A recently found space rock is schlepping along with Earth around the sun. This Trojan asteroid is only the second one discovered that belongs to our planet. And it’s probably a visitor.
Trojan asteroids, which are also found accompanying Mars, Jupiter and Neptune, hang out in two locations near a planet where the gravitational pulls of that planet and the sun balance each other (SN: 10/15/21). Because of this balancing act, these locations are stable spots in space. In 2010, astronomers discovered the first known Earth Trojan — called 2010 TK7 — orbiting within one of these two regions, known as L4, tens of millions of kilometers from Earth and leading our planet around the sun (SN: 8/2/11).
Now, researchers have found another one. Dubbed 2020 XL5, this roughly 1-kilometer-wide asteroid is also at L4, astronomer Toni Santana of the University of Barcelona and colleagues report February 1 in Nature Communications.
The space rock was first spotted in December 2020, and follow-up observations suggested that it might be at L4. To confirm this, Santana and colleagues observed the asteroid using ground-based telescopes in 2021. Measurements of its brightness let the researchers estimate the asteroid’s size — about three to four times as wide as 2010 TK7. They also scoured archival data and found the object in images dating to 2012.
There is no doubt this is an Earth Trojan, Santana says. That decade-worth of observations let the team calculate the rock’s orbit thousands of years into the future, confirming the asteroid’s nature. It will hang around at L4 for at least 4,000 years, the team predicts. 2010 TK7, for comparison, will stick around for some 10,000 years.
Now that scientists know of two just-visiting Earth Trojans, they can envision more. The fact that the team discovered a second object means that 2010 TK7 isn’t a rarity or loner, Santana says. It might be part of a family or population. For the first time, a spacecraft is headed to Jupiter’s odd Trojan asteroids. What Lucy finds there could provide a fresh peek into the history of the solar system.
Lucy will profoundly change our understanding of planetary evolution in our solar system, Adriana Ocampo, a planetary scientist at NASA Headquarters in Washington, D.C., said at a news briefing October 14.
The mission is set to launch from the Kennedy Space Center at Cape Canaveral, Fla., as early as October 16. Live coverage will air on NASA TV beginning at 5 a.m. EDT, in anticipation of a 5:34 a.m. blast off.
The Trojan asteroids are two groups of space rocks that are gravitationally trapped in the same orbit as Jupiter around the sun. One group of Trojans orbits ahead of Jupiter; the other follows the gas giant around the sun. Planetary scientists think the Trojans could have formed at different distances from the sun before getting mixed together in their current homes. The asteroids could also be some of the oldest and most pristine objects in the solar system.
The mission will mark several other firsts, from the types of objects it will visit to the way it powers its instruments. Here are five cool things to know about our first visit to the Trojans.
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