Astronomers in Chile used the powerful 570-megapixel Dark Energy Camera (DECam) to find an asteroid with the shortest orbital period of any known asteroid in the Solar System. They did this just ten days ago. Every 113 days, the asteroid, which is about 1 kilometer across, comes as close to the Sun as 20 million kilometers (12 million miles or 0.13 au).
Asteroid 2021 PH27 has the shortest period and the shortest
semi-major axis of any known asteroid in our Solar System. Only Mercury has a
shorter period and a shorter semi-major axis. Since the asteroid is so close to
the Sun, it feels the strongest effects of general relativity of any known
object in the Solar System.
Scott S. Sheppard of the Carnegie Institution of Science
found the asteroid 2021 PH27 by looking at data from the Dark Energy Camera
(DECam) on the Vctor M. Blanco 4-meter Telescope at the Cerro Tololo
Inter-American Observatory (CTIO) in Chile. Ian Dell’antonio and Shenming Fu of
Brown University took pictures of the asteroid in the evening sky on August 13,
2021, as it was getting dark.
Sheppard worked with Dell’antonio and Fu while using DECam
to make observations for the Local Volume Complete Cluster Survey. This survey
is looking at most of the large galaxy clusters in the nearby universe. They
took a break from looking at some of the biggest objects millions of
light-years away to look for asteroids much closer to Earth.
DECam is one of the best wide-field CCD imagers in the
world. It was made for the DOE-funded Dark Energy Survey (DES), built and
tested at the DOE’s Fermilab, and ran by the DOE and NSF from 2013 to 2019. At
the moment, DECam is used for a wide range of science programs. The Community
Science and Data Center is in charge of taking care of the DECam science
archive (CSDC). The NOIRLab at NSF runs the CTIO and CSDC programs.
The best time to look for asteroids that are inside Earth’s
orbit, in the direction of Mercury and Venus, is at twilight, just after sunset
or just before sunrise. Any stargazer will tell you that Mercury and Venus
never seem to get very far from the Sun in the sky and are always easiest to
see near sunrise or sunset. Asteroids that orbit close to the Sun are the same.
After 2021 PH27 was found, David Tholen of the University of
Hawai’i measured its position and figured out where it could be seen the next
night. After that, on August 14, 2021, DECam and the Magellan Telescopes at the
Las Campanas Observatory in Chile both looked at it again. Then, on the evening
of the 15th, Marco Micheli of the European Space Agency used the Las Cumbres
Observatory network of 1- to 2-meter telescopes to look at it from CTIO in
Chile and from South Africa. Other astronomers used DECam and Magellan to look
at the newly found asteroid as well.
Sheppard says that even though astronomers’ telescope time
is very valuable, the international nature of their work and their love of the
unknown make them very willing to put their own science and observations on
hold to follow up on new, interesting discoveries like this one.
Planets and asteroids move in oval-shaped orbits around the
Sun. The widest part of the oval is called the semi-major axis, and it has a
radius. The semi-major axis of 2021 PH27 is 70 million kilometers (43 million
miles or 0.46 au), which gives it a 113-day orbital period on an elongated
orbit that crosses both Mercury and Venus’ orbits.
It may have started out in the main Asteroid Belt between
Mars and Jupiter but moved closer to the Sun because of gravitational
disturbances from the inner planets. Its high orbital inclination of 32
degrees, on the other hand, suggests that it might be an extinct comet from the
outer Solar System that was pulled into a closer, shorter-period orbit by one
of the terrestrial planets as it passed by. As more observations are made of
the asteroid, we will learn more about where it came from.
Its orbit is probably also unstable over long periods of time.
In a few million years, it will probably either crash into Mercury, Venus, or
the Sun, or it will be pushed out of the inner Solar System by the gravity of
the inner planets.
Astronomers have a hard time finding asteroids in the inner
solar system because the Sun’s light often hides them. When asteroids get so
close to the Sun, they are put under a lot of stress, including thermal stress
from the Sun’s heat and physical stress from the Sun’s gravitational tides.
Some of the asteroids that are not as strong could break up because of these
stresses.
“The number of asteroids that are close to Earth and Venus
compared to those that are far away will tell us about their strength and
make-up,” says Sheppard. If the number of asteroids in the same orbit as 2021
PH27 seems to be decreasing, it could tell astronomers how many near-Earth
asteroids are loosely held together piles of rubble instead of solid chunks of
rock. This could affect how we deal with asteroids that might be headed for
Earth and how we try to stop them.
“Understanding the population of asteroids inside Earth’s
orbit is important to finish the census of asteroids close to Earth,” says
Sheppard. “Some of the most likely Earth-impacting asteroids may come close to
Earth during the day, but they are hard to find in most surveys because they are
done at night, when the Sun isn’t out.” He also says that since 2021 PH27 is so
close to the Sun, “its surface temperature gets to almost 500 degrees C (around
900 degrees F) at closest approach, hot enough to melt lead.”
Because 2021 PH27 is so close to the Sun, its effects from
general relativity are the strongest of any known Solar System object. This
reveals itself as a slight angular deviation in the asteroid’s elliptical orbit
over time, a movement called precession, which amounts to about one arcminute
per century.
The asteroid is now moving behind the Sun, which we can see
from where we are. This is called solar conjunction. It is expected to return
to visibility from Earth early in 2022, when new observations will be able to
determine its orbit in more detail, allowing the asteroid to get an official
name.