Fiery re-entry of China's Chang Zheng 7 rocket over the Western US

Fiery re-entry of China's Chang Zheng 7 rocket over the Western US

A slow moving bright streak of light burst across the night sky over California, Utah and Nevada around 21:36 PTD on Wednesday, July 27 (04:36 UTC on July 28). Images and videos of the event immediately sent social media into a frenzy. The event lasted for over 30 seconds and ended with a loud boom.

At first, people thought the bright fireball was a meteor, but after analyses of multiple videos recorded from Utah to California, it was established the fiery event was caused by Chinese space junk. At 04:40 UTC, the second stage of the first Chang Zheng 7 rocket, launched June 25, re-entered Earth's atmosphere and burned over the Western United States.

"The launch of the rocket heralded a new era in Chinese rocketry," said Jonathan McDowell, an astronomer at the Harward-Smithsonian Center or Astrophysics. "After a month in low orbit, it re-entered Earth's atmosphere and was probably traveling about 29 000 km/h (18 000 mph). When those on the ground spotted the trail of light, it was probably about 80 km (50 miles) from the Earth's surface."

"The main body probably melted, but a few small pieces of metal may eventually reach the ground," McDowell added.

McDowell agreed that although re-entry is common, such dramatic visibility is not.

“Something this big enters in an uncontrolled way probably once a month,” McDowell said. “Mostly they fall into the ocean. The chance that you get one at night, over the US at the time when people look at the sky, is relatively low.”

The video below shows the launch of Chang Zheng 7.

The second stage of Chang Zheng 7 is about 11.5 meters (37 feet) in length, 3.35 meters (11 feet) in diameter and weighs around six metric tons without fuel. According to the Joint Space Operations Center, its Rocket Body re-entered at 4:36 UTC (+/-1 minute) near the California-Nevada border, however, the two-minute uncertainty window represents a 900-km (560-mile) track along which orbital decay took place. 

According to the SpaceFlight101, as plasma built up around the stage, starting around 104 km (65 miles) in altitude, it became visible as a small dot moving across the sky. Hitting the dense layers of the atmosphere, heat started building up on the stage and drag at the high entry speed of 7.8 km/s quickly built up to a destructive force, causing the rocket body to break apart.

Components that survive re-entry usually impact 800 to 1 300 km (500 to 800 miles) downrange from the Orbital Decay Point.

Parts likely to survive re-entry include the combustion chambers of the YF-115 engines that are designed to withstand high temperatures and represent some of the densest components on the rocket stage. During the flight, the tanks of the stage are pressurized with Helium stored in small spherical tanks that have been known to frequently survive re-entry of Russian and American rockets.

Assuming re-entry in the center of the two-minute uncertainty window would place surviving pieces along a stretch from North Central Colorado to southern Nebraska.

Featured image copyright: Ian Norman

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