SunElliptical orbitCircular orbitHyperbolic
orbit70 ASTRONOMY • AUGUST 2020
ASK ASTRO Astronomy’s experts from around the globe answer your cosmic questions.
QI
HOW DO ASTRONOMERS
DETERMINE IF ASTEROIDS OR
COMETS ARRIVE FROM OTHER STARS?
Douglas Kaupa
Council Bluffs, IowaAI
We recognize that an asteroid or comet is
interstellar from its trajectory. The trajectory
of a n objec t i s computed by compa r i ng prec i se me a su re-
ments of its changing position on the sky to the possible
trajectories that account for all known forces acting on
it, such as the gravity of the Sun and the planets. The
calculated trajectory we use is the one that best matches
the object’s observed motion.
Objects that reside within our solar system (planets,
asteroids, and comets) all move approximately along
closed elliptical orbits, each with a perihelion (closest
point to the Sun) and an aphelion (farthest point from
the Sun). Objects move fastest at perihelion, and then
solar gravity slows them down as they try to race away
from the Sun. Eventually, they reach aphelion and solar
gravity brings them back closer to the Sun again.
Despite being computed the same way as the trajec-
tory of asteroids and comets, the trajectory of an inter-
stellar object is different. It is essentially an open-ended
hyperbola with a perihelion point but no aphelion.
Interstellar objects have such high velocities that, on the
outbound leg, the Sun’s gravity is not strong enough toInterstellar
visitors
slow them down completely. Therefore, the trajectory
shows that the object originated somewhere else in the
universe, traveled through interstellar space, and
entered the solar system. The object then reaches peri-
helion before eventually leaving the solar system.
Davide Farnocchia
Navigation Engineer, Jet Propulsion Laboratory,
Caltech, Pasadena, CaliforniaQI
WHY DOES SUPERNOVA 1987A
LOOK LIKE A RING INSTEAD OF
A SPHERICAL BALL OF DEBRIS?
James Johnson
IndianapolisAI
Supernova 1987A is a supernova remnant in
the Large Magellanic Cloud, the Milky Way’s
largest satellite galaxy. The remnant was created when a
massive star exploded — an event we saw here on Earth
in February 1987. Since then, the remnant has evolved
as the energy from the exploding star travels outward,
slamming into nearby gas and dust.
But if the explosion sent material and energy in all
directions, then why does the remnant appear to have
three rings, instead of looking like a sphere? The answer
lies in what happened before the supernova, in the
behavior of the progenitor star doomed to explode. As
that star neared the end of its life, it evolved from a red
supergiant star into a blue supergiant star. At the same
time, the star’s winds changed from dense and slow-
mov i ng to t h i n a nd fa st-mov i ng. When t he fa st-mov i ng
wind collided with the slower-moving wind that was
ejected before it, it caused material to pile up aroundThe star responsible for Supernova 1987A ejected more material
in some directions than others. This artist’s impression shows
how the supernova remnant might look from a different
viewpoint than we have on Earth. Rings of material are now
lighting up as shock waves from the supernova pass through
them. ESO/L. CALÇADAComets and asteroids
in our solar system
orbit the Sun with
closed elliptical
orbits. But objects
that come from other
star systems have
open-ended,
hyperbolic orbits. This
means they not only
originated elsewhere,
but also are traveling
too fast for the Sun
to capture them, so
they’ll eventually
escape back into
interstellar space.
ASTRONOMY: ROEN KELLY
ORBITAL TRAJECTORIES