The Mesosphere, Lower Thermosphere and Ionosphere (MLTI) region of the atmosphere to be studied by TIMED is located between 60 and 180 kilometres (37 and 112 mi) above the Earth's surface, where energy from solar radiation is first deposited into the atmosphere. This can have profound effects on Earth's upper atmospheric regions, particularly during the peak of the Sun's 11-year solar cycle when the greatest amounts of its energy are being released. Understanding these interactions is also important for our understanding of various subjects in geophysics, meteorology, aeronomy, and atmospheric science, as solar radiation is one of the primary driving forces behind atmospheric tides. Changes in the MLT can also affect modern satellite and radiotelecommunications.
Scientific instruments
The spacecraft payload consists of the following four main instruments:
Global Ultraviolet Imager (GUVI), which scans cross track from horizon to horizon to measure the spatial and temporal variations of temperature and constituent densities in the lower thermosphere, and to determine the importance of auroral energy sources and solar extreme ultraviolet sources to the energy balance in that region.
TIMED Doppler Interferometer (TIDI), designed to globally measure the wind and temperature profiles of the MLT region.
Sounding of the Atmosphere using Broadband Emission Radiometry (SABER), multichannel radiometer designed to measure heat emitted by the atmosphere over a broad altitude and spectral range, as well as global temperature profiles and sources of atmospheric cooling.
The data collected by the satellite's instruments are made freely available to the public.[3]
TIMED experienced minor problems with attitude control when, after launch, the magnetorquers failed to slow the spacecraft's spin as intended. An engineer installing the magnetorquers had mistakenly recorded the reverse of their actual polarities, which generated a sign error in the flight software. The problem was fixed by temporarily disabling the orbiter's magnetic field sensor and uploading a software patch to fix the sign error.[4] In a separate incident, another software update fixed a problem caused by faulty testing of the Sun sensors. After these corrections, the attitude control system functioned as intended.[4]
Kosmos 2221 conjunction
At approximately 06:30 UTC on 28 February 2024, TIMED passed within 10 meters of the defunct Kosmos 2221 satellite. As neither TIMED nor Kosmos 2221 can be maneuvered, the conjunction was unavoidable. LeoLabs, a satellite tracking company, had estimated a satellite collision probability of as high as 8% prior to the encounter. A collision between the two satellites, both traveling at hypervelocity speeds relative to each other, was projected to generate between 2,500 and 7,500 fragments of space debris, a figure potentially exceeding that of the 2009 satellite collision between Iridium 33 and Kosmos 2251. This close miss was particularly concerning to NASA, which highlighted the event at the 39th Space Symposium in Colorado Springs in a broader speech on NASA's new space sustainability strategy plan.[5]
Scientific results
TIMED has improved scientific understanding of long-term trends in the upper atmosphere. The SABER instrument has collected a continuous record of water vapor and carbon dioxide levels in the stratosphere and mesosphere.[6][7]
SABER is able to collect 1,500 water vapor measurements per day, a vast improvement from previous satellites and ground-based observations.[8] SABER had a flaw in its optical filter that caused it to overestimate water vapor levels; this error was discovered and the data were corrected.[9] Based on the corrected data, SABER found that between 2002 and 2018, water vapor levels in the lower stratosphere were increasing at an average rate of 0.25 ppmv (around 5%) per decade, and in the upper stratosphere and mesosphere, water vapor levels were increasing at an average rate of 0.1-0.2 ppmv (around 2-3%) per decade.[10] Growth in methane levels is thought to be partially responsible for the growth in water vapor levels, as methane oxidizes into carbon dioxide and water vapor, but changes driven by the solar cycle may also be responsible.[11]
SABER has also monitored carbon dioxide levels in the upper atmosphere. The instrument found that carbon dioxide levels in the upper atmosphere are increasing: at an altitude of 110 kilometres (68 mi), CO2 levels were rising at an average rate of 12% per decade.[12] This rate is faster than what has been predicted by climate models, and suggests that there is more vertical mixing of CO2 than previously thought.[13]
By collecting upper atmosphere data, TIMED assists the modeling of environmental impacts. Water vapor and carbon dioxide are greenhouse gases and their growth in the upper atmosphere must be factored into climate models. Additionally, upper atmosphere water vapor contributes to ozone depletion.[14]
^ abHarland, David M.; Lorenz, Ralph D. (2006). Space Systems Failures: Disasters and Rescues of Satellites, Rockets, and Space Probes. Berlin: Springer. pp. 214–215.
Launches are separated by dots ( • ), payloads by commas ( , ), multiple names for the same satellite by slashes ( / ). Crewed flights are underlined. Launch failures are marked with the † sign. Payloads deployed from other spacecraft are (enclosed in parentheses).
