El próximo vuelo del Discovery y la Astrofísica Espacial
Miguel Mas Hesse
LAEFF-CSIC
Dentro de unas semanas será lanzado de nuevo el «Space Shuttle» Discovery, de la NASA, camino de la Estación Espacial Internacional (ISS). El éxito de este vuelo es fundamental para la investigación espacial en general, ya que los vuelos del Shuttle son necesarios para completar la ISS. Pero el éxito es también imprescindible para el futuro de la Astrofísica Espacial, ya que de él depende el que el Telescopio Espacial Hubble (HST) pueda seguir operativo unos años más.
¿Cómo es posible que nos encontremos en una situación en que tanto el futuro de la ISS como el del HST dependan del lanzamiento de un Space Shuttle, algo que considerábamos trivial hasta hace poco tiempo? Hagamos un poco de historia. El acceso del Hombre al Espacio se materializó en los años 60 del siglo pasado por medio de pequeñas cápsulas presurizadas que eran impulsadas por potentes cohetes. Estas cápsulas eran muy pequeñas, y en los primeros tiempos tan sólo dejaban espacio para 1 o 2 astronautas sentados, que tenían que permanecer en esa posición durante el desarrollo de la misión. Por el lado americano este concepto llegó a su punto culminante con la combinación de las cápsulas Apollo, que incluían un módulo de servicio más amplio, y los lanzadores Saturno V, fruto del ingenio del hoy ya mítico Wernher von Braun. El Saturno V es el cohete más potente y más fiable (ningún fallo) que ha fabricado el Hombre, más potente incluso que los «Space Shuttle» modernos.
El Apollo 13 en su plataforma de lanzamiento
Pero el coste de cada lanzamiento era prohibitivo. Por este motivo, a comienzos de los años 70 NASA decidió desarrollar un sistema de lanzaderas espaciales que fueran potentes, versátiles, y mucho más baratas. El concepto básica fue la capacidad de reutilización, con el objetivo de reducir costes. En el Shuttle se aprovechan casi todos los elementos, excepto el tanque exterior de combustible. Tanto la nave como los motores principales regresan a Tierra y pueden volar múltiples veces. Sin embargo, lo que parecía una buena idea no resultó tan buena después de todo. Para que un Shuttle vuelva a volar repetidamente a lo largo de los años en condiciones de seguridad hacen falta revisiones y mejoras tan profundas que el coste de cada misión sigue siendo muy elevado.
Lanzamiento de la misión Discovery STS141
Los soviéticos optaron por un concepto más clásico: siguieron desarrollando sus cápsulas tipo Soyuz, que son lanzadas por cohetes fabricados en serie a un coste relativamente bajo. En los años 80 intentaron copiar el diseño de los Shuttle americanos, pero su nave Buran acoplada a un cohete Energya sólo realizó un vuelo de prueba. La crisis de la URSS impidió el desarrollo del sistema, pese a sus ventajas.
Los Shuttle americanos han realizado ya numerosas misiones con éxito. Pero desgraciadamente adolecen de defectos de diseño que los hacen más vulnerables de lo que sería aceptable. No olvidemos que el diseño básico es de los años 70 del pasado siglo. El primer defecto grave afectó a los motores de combustible sólido: un error de diseño en unas simples juntas de caucho provocaron que el Challenger estallara durante el lanzamiento en enero de 1986. A pesar de la gravedad del accidente, el defecto era relativamente simple y fue corregido rápidamente. Pero hay otro problema de diseño mucho más serio: el Shuttle despega acoplado al gigantesco depósito de hidrógeno y oxígenos líquidos, que sobresale claramente por encima de la nave. Este depósito se encuentra a temperaturas criogénicas por lo que, a pesar de los aislantes, se forma hielo en su superficie cuando se encuentra en la rampa de lanzamiento. Al encender los motores, gruesos bloques de hielo se desprenden y golpean contra la estructura del Shuttle, produciendo daños en el aislamiento térmico. Este problema ha ocurrido desde los primeros días del Shuttle, y siempre se consideró normal que algunas losetas térmicas se desprendieran. Pero en el último lanzamiento del Columbia, en 2003, un bloque de hielo dañó el borde de ataque de una de las alas. En la reentrada, el gas a altas temperaturas penetró por allí y llevó a la destrucción completa de la nave. Este problema podría haber sucedido, de hecho, en cualquier otro vuelo anterior.
Astronautas realizando operaciones de mantenimiento en el HST en 1997
NASA reaccionó rediseñando el tanque de combustible. Durante el último vuelo del Discovery el pasado otoño se probó el nuevo diseño, pero para desesperación de los ingenieros de NASA, gruesos bloques de hielo volvieron a caer sobre el Shuttle, forzando una reparación en órbita de los paneles dañados. Dentro de unas semanas se procederá a una nueva prueba con el Discovery. En esta ocasión los ingenieros esperan que los problemas se hayan solucionado.
No sólo los ingenieros lo esperan. Mientras el problema no esté bajo control, NASA decidió que los Shuttle sólo volarían a la ISS, de manera que si se detecta algún daño los astronautas podrían refugiarse allí y retornar mediante una cápsula Soyuz que permanece permanentemente atracada. Esto implica que hoy por hoy NASA no autoriza que el Shuttle visite el HST, una visita necesaria para reemplazar los giróscopos e instalar nuevos instrumentos. Sin esta visita será difícil que el HST pueda continuar sus operaciones mucho más allá de 2008. De hecho, sin el empuje periódico del Shuttle, el HST acabará cayendo al océano de manera descontrolada en un plazo relativamente corto.
El Shuttle Discovery atracado en la ISS durante su último vuelo
Esperemos por lo tanto que NASA vuelva a poner en operación su flota de Shuttles, que sean capaces de terminar la construcción de la ISS, y que puedan volver a dejar el HST en condiciones óptimas de funcionamiento dentro de un par de años.
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