In June 2019, the automated investigation ATLAS (Asteroid Terrestrial impact Last Alert System) found a new object moving against the background stars. Initially called 2019 LD2, it was thought to be an asteroid orbiting the sun near Jupiter. However, an amateur astronomer noted that it appeared hazy, not point-like, meaning it looked more like a comet: icy material on the surface that turns into a gas as it warms up by the sun.
When checking the archived images, astronomers determined that it had been “active” for at least several months. The object’s name was then changed to P / 2019 LD2, indicating the status of a periodic comet.
Images from other observatories confirmed this, including Hubble. When they looked at the comet in April 2020, they saw that it had a fairly large tail, which stretched for about 600,000 kilometers, almost twice the distance from the Moon to Earth! Mind you, the core – the solid part of the comet – is probably only about 4 kilometers wide.
Calculations show that around that time it was losing about 80 kilograms of water ice per second. It also emitted gases such as carbon monoxide (about 50 kilos / second), carbon dioxide (7 kilos / second) and diatomic carbon (two carbon atoms bonded together at a rate of 40 grams per second).
That may sound like a lot, but it shows only started to degas like this … and it won’t be long. Its status as a periodic comet is only temporary. Extreme temporary: Follow-up measurements to determine its orbit showed it to be in a similar orbit to Jupiter, and there is an excellent chance that in the distant future the giant planet’s mighty gravity will completely eject the comet from the solar system.
When that happens, it becomes an interstellar comet like 2I / Borisov or ‘Oumuamua, interstellar objects that have both recently passed through our solar system (and which, I note, are not alien spaceships).
That’s appropriate, since it likely began to live in the outer reaches of the solar system as well.
It’s likely that P / 2019 LD2 started as what a Trans-Neptunian object, an ice-cold body that orbits the sun in the Kuiper Belt, beyond Neptune. Over time, very gentle nudges from Neptune’s gravity forced it into a smaller orbit closer to the sun. Eventually, it got so close that Neptune was able to pull on it much harder, changing its orbit significantly and putting it in orbit between Jupiter and Neptune (about 800 million to 3 billion kilometers from the sun). Objects in such jobs are called Centaurs
Centaurs are interesting. Over time, the gas giants tend to change their orbits even more. In general, they get too close to one of the planets after a few million years in this part of the solar system. Either they are dropped into the inner solar system (and become what we call Jupiter Family Comets) or they are thrown out of the solar system altogether. That is why we call them transition objects
What will be the fate of P / 2019 LD2? And where did it originally come from?
Time observations of an object can be used to determine its trajectory, which can then be projected into the past and the future. The problem is that we cannot measure the trajectory exactly; there is always some uncertainty in it. The farther you try to predict its position in the future (or predict its position in the past), the fuzzier it gets, the greater the volume of space it occupies. That makes these kinds of forecasts difficult.
To get around this, astronomers did something clever: they simulated its orbit using a so-called Monte Carlo technique. They take the physical characteristics of the orbit (the shape, the distance from the sun, the slope, and so on) and change them all very slightly, creating a slightly different orbit. They then run that into the past and the future and see what it does. They do this over and over, creating a virtual cohort of objects with each marginally 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 didn’t enter Jupiter’s space until about 2.5 years ago! Before that, it was a standard Centaur, but it was pushed into its current orbit very recently.
And its future? They thought it likely will only remain in its current job 8 or 9 yearsAfter that, it will likely fall to the Inner Solar System and become a Jupiter Family Comet. This means that it only makes a pit stop near Jupiter.
Even that is temporary. In 340,000 years, 50% is ejected from the solar system, which increases to 95% in 4 million years.
It is likely that over the age of the solar system, billions of objects like this were ejected. And there are billions of stars like the sun … which is why astronomers think the galaxy is full of rogue interstellar ice balls like P / 2019 LD2, and why it’s not surprising that we see them passing through our solar system too.
In the distant future, will some alien scientists see LD2 pass through their own system? What would they think? It’s nice, and strangely reassuring, to know that bits of our neighborhood will be among the stars, ranging from citizens of our solar system to citizens of the galaxy.
That’s pretty cool that it worked out this way, considering they are named after mythical half-human / half-horse creatures.