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Why NASA is dropping three probes through Earth’s atmosphere today

An uncrewed Cygnus supply ship will undock from the International Space Station (ISS) today, Saturday, November 20. Packed with trash and unneeded gear from the station, the craft is set to burn up harmlessly in Earth’s atmosphere over the Pacific Ocean. But before it is destroyed, the craft will perform one more scientific investigation by releasing a set of three probes which will collect data on the effectiveness of heat shielding and transmit it to the ground as they speed through the planet’s atmosphere.

Northrop Grumman’s Cygnus space freighter pictured arriving at the International Space Station on Aug. 12, 2021. Cygnus will depart from the space station on Nov. 20, 2021.
Northrop Grumman’s Cygnus space freighter pictured arriving at the International Space Station on Aug. 12, 2021. Cygnus will depart from the space station on Nov. 20, 2021. NASA

The three capsules are part of an experiment called the Kentucky Re-Entry Probe Experiment (KREPE), which is designed to be an affordable way to test heat-shielding materials.

Currently, the design of heat shields — technically called Thermal Protection Systems (TPS) — is mostly based on mathematical models of what re-entry conditions are expected to involve. In order to be on the safe side, TPSs are often overestimated in terms of their size, so the shielding takes up more space and mass than might be necessary. With the KREPE experiment, engineers can gather more data on the actual conditions of re-entry which could help make shielding slimmer and lighter.

The capsules used in the KREPE test have sensors placed in them at different depths within the heat shielding. When the Cygnus is ready to depart from the ISS, the astronauts put the capsules into a dormant mode so they are ready and waiting while the craft undocks and leaves the station. The capsules search for signals that the Cygnus craft is entering the atmosphere and is starting to break apart, such as spikes in temperature and particular acceleration. When they get these signals, the capsules are activated and start collecting data. The activation also turns on a communication system, allowing the capsules to broadcast the data down to the ground.

The capsules should continue collecting data as they fall through the atmosphere before finally splashing down in the ocean. The findings could help not only in the development of heat shielding materials for space, but also for purposes here on Earth.

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