What Is a Solar Conjunction and How Does it Affect Space Missions?

In a solar conjunction, two objects are on opposite sides of the Sun. What is so special about this event, and how does it affect space missions exploring the Solar System? Let’s find out.

In most cases, solar conjunctions are not exact. When seen from one object, the other object and the Sun are at a small angular distance, appearing very close to each other. But light can still travel through each other without being blocked by the Sun. So why is it still a problem for spacecraft?

Very often, the Sun ejects material into outer space. They range from small outbursts of matter to massive coronal mass ejections. However, they have one thing in common — the particles are ionized, forming plasma. When signals are sent to space probes, they travel in electromagnetic radiation (a.k.a, light). However, these signals get disrupted by the ionized particles when they encounter the cloud of plasma ejected by the Sun. Therefore, the data can be corrupted when they reach the other side of the communication.

If we send commands to a spacecraft in solar conjunction with Earth, the commands received may differ from what was sent from Earth. At best, the spacecraft does not recognize the instructions and will produce an error that it will send back home. However, in the worst-case scenario where the space probe accepts the wrong command, it could do something potentially threatening to a mission, like causing it to miss the target by manipulating the duration of the engine burn. That’s why mission operators don’t allow communication to the spacecraft during the solar conjunction period. It’s simply too unsafe to do so.

If we cannot talk to the space probe, what will it do during solar conjunction? Before the event happens, engineers will send a few weeks’ worth of instructions to the space probe. This allows it to operate autonomously at low risk while still collecting some data as its waits for the conjunction period to elapse. After the event occurs and the Sun no longer poses a threat to the data packets sent back to Earth, the spacecraft will resume operations and send the measurements collected on the storage devices of the space probe.

Apart from actual solar conjunctions, being close to one can also be a problem for telescopic observations. Besides sending out plasma that interferes with signals, it also sends out vast amounts of light at different wavelengths. It’s so luminous that, even at one astronomical unit away, looking at it directly can still permanently harm your eyes. Thus, the Sun is bright enough to outshine many objects near its location in the celestial sphere, making it challenging to observe these objects.

In fact, one of the loopholes in the system of detecting near-Earth asteroids is precisely due to this. That susceptibility is the lack of asteroid discoveries closer to the Sun than Earth. While hundreds of thousands of asteroids are found in the asteroid belt, and thousands of them near the orbit of Earth, only 28 of the known minor planets reside entirely inside Earth’s orbit. This could mean that large asteroids closer to the Sun than Earth may escape detection until very recently, making it a significant vulnerability in asteroid impact prediction and prevention.

To recap, we’ve learned what a solar conjunction is, how it affects space missions, and how it makes objects hard, if not impossible, to observe. Remember that when planning mission operations or telescope observations, it’s essential to consider the Sun’s position. If we have missed something we should have included, feel free to comment below to help improve this article.