satellite should remain on the satellite if it does not have a direct connection with the next satellite or with the Earth, and wait Until a communication channel is found - direct or indirect - and then the information will be transferred.

This requires large memory banks in the relay satellites that will make up the planetary internet, as well as a different operating logic than is customary in distributed computer communication on Earth. The method is similar to the principle found in the use of communication satellites in low orbit around the Earth, known as store and forward (this is similar to a basketball player who cannot attempt to score a basket from his current position on the court, who passes the ball to another player who can do so, or who is located where he in turn will pass the ball to another player who will try to score). This mode ensures that the transmitted information will not be lost if there are communication interruptions, and the price paid is only a delay in receiving it.

A planetary internet will consist of several satellites located in different orbits and locations in the vastness of the solar system, each of which will be able to receive information and transmit it to each of the components of the space network to which it will have a line of sight. In the space communication carried out today, every move of communication with a spacecraft requires the involvement of a person and a very careful planning that will ensure the creation of the connection, its quality and the period of time in which the communication with the spacecraft will take place. When the planetary Internet operates, a significant part of the operations required to create communication will be performed automatically without human involvement, and with much greater availability than in point-to-point communication.

A space internet will enable the existence of innovative space missions, which can include the simultaneous landing of several spacecraft at several landing sites, integration between spacecraft in different orbits around the planet, research using various gliding systems (such as UAVs, unmanned aerial vehicles, or balloons in the context of Mars). Moreover, a space internet will be able to serve with higher reliability manned missions on the surface of the moon and in the more distant future - on the surface of Mars and in the area of ​​asteroids.

A space internet must also take into account the distortions of the signals themselves, which result from their transmission over a platform moving at high speed. The Doppler effect must be taken into account when processing the signals and propagating them further. In the past, this detail was almost forgotten during the Huygens spacecraft mission, which was launched from the Cassini spacecraft to Saturn's moon, Titan. Only the vigilance of a young technician drew attention to the problem, and part of the spacecraft's communications system was reprogrammed ahead of time.

What bandwidth will you get on Mars?

Mini-grids will be deployed in the future around centers of human activity - mainly around the moon and around Mars. A few communications satellites in a variety of suitable orbits could support stationary and mobile robotic research activity on the surface, as well as tools that would orbit the Moon or Mars in lower orbits.

The principles of a planetary internet would also be applicable in regional networks around Mars or the Moon. Planetary Internet is not only essential for supporting the activity of research spacecraft and transferring data from and to them. During human activity on the moon or on Mars, there will be a need for ongoing monitoring of the health status of the astronauts - and this will be easily done by a local communication network.

Also, the communication satellites that will make up the space communication networks must process the information already on the satellite, and not just transfer raw information to the earth - this is demonstrated nowadays in various communication satellites and is known as On Board Processing for short. Processing on the satellite computers may greatly streamline the process.

First, processing information on the satellite enables better data compression and the use of a variety of radio frequencies, saving electricity in the spacecraft itself and using communication beams in a more economical configuration. Furthermore, in various missions, the spacecraft has X transmission time to Earth. The more processing is done in the spaceship itself, the more and more useful the information that will reach the earth.

Also, processing information on the satellites themselves will send processed information to Earth that can be worked on immediately. Second, raw information is stored on servers, processed and most of it is not seen by the researchers and will not be seen - since the research projects are limited in time and a significant part of the cost is borne by the people on the ground. Therefore, it is desirable to avoid as much as possible the usual process of sending raw information and processing it while it is on Earth. Thirdly, in such an operation it is possible to distinguish patterns of change, something that serves meteorological satellites, for example.