In the automated survey ATLAS (Asteroid Terrestrial-Impact Last Alert System) a new object was found in June 2019 moving against the background stars. Initially named LD2 of 2019, it was thought to be an asteroid orbiting the Sun near Jupiter. However, an amateur astronomer notes that it looks vague, not pointy, which means it looks more like a comet: icy material on the surface turns into a gas when heated by the sun.
Astronomers checked the archival footage and determined that it had been ‘active’ for at least a few months. The name of the object was then changed to P / 2019 LD2, indicating its status as a periodic comet.
Images by other observatories have confirmed this, including Hubble. When they looked at the comet in April 2020, they saw it running a rather large tail, stretching about 600,000 kilometers, almost twice the distance of the Moon from Earth! Note that the core – the fixed part of the comet – is probably only about 4 kilometers wide.
Calculations show that it lost about 80 kilograms of water ice per second. It also repelled gases such as carbon monoxide (about 50 kilograms / second), carbon dioxide (7 kilograms / second) and diatomic carbon (two carbon atoms bonded together, at a rate of 40 grams per second).
It may sound like a lot, but it turns out only started gassing like that … and it would not last very long. Its status as a periodic comet is only temporary. Extremely temporary: follow-up measurements to determine its orbit have found that it is actually in a similar orbit to Jupiter, and there is an excellent chance that the mighty gravitational pull of the giant planet in the distant future completely removes the comet from the solar system will throw.
If that happens, it’s going to be an interstellar comet like 2I / Borisov or ‘Oumuamua, interstellar objects that have both recently moved through our solar system (and which, I’ll notice, are not alien spaceships).
This is appropriate because it probably started living in the outer parts of the solar system as well.
It is likely that P / 2019 LD2 started as called a Trans-Neptunian object, an icy body orbiting the sun in the Kuiper Belt past Neptune. Over time, many gentle thrusts through the gravitational pull of Neptune propelled it into a smaller orbit, closer to the Sun. Eventually it came close enough that Neptune could jerk it much harder, change its orbit significantly and place it in an orbit between Jupiter and Neptune (from about 800 million to 3 billion kilometers from the Sun). Objects on such orbits are called Centaurs.
Centaurs is interesting. Over time, the gas giants tend to change their orbits even more. In general, after a few million years in this part of the solar system, they come too close to one of the planets. Either they fall into the inner solar system (and become what we call Jupiter Family Comets) or are completely thrown out of the solar system. That’s why we call it transition objects*.
What will be the fate of P / 2019 LD2? And where did it originally come from?
Observations over time of an object can be used to determine its orbit, which can then be projected into the past and the future. The problem is that we can not measure the trajectory exactly; there is always some uncertainty in it. The further you try to predict its position in the future (or predict its position in the past) the darker it becomes, the greater the amount of space it can take up. This makes this kind of prediction troublesome.
To circumvent this, astronomers have done something clever: they simulate the orbit using a Monte Carlo technique. They take the physical properties of the orbit (the shape, the distance from the sun, the tilt, etc.) and then change each one very slightly and create a slightly different orbit. They then walk into the past and the future and see what it does. They do this again and again and create a virtual group of objects, each with slightly different paths. This way you get a more statistical idea of what the history and future of the object was and will be.
What they found for P / 2019 LD2 is that it probably only came into Jupiter’s space about 2.5 years ago! Previously, it was a standard edition of Centaur, but he recently pushed into its current orbit.
And its future? They probably think it will only stay in its current orbit 8 or another 9 years. After that, it will probably fall into the inner solar system and become a Jupiter Family Comet. That means it only makes a pit stop near Jupiter.
Even that is temporary. It is carried out of the solar system in 340,000 years, rising to 95% within 4 million years.
It is likely that billions of objects like these were thrown out over the age of the solar system. And there are billions of stars like the sun … that’s why astronomers think the galaxy is loaded with rogue interstellar ice balls like P / 2019 LD2, and why it’s not so surprising that we see how it goes through our solar system.
Will some alien scientists see in the distant future that LD2 goes through their own system? What would they make of it? It is nice, and strangely reassuring, to know that pieces of our environment will be scattered among the stars, from citizens of our solar system to citizens of the galaxy.
*It’s pretty cool, it worked out that way, since they’re named after mythical half-human / half-horse creatures.