Every now and then comes a day when you read a news title and have to re-read it twice to be certain you haven’t misread something. A rocket colliding with the Moon? Another science fiction plot entering the realm of reality, but it does not sound very cool this time. At least not to those involved in scientific space research and activities, or to those in love with the night sky and solar system as it was before the age of “space pollution”. Two of our senior scientists, involved in ESA Science and Earth Observation missions, Didier Fussen and Johan De Keyser share what the news looks like from their experienced perspectives.
What are we talking about?
Edit of February 14, 2022: Initially, the space debris had been identified as a booster of a SpaceX Falcon 9 rocket from a mission launched towards Lagrange point L1 in 2015. For this reason, this article goes into detail about space missions towards the Langrange points L1 and L2, for this remains relevant in the space debris issue. The piece of debris appears to have been misidentified. There is strong evidence that it actually concerns a booster from the Chang'e 5-T1 lunar fly-by mission, launched by the China National Space Administration in 2014. The time of impact with the moon remains accurate.
If you have missed the news story, here is a short summary. In 2015, SpaceX launched a weather satellite aboard a Falcon 9 rocket, towards Lagrange point L1 in the Sun-Earth system. This is one of five points where the gravitational forces exerted by the Sun and the Earth balance each other out, and an object like a spacecraft can remain in a more or less stable position within the system, with minimal adjustments. The recently launched James Webb Space Telescope, for instance, has been placed in orbit at Lagrange point L2 (see figure 1). Sending missions to these points also has several other advantages besides stability. For example, it allows for constant alignment with regards to the Earth and the Sun, which is very important in order to remain in contact with the mission.
Dr Didier Fussen:
You have to know that L1, L2, L3 are intrinsically unstable, as if you would put a marble on a saddle. In other words, any satellite there has to ‘orbit’ around the point (it is called a halo orbit) with some propulsion needed until the fuel tank is empty. Once this happens, the satellite (or the rocket element) will drift toward the Sun-Earth-Moon system in a very complex (dynamically chaotic) way; the final trajectory being uncertain until the end.
The Lagrange points, even the closest ones, are quite far from Earth (L1 being at 1 500 000 km from Earth), and a lot of energy is needed to get them there. This energy is supplied by rocket boosters, usually in several stages that fall away once they have done their job. The useless boosters are left behind and can either return to Earth where they burn up in the atmosphere, or they find themselves on the chaotic course Dr Fussen mentioned above.
One of the boosters of SpaceX’s Falcon 9 launched in the direction of L1 has been on such a chaotic trajectory ever since 2015. For a long time, it was unclear where it was going to go next, but as it is nearing the end of its course, scientists are now able to pinpoint the time and location of impact with the Moon; namely on March 4 around 1:25 pm (Belgian time) on the hidden side of the Moon, with some degree of uncertainty because of the influence of solar radiation.
Dr Didier Fussen:
This is not an unusual fate for a booster being sent towards Lagrange point L1. The Moon is always a possible attractor for such debris. The collision velocity (almost 9300 km/h) is about the escape velocity from the Moon, meaning that the debris is coming from far away.
Seeing as it isn’t unusual for a lost object to get on a collision course with the Moon, how come we haven’t heard of such an occurrence before? There are actually few space missions sent to Lagrange points, since they are so far and it is thus very expensive to transport massive bodies to such distances. Most satellites are launched into orbits close to and around our own planet, where debris – if the mission is carefully planned – can burn up quickly in our atmosphere and disappear.
What does this oncoming collision signify within the growing influence of humanity in the solar system?
Close to Earth
Dr Johan De Keyser:
Most of the stuff that we earthlings launch does not get very far. All objects in Low Earth orbit, Middle Earth orbit, Geosynchronous orbit etc. will ultimately fall back to Earth (though perhaps after a very, very long time). It is therefore close to Earth that the space debris problem is the biggest – think of the Starlink constellation with ultimately thousands of satellites, and of other constellations that are currently being deployed. They increase the risk of collisions, especially for the ISS or the Chinese space station, which are in a not-too-high altitude orbit. This is not funny.
It is not funny for (amateur) astronomers on the ground either. If I look up at the sky on a clear night, I see numerous satellites pass by (more than airplanes, and I live under an approach corridor for Brussels international airport) and this bothers me. It was definitely not like that a few decades ago.
Dr Johan De Keyser:
But what happens when we launch something to the Moon or towards an interplanetary target? This is usually only done for science and exploration missions, not so much for commercial missions.
First of all, we need the full performance of the launcher to get even a small payload towards the target. As scientists, we obviously always want that payload to be larger because that improves the science return of the mission. Therefore, the satellite is kept as light as possible and it is the last stage of the launcher itself - or possibly a kick stage if there is one – that has to give most of the final push to get onto the desired trajectory.
At that point, the last stage or the kick stage are stranded in some orbit that just falls short of the satellite trajectory, but for many interplanetary missions, that trajectory is still an interplanetary one. Most of these objects end up in an orbit around the Sun.
Then there are the missions to the Sun-Earth Lagrange points L1 and L2. These are in between interplanetary missions and Earth orbit: The L1 and L2 points are due to the interplay between the Sun's gravity and the Earth's. Upper stages that push payloads most of the way to L1 and L2 may therefore still return to Earth - or to the Moon (L1 and L2 are much farther away from Earth than the Moon is). Orbits around L1 and L2 need a little bit of active stationkeeping. Defunct satellites, once stationed at L1 and L2, may come back too, but can also end up in an orbit around the Sun.
As a side note, the famous (and not very scientifically useful) Tesla Roadster that was launched by SpaceX in 2018, is still roaming about. A study has been performed to try and determine its trajectory through our solar system. It found that the Roadster will regularly cross the Earth's orbit, even in the vicinity of the Earth. The impact probability with Earth (or the Moon) is not null and increases with time: 6% after 1 million years, 11% after 3 million years, and within the next 15 million years, there is a 22% chance of impact on Earth and a 12% chance of impact with Venus or with the Sun (Rein et al., 2018). Admitting that a million years is a long time, it remains a question whether we wish to send out objects with no scientific relevance and with a risk of affecting the solar system, of which humanity only makes up the smallest fraction of its space and history.
- The debris problem is most pronounced in near-Earth space. Because there are many objects launched to this destination, it is rather crowded there, and the lifetime in orbit of these objects can be quite long (tens of years).
- For interplanetary missions, it is not the actual spacecraft, but parts of the launchers that may pose a debris problem in near-Earth space. The spacecraft themselves often end up in an orbit around the Sun (where it is not very crowded); collisions with another planetary object are avoided for planetary protection reasons, but are sometimes carried out for scientific purposes (though landings are scientifically more rewarding, of course).
- For the in-between category of missions to L1 and L2, both launcher parts and the actual satellites may pose a problem in near-Earth space.
Dr Johan De Keyser:
A collision with the Moon is in my view not a matter of space debris, but a matter of planetary protection: do we want all this stuff to impact the Moon? Do we wish to scatter our litter on the Moon's surface? Since there is almost no weathering of the lunar surface, the marks of it are there to stay for thousands of years!
Rein, H., Tamayo, D., Vokrouhlický, D.: The Random Walk of Cars and Their Collision Probabilities with Planets. Aerospace 5(2), 57 (2018). https://doi.org/10.3390/aerospace5020057