SMOS

SMOS (Soil Moisture and Ocean Salinity) is an Earth observation satellite operated by the European Space Agency (ESA) and catalogued under NORAD ID 36036, international designator 2009-059A. Launched on 1 November 2009 into a sun-synchronous orbit, the spacecraft has been circling Earth continuously ever since, gathering data fundamental to understanding how water moves through the planet's interconnected systems. With a mass of 630 kg, SMOS occupies a modest footprint among Earth observers, yet the measurements it makes address scientific questions that have broad implications for meteorology, hydrology, and climate science.

Mission and Purpose

SMOS was developed as part of ESA's Living Planet Programme, an initiative conceived to address pressing questions about how Earth's natural systems function and how they are changing over time. The Living Planet Programme frames Earth observation not merely as a technical exercise but as a sustained scientific commitment, and SMOS represents one of its dedicated "Earth Explorer" missions — spacecraft designed to target specific, well-defined gaps in global environmental monitoring.

The central objective of SMOS is to measure two variables that, despite their apparent simplicity, are notoriously difficult to observe consistently at a global scale: the moisture content of soil and the salinity of the ocean surface. Soil moisture — the proportion of water held within the uppermost layer of the ground — governs how rainfall is partitioned between runoff and absorption, directly influencing river flow, groundwater recharge, and the amount of water returned to the atmosphere through evaporation and plant transpiration. Because this single variable sits at a crossroads between the atmosphere, the land surface, and the biosphere, reliable global measurements of it have long been a priority for Earth scientists.

Ocean salinity, measured at the sea surface, is equally consequential. Variations in salinity affect the density of seawater, which in turn drives the large-scale ocean circulation patterns responsible for redistributing heat around the planet. Changes in salinity can signal shifts in the global water cycle — increases in evaporation in some regions, or freshwater influx from precipitation and melting ice in others. Prior to missions like SMOS, continuous, synoptic salinity observations from space were essentially unavailable.

Beyond these core measurements, SMOS data contribute to improved weather forecasting by giving numerical prediction models better information about the state of the land surface. Soil moisture, in particular, has a significant influence on how the boundary layer of the atmosphere develops on any given day, affecting temperature extremes and precipitation likelihood. The satellite's data also support the monitoring of snow and ice accumulation, which matters both for water resource management and for tracking changes in polar and high-altitude environments.

Orbit and Tracking

SMOS operates in a sun-synchronous orbit (SSO), a class of near-polar orbit carefully chosen to ensure that the satellite crosses any given point on Earth's surface at approximately the same local solar time on every pass. This consistency is essential for Earth observation missions: by holding the illumination geometry and surface conditions as constant as possible from one overpass to the next, scientists can compare measurements made weeks or months apart with far greater confidence than would be possible with a drifting orbit geometry.

The orbit parameters recorded in current tracking data place the apogee at 764 km and the perigee at 763 km above Earth's surface — a difference of just one kilometer, indicating a very nearly circular orbit. The inclination is 98.4°, which is the slightly retrograde tilt characteristic of sun-synchronous trajectories at this altitude. At these figures, SMOS completes one full orbit approximately every 100.0 minutes, meaning it circles Earth roughly 14 to 15 times each day.

The near-perfect circularity of the orbit is not incidental. A highly elliptical orbit would cause the satellite's altitude — and therefore its ground resolution and signal path length — to vary substantially over the course of each pass, complicating the interpretation of the passive microwave measurements that form the basis of the SMOS science return. Maintaining a stable, circular orbit keeps instrument performance predictable and calibration manageable over the mission's operational lifetime.

Tracking data for SMOS are maintained by the United States Space Surveillance Network and published in the form of two-line element sets (TLEs), which are updated regularly as ground-based radar tracks the satellite's precise position. LowEarth sources these elements to compute current and predicted passes, allowing users to determine when SMOS will be overhead at any given location.

Design and Operator

SMOS was manufactured by Alcatel Space and is operated by the European Space Agency. With a launch mass of 630 kg, it falls within the medium-small range for an ESA science mission. The spacecraft carries a passive microwave imaging radiometer that operates in the L-band portion of the radio spectrum — a frequency range particularly well suited to sensing surface soil moisture and ocean salinity because microwave emissions at these wavelengths penetrate shallow depths into the soil and are sensitive to the dielectric properties of water, which change dramatically with salinity and moisture content.

Rather than a conventional dish or scanning antenna, the instrument uses a synthetic aperture approach with a Y-shaped array of small receiving elements. This configuration allows the satellite to achieve useful spatial resolution without a physically enormous antenna — a critical engineering constraint given the limits of what can be launched and deployed in orbit. The instrument's sensitivity to subtle differences in microwave brightness temperature is what allows scientists to infer soil moisture and salinity from orbit.

ESA, headquartered in Paris and with operational centers across Europe, manages SMOS as part of its broader portfolio of Earth Explorer and Earth Watch missions. The Living Planet Programme under which SMOS operates was designed with the explicit goal of giving researchers access to Earth system observations that could not be obtained through existing meteorological or commercial satellite assets.

Scientific Significance and Current Status

Since its launch in late 2009, SMOS has demonstrated that global, repeated measurements of soil moisture and ocean salinity from space are not only feasible but scientifically productive. The dataset it has accumulated now spans well over a decade, making it one of the longer continuous satellite records of these particular variables. Long time series of this kind are especially valuable because they allow researchers to study interannual variability, detect trends, and validate the models used to project future changes in the climate system.

Soil moisture data from SMOS have been used in drought monitoring, flood forecasting, and agricultural assessment — applications where the difference between a timely, accurate measurement and an outdated estimate can carry real practical consequences. Ocean salinity measurements have contributed to research on freshwater balance, thermohaline circulation, and the response of the global ocean to a changing climate.

The mission has also served as a proof of concept that encouraged subsequent investment in similar observational capabilities. The scientific and technical lessons learned from SMOS have informed the design of later Earth observation instruments and missions, both within ESA and internationally.

As of the time this entry was compiled, SMOS remains in orbit and has not undergone atmospheric reentry. No decay or reentry date is recorded in the catalog, and the satellite continues to be tracked as an active payload. Mission operational status and any current data-collection activities are not publicly detailed in the tracking catalog record.

How to Spot SMOS

SMOS is a relatively small spacecraft at 630 kg and does not carry any reflective surfaces — such as the large solar panels found on some communications satellites or the mirror-like arrays of imaging platforms — that would make it especially prominent in the night sky. It is not considered a particularly bright object, and casual naked-eye observers are unlikely to notice it without specific guidance.

That said, low Earth orbit satellites of this class are occasionally visible as steadily moving points of light in the minutes after sunset or before sunrise, when the observer is in darkness but the satellite is still illuminated by sunlight. SMOS passes overhead roughly every few hours at any given mid-latitude location, and its 100.0-minute orbital period means that consecutive visible passes can occur within a single observing window on nights near the summer solstice at higher latitudes.

To find out whether SMOS will be visible from your location, use the pass prediction tools on this page, which draw on current tracking elements to calculate elevation, azimuth, and magnitude estimates for upcoming passes. Look for a steady, non-blinking point moving smoothly across the sky — unlike aircraft, satellites do not have flashing navigation lights and travel in long, undeviating arcs from horizon to horizon.