Free Astronomy Magazine May-June 2022

MAY-JUNE 2022 T his graphic shows where the Earth Trojan asteroid 2020 XL 5 would appear in the sky from Cerro Pachón in Chile as the asteroid orbits the Earth-Sun Lagrange point 4 (L4). The arrows show the direction of its motion. The SOAR Telescope appears in the lower left. The asteroid’s apparent magnitude is around magnitude 22, beyond the reach of anything but the largest telescopes. [NOIRLab/NSF/AURA/J. da Silva] ! supported the Trojan asteroid’s iden- tification, the new results make that determination far more robust and provide estimates of the size of 2020 XL 5 and what type of asteroid it is. “SOAR’s data allowed us to make a first photometric analysis of the ob- ject, revealing that 2020 XL 5 is likely a C-type asteroid, with a size larger than one kilometer,” says Santana- Ros. A C-type asteroid is dark, con- tains a lot of carbon, and is the most common type of asteroid in the Solar System. The findings also showed that 2020 XL 5 will not remain a Trojan asteroid forever. It will remain stable in its position for at least another 4000 years, but eventually it will be grav- itationally perturbed and escape to wander through space. 2020 XL 5 and 2010 TK 7 may not be alone — there could be many more Earth Trojans that have so far gone undetected as they appear close to the Sun in the sky. This means that searches for, and observations of, Earth Trojans must be performed close to sunrise or sunset, with the telescope pointing near the horizon, through the thickest part of the at- mosphere, which results in poor see- ing conditions. SOAR was able to point down to 16 degrees above the horizon, while many 4-meter (and larger) telescopes are not able to aim that low. “These were very challenging obser- vations, requiring the telescope to track correctly at its lowest elevation limit, as the object was very low on the western horizon at dawn,” says Briceño. Nevertheless, the prize of discover- ing Earth Trojans is worth the effort of finding them. Because they are made of primitive material dating back to the birth of the Solar System and could represent some of the building blocks that formed our planet, they are attractive targets for future space missions. “If we are able to discover more Earth Trojans, and if some of them can have orbits with lower inclina- tions, they might become cheaper to reach than our Moon,” says Briceño. “So they might become ideal bases for an advanced exploration of the Solar System, or they could even be a source of resources.”