A recently discovered object sharing Earth’s orbital path around the sun could in fact be a Trojan asteroid, astronomers have discovered.
If confirmed, it will only be the second object of its kind identified so far, suggesting more of these hidden asteroids may be lurking in Earth’s gravity pockets.
Trojan asteroids are space rocks that share the orbital path of larger planetary bodies in the solar system and hover in gravitational stable regions known as Lagrangian points.
These are cavities where the gravity of the planet and the sun are in perfect balance with the centripetal force of every small body in that area to actually hold it in place.
Each two-body system has five Lagrange points, as shown in the diagram below. There are five between the Earth and the Moon; and five more between the earth and the sun.
These are actually quite useful – we can put spacecraft in them and be reasonably sure that they will stay put. For example, the James Webb space telescope will go into the Earth-Sun L.2 Lagrangian.
(NASA / WMAP Science Team)
Lagrangians can also capture space rocks, however, and the phenomenon is well known in the solar system.
Jupiter has the most Trojans, with over 9,000 documented, but the other planets can’t live without them. Neptune has 28, Mars has 9, and both Uranus and Earth have confirmed one each.
Earth’s confirmed trojan, called 2010 TK7, is a piece of rock about 300 meters wide, hanging around the Earth-leading L.4 Lagrangian in an oscillating tadpole-shaped orbit known as libration.
The new object, called 2020 XL5first observed in November and December last year, appears similar.
According to amateur astronomer Tony Dunn, who calculated the object’s orbit using NASA’s JPL-Horizons software, it also librates around the Earth-Sun L4 Lagrangian, in a loop close to the orbit of Mars and intersecting the orbit of Venus.
In the gif below, the asteroid’s orbit is teal, with Earth in blue and Mars in orange. Venus and Mercury are both white.
(Tony Dunn / Twitter)
Earth has a known Trojan asteroid, 2010 TK7. But the newly discovered 2020 XL5 is a good candidate because it is librating around Earth’s L4 point, and will continue to do so for thousands of years. pic.twitter.com/mXUyFIIOSw
– Tony Dunn (@ tony873004) January 28, 2021
Because it is as close to Venus as 2020 XL5 is a Trojan, and may not be stable over long timescales. According to Dunn’s simulations, the asteroid will pass above and below Venus’ orbital plane for a few thousand years when it intersects, keeping the planet from disrupting its orbit.
Ultimately, however, gravitational interactions should move it off the L.4 point. This is supported by simulations from amateur astronomer Aldo Vitagliano, the maker of the Solex and Exorb orbital determination software.
“I can confirm that 2020 XL5 is currently a fairly stable Earth Trojan (I mean stable on a timescale of 2-4 millennia), ”he wrote on the Minor Planets Mailing List.
“I downloaded the nominal elements and their covariance matrix from the Neodys site, generating 200 clones of the body. All 200 clones integrated up to 4500 AD, although they are spread over an orbital arc of more than 120 degrees. , keep librating around the L.4 point. The first clone jumps over the L3 point around AD 4500, and by AD 6000 many of them have made the leap and a few of them are librating around the L5 point.”
2010 TK7 is also not necessarily stable in its current position in the long term. A 2012 analysis found that it only became a trojan about 1,800 years ago and is likely to move away from the L.4 point in about 15,000 years, in a horseshoe-shaped orbit, or in L.5.
Although there is only one data point left, 2020 XL5 can help us figure out how to search for other potential Earth Trojans. We did – both the OSIRIS-REx and Hayabusa2 spacecraft have the L4 and I5 points respectively in 2017 as they progressed towards their respective goals, but found nothing. Searches from Earth have been almost as fruitless.
That does not necessarily have to be surprising. Any objects that the Lagrangians inhabit would move around a lot, leaving a very large chunk of air to search for relatively small objects. Also from Earth, its placement with respect to the sun makes detection a challenge.
Scientists have ruled out that a stable population of native Trojans has been hiding since the dawn of the solar system.
Nevertheless, even with the current observation limitations, scientists have estimated that we could detect hundreds of Trojans on Earth that are comparable in size to 2010 TK7. Getting a better sense of how they move around the Lagrangians can help us determine where to look in the sky.
What we find – be it a lot of Trojans or a lot of nothing – will no doubt tell us more about the dynamics of our solar system.
H / T: Sky & Telescope