Probe Towing

Earth satellites generally orbit well above an altitude of 200 km (120 miles), because below that height the atmosphere is so dense that aerodynamic drag quickly slows a spacecraft, making it fall back to Earth. Gas-filled balloons, on the other hand, can reach altitudes up to about 50 km (30 miles) and stay there for days, but can go no higher. The zone in the middle, between 50 and

100 km, can be reached by sounding rockets that zoom up and then fall back down. However, these rockets stay in the upper atmosphere for only a couple of minutes. At the moment there are no possibilities to reach the upper atmosphere and conduct experiments there for extended periods.

A large satellite in a sufficiently high and thus virtually drag-free orbit could be used to tow a smaller probe on a long tether through the upper layers of the atmosphere. The aerodynamic drag on the probe and tether would start to slow down the mother satellite, but the higher its mass, the more difficult it is to decelerate. A rocket engine can be used to temporarily compensate for the drag, and once the scientific measurements of the atmosphere are done, the tether can be cut so that the large satellite can remain in orbit.

This concept can in principle be applied at all planets with an atmosphere: Earth, Venus, Mars, and the giant outer planets Jupiter, Saturn, Uranus, and Neptune. Small tethered probes could be lowered into an alien atmosphere to take gas samples or even to collect Mars dust that is blown up to high altitudes by the wind.

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