The “Zhurong” on Mars, do you need to sleep at night?

The "Zhurong" on Mars, do you need to sleep at night?

On May 15, 2021, Tianwen-1 successfully landed on Mars to start its scientific exploration journey, and on May 22, China’s first Mars rover, Zhurong, drove to the surface of Mars and carried out environmental sensing, movement, scientific exploration, etc. On June 11, the first scientific images of the Tianwen-1 rover landing on Mars were released.

In the previous lunar exploration missions, the lunar rovers Yutu-1 and Yutu-2 would wake up every month from their dormant state on lunar nights – it could not work on cold lunar nights and had to sleep on the equivalent of Earth days for about 15 days each time. And to ensure that Yutu-2 can wake up properly, the lunar exploration project team has made a lot of efforts and accumulated a lot of experience.

Now that Zhu Rong has been on Mars, does Zhu Rong also have to sleep during the night on Mars? And how does the Mars exploration team see these problems? This article will tell you that “sleeping” is not an easy task.

Does the rover need to sleep too? If you want to get straight to the conclusion, the answer is: probably yes. If you want to know why, you’ll need a little patience.

The "Zhurong" on Mars, do you need to sleep at night?

The actual picture of the Wishbone

I. Talking from the Moon

The story starts 17 years ago, when our lunar rover development team started to consider designing China’s first lunar surface rover – actually a lunar rover – and planned to send it to the Moon. There were many problems, and one of them was how to spend the lunar night.

Before us, the Soviet Union had launched two lunar rovers that worked for 10 months and 4 months, respectively. Their method was to use the heat generated by the decay of the isotope polonium 210 to help the rovers get through the cold, long lunar nights.

This sentence needs three notes added to it. First, how cold were the lunar nights? Actually, the Soviets did not know the temperature of the lunar night at that time, and after analysis, it was estimated to be about minus 150 degrees Celsius. The equipment on the lunar rover could not stand such temperatures, and thermal control measures were needed to make the equipment more comfortable during the lunar night. It will be 50 years before the Chinese Chang’e-4 lander uses direct measurements using temperature difference power generation technology to reach temperatures below minus 190 degrees Celsius on the lunar night.

Second, how long is the night on the Moon? Earth’s surface night typically lasts about 12 hours (not typically referring mainly to Earth’s polar regions), while the Moon’s rotation is much slower. We all have an idea that a lunar month is either 29 days or 30 days, which is a lunar day. That means that the night of the Moon corresponds to about 15 Earth days.

If the time is short, people can use batteries to heat the equipment at night, for example, artificial satellites flying around the Earth will also briefly into the shadow of the Earth, is to use batteries to solve this problem. But the moon night time so long, if the use of batteries, that the required capacity of the weight of the battery almost half of the total weight of the lunar rover, which is obviously not a good solution.

The third point to clarify is that isotope decay is a relatively mild nuclear reaction in which atomic nuclei emit rays that transform into another kind of nucleus. The moon rover uses the heat that is slowly released from the material during the nuclear reaction. The half-life of polonium 210 is 138 days, which means that after 138 days, half of it has decayed into another element; after another 138 days, only a quarter remains. The decay process generates heat, which becomes less and less as time passes.

The first Soviet lunar rover, which worked for ten months, ended up being the one where the heat released by isotopic decay decay decayed to the point where it was not enough to maintain the temperature conditions needed to keep the rover going at night. The second Soviet lunar rover worked for only four months, this time not because of thermal decay, but because the solar cell cover touched the ground when the vehicle moved, and when the cover was closed at night, it spilled moon soil on the body of the vehicle, affecting the heat dissipation of the vehicle, which eventually led to the failure of the lunar rover due to high temperature.

Before us, the Americans also had three astronauts riding on the moon rover, but only work briefly in the moon day, do not need to spend the night, and this article is not related to the topic, not to repeat.

If we do not solve the problem of overnighting, our rover can only work on the lunar surface for 7~10 days. It is a pity that it can only work for one week after so much effort, so we treat how to spend the lunar night as a key technical problem that must be solved at the beginning of the lunar rover design.

II. Program formation

Our first idea was to use a big battery. However, it was calculated that the lunar rover would need no less than 50 watts of energy at night for 15 days, so the weight of the battery reached 50 kg, which is nearly half of the total weight of the rover.

This method does not work, it seems to use isotopes. The Soviets use polonium 210 which releases more heat per unit mass, but decays too quickly to make the lunar rover work for a long time, and other isotopic radioactive sources have to be found.

Plutonium-238 has a half-life of about 88 years, and the heat reduction can be disregarded over the lifetime of the lunar rover, which is clearly the better technical route.

As can be seen in the graph below, the polonium 210 heat source has decayed to an initial 22% of the heat released after 10 months from production, while the plutonium 238 heat source has only decayed to an initial 97% after 48 months, a very small change. Considering the heat source production out to launch and then to work on the lunar surface, the plutonium-238 heat source is clearly a more suitable choice. The Jade Rabbit II lunar rover has been working on the back of the Moon for more than two years now, and no diminishing of the heat provided to the rover on lunar nights has been measured yet.

The "Zhurong" on Mars, do you need to sleep at night?

How big a heat source is appropriate is another issue that needs to be considered. Theoretically put a suitable heat source in the car the problem is solved, but this part of the heat in the daytime of the moon will also be constantly released. And the temperature of the lunar day landing site is very high, will be close to 100 degrees Celsius, coupled with the sunlight, considering how to dissipate the heat generated by the work of the equipment is already very difficult, but also add the heat energy released by the isotope during the day, all to be dissipated from the vehicle, we really can not find a solution.

Therefore, we proposed to put the heat source outside the car body, and only bring the heat inside the car body at night when needed. Along this line of thought, the designers researched and developed a two-phase fluid circuit. During the daytime, the circuit is closed and the heat is dissipated outside the vehicle body by radiation. Although the heat source will be hotter, it has little to do with the equipment inside the lunar body; at night, the valve is opened and the fluid circuit starts to work, and the heat source heats the work substance ammonia from liquid to gaseous state, and the ammonia gas enters the lunar body along the pipeline, heats the equipment and then turns into liquid again, and so on.

The ingenious thing is that such a mass flow does not need pumps and other driving equipment, but only the gravity of the moon to drive the operation of the circuit, to ensure that the circuit can work reliably in the moon night.

The "Zhurong" on Mars, do you need to sleep at night?

Jade Rabbit lunar rover

In fact, this part of the thermal energy can also be partially converted into electricity using temperature difference power generation technology, only the conversion efficiency is not high, about 5%. This way, the lunar rover can always have a small power device to power up at night. Traditionally, satellites are always powered up after orbit, and a power failure means that the satellite is disabled.

During the technical discussion, a veteran expert was more open-minded and offered a different opinion. He said that with heat, the lunar rover would not freeze, that power generation was not necessary, and that it was perfectly possible for the rover to wake itself up and continue working according to changes in sunlight.

Carefully refining the idea of light wake-up, it is true that it is simpler and more reliable to do the system this way. Although it cannot achieve goals such as measuring the moon’s nighttime temperature, the light wakeup idea does have advantages in a situation where weight resources are so tight.

Thus, the two solar wings of the lunar rover were given more functions. The right solar wing is movable, and its primary function is to generate electricity, turning it into a quilt for the lunar rover to cover the body at night when it is not needed. If the temperature of the equipment is too high at noon on the Moon, this solar wing can be erected and turned into a sunshade, or “right wing triple use”.

The left side of the solar wing will be unfolded in place once it reaches the moon and cannot be moved again. In addition to the function of power generation, this solar wing is deliberately designed to tilt down 10 degrees, in order to receive the sunlight rising from the east early in the morning of the moon. The solar wing emits current to rise in the process, once it reaches a specific current value, it will drive the relay on board to act, so that the lunar rover wakes up and starts a new day’s work. At this time, the left solar wing will be used to generate electricity, referred to as “left-tilt dual-use”.

First wakeup

On December 27, 2013, after the sunlight altitude angle on the lunar surface dropped below 10 degrees, the ground team performed a series of actions to start controlling the hibernation of Yutu.

The first step was to find a suitable hibernation site. The heading angle requirement is 161-169 degrees, which means that the nose is facing east of south (due east corresponds to 90 degrees, due south corresponds to 180 degrees), so that the left solar wing is basically facing east, waiting for the early morning sunlight. The front end is a little to the east to avoid the morning sunlight shining from the rear of the car to the isotope heat source. The nose heading can be completely controlled by turning the vehicle in place, and the control target range is relatively wide and easy to achieve.

The vehicle pitch angle requirement is also relatively wide (1~14 degrees), requiring a high nose and low tail to facilitate the fluid circuit to circulate the work mass up under the lunar gravity drive, and it is relatively easy to find such a terrain.

The most difficult part is the side tilt angle requirement of -2~1 degree, the allowable range is only 3 degrees, and the lunar rover does not have the ability to adjust the side tilt angle, need to find the terrain that meets the requirements, and then drive the vehicle over. The requirement is so strict because the left side downward tilt 10 degrees of the solar wing will be in the solar altitude angle of 5 degrees, that is, in the sun relative to the battery sheet 15 degrees angle when the appropriate size of the current, the lunar rover wake up.

If the left tilt of the rover exceeds the limit, it means that it wakes up earlier, the surrounding environment is too cold to continue working, and the temperature of the battery may not have risen to the level of normal discharge; conversely, the right tilt exceeds the limit, it means that it wakes up too late, and the temperature of the equipment inside the rover will become too high under the joint “baking” of sunlight and isotope heat source.

The day before the rover is scheduled to hibernate, we start looking around the rover for the most likely lunar night mooring site. The engineers controlled the lunar rover to drive over to it, and it measured -4.0 degrees of lateral tilt, not yet compliant. The clock ran out, forcing us to use our last resort – plowing.

By rotating the right wheel back and forth, the lunar rover drained the soil and allowed the wheels to sink. It was estimated that with the right wheel sinking 4.5 cm, the rover’s lateral tilt would change by 3.2 degrees to -0.75 degrees, at which point it met the dormant attitude requirement. Finally, we issued a series of commands to control the hibernation of the lunar rover, and the Jade Rabbit went to sleep.

It was expected that the lunar rover would be awakened only at 4 a.m. on January 11, 2014, and the Jade Rabbit would sleep for half a month during this sleep. On the 11th, at 3:00 a.m., several of us took the elevator to the operation hall, but we didn’t expect that there were many people around the entrance of the hall.

Quietly asked, many people arrived at 2:00, just now still talking about how the designer has not come, so calm? We hurriedly explained that the sunlight on the moon oblique shine on the left side of the solar panel, the sun takes 2 hours for every degree of elevation, must be to the specified 5 degrees of solar altitude angle, the solar wing issued by the current is enough to wake up conditions, it is impossible to advance 2 hours. “Why didn’t you say so earlier”, someone complained while dispersing.

Time passed quickly, four o’clock arrived, and the designer was sitting in his seat, waiting for news from 380,000 km away, but there was no movement yet. Ten minutes later, a carrier signal was received on a specific frequency, and the hall cheered as the lunar rover woke up on its own for the first time!

Later, the Yutu-1 lunar rover worked for another two and a half years, but due to problems with the institutional control, the solar wings could not move, and the wake-up of the rover became based on the power on the two solar wings, and woke up after meeting the work needs of the platform equipment, which was several hours later than the initial design of the autonomous wake-up time.

IV. Hibernation and wake-up of Yutu-2

After deciding that Chang’e-4 would perform the first human soft landing and roving survey mission on the back of the Moon, the designers wondered what improvements to make to the lunar rover. In terms of hibernation and wake-up, the first lunar rover hibernation was implemented by ground command control, and after reaching the back of the Moon, the ground and the rover on the back of the Moon could not contact each other directly, and could only rely on the Magpie Bridge relay star to help deliver messages. Here there is a little bit of hidden danger, in case the signal is blocked when the rover is going to hibernate, or the relay star cannot provide relay service at that time for some reason?

Therefore, the designers considered that Yutu II should not only wake up independently, but also upgrade to have an autonomous hibernation function. When the sun is slanting in the west, the current from the solar wing is constantly monitored, and when a specific current value is reached, the lunar rover will control the right solar wing to close itself, which is equivalent to covering itself with a quilt and preparing for the night.

The back side of the Moon, which is relatively mountainous, was originally designed to wake up when the altitude angle of the Sun was greater than 5 degrees. In the back landing point, if the mountain in the east is higher than 5 degrees elevation angle, it means that once the lunar rover comes out from the darkness, the temperature of the rover has not yet risen, especially the electrolyte temperature in the battery is still too low, but at this time the solar wing current has met the requirements, the rover will wake up, and the battery needs to output electric power immediately to cooperate with the wake up, but at this time the battery is still frozen to lose electricity, which will finally lead to the failure of the lunar rover wake up, or even permanent Failure.

The clever designer came up with a way to adjust the lunar rover’s heading when it sleeps. Before hibernation, it is necessary to adjust the orientation of the rover according to the height of the eastern mountains, to ensure that no matter how much the height of the mountains, wait until the sun rises 2 degrees again, has raised the temperature of the lunar rover and the surrounding soil, the intensity of sunlight shining on the left solar wing to meet the wake-up requirements, at which time the lunar rover wakes up, there is no difficulty.

The program is good, a new problem comes, must know the height of the Eastern mountains before hibernation, in order to know which direction the head of the rover towards sleep appropriate. Therefore, just after arriving at the lunar surface, engineers organized to take pictures of the altitude angle of the horizon, and the results came in that the height of the Eastern Mountains was just less than 3 degrees, so everyone was relieved.

The "Zhurong" on Mars, do you need to sleep at night?

Taking a picture of the altitude angle of the horizon

Now Yutu-2 has been working on the back of the Moon for more than two years, becoming the longest working lunar rover on the lunar surface, traveling more than 700 meters, and the hibernation and wake-up process has been carried out more than 30 times, a design that has proven to be reliable.

V. Hibernation and wake up of Mars rover

When we first designed the “Zhurong” Mars rover, we felt that we did not need to design a special hibernation and wake-up function for it, because the Mars day is 24 hours and 40 minutes (very close to the Earth’s day, not to talk about the polar regions of the polar day and night phenomenon) – using battery power, there is no problem to spend the night on Mars.

Interestingly, the orbit of Mars around the Sun is not as circular as the Earth’s orbit, and a strict analysis of the length of a Martian day will vary by a few minutes, something that must be taken into account when designing a Mars rover.

As the analysis became more in-depth, we found new problems. It is possible for dust storms to occur on Mars, and this is when the energy supply for the rover will have problems. The duration of such dust storms can be long or short, and the batteries are not enough to support the rover all the time.

Dust storms are one reason why the rover may be dormant, and another reason is the Martian winter. Due to the 25.2 degree angle of intersection between the rotation axis and the direction of the orbital plane (23.5 degrees on Earth, again very similar), Mars also has four seasons, although each season is nearly six months long, twice the length of a season on Earth.

The latitude of the Mars rover landing site is located near the Tropic of Cancer because it has better light conditions during the 3-month lifetime. But if after a year, the direct sunlight point slowly shifts south to south of the equator, the Mars rover work site will gradually usher in the northern hemisphere winter, the sun will no longer pass over the roof of the rover, and it will start to decline when it rises to an altitude angle of about 40 degrees in the south, the light conditions become worse, the maximum temperature of the Mars surface gradually decreases, and the Mars rover will be short of energy at this time.

The Mars rover is far away from Earth, so if the situation occurs by the ground to determine the hibernation disposal will certainly be too late, so the autonomous hibernation wake-up technology used on the Yutu was again put to use, but this time the method became more complex. Because the hibernation time cannot be predicted in advance, the Mars rover will not directly feel the dust storm blowing, nor will it directly know that the Martian winter has arrived.

The way to deal with this is that every day when the sun is about to set, the Mars rover will have to determine if the battery is in a near full charge when it starts to discharge. If the current battery charge is not ideal to get through the night, it will have to decide to hibernate and save as much battery power as possible.

When is the right time to wake up? One condition is the solar wing power generation enough, enough means, not only to meet the needs of the Mars rover work after waking up, but also to ensure that there is a part of surplus energy to charge the battery, to ensure the needs of the fire night. Another condition is the temperature of the battery, because the battery temperature is too low, there is power can not be charged. The technical implementation means that the current judgment and temperature judgment are made into a hardware circuit, and only after the two conditions are met, the Mars rover will wake up on its own.

Another important thing after waking up is to let the ground know as soon as possible, but energy is still very tight at this time, in order to save electricity, the Mars rover can not always release the news that it has woken up, how can this be done? The designers of the Mars rover came up with an old-fashioned way of looking at the sun to estimate the time. After waking up the rover, if you find a long time no one care, will look at the sun to determine the time of Mars, and then only in a specific period of time to send “I’m awake”, the ground engineers will know when and Mars rover contact the probability of the greatest.

It is like two people going on an expedition and agreeing to send a contact signal at a specific time in case they lose contact, so that the possibility of finding each other is greatly increased.

The "Zhurong" on Mars, do you need to sleep at night?

The “China footprint” image. “The image of the landing platform taken by the Zhu Rong rover about 6 meters south-southeast of the landing platform, showing the five-star red flag on the landing platform and the details of the Martian surface.

Finally, back to the question mentioned in the opening paragraph: Does the Mars rover need to sleep too? A: Probably also need. However, the designers still hope that in the first three months of the rover’s work, the surface of Mars will not blow up dust storms, sunlight is sufficient, and the hibernation wakeup function is not used, so that the rover can go as far as possible on the surface of Mars and get more exploration data.

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