How Do Our Probes Locate and Orient Themselves in Space?

The challenges of space travel are very different from those faced when traveling around the globe. One of the most important pieces of information for travelers, both in space and here on Earth, is location. Here, even before the invention of GPS, our ancestors were able to navigate long distances, almost without getting lost, using only a compass and the stars. But out there, things are much more complicated.

Have you ever imagined how difficult it would be to find your way around a place with no landmarks, no streets, no signs and no GPS signal? In space, there is no above or below, the distances are astronomical and even Earth, our most familiar reference point, becomes a distant blue dot surrounded by the immensity of the cosmic void. So how do our scientists manage to determine the location and orientation of our spacecraft on their journeys through the cosmos?

The answer lies in a fascinating combination of physics, mathematics and high-precision engineering. Spacecraft, even on their lonely journeys through the outer reaches of the solar system, are not just drifting through space. They are always orbiting something, whether it’s the Earth, the sun or an asteroid they’re studying. Even the Voyagers, which left our solar system, are now orbiting the center of our galaxy, the Milky Way. And to ensure that these spacecraft reach their destinations, scientists need to plot precise trajectories, monitor them constantly and correct them whenever necessary.

At the heart of this monumental task is the work of the spacecraft navigation team, which is divided into three main areas: reference trajectory design, orbit determination, and flight path control. It’s like planning a car trip: first, you define your destination and plot the route on a map (reference trajectory). During the journey, you need to know where you are and whether you’re on the right track (orbit determination). And if you go off course, you need to make corrections to get back on track (flight path control).

The reference trajectory is the spacecraft's flight plan, a detailed itinerary that takes into account the laws of physics, the position of the planets, the gravitational influence of the Sun, and other factors that may influence the journey. We have already seen that this trajectory is the so-called transfer orbitcalculated from the laws of physics and mathematics to make the best use of the Sun's gravitational energy.

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But, just like on a car trip, unexpected events can happen. The spacecraft may deviate from its planned route due to the gravitational influence of other celestial bodies, the pressure of solar radiation, or even minor failures in its propulsion systems. That's where the tracking team comes in.

Using data sent by the spacecraft, such as radio signals and images, scientists can determine the exact position of the spacecraft in space. To do this, they mainly use telemetry. The spacecraft sends radio signals to Earth, which are picked up by giant antennas, such as those on NASA's Deep Space Network. By analyzing the signals picked up by these antennas, scientists can determine the spacecraft's distance, speed and direction.

Another technique used is Very Long Baseline Interferometry (VLBI). This technique uses multiple radio antennas to observe the spacecraft simultaneously, combining the received signals to obtain an even more precise location. It is like triangulating the position of an object using multiple viewpoints.

And just as our ancestors used the stars to navigate, some spacecraft are equipped with cameras that can capture images of stars and planets. A software system can compare these images with star maps to determine the spacecraft’s orientation in space. This technique, called optical navigation, is especially useful for missions where precision in the spacecraft’s orientation is critical. Thanks to optical navigation, NASA’s DART spacecraft reached the small asteroid Dimorphos at about 22,000 kilometers per hour.

But knowing and maintaining the orientation of a probe during a space trip is not only important for precision missions. Imagine, for example, if after capturing the fantastic images of Saturn and its moons, the Cassini probe was unable to point its antennas towards Earth to transmit this precious data to us. The correct orientation of the spacecraft is essential for its communication with Earth, but also for correctly capturing data and carrying out its maneuvers.

To orient themselves in space, spacecraft use a combination of sensors, such as gyroscopes, accelerometers, and star sensors. Gyroscopes measure the rotation of the spacecraft, accelerometers measure changes in its speed, and star sensors identify familiar stars as landmarks in the sky. Based on this data, the spacecraft's attitude control system adjusts its orientation using small thrusters or reaction wheels, ensuring that the spacecraft is always “looking” in the correct direction.

These methods, combined with increasingly sophisticated space navigation software, allow scientists to guide our spacecraft with pinpoint accuracy on their journeys through the cosmos. It’s like having an interplanetary GPS, capable of taking us to the far reaches of the Solar System and beyond.

And so, guided by the stars like our ancestors thousands of years ago, we now sail the cosmic oceans. The conquest of space is a testament to human ingenuity, to our relentless pursuit of unlocking the mysteries of the universe and our ability to use science to achieve what once seemed impossible. Every radio signal received from a distant spacecraft, every image captured of a distant planet, every maneuver performed with pinpoint precision, brings us a little closer to understanding our place in the vastness of the cosmos.—