Humanity’s Footsteps in Space: From Satellites to Observatories

We are living in exciting times. Human civilization has advanced to the point where we can step off Earth and venture into outer space, establishing a lasting presence through satellites, observatories, and manned stations. The sky is no longer empty. Plans are underway to build the first human outposts on the Moon and Mars, potentially paving the way for permanent human settlements beyond our planet.

Fig. 1. Artemis 2, launched in April 1, 2026, carried the Orion crew capsule with four asctonauts on a 10-day journey around the Moon and back. Credit: NASA/Joel Kowsky.

We are rapidly evolving into a spacefaring civilization, and as our ambitions grow, so does our collection of spacecraft. Each is designed for a specific purpose. Some provide internet connectivity, others monitor Earth's climate, study distant galaxies, and transport astronauts into orbit. There are many ways to classify them: by mission, destination, or orbit. Yet perhaps the most fundamental distinction, involving both engineering and ethics, is human occupancy.

Crewed spacecraft

The presence of humans transforms every aspect of spacecraft design. Space is inherently hostile to us and all organic forms. A crewed spacecraft requires radiation shielding, breathable air, water, food, and waste management to ensure the survival of its occupants. Every kilogram launched into space comes at considerable cost, making human spaceflight one of the most demanding engineering projects ever attempted.

Space stations are orbiting laboratories where researchers study how microgravity affects biological organisms, materials, and physical processes. Unlike rockets, stations don't need engines and maintain their orbits by inertia. Currently, we have two fully operational stations: the International Space Station (ISS) and China's Tiangong space station. Both are located at an altitude of about 400 km, in low Earth orbit (LEO), which facilitates regular crew rotations.

Fig. 2. The International Space Station (left) and China’s Tiangong space station (right).

These stations, which include workshops and living quarters, are too large to be launched as a single unit (Fig. 2). Consequently, they were built from separate modules, much like prefabricated homes. These modules were manufactured on Earth and then sent into orbit one by one, where they were assembled by astronauts and robots who moved with their project (Fig. 3).

Fig. 3. Artist’s composite depicting astronauts and robotic arms assembling a modular space station in orbit.

Space capsules are designed for short-term occupancy. Much smaller than space stations, they can be launched on a single rocket to carry astronauts into space and return them to Earth. While their primary role is to shuttle crew to and from space stations, they have also been used in more ambitious projects, such as the Apollo and Artemis missions to the Moon. As we venture farther into the Solar System, they will be used to transport humans to Mars and other destinations.

Robotic spacecraft

The contrast is evident when we examine uncrewed spacecraft built exclusively for scientific objectives. Without the need to sustain human life, these spacecraft are smaller and lighter, can travel farther, function for longer, and endure conditions humans can't withstand. Thanks to technological advances, robotic spacecraft can perform a wide range of tasks, divided into several broad categories.

Satellites are roughly the size of a refrigerator, with solar panels extending a few meters on either side. We have built an impressive network of about 16,000 satellites around Earth alone to support modern civilization. They are used for weather forecasts, monitoring Earth’s magnetic field, military surveillance, and particularly navigation and communication.

Fig. 4. Earth’s orbital zones: LEO, MEO, GEO, and HEO, compared with the Moon's orbit.

1. Navigation satellites act like guiding stars. A satnav device in your phone or vehicle receives signals from several satellites overhead and uses them to calculate your position on Earth. It can then place you on the map, plan your route, and update your position as you move. Most navigation satellites operate in Medium Earth Orbit (Fig. 4).

   
   

2. Communication satellites relay internet, television, telephone, and other data between distant locations. To overcome the Earth's curvature and other geographical obstacles, signals are sent to a satellite, which retransmits them to another ground station or user. These satellites operate in various orbits, but the geostationary orbit (GEO) is often preferred for its wide-area coverage.

   
   

3. Decommissioned satellites are often relocated to a graveyard orbit, located several hundred kilometers above the geostationary belt. In the vacuum of space, where atmospheric drag is nearly nonexistent, these silent relics will continue orbiting Earth for thousands or even millions of years, serving as archaeological records of our earliest ventures into space.

4. The DSCOVR satellite holds a unique position among satellites serving our planet due to its enormous distance from Earth, about four times farther away than the Moon. Located at the L1 Lagrange point (Fig. 5), which is between the Sun and Earth, it can continuously monitor the solar wind, which influences geomagnetic storms, climate variations, and other phenomena. Unlike the above satellites, the DSCOVR doesn't orbit Earth; instead, it orbits the Sun alongside Earth.

   
   

5. Lunar orbiters. Several satellites currently orbit the Moon, studying its climate and topography, and scouting for water ice. The scientific data is transmitted to the labs on Earth in preparation for future human expeditions. The Moon is our closest neighbor and most likely gateway to the rest of the solar system. It will remain under continuous surveillance for many years to come.

   
   

6. Planetary orbiters. Satellites currently orbit all planets except the most distant, Uranus and Neptune. For the foreseeable future, orbital surveillance might remain the only safe option even for robots. The extreme heat of Mercury, the crushing pressure of Venus, the powerful radiation of Jupiter, and Saturn's violent winds pose serious challenges, making entry into the atmospheres of these planets virtually impossible.

   
   

Fig. 5. Lagrange points in the Sun–Earth system, with labels added. Original diagram by Xander89, via Wikipedia.

Space observatories are unmanned labs usually equipped with telescopes. For instance, the Hubble Space Telescope, known for capturing the Pillars of Creation, is about the size of a large school bus and orbits the Earth. Another example is the more advanced James Webb Space Telescope, which is about the size of a tennis court and is positioned at the Lagrange point L2, orbiting the Sun in tandem with Earth (Fig. 6).

One of the recent space observatories is Euclid, launched in 2023 to investigate dark matter and dark energy. By observing billions of galaxies and constructing a three-dimensional map of the Universe, scientists will be able to understand how cosmic structures form and why the Universe's expansion is accelerating. Away from the Earth's atmosphere, the space observatories can obtain clearer images and more accurate measurements.

Fig. 6. Hubble Space Telescope (left) and James Webb Space Telescope (right).

Space probes travel beyond Earth to explore other members of the Solar System. Depending on their mission, they may fly past a planet, enter orbit around it, descend through its atmosphere, land on its surface, or deploy rovers to explore the terrain. Historic examples include the Voyager probes, which have now entered interstellar space, New Horizons, which revealed Pluto in unprecedented detail, and the Parker Solar Probe, which repeatedly flies through the Sun's outer atmosphere to study our home star.

Together, these crewed and uncrewed spacecraft have become humanity's eyes and hands in space. They venture where most of us can't yet go, extending our reach across the Solar System and preparing the way for future, even more daring missions. Humanity's presence in space is firmly established and continues to grow. Through knowledge and determination, we have created an entire ecosystem of spacecraft, each extending our capabilities in a different way.