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How to Land on Another Planet

Over the past few decades, we’ve learned so much about our star system and observed unique phenomena on so many other worlds and asteroids. At first, this knowledge could only be attained through terrestrial telescopes. At the time, this opened up the heavens, but soon, scientists realized that a stronger source was needed. In 1990, the Hubble Space Telescope was launched into orbit and was named the first optical telescope in space. 

But the evolution didn’t stop there. Soon, scientists realized that the only way to get the detailed data they required was through a mission to a celestial object itself. At first, only orbiters were sent, but soon enough, unmanned missions began to land and explore the surface of our moon, Mars, and even other moons. All of this combined paved the way to the manned missions, such as the famous Moon landing. 

If these missions have taught scientists anything, it’s that our solar system is filled with unique characteristics. From Jupiter and Saturn’s raging, fierce storms, to Uranus’s eerily silence, each planet and moon has completely different surfaces and atmospheres. 

To land on another planet takes years of planning, and one of the main things they need to account for is the atmosphere. Usually, the atmosphere is used to slow down the spacecraft, but places such as our Moon barely has an atmosphere. To accommodate for these various challenges, a process, known as EDL (Entry Descent Landing), is often used as a skeletal plan for the final stages of the mission’s journey to the surface of another world. 

Here, we’re going to take a look at the main stages and what happens at each stage to prepare a rover for a safe landing. 

Preparations 

The entire process of EDL is quite intricate and requires the most precise timing for the execution to even work. To start off the process, around 10 minutes before atmospheric entry, the spacecraft sheds its cruise stage. This includes its solar panels (the rover has a set of its own to gather energy during its time on the surface), the radio, and fuel.

  After this weight has been released, the thrusters are fired to ensure the spacecraft’s heat shields are pointing in the right direction so they can absorb the majority of the heat caused by friction during atmospheric entry. The thrusters also ensure that the spacecraft will hit the atmosphere at precisely the right angle. Even a few degrees off and it could be burnt to a crisp.

Atmospheric Entry

Now, the spacecraft is ready to enter the atmosphere. As an example, let’s assume that that a spacecraft is descending through the Marian atmosphere. As it descends, a large amount of friction is created because of the high speeds at which the spacecraft is going at. This friction translates into an immense amount of heat. The peak temperature (around 80 seconds in) is around 2,370F (1300C). However, the heat shield ensure that the inside only heats up to room temperature. 

As the craft makes the bumpy trip down, the thrusters continuously fire to ensure that it doesn’t stray from the flight path. Small pockets of air with different densities can nudge the craft off the path, and even a seemingly small push could result in a landing miles away from the designated spot. 

Parachute Deployment

The drag forces of the atmosphere slow the craft down to under 1000 miles per hour (1600 km/hr). Once this speed is reached, the parachute can be deployed, but it must be done at the exact right time to ensure a safe landing. 

The Perseverance Mars rover used a new technology at this stage called “Range Trigger.” It allowed the craft to calculate its altitude and the most optimal time to open the parachute. 

The parachute itself is huge, measuring to around 70 feet (21.5 m) in diameter. 

Zeroing In On a Landing Spot

20 seconds after parachute deployment, the heat shield separates and falls off. This is the first time the equipment is fully exposed to the atmosphere. At this point, they begin to turn on and scan the ground, searching for a suitable place to land. 

The Perseverance rover uses another new piece of technology called “Terrain Relative Navigation.” This allows the rover to scan the surface and compare it to pictures already in its computer to find the best place to land. This method allows landings in difficult terrains (such as the Jezero Crater). 

Powered descent

Mars, for example, has a pretty thin atmosphere, so the speed of the spacecraft is not slow enough (even with the parachute) for a landing. In order to reach the speed it needs, the rover cuts itself from the parachute and uses its thrusters to lower itself down the final few hundred feet. 

Landing

There are two main types of landings for these spacecrafts. 

The first method uses airbags. Essentially, when the rover cuts free from the parachute, it’s dropped from the main capsule and protected by a balloon-like design that absorbs the impact of the fall. The rover and airbag bounce until coming to a stop, at which point the balloon is deflated and the rover can safely come out. However, these landing can be quite imprecise. For instance, Spirit bounced around 28 times before coming to a stop 300 years from the point of its impact. Additionally the airbags are quite large and heavy, meaning that theirs less space for other instruments. 

The second (and newer) method uses a Sky Crane. During the spacecraft’s final descent, the speed is lowered to around 1.7 mph (2.7 kph). With around 12 seconds till impact, the descent stage powers the rover onto the ground using a set of 21 ft (6.4 meters) long cable. While being lowered, the rover unhooks it’ legs and wheels and prepares for touchdown. Once the landing has been confirmed, the cables are cut and the sky crane flies away to crash onto the surface a safe distance away. 

In the case of the Martian missions, these steps are known as the 7 minutes of terror. During the 7 minutes in which all these procedures occur, the team has no control over the rover and almost all communications are cut. Only after the rover has touched down and the signal has reached Earth will the team know the ultimate fate of the mission. 

Sources :
NASA