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天文与航天科普，每篇讲清楚一个现象、一次任务或一段历史。

Earth's night side seen from the International Space Station, a bright band of airglow along the horizon and a thin orange plasma trail from a Progress cargo ship reentering below
Spaceflight

Atmospheric Reentry:
Why Capsules Don't Burn Up

At nearly 8 km/s, a capsule is heated less by friction than by air it compresses to over 10,000 K. A blunt shape, an ablative heat shield and a narrow corridor bring it home.

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Hubble image of the Crab Nebula, orange and yellow filaments of gas wrapped around a bluish-white interior
Astronomy

SN 1054:
The Guest Star in the Song Dynasty Records

In 1054, Song dynasty astronomers logged a guest star seen in daylight for 23 days. Nearly 900 years later, it was confirmed as the supernova that created the Crab Nebula.

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Saturn and its rings backlit by the Sun, seen by Cassini from within the planet's shadow
Astronomy

The Roche Limit:
Why Saturn's Rings Neither Scatter Nor Grow Into Moons

Within about 2.2 Saturn radii, tides tear an icy moon apart and stop fragments from regrouping. Saturn's main rings end right at this line, the Roche limit.

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Artist's impression of ʻOumuamua, a long, dark, elongated object
Astronomy

Interstellar Visitors:
Three Messengers We Only Got a Glimpse Of

Since 2017 we have confirmed three objects from other star systems: ʻOumuamua, Borisov and 3I/ATLAS. Each passed through once, too fast for the Sun to hold.

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Part of the Hubble Deep Field, a black background crowded with spiral, elliptical and irregular galaxies
Astronomy

The Hubble Deep Field:
Ten Days Staring at "Empty" Sky

In December 1995, Hubble spent ten days on a seemingly blank patch of Ursa Major, taking 342 exposures. The image held about 3,000 galaxies and changed how we see galaxy evolution.

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Webb's view of the Cosmic Cliffs in the Carina Nebula, an orange-brown ridge of gas and dust beneath a blue, star-filled sky
Spaceflight

The Webb Telescope:
Why Go 1.5 Million km Away to See Infrared

Cosmic expansion stretches the light of the earliest galaxies into the infrared. To see it, Webb carried a 6.5 m gold-coated mirror and a five-layer sunshield to Sun–Earth L2.

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The 70-meter DSS-14 antenna at Goldstone silhouetted against a desert sunset in California
Spaceflight

The Deep Space Network:
Hearing a Whisper from 25 Billion km Away

Voyager 1 transmits on about 22 watts, and its signal reaches Earth at around 10⁻¹⁸ W. Three complexes 120° apart, 70 m dishes and antenna arraying are why we can still hear it.

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Phobos imaged by Mars Reconnaissance Orbiter, with Stickney crater at lower right
Astronomy

Phobos:
The Moon Falling Into Mars, and the Visitor Now on Its Way

Phobos orbits just 6,000 km above Mars and is slowly falling toward it. Japan's MMX spacecraft launched on October 20, 2026, to bring a sample of it back in 2031.

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Europa imaged by the Galileo spacecraft, its icy surface laced with reddish-brown cracks
Astronomy

Europa:
An Ocean Under the Ice With Twice Earth's Water

Europa's surface sits near -160°C, yet under its ice there is very likely an ocean holding about twice the water of Earth's oceans. Europa Clipper reaches Jupiter in 2030.

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