On February 22, a Falcon rocket launched the Indonesian communications satellite from Cape Canaveral Nusantara Satu . While the satellite, as usual in the Falcon 9, was the main payload for the mission, the rocket had some stowaways. These secondary payloads are typically experiments or spacecraft that are too small or too weak to warrant their own rocket, such as CubeSats. But despite the flights in the economy seats, one of the secondary payloads on this launch is a stroke of fate: Israel's Beresheet the first privately funded mission to try to land on the Moon.
But different The Apollo missions, which only took three days to reach our next heavenly neighbor, Beresheet takes a much calmer course. It will take over a month for the probe to reach the moon, and it will take a few weeks for it to finally make a powerful descent towards the Sea of Tranquility, not far from where Apollo 1
At first glance this may seem strange. If the journey took only a few days with the 1960s technology, it would seem that a modern rocket like the Falcon 9 would make better times. In reality, however, the pace is dictated by budgetary constraints on both the vehicle itself and the booster that has carried it into space. It could be argued that the orbital maneuvers involved in this "scenic route" towards the Moon are more complicated than the direct trajectory of the manned Apollo missions, but it promises a whole new class of moon probes. If you are in no particular hurry and are trying to save some money, it may be better for your lunar mission to take the long road.
Orbital Mechanics for Beginners
Before We Can Understand the Different Methods To reach the moon, we need to know a little bit how the space probes navigate. Specifically, how a spacecraft orbiting one body can transition to orbit around another. As you might expect, these concepts in the literal rocket technology are a bit difficult to master.
Therefore, we will take some liberties here to make things a little easier to capture the mind. Let's forego the third dimension first and imagine that both the start and destination lanes are on the same level. Let's say the orbits are nicely concentric, meaning that the orbit we are going to transition into is essentially the same as our current orbit, only at a higher elevation from our starting position.
Obviously the moon does not have a perfectly flat and circular orbit around the earth, but it is not a terrible analogy. Once the basics of orbital transmission are understood, it is not a big jump to see how they could be applied to the actual orbit of the moon around our planet. However, applying these concepts to distant targets in our solar system is left as an exercise for the reader.
The other important thing to understand is how changes in speed affect orbit. When the speed in the direction of travel is increased, the altitude of the orbit on the side is increased from of the combustion. The height and position at which this speed increase would then become the lowest point or periapsis in a now elliptical orbit. The principle works in reverse order when the spacecraft fires its engine against the direction of motion.
To put this into the perspective of our 2D model: When a spacecraft fires its engine on the "right" side of the orbit, the altitude change will be on the left side. In this way, the peak height of the orbit, referred to as apoapsis, can be increased until it intersects with that of the orbit of the target. The spacecraft then drifts along this temporary transfer orbit (19459004) to the Apoapsis, where it fires the engine again to perform a so-called "circulation combustion". This maneuver increases the periapsis to that of the target and completes the transition of the vehicle into the new orbit.
The Express Route
When the Apollo missions sent people to the moon, they obviously wanted to get there as fast as possible. The living conditions aboard the spacecraft were not only cramped, but also, the longer the men stayed in space, the more supplies they needed to sustain them. To minimize the time needed to travel to and from the Moon, the Apollo spacecraft was transferred from the underground orbit (LEO) directly to the Moon's orbit.
After launch, the Apollo probe was put into a LEO "Parkbahn", where the crew checked out the vehicle's systems and prepared for the upcoming ride. Once the spacecraft was in the correct position, they performed the Trans-Lunar Injection (TLI) maneuver, a motor fire that increased the speed so much that its orbit would intercept that of the moon. On the way, a few course corrections were made and some braking maneuvers to slow down and enter the lunar orbit, but overall the trajectory is not far from the simplified model we saw earlier.
This type of TLI maneuver is ideal because it will get you to the moon as fast as possible, but it will only work if the vehicle engine is strong enough to accelerate to the required speed within the appropriate window. In the Apollo missions, the Rocketdyne J-2 engine increased the spacecraft's speed by just over three kilometers per second in just six minutes.
Slow and steady
But what if you have none? powerful J-2 engine of the Apollo spaceship, let alone the mighty Saturn V, to lift you into space? What if your spacecraft is the size of a bathtub and the secondary payload status means the booster missile can not send it directly to the destination lane? In that case, you have to get creative.
To reach the Moon, Beresheet will eventually use multiple orbits to slowly lift its apoapsis over the next month. In each orbit around the earth, the spacecraft, when approaching the planet closest, performs a relatively short engine burn. Each burn not only increases the apoapsis slightly higher but also increases the orbital period (how long it takes to complete an orbit). In other words, with each combustion of the engine, the time until the next combustion is postponed further. Once on the moon, the spaceship performs a similar routine to lower itself to the lunar surface.
The compound nature of these maneuvers makes it easy to see why the journey takes so long. Should Beresheet miss any of its planned burns (which indeed has already happened), the transit time could be further extended. On the plus side, the vehicle can reach the moon with a very small engine, and ground controllers have plenty of time to tune the flight path accurately and solve potential problems.
Make Luna Great Again
Despite the incredible success of the manned Apollo missions, humanity has never taken the next steps to make full use of our natural satellite. In the following decades, only a handful of vehicles ventured back to the moon, and most of them were never on the surface. But thanks to a rapidly developing Chinese space program and the current government in the United States, which is committed to a return to the moon, we might be able to finish the work that we started in the 1960s.
If this is the case These "economy" flights to the moon may become more frequent. Starting as a secondary payload and using low-thrust orbital transfers could be an alternative to expensive direct flights. Payloads that are not particularly time-sensitive, such. As building materials, spare parts or food with long shelf life, could be sent regularly with excessive capacity to the moon; potentially creating a completely new market for the first company to develop a small, all-purpose lunar craft. Maybe one day Amazon Prime will be able to add Sea of Serenity to its list of supported sites.