Two long-running space missions now reaching the end of their lives embarked years ago on very different missions, but today they share one significant attribute: each made important contributions in its respective field of space activity.

From a mission-objective standpoint, the Ulysses solar observatory and the Midcourse Space Experiment, or MSX, had little in common. Ulysses was a scientific endeavor and a model of international cooperation; MSX was a military capability demonstration launched by the forerunner to the U.S. Missile Defense Agency. But both share the distinction of being trailblazers – each in its own way – and of far exceeding expectations, both in terms of longevity and productivity.


was one of the first significant scientific collaborations between NASA and the European Space Agency. Not only was it a huge success from the

standpoint of the scientific data it produced, it was a case study in overcoming adversity.

Launched in 1990 aboard NASA’s Space Shuttle Discovery, the European-built, nuclear-powered spacecraft has orbited the sun around its poles for more than 17 years, or 12 years longer than its design lifetime. During that time Ulysses experienced a variety of challenges that, were it not for the resourcefulness and ingenuity of the ground operations team, could have ended its mission as early as a few weeks after its launch.

As it turned out, Ulysses lasted through one-and-a-half solar cycles, providing astronomers with an unprecedented wealth of information on the effects of solar activity on the surrounding space and all that lies within it. The mission was extended three times and the probe just recently passed over the sun’s north pole at the beginning of the current 11-year solar cycle. Scientists trying to unlock the mysteries of the sun and the heliosphere – the magnetic influence of the sun that engulfs the solar system – will be using

Ulysses data for years to come.

The MSX had a diverse set of optical payloads and objectives, but its primary original mission was to demonstrate the feasibility of tracking ballistic missiles as they pass through space during the midcourse portion of flight. This phase of the mission was carried out within a year of launch, ending after the cryogenic coolant for its infrared sensor had evaporated. The satellite also found use as an astronomy and atmospheric measurement platform.

But it was in a largely makeshift role that the MSX satellite truly made its mark: helping the U.S. military keep close tabs on an increasingly crowded and – in the minds of many – increasingly dangerous orbital medium. The space surveillance mission began as a U.S. Air Force experiment using the MSX’s Space Based Visible sensor, but evolved into an operational activity that kept the spacecraft in service for more than a decade after its other activities had ended.

During that period, the Pentagon, which traditionally has relied on ground-based systems for space surveillance, came to appreciate the pivotal role of satellites in that vital mission, particularly for gauging potential threats to geostationary-orbiting satellites operating 36,000 kilometers above the Earth’s surface. The Air Force is now building its first dedicated space-surveillance satellite and it is hard to imagine not having such a capability on orbit on a permanent basis in the future.

According to the Air Force, the MSX is being retired because the Space Based Visible sensor is no longer effective.

For Ulysses, the culprit is its plutonium-fueled radioisotope thermoelectric generator, which no longer provides enough heat to keep the spacecraft’s hydrazine thruster propellant – necessary to point it in the right direction for sending and receiving radio transmissions – from freezing.

That both missions are ending at roughly the same time is purely coincidence. That they are going out on a high note after so many years is not coincidence but rather testimony to the adaptability and professionalism of the teams that built and operated them.