JupiterMarsEarthVenus
Mercury62 ASTRONOMY • FEBRUARY 2022
ASK ASTRO Astronomy’s experts from around the globe answer your cosmic questions.
QI
WHAT IS THE LARGEST ASTEROID
AND COMET CONSIDERED TO BE
A NEAR-EARTH OBJECT? HOW LARGE
ARE THEY?
Doug Kaupa
Council Bluffs, IowaAI
Doomsday impactors are a staple of modern
science fiction. The most famous real-world
example of an impact wiping out much of the life on
Earth is, of course, the object that took out the dino-
saurs some 66 million years ago.
Thankfully, events of that magnitude are fairly
uncommon, happening about once every few hundreds
of millions of years. But the possibility is there, so space
agencies around the globe have taken to monitoring the
skies for near-Earth objects (NEOs). These objects are
comets and asteroids whose orbits take them within
1.3 astronomical units (AU; where 1 AU is the average
distance between Earth and the Sun) of our star.
On that note, the largest comets and asteroids tend
to stay far enough away that we have very little to worryHeavenly
hazards
about. The largest asteroid considered a NEO is
1036 Ganymed. This asteroid is about 23 miles
(37 kilometers) in diameter. With a closest solar
approach of 1.24 AU, 1036 Ganymed is just within the
minimum distance to categorize it as an NEO.
The largest comet in the sky today is
109P/Swift-Tuttle. At 16 miles (26 km), Swift-Tuttle is
about twice the size of the object believed to have wiped
out the dinosaurs. However, this comet poses little
threat to us, as it makes its closest solar approach of
0.95 AU every 133 years. And, in fact, we have it to thank
for the Perseid meteor shower.
It’s actually the little guys that we need to be more
concerned about, as they’re more likely to sneak past
our detection. Earth’s closest encounter on record with
a known NEO was with 2020 VT 4 , which passed a mere
232 miles (375 km) above Earth’s surface Nov. 13, 2020.
The asteroid was spotted only after it made its closest
approach. The previous closest asteroid, 2020 QG, had
skimmed past Earth just three months before 2020 VT 4 ;
it, too, was not seen prior to its close pass.
Caitlyn Buongiorno
Associate EditorQI
THE MOON CAN APPEAR RED
DURING A TOTAL LUNAR ECLIPSE.
WHAT DOES THE ECLIPSED MOON LOOK
LIKE AFTER A RECENT ERUPTION LEAVES
A LARGE AMOUNT OF VOLCANIC ASH IN
THE ATMOSPHERE?
Shawn Trueman
Rochester, MinnesotaThe orbits of over
1,400 potentially
hazardous asteroids
(PHAs) were mapped
in this illustration,
made in 2013.
Nine years later, an
additional 800 PHAs
have been identified.
NASA/JPL-CALTECH
The January 2019
lunar eclipse was a
brilliant red. This may
have been partially
due to an eruption
the month before
on Anak Krakatau,
Indonesia. JAMIE COOPERTRACKING
NEARBY OBJECTS