|When a spacecraft such as the Space Shuttle leaves orbit and reenters the atmosphere as it travels to a landing site, there is a critical period of time when all communications between the spacecraft and ground are lost. This phenomenon is due to the tremendous heating experienced by the craft during reentry and is termed 'reentry blackout'.|
In low Earth orbit the Space Shuttle or similar vehicle is travelling at almost 8 km per second. To land safely on the ground this speed must be reduced to zero by making use of atmospheric drag. What NASA calls the 'entry interface' commences when the Shuttle descends to around km altitude. It does this by deorbit thruster firing. This rocket burn is performed opposite to the direction of travel. The result of this is a reduction of altitude rather than a reduction in speed. The speed changes little due to an exchange of gravitational potential energy for kinetic energy.
At around 120 km altitude, the atmospheric drag increases significantly and the resultant heating increases as the shuttle kinetic energy is exchanged for thermal energy.
A shockwave forms just in front of the nose and underside of the spacecraft. Between this shock and the vehicle itself temperatures may reach 10,000 to 12,000 Kelvin. (The heat resistant surfaces of the shuttle only reach a maximum of 1600 K themselves.) This very high temperature ionises the gas close to the shuttle forming a plasma cloud or miniature ionosphere around the spacecraft. The plasma frequency (that frequency below which radio communications is not possible) may rise to many gigahertz around the lower parts of the vehicle. This gives rise to a communication blackout for direct communications between the Shuttle and ground control. This typically lasts from 25 to 12 minutes prior touchdown, a total outage of 12 to 13 minutes.
Maximum heating of the orbiter occurs during this time frame, at an altitude of 70 km and about 20 minutes prior to touchdown. Unfortunately this is also the most critical time of reentry, and if any problems occur during this phase of flight, the communications blackout prevents any diagnostic telemetry from reaching the ground. Such was the case with the catastrophic breach of the Space Shuttle Colombia's hull during reentry on the first of February 2003.
The figure below illustrates a typically reentry, showing the variation of altitude and velocity.
The Saha equation may be used to calculate the ionisation of the gas surrounding a deorbiting vehicle as a function of temperature:
The plasma frequency is given by fp[Hz] = 9 Ne0.5
At an altitude of 70 km, where maximum Space Shuttle heating occurs, the atmosphere is still about 20% oxygen and 80% nitrogen. The Saha equation must be applied to these constituents separately. When this is done, the results are shown in both tabular and graphical form below.
|T (Kelvin)||Ne /m3||fp|
The Saha model becomes strictly invalid after 8000 Kelvin, but the curve can be extended smoothly up to the point of complete first ionisation, when the number of ions/electrons equals the total atmospheric number density [ 2x1021 per cubic metre at 70 km ]. Of course, higher temperatures will result in more than one electron being removed from the atoms, with a further increase in plasma frequency, but we will ignore this in the current approximation.
For a given sheath temperature, the frequencies above the curve will propagate to and from the spacecraft, whereas the frequencies below the line will not. For sheath temperatures of 10,000 to 12,000 K, we see that a frequency in excess of 350 GHz would be needed for direct Shuttle communication with the ground at the time of maximum heating.
There are however, ways in which communication is possible during this critical reentry time. The sheath temperatures on the top surface of the Shuttle are much less than those on the bottom which, at a descent angle of about 22 degrees, is where the shock is formed. Communications from the topside via a satellite relay in the GHz range may be possible. Other options which have been tried are water injection into the plasma, and also the use of very narrow width pulsed signals, both with some success.
Australian Space Academy