How did astronauts figure out where they were on the way to the Moon with enough precision to land on the lunar surface? In other words, how did NASA’s astronauts navigate in space? Sailors would check their position using a sextant, the Sun, Moon, planets, stars, compass, a knotmeter, and a clock to break the question of navigation into time, rate, and distance. With those tools, you can calculate your position at any point.
But instead of using a compass, astronauts relied on a gyro-stabilized “stable table” to keep track of direction. So, instead of using a knotmeter, accelerometers tracked every little change in speed.
Instead of water currents, gravity produces unfelt changes to the course, so the computer running the math model of gravity created by Isaac Newton. Otherwise, the astronauts’ situation was very similar to that of sailors, which means instead of a marine sextant, astronauts used a space sextant.
How Do Spacecrafts Navigate In Space?
NASA
How does a space probe like Voyager 2, which was launched in 1977, visit the four outer planets and travel over 17 billion kilometers over a space of 40 years with almost nothing in the way of fuel? Apollo astronauts used three navigation systems to determine the proper flight paths to the Moon and back to Earth. These systems were used jointly or separately. Together, they formed the Primary Guidance and Navigation System.
By the time Voyager 2 reached Neptune, it had swung by Jupiter, Saturn, and Uranus, traveled 7 billion kilometers, and was still within 100 kilometers of its target, all with mid-1970s technology. The movie’s spacecraft just seemed to fly where they wanted and get there in no time, but in our version of reality, it’s somewhat more complicated and takes much longer to get around.
Navigating The Solar System
Can you imagine Spock saying to Kirk, “We’ve just passed Pluto almost home, only nine years to go?” Just in case you missed the relevance of that, it took nine years for the New Horizons probe to get from Earth to Pluto, a distance of about five billion kilometers, and that was one of our fastest spacecraft.
It might seem like an impossible task, but when you know how space and physics work, it becomes a set of procedures, science fact instead of science fiction, and a key to all of this is knowing how gravity works and how it affects not only you and me but also everything in the universe.
The German mathematician Johannes Kepler first worked out the laws of planetary motion 400 years ago. Isaac Newton then used these as a basis for Newton’s motion laws and the creation of Classical Mechanics.
Predict The Movement of Everything in Space
How can we predict the movement of everything in the solar system and beyond, including planets, comets, asteroids, and spacecraft, with incredible accuracy? Newton’s first law states that an object at rest or traveling a straight line will stay that way unless a force acts upon it.
A rock, for example, on the ground, won’t move by itself unless something else picks it up or pushes it along. If that same rock were in space and moving in a straight line, it would not change its speed or direction of travel unless an external force acts upon it.
Interstellar Navigation
There is always a force acting on a moving body in space, and that force is gravity, be it from the Sun, a planet, or even another rock. Anything with mass exerts a gravitational force. The larger the mass, the larger the force. The other component of a moving object is its speed. Newton’s second law states that an object’s speed will change when a force is applied to it. This is also reversible, so a force is generated when its speed changes.
This is also why an asteroid traveling at 17 km/s and hitting the Earth can be so devastating. The sudden change in its velocity can release a huge amount of kinetic energy. If you fire a projectile on earth parallel to the ground, it will eventually fall under the influence of gravity back to the ground.
Earth’s Gravity Continuously Pulls
If you fire your projectile fast enough and maintain that speed, it’s still traveling in a straight line. Earth’s gravity continuously pulls on it, and when the curvature of its trajectory matches that of the Earth, it’s now said to be in orbit around the Earth. In other words, the force of a projectile trying to go in a straight line is matched by that of gravity pulling it back to Earth.
This is how satellites and space stations stay in orbit, but they are also affected by the tiny amount of drag of a fragile atmosphere high above the Earth. This slows them down, and as the force keeping them in their orbits becomes smaller, the balance between this and gravity gradually tips towards gravity.
Apollo 11 astronauts used this star chart while training for their 1969 lunar landing mission. It shows a select group of stars’ locations, names, and code numbers. The astronauts would key those numbers into their Apollo Guidance Computer while taking readings with a sextant.
As it pulls them down further, the atmospheric drag becomes even greater as they get lower, reducing the speed even more. Without a periodic boost in speed to increase their orbit, they will eventually come back to Earth.
The Hohmann Transfer Approach
If, however, a spacecraft increases its speed, the orbit will become larger and more elliptical. Still, it will always return to pass through the point where the speed was originally boosted. If our craft’s speed is increased enough, it will escape the pull of Earth’s gravity and enter an orbit around the Sun.
If it increases its speed, it will increase the size of its orbit. If we get the speed boost correctly timed with an approaching planet in what’s called an “opportunity,” we can get the orbit of our spacecraft to intersect the orbit of a planet and a method known as the Hohmann transfer approach.
It is one of the most common ways to move from one moving body to another. However, there are now more efficient but much longer ways, such as the low thrust transfer and interplanetary transport network methods.
How Can We Use The Planet’s Gravity to Slingshot?
Once there, our spacecraft can either enter into an orbit around the planet, or we can use the planet’s gravity to slingshot around it or use gravity assist, as it’s known, and increase the craft speed relative to the Sun.
Gravity assists work by using a planet’s gravity to pull on our spacecraft as it flies close by and can be used to increase or decrease a spacecraft’s speed, make its orbit larger or smaller, and change its travel direction. If our craft is flying in the direction of motion of the planets, it will speed up.
If it flies in an opposing direction, it will decrease speed. Depending on how it approaches the planet, its course can be changed dramatically and even leave traveling in the opposite direction.
But there is no such thing as a free lunch, and to obey the law of conservation of energy, what energy our craft gains, the planet must lose when the voyagers used Jupiter to increase their speed to get to Saturn, Jupiter’s orbit around the Sun slow but only by about one foot per trillion years.
Space Navigation Methods
We can use this gravity assist method to move from planet to planet further and further away, increasing our craft speed as we go until it reaches escape velocity, the point where we’ll be traveling fast enough to escape the pull of the sun and leave off a solar them just like Voyager 1 has already done.
But the Sun’s gravity will still pull on the craft and slow it down. In fact, the Sun’s gravitational effect extends out about two and a half light-years, and it will take Voyager traveling at over 60,000 km/h 40,000 years to reach the point where the Sun’s gravity no longer dominates.
How Can We Use Thrust For Navigation?
Newton’s third law states that every action has an equal and opposite reaction. Basically, the thrust from an engine pushing backward moves a craft forwards. Some people think that a rocket’s thrust pushes against the ground or the atmosphere, and thus, they can’t work in space.
This is clearly not the case, as our rockets and thrusters don’t stop working once they are in space when there is nothing for them to push against. We use this thrust to increase or decrease speed and, as such, change our spacecraft’s orbit and move it in its X from Y planes with thrusters to orient its antenna with earth or point its cameras towards a target.
Once we know how gravity affects our spacecraft and that we can use it to move from planet to planet, the next thing we need is an accurate model of the solar system. This will show us where the planets will be in relation to the Sun and each other and other objects like comets and asteroids.
How Can We Use Celestial Mechanics For Space Navigation?
This model is created from the planetary ephemeris, which is like a timetable for all the major bodies in the solar system and gives their positions relative to the Sun for any given time, both in the past and the future.
This data has been built up over centuries. We were the first ones being created by the Babylonians as far back as 1200 BC. Using celestial mechanics makes it possible to calculate ephemeris for several centuries into the future. Because space missions last for years or even decades, like the Voyager ones, it would be impossible to plan missions without knowing where the planets would be in the years ahead.
However, these ephemerides are not perfect due to the gravitational effect of unknown asteroids and maybe an as-yet-unknown Planet X far beyond Pluto. NASA has updated its ephemerides almost every year for the last 20 years as new data has come to light.
Spacecraft and Position of Planets
So, knowing how our spacecraft will move in space and the position of the planets well into the future allows navigators to plot a course for our spacecraft with incredible accuracy. This can be seen with the Voyager missions. They used planetary ephemeris to find a once-in 175-year alignment in Jupiter, Saturn, Uranus, and Neptune.
How Can We Use Gravity?
This was discovered by Gary Flandro in 1964 whilst working at JPL and allowed the planers to come up with the Grand Tour. This would allow one spacecraft to visit all four planets using gravity assist and cut their mission time from 40 years to less than ten if they launched in 1977.
The original Grand Tour was to include Pluto, but it was left out due to funding limitations. However, the New Horizons probe visited Pluto in 2015. Voyager 2 was the first to set off in 1977 on another grand tour of the four outer planets and eventually traveled out in the plane of a solar system. This same gravity assist technique has since been used on the Galileo, Cassini, and New Horizons missions.
“Pale Blue Dot”
Voyager 1 launched three weeks after Voyager 2 on a quicker route to visit Jupiter and Saturn and do a flyby of Saturn’s moon Titan. But this would then put it on an upward trajectory and out of a plane of a solar system to interstellar space. On its way out, it was turned around so its camera could face back to earth and take one last set of photos.
These were the farthest images of the solar system ever taken, and one of them captured Earth’s place in it. Covering just 0.12 pixels in size in the middle of a lens flare, the famous “Pale Blue Dot,” as Carl Sagan called it, was taken 6.4 billion kilometers away, looking down at a 32-degree angle onto the solar system.
Navigating in Space
Now we have a plan. We still need something to guide our spacecraft along its planned trajectory. For this, they use an inertial navigation system. Basically, this is a highly accurate system of gyroscopes, accelerometers, and other sensors that can detect a craft’s movement in any direction in space. Using this information, the navigators can work out if the craft is on course.
However, inertial navigation systems are mechanical devices and, as such, suffer from what is known as integration drift tiny errors in the gyroscopes and sensors. This is compounded over time because they calculate their position as they move along from the last previously calculated position. The longer they go, the more the errors build up.
Fixed Reference System is Needed
The error in a good system is less than 1.1 kilometers per hour, so if a journey to Mars lasted eight months, which will be 5760 hours, then the error would be about 6300 km by the time it reached Mars, far too much when you have to enter an orbit with an accuracy of just a few kilometers.
Another fixed reference system is needed to compensate for the integration drift, and this is the Stars. Just as marine navigators used a sextant to work out their position, spacecraft use optical sensors and cameras to determine their position and reset the inertial navigation systems.
On Apollo Missions, The Crew Used a Space Sextant
On the Apollo missions, the crew used a space sextant to correct the onboard navigation system’s drift. They also used a star tracker on the Voyager probes that could look for a very bright guide star, which was Canopus in the voyager’s case. The tracker also had a Sun sensor that could be used in conjunction with a radio signal from Earth.
Newer spacecraft have more sophisticated systems that use cameras to look for known objects like planets, comets, asteroids, and the target itself. Even with the best-planned course, things will vary along the way. Other forces can also affect a craft deep in space.
For example, the solar wind flow of charged particles from the Sun can gradually change the course of a spacecraft, and this has to be corrected, and timing is everything.
How Do Astronauts Determine Where They Are When in Space?
Our spacecraft must arrive at particular points in space along the journey within a tiny window of time. Traveling at 30 km/s and approaching a planet to use its gravity to swing by and change course, if you are out by more than a few minutes or so, it could mean the difference between being sucked into the planet by gravity or undershooting the planned course.
NASA uses the Deep Space Network to communicate and work out a craft’s distance and speed. This network of radio telescopes is spread worldwide so that at least one is always in contact with a spacecraft.
How Do You Use Radio Signals For Navigation?
By sending a radio signal to the craft and having it return the signal and using the Doppler effect and a highly accurate atomic clock, the slight difference between the two signals can be used to calculate its distance from the earth to living 3 meters and its speed to within 180 millimeters per hour. Combining all this information, we are now able to send space probes with incredible accuracy.
Theories Developed Hundreds of Years Ago
So much so that we can now land on a comet as we did with the Rosetta probe and its Philae lander and take close-up pictures of Pluto within a two-hour time window, nine years after launch and 5 billion kilometers away and when we only had one-third of Pluto’s orbit mapped. Five spacecraft have now achieved escape velocity using these methods we’ve spoken about and are now the farthest objects created by man.
Pioneer 10, 11 Voyagers 1 & 2 & New Horizons. It’s incredible to think that all of this was done based on theories that were developed hundreds of years ago by observation only and the desire to figure out how the heavens worked long before we even thought it was possible to get into space, let alone use gravity as our main engines.
IBM Houston Programmers
(The picture above). Standing here in front of a model of the Apollo 11 Lunar Module. To the left, IBM Houston programmers Susan Wright, Mitch Secondo, rear, and David Proctor overlooking the equations they have entered into NASA’s computers at the famous Manned Spacecraft Center.
Most of these formulas were taken from the complex mathematics applied by ground computers that guided Apollo 11 American astronauts Neil Armstrong and Buzz Aldrin to their lunar touchdown. Credit: IBM.
Check out this article that reveals the inside of the Apollo Saturn V rocket and its significant components. See for yourself these fantastic drawings. You will be amazed.
