Astronomy - USA (2021-12)

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close to the Sun can reliably use solar panels. But as a

probe moves further out in the solar system, the Sun’s

rays become too weak. In that case, it can use

radioisotope thermoelectric generators, which is a

fancy way to say nuclear batteries. As radioactive mate-

rial decays, it releases heat that can be converted into

electricity for the craft.

Caitlyn Buongiorno

Associate Editor

QI

WHAT HAPPENED TO THE DEBRIS

FROM THE COLLISION THAT

FORMED THE CALORIS BASIN ON

MERCURY? WHY DIDN’T IT FORM A

MOON?
Fabian Marson
Wyndham Vale, Australia

AI

Mercury’s Caloris Basin is about 950 miles

(1,525 kilometers) across. For comparison, the

state of Texas is 773 miles (1,244 km) wide. The Moon

has a similarly sized impact crater, Mare Orientale.

Researchers estimate that the collision that created

the Caloris Basin occurred some 3.8 billion years ago,

when a small body about 93 miles (150 km) across

crashed into the planet. The impact sent seismic waves

rumbling around the planet and even through its core.

These tremors disrupted the surface on the opposite

side of Mercury, jumbling it into what researchers call

“weird” terrain.

As to why the impactor didn’t create a debris disk

that eventually coalesced into a moon like Earth’s, the

simple answer is that Mercury is too close to the Sun

and its mass is too small. Every object has a region

around it in which its gravity dominates, known as the

Hill sphere. In order for a satellite

(natura l or human-made)

to stay in a stable

orbit around a body, that satellite needs to be within the

Hill sphere. In our solar system, a planet’s Hill sphere

is dictated by its orbit (more specifically, its orbit’s

semi-major axis — the long semidiameter of an oval),

its mass, and the Sun’s mass. In Mercury’s case, its Hill

sphere is so small that any debris f lung up from the

impact that created the Caloris Basin either careened

out of the Hill sphere and into the Sun, or crashed back

into the planet, rather than lucking into a stable orbit

and forming a moon.

Caitlyn Buongiorno

Associate Editor

QI

IS SIRIUS PART OF THE MILKY

WAY? DOES IT HAVE ANY KNOWN

PLANETS?

Thomas Steiner

Greendale, Wisconsin

AI

Sirius is the brightest star in the Northern

Hemisphere. In order to see a pointlike star

with your naked eye, it must be relatively close to us;

this means that all the stars you can see in the night sky

are within the Milky Way Galaxy. Resolving individual

stars in other nearby galaxies, such as Andromeda

in the Northern Hemisphere or the Large and Small

Magellanic Clouds in the Southern Hemisphere,

requires professional equipment like the Hubble Space

Telescope.

Sirius is about 8.6 light-years from Earth. There are

two identified stars in the Sirius star system, a main

sequence spectral type A star and a faint white dwarf,

designated Sirius A and B, respectively. No planets have

been ident i f ied i n t he s y stem so fa r. C ombi n i ng Hubble

observations with historical measurements dating back

to the 19th century, researchers have ruled out the pos-

sibility of any brown dwarfs or any exoplanets larger

than 15 times the mass of Jupiter. However, looking at

our own solar system’s plethora of smaller planets, it’s

not a stretch to imagine that there could be a consider-

able number of exoplanets evading detection in this

system.

Caitlyn Buongiorno

Associate Editor

Japanese amateur

astronomer Akira

Fujii captured this

close-up shot of the

sky’s brightest star.

The Sirius star

system contains two

stars, but so far no

identified planets.

AKIRA FUJII