In June 2019, the automated ATLAS (Asteroid Terrestrial-impact-Last Alert System) survey found a new object moving against the background stars. Initially called LD2 2019, it was thought to be an asteroid orbiting the Sun near Jupiter. However, an amateur astronomer noticed that it looked blurry, not like a dot, meaning it was more like a comet: the icy material on the surface turns to gas as the Sun heats it up.
Checking the archived images, astronomers determined she had been “active” for at least several months. The object’s name was changed to P / 2019 LD2, indicating its periodic comet status.
Images from other observatories confirmed this, including Hubble. When they looked at the comet in April 2020, they saw it sporting a rather large tail that stretched for about 600,000 kilometers, almost twice the distance from the Moon to Earth. Note that the nucleus – the solid part of the comet – is probably only about 4 kilometers in diameter.
Calculations show that at that time he was losing about 80 kilograms of water ice per second. It also gave off gases such as carbon monoxide (about 50 kilos / second), carbon dioxide (7 kilos / second) and diatomic carbon (two carbon atoms together; at a rate of 40 grams per second).
It may sound like a lot, but it turns out not more it started to wear out like that … and it won’t be for long. Its status as a periodic comet is only temporary. Extremely temporary: tracking measures to determine its orbit found it to be in an orbit similar to Jupiter, and there is an excellent chance that, in the distant future, the powerful gravity of the giant planet will completely launch the comet from the system solar.
When that happens, it will become an interstellar comet like 2I / Borisov or ‘Oumuamua, interstellar objects that have recently passed through our solar system (and which, I will note, are not alien spacecraft).
This is appropriate, as he probably also began living on the outside of the solar system.
P / 2019 LD2 is likely to start as what is called a Transneptonian object, an icy body orbiting the Sun in the Kuiper belt in front of Neptune. Over time, very gentle thrusts by Neptune’s gravity propelled it into a smaller orbit, closer to the Sun. Eventually, it got close enough that Neptune could pull it much harder, substantially changing its orbit, placing it in an orbit between that of Jupiter and Neptune (about 800 million to 3 billion kilometers from the Sun). They are called objects in orbits like this Centaurs.
Centaurs are interesting. Over time, gas giants tend to change orbit even more. In general, after a few million years in this part of the solar system, they get too close to one of the planets. Either they drop into the inner solar system (and become what we call comets of the Jupiter family) or are completely thrown out of the solar system. That’s why we call them 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 future. The problem is that we cannot measure the orbit exactly; there is always uncertainty. The farther you try to predict your position in the future (or precede your position in the past), the more diffuse you will get, the greater the volume of space it could take up. This makes this type of forecast difficult.
To solve this, astronomers did something clever: they simulated their orbit using what is called a Monte Carlo technique. They take the physical characteristics of the orbit (shape, distance from the Sun, tilt, etc.) and then change each very slightly, creating a slightly different orbit. They then head to the past and the future and see what it does. They do this over and over again, creating a virtual cohort of objects each with marginally different paths. In this way, a more statistical idea of what was and will be the history and future of the object is obtained.
What they found for P / 2019 LD2 is that it probably only entered Jupiter space about 2.5 years ago. It used to be a standard-themed Centaur, but it was recently put into its current orbit.
And your future? They found that it is likely to only remain in its current orbit for 8 or 9 more years. After that, it will probably fall into the inner solar system, becoming a comet of the Jupiter family. That means it only makes a pit stop near Jupiter.
Even this is temporary. It has a 50% expulsion of the solar system in 340,000 years, which rises to 95% in 4 million years.
It is likely that during the age of the solar system, billions of objects like this have been ejected. 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 isn’t it weird that they also we see it pass through our solar system.
Will some foreign scientists in the distant future see LD2 go through their own system? What would they do with it? It’s fun and strangely reassuring to know that chunks of our neighborhood will scatter among the stars, from citizens of our solar system to citizens of the galaxy.
*Which is pretty cool it worked that way, as they are called mythical half-human / half-horse creatures.