Redirecting Space Probes: Why Fuel Is Essential

Space probes need to change direction to navigate accurately and reach their targets. Spacecraft are typically launched on a set trajectory, and smaller rockets attached to the probe are often not powerful enough to alter its course significantly. To change direction, a probe can use the gravity of a planet or moon to adjust its orbit or utilise a gravitational assist. Alternatively, it can fire small attitude rockets to change the direction it is pointing, before the main thruster pushes it in the new direction. The location and heading of the probe must be known perfectly for this course correction to be successful.

Spacecraft use sensors and trackers to determine their position and orientation

Spacecraft use a variety of sensors and trackers to determine their position and orientation. This process is known as spacecraft attitude determination and is crucial for navigation and control. Here are some of the commonly used methods:

Ground Tracking Stations

Ground-based tracking stations are part of a Deep Space Network. These stations send radio signals to the spacecraft, which then sends the signals back. By measuring the time it takes for the signal to return, the tracking station can calculate the distance to the spacecraft. If multiple stations perform this task simultaneously, the spacecraft's position can be triangulated.

Onboard Sensors and Star Trackers

Spacecraft also employ onboard sensors, such as star trackers, to determine their position and orientation. Star trackers capture images of stars and use their positions to calculate the spacecraft's orientation. These images are processed to identify individual stars, and pattern recognition algorithms are used to match the star patterns with a star catalog stored in the spacecraft's memory. This information is then used to adjust the spacecraft's orientation and trajectory.

Gyroscopes and Accelerometers

Gyroscopes and accelerometers are onboard sensors that measure changes in the spacecraft's direction and speed. By integrating these measurements over time, changes in position can be calculated. However, this method may accumulate errors over time.

Global Positioning System (GPS)

For spacecraft in Low Earth Orbit (LEO), GPS technology can be used to determine their position, similar to how it is used on Earth. The spacecraft receive signals from multiple GPS satellites and calculate their distance based on the time it takes for the signals to arrive. However, GPS has limited use beyond a certain altitude due to signal strength limitations.

Onboard Cameras and Radar

Spacecraft that are designed to land on other planets or dock with other spacecraft may use onboard cameras and radar to determine their position and speed relative to the target.

Celestial Navigation

Spacecraft venturing beyond Earth's orbit sometimes employ celestial navigation, a technique inspired by ancient mariners. By observing the positions of celestial bodies such as the Sun, Moon, planets, and stars, the spacecraft can determine its position and orientation. This method involves complex calculations and is particularly important for deep space missions where other navigation methods may not be reliable.

Sun Sensors and Earth Sensors

Sun sensors detect the direction of the Sun relative to the spacecraft, contributing to attitude determination. Earth sensors, usually infrared cameras, sense the direction to Earth and provide orientation information.

These methods are often used in combination, with data from different sources being processed to estimate the spacecraft's position and velocity accurately. This complex process requires sophisticated technology and careful calibration.

The direction of a spacecraft is described as an orbit

Prograde and Retrograde Orbits

The direction a spacecraft travels in orbit can be direct, or prograde, in which the spacecraft moves in the same direction as the planet rotates. Orbits can also be retrograde, meaning the spacecraft goes in the direction opposite the planet’s rotation.

Geosynchronous Orbits

A geosynchronous orbit is a prograde, low-inclination orbit with a period of 23 hours, 56 minutes, and 4 seconds. A spacecraft in geosynchronous orbit appears to remain above Earth at a constant longitude, though it may seem to wander north and south. The spacecraft returns to the same point in the sky at the same time each day.

Geostationary Orbits

To achieve a geostationary orbit, a geosynchronous orbit is chosen with an eccentricity of zero and a low inclination. The orbit can then be called geostationary. This orbit is ideal for certain kinds of communication satellites and meteorological satellites.

Polar Orbits

Polar orbits are 90-degree inclination orbits, useful for spacecraft that carry out mapping or surveillance operations. The orbital plane is nominally fixed in inertial space, so the planet rotates below a polar orbit, allowing the spacecraft low-altitude access to virtually every point on the surface.

Sun-Synchronous Orbits

A walking orbit whose parameters are chosen such that the orbital plane precesses with nearly the same period as the planet’s solar orbit period is called a Sun-synchronous orbit. In such an orbit, the spacecraft crosses periapsis at about the same local time every orbit.

Halo Orbits

A halo orbit is when a spacecraft with negligible mass orbits along with a more massive body that is already in a near-circular orbit. In this case, the combined gravitational forces of the two larger bodies can maintain the smaller body rotating in a constant relative position as they orbit.

Radio signals are transmitted to a probe to determine its distance and speed

Radio signals are transmitted to a space probe to determine its distance and speed. This is done by measuring the Doppler shift of a coherent downlink carrier, which provides the radial component of a spacecraft's Earth-relative velocity.

The distance of a space probe can be calculated by sending a radio signal from Earth, which is then received by the probe and returned. The time it takes for the signal to make a round trip is recorded, and the distance is determined by multiplying the speed of light by half the round-trip time.

The Deep Space Network (DSN) is used to calculate the direction, distance, and speed of a space probe. This is done by measuring the angle of the radio signal from the probe, and using the Doppler shift and round-trip time of the signal to calculate its velocity and distance from Earth.

The DSN can also be used to send commands to a space probe to correct its trajectory. This is done by first calculating the required change in velocity (delta-V) to get the probe back on course. This information is then sent to the spacecraft engineering team, who use it to adjust the probe's trajectory by firing its engines or thrusters.

Spacecraft can use the gravity of a planet or moon to change direction

Spacecraft are often launched with huge rockets, which set them on a specific trajectory. Smaller rockets attached to the spacecraft are usually not powerful enough to significantly alter this initial trajectory. As a result, spacecraft generally require an external force to change direction. One way to achieve this is by utilising the gravity of a planet or moon for an orbit or swing-by, allowing the spacecraft to change direction without relying solely on its own propulsion systems. This technique, known as a "gravity assist", has been employed by several robotic spacecraft to reach their targets.

A gravity assist manoeuvre takes advantage of the gravitational pull and relative movement of a planet or other astronomical object to alter the path and speed of a spacecraft. By entering and exiting the gravitational sphere of influence of a planet, a spacecraft can increase or decrease its velocity relative to the Sun. This is similar to a tennis ball bouncing off the front of a moving train. The ball acquires the train's velocity, resulting in a higher speed relative to the train platform.

The Voyager missions provide a classic example of gravity assist. Launched in 1977, the Voyager spacecraft used the gravity of Jupiter to boost their trajectory towards Saturn. Similarly, the Galileo spacecraft utilised gravity assists from Venus and Earth to reach its destination, Jupiter.

Gravity assists are particularly useful for saving propellant and reducing expenses. They can also be employed to decrease the required amount of rocket propellant for orbit insertion. For instance, the Galileo spacecraft used a gravity assist flyby in front of Jupiter's moon Io to decrease its energy relative to Jupiter. This manoeuvre reduced the mass of rocket propellant needed.

In addition to planets, moons can also be utilised for gravity assists. The Ulysses spacecraft, launched in 1990, used a gravity assist from Jupiter to eliminate the speed it inherited from Earth's orbit and gain the necessary speed to orbit the Sun's polar regions.

Small rockets are fired to change the direction a spacecraft is pointing

Spacecraft are usually equipped with a reaction control system (RCS). These are typically located in specific locations around the exterior of the spacecraft to allow for its orientation to be changed.

The process of changing direction is based on Newton's third law of motion: for every action in nature, there is an equal and opposite reaction. So, when a rocket pushes gas out of its nozzles with great force, that gas pushes back on the rocket with an equal force, causing the rocket to accelerate in the opposite direction.

Attitude adjustments can be made in several ways. One method is to use small rockets, or thrusters, placed around the spacecraft. Firing a thruster causes the spacecraft to begin moving in the opposite direction. To stabilise the spacecraft, an equal burn must be made in the opposite direction.

Another method is to use reaction wheels. These are wheels mounted in the x, y, and z directions of the spacecraft, which can be spun up or down and have their acceleration changed to control the position of the spacecraft.

A third method is to use magnetometers. This involves using loops of wire embedded in the spacecraft's solar panels to create an electric field that interacts with the Earth's magnetic field, allowing the spacecraft's attitude to be changed.

Once the orbit determination team has a good estimate of the spacecraft's current location, the flight path control team evaluates how far the spacecraft has drifted from its planned trajectory. They then design a manoeuvre to correct the spacecraft's course. This involves creating a ΔV (delta-V) vector, which represents the direction and magnitude of the required change in velocity. The ΔV vector is then decomposed into requirements for spacecraft pointing and rocket-engine or thruster firings.

The spacecraft team then creates a set of commands to accomplish the correction, performs any necessary testing, and uploads the commands to the spacecraft, which then performs the manoeuvre.

After the manoeuvre, the orbit determination team takes more tracking data to verify the spacecraft's new trajectory, and the cycle repeats.

Frequently asked questions