Using the powerful 570-megapixel Dark Energy Camera (DECam) in Chile,
astronomers just ten days ago discovered an asteroid with the shortest
orbital period of any known asteroid in the Solar System. The orbit of the
approximately 1-kilometer-diameter asteroid takes it as close as 20 million
kilometers (12 million miles or 0.13 au), from the Sun every 113 days.
Asteroid 2021 PH27, revealed in images acquired during twilight, also has
the smallest mean distance (semi-major axis) of any known asteroid in our
Solar System—only Mercury has a shorter period and smaller semi-major axis.
The asteroid is so close to the Sun's massive gravitational field, it
experiences the largest general relativistic effects of any known Solar
System object.
The asteroid designated 2021 PH27 was discovered by Scott S. Sheppard of the
Carnegie Institution of Science in data collected by the Dark Energy Camera
(DECam) mounted on the VÃctor M. Blanco 4-meter Telescope at Cerro Tololo
Inter-American Observatory (CTIO) in Chile. The discovery images of the
asteroid were taken by Ian Dell'antonio and Shenming Fu of Brown University
in the twilight skies on the evening of 13 August 2021. Sheppard had teamed
up with Dell'antonio and Fu while conducting observations with DECam for the
Local Volume Complete Cluster Survey, which is studying most of the massive
galaxy clusters in the local Universe. They took time out from observing
some of the largest objects millions of light-years away to search for far
smaller objects—asteroids—closer to home.
One of the highest-performance, wide-field CCD imagers in the world, DECam
was designed for the Dark Energy Survey (DES) funded by the DOE, was built
and tested at DOE's Fermilab, and was operated by the DOE and NSF between
2013 and 2019. At present DECam is used for programs covering a huge range
of science. The DECam science archive is curated by the Community Science
and Data Center (CSDC). CTIO and CSDC are programs of NSF's NOIRLab.
Twilight, just after sunset or before sunrise, is the best time to hunt for
asteroids that are interior to Earth's orbit, in the direction of the two
innermost planets, Mercury and Venus. As any stargazer will tell you,
Mercury and Venus never appear to get very far from the Sun in the sky and
are always best visible near sunrise or sunset. The same holds for asteroids
that also orbit close to the Sun.
Following 2021 PH27's discovery, David Tholen of the University of Hawai'i
measured the asteroid's position and predicted where it could be observed
the following evening. Subsequently, on 14 August 2021, it was observed once
more by DECam, and also by the Magellan Telescopes at the Las Campanas
Observatory in Chile. 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 observe it from CTIO in Chile and from South Africa,
in addition to further observations from DECam and Magellan, as astronomers
postponed their originally scheduled observations to get a sight of the
newly found asteroid.
"Though telescope time for astronomers is very precious, the international
nature and love of the unknown make astronomers very willing to override
their own science and observations to follow up new, interesting discoveries
like this," says Sheppard.
Planets and asteroids orbit the Sun in elliptical (or oval-shaped) orbits,
with the widest axis of the ellipse having a radius described as the
semi-major axis. 2021 PH27 has a semi-major axis of 70 million kilometers
(43 million miles or 0.46 au), giving it a 113-day orbital period on a
elongated orbit that crosses the orbits of both Mercury and Venus.
It may have begun life in the main Asteroid Belt between Mars and Jupiter
and got dislodged by gravitational disturbances from the inner planets that
drew it closer to the Sun. Its high orbital inclination of 32 degrees
suggests, however, that it might instead be an extinct comet from the outer
Solar System that got captured into a closer short-period orbit when passing
near one of the terrestrial planets. Future observations of the asteroid
will shed more light on its origins.
Its orbit is probably also unstable over long periods of time, and it will
likely eventually either collide with Mercury, Venus or the Sun in a few
million years, or be ejected from the inner Solar System by the inner
planets' gravitational influence.
Astronomers have a hard time finding these interior asteroids because they
are very often hidden by the glare of the Sun. When asteroids get so close
to our nearest star, they experience a variety of stresses, such as thermal
stresses from the Sun's heat, and physical stresses from gravitational tidal
forces. These stresses could cause some of the more fragile asteroids to
break up.
"The fraction of asteroids interior to Earth and Venus compared to exterior
will give us insights into the strength and make-up of these objects," says
Sheppard. If the population of asteroids on similar orbits to 2021 PH27
appears depleted, it could tell astronomers what fraction of near-Earth
asteroids are piles of rubble that are loosely held together, as opposed to
solid chunks of rock, which could have consequences for asteroids that might
be on a collision course with Earth and how we might deflect them.
"Understanding the population of asteroids interior to Earth's orbit is
important to complete the census of asteroids near Earth, including some of
the most likely Earth impactors that may approach Earth during daylight and
that cannot easily be discovered in most surveys that are observing at
night, away from the Sun," says Sheppard. He adds that since 2021 PH27
approaches 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's massive gravitational field, it
experiences the largest general relativistic effects 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 entering solar conjunction when from our point of view
it is seen to move behind the Sun. 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.
Tags:
Space & Astrophysics