How does a spacecraft flying in space find its way home?

After a spacecraft enters space, there is one unshakable task that needs to be performed every day, which is to continuously confirm the distance between itself and the ground.

This is to allow the ground control center to better understand the location of the spacecraft, otherwise it would not be easy to find the way home if you get lost in space.

Therefore, the control center on the surface of the earth will continuously send signals to the spacecraft (the signal travels at the speed of light), and when the spacecraft receives it, it will also reply.

Then, by measuring the time required for two-way transmission of the signal and combining it with the distance calculation formula - speed multiplied by time, the ground control center can calculate the flight trajectory, position and direction of the spacecraft.

It may seem complicated, but if you simplify it, it is actually very similar to our daily life.

Assuming that your workplace is a 10-minute walk from your home, and you know that you can walk 400 meters in 1 minute, you can roughly calculate the distance between your home and the company. When you leave home for 7 minutes, you will know how far you are from the company.

We usually measure this 7 minutes using the clocks we use in our daily lives.

However, the two-way transmission time between the spacecraft and the ground control center cannot be accurately measured by the clocks we use in daily life.

According to space travel standards, clocks used to keep time must have very good stability, which means that the clock can continue to accurately measure a unit of time over a certain period of time. For example, its measurement of the length of a second must be the same over days or even weeks.

Most of our current clocks generally use quartz crystal oscillators to keep time.

These oscillators use the "piezoelectric effect" of quartz crystals. When voltage is applied to them, the quartz crystals vibrate at a precise frequency . This vibration is similar to the swing of an ancient pendulum clock, thus outlining the footprints of time.

Unfortunately, quartz clocks are not very stable. Even the best quality quartz oscillators can be off by a billionth of a second after just one hour, and after six weeks they can be off by a full microsecond .

This would produce huge errors in measuring the position of a fast-moving spacecraft.

Therefore, space timing uses the most accurate clock on Earth today - the atomic clock .

What is an atomic clock?

Since the 1950s, the gold standard for timekeeping has been ground-based atomic clocks.

1948 World's first atomic clock

In 1967, the General Conference on Weights and Measures adopted a resolution to change the original definition of the time unit "second" based on the macroscopic periodic motion of celestial bodies to the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine energy levels of the ground state of the cesium 133 atom.

In simple terms, we know that an atom is made up of a nucleus (protons and neutrons) surrounded by electrons.

These electrons orbiting the nucleus are not stable. If they are hit by energy in the form of microwaves, they will rise to higher orbits (energy levels) around the nucleus .

However, the excitation energy between two orbits (energy levels) is fixed . It cannot be too much or too little. Therefore, electrons must receive exactly the right amount of energy to complete the transition. Fortunately, microwaves have a specific frequency .

In addition, the energy required to change an electron's orbit (energy level) is unique to each element and is consistent across the universe . For example, the frequency required to change the energy level of an electron in a hydrogen atom is the same for every hydrogen atom in the universe.

It is precisely because the energy difference between these orbits (energy levels) in atoms is very accurate and stable that atomic clocks can achieve timing capabilities far exceeding those of quartz clocks.

Atomic clocks for space navigation

Although atomic clocks can be used to obtain the precise time required for two-way signal transmission, there is currently an embarrassing problem.

This two-way signal transmission method means that no matter how far the spacecraft is from the earth, it must wait for the signal carrying the earth's instructions to be transmitted across the ultra-long distance in the vast universe before taking the next step.

This scene is not made up in our imagination. This happened before the Curiosity spacecraft landed on Mars. It took 14 minutes for Curiosity to receive the "confirmed landing" signal sent by the control center on Earth.

This delay is an average wait time: it depends on the positions of Earth and Mars in their orbits around the sun.

And this problem is not only embarrassing, it will also have a relatively large impact on future manned space missions to other planets.

Therefore, NASA experimented with a method: installing an atomic clock directly on a spacecraft, also known as a deep space atomic clock .

At this time, the spacecraft only needs to receive the signal from the ground control center, and the atomic clock on it can accurately and timely obtain the time taken for the signal to be transmitted. Then, the astronauts on the spacecraft can calculate their own position and trajectory, and determine their direction in space.

In fact, installing atomic clocks on spacecraft is not new. Current navigation satellites (such as China's Beidou satellites) are equipped with atomic clocks.

When we use the navigation software on our mobile phones, the atomic clock on the satellite can calculate our position on the earth based on the time required for the mobile phone signal to be transmitted to the satellite, and then provide navigation function in combination with the 3D map.

As for why the atomic clocks on satellites are not used, it is mainly because the atomic clocks on satellites are not stable enough. Although atoms are in a vacuum environment, they may still be affected by external factors such as temperature and magnetic field, resulting in frequency errors .

However, the deep space atomic clock does not use neutral atoms, but mercury ions that have their own electrons. In this way, the mercury ions will be protected by the "ion trap" to reduce external influences.

According to NASA ground tests, the stability of the mercury ion deep space atomic clock is 50 times higher than that of the atomic clock on the GPS satellite.

For missions to distant destinations such as Mars or other planets, this high-precision atomic clock will free up spacecraft and make automatic navigation in space possible.

Perhaps humans will be able to land on Mars in our lifetime.