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Exactly 20 years ago today, Dec. 7, 2001, a Delta II rocket carrying NASA’s spacecraft took off from the California coast on a journey to study the least explored and understood part of Earth’s atmosphere — the upper climes 40-110 miles (60-180 kilometers) above the surface, where the Sun’s energy first enters Earth’s environment.

Originally slated to operate for just two years as the first mission under NASA’s Solar Terrestrial Probes program, TIMED (Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics) opened a new era of better, faster and cheaper spacecraft for exploration. It was an effort pioneered by scientists and engineers at the ����ֱ�� Applied Physics Laboratory (APL) in Laurel, Maryland, which jointly developed the TIMED mission with NASA’s Goddard Flight Space Center in Greenbelt, Maryland.

“TIMED raised the bar for spacecraft engineering and design,” said Mike Ryschkewitsch, head of APL’s Space Exploration Sector and former NASA chief engineer. “From its onboard autonomy to its redundant subsystems to its use of GPS for onboard tracking, navigation and event-based commanding, TIMED embodies creative thinking that represented true innovation in our field. Over two decades ago, a small APL team designed, delivered and sustained a low-cost, durable spacecraft that exceeded all expectations in its exploration of a region above our own planet that, before TIMED, was a complete mystery. TIMED is a spacecraft and mission that APL can be proud of.”

Called the gateway to space, Earth’s mesosphere, lower thermosphere and ionosphere were truly a mystery to scientists. At altitudes too high for airplanes or balloons but too low for satellites without burning up, this tenuous blanket of charged particles in the upper atmosphere — where auroras dance in fantastic light shows at the poles, yet other forms of space weather can affect global navigation systems and radio signals — was for the longest time inaccessible.

TIMED changed that. By orbiting above the atmospheric region with four powerful remote sensing instruments, TIMED circumvented the earlier problem. And now with its unrivaled 20-year dataset, scientists have a much clearer idea of how this part of the atmosphere responds not only to sudden changes from the Sun and space environment above but also to the ground and people below.

“Our picture has really changed from the time that TIMED was first conceived to now,” said Sam Yee, an atmospheric scientist and the APL project scientist for the TIMED mission. “What we’re seeing now is that the entire atmosphere doesn’t behave in discrete parts — an upper part and a lower part, a high-latitude part and a low-latitude part — but as one coupled system, and not only to itself but to the surface as well.”

Small atmospheric fluctuations near the ground, TIMED data has shown, can propagate upward to create massive waves that spread hundreds of miles across the upper atmosphere. For example, waves of warm air rising up along the windward sides of mountains not only create chains of clouds that blanket mountain peaks with snow but continue up hundreds of miles into the atmosphere where they spread out. Even the relatively tiny movements of earthquakes can create waves in the lower atmosphere that cause disturbances as they reach the upper atmosphere.

“That was all sort of a surprise,” said , a space scientist at APL and APL’s lead investigator of TIMED’s Global Ultraviolet Imager instrument. “But there are all kinds of these effects that we’re just starting to unravel, all pioneered by TIMED.”

TIMED data has shown that the amount of carbon dioxide — the greenhouse gas most culpable for climate change — has increased in the upper atmosphere like it has in the lower atmosphere. But while the dense concentration of carbon dioxide close to the ground reflects heat and traps it near the surface so the region warms, the much sparser (but growing) concentration in the upper atmosphere counterintuitively cools that region. With so few molecules to reflect and trap the heat at the edge of space, the carbon dioxide instead sends it into space.

TIMED’s insights extend even beyond Earth. Paxton noted that this gateway region between the planet’s surface and space controls much of how an atmosphere changes over time. The change in composition and physical behavior of the different regions “in a real sense controls the evolution of planetary atmospheres,” he said. “So we have this laboratory — our own atmosphere — for understanding how other planets evolved, from Venus and Mars to planets in other solar systems.”

“TIMED has made many discoveries and shown that low-cost missions can make an enormous contribution to basic research,” added TIMED Project Scientist Diego Janches of NASA’s Goddard Flight Space Center. “With nearly 3,000 papers referencing TIMED data that researchers and institutions around the world published, TIMED is a major success story in our constant search for understanding our home in space.”

Myriad mysteries about the upper atmosphere still remain, however. Scientists understand the physics of what’s happening in the atmosphere, Paxton said. They can write down equations that describe the phenomena and make predictions from them. But there are still many mysteries about these physical processes that scientists “parameterize” by capturing, as well as they can, complex behavior in simple terms. One such term, for example, describes how waves in the lower atmosphere grow as they rise into the upper atmosphere until they eventually collapse and become turbulent, much as a wave grows taller as it approaches and then breaks on the shore.

“We can write down the equations and make our best guess, but we’re not really sure how that happens,” Yee said. “So we do know a lot more from TIMED, but there are still many newly raised questions to be answered.”

Perhaps only TIMED will tell.