Lunar Reconnaissance Orbiter: A guide to NASA's venerable lunar orbiter

Graphic illustration of the Lunar Reconnaissance Orbiter above the surface of the moon with a partly illuminated Earth in the background.
The Lunar Reconnaissance Orbiter (LRO) is an uncrewed spacecraft currently orbiting and studying the moon (Image credit: NASA/GSFC)

NASA's Lunar Reconnaissance Orbiter (LRO) has studied the moon up close since its launch in 2009 and continues to do so to this day.

When LRO launched, its mission planners intended the mission to last two years, with a primary goal of producing a 3D map of the moon's surface, according to NASA. Instead, LRO has lasted well over a decade — and its lifespan is far from over.

The mission particularly focuses on the moon's poles to search for water or ice thought to exist there.

Related: How far is the moon from Earth? 

What is the LRO?

As its name indicates, LRO is an uncrewed lunar orbiter.

LRO launched on June 18, 2009. It arrived in lunar orbit five days later and settled into its final orbit by September. When it launched, LRO was NASA's first lunar orbiter since Lunar Prospector in 1998. LRO was launched as an initial step and support for NASA's Constellation program, which planned to put humans on the moon by 2020.

Unfortunately, budget cuts meant that Constellation did not materialize. Still, LRO orbited on, evolving into a more dedicated science mission.

But NASA's priorities continued to shift. Now, LRO has found new life as a forward scout for the Artemis program, NASA's project to return humans to the moon's surface

Where is the LRO?

LRO lives in a circular orbit, roughly 31 miles (50 kilometers) above the lunar surface, according to NASA. When it reached the moon in 2009, this was the closest to the moon that any orbiter had come. The probe takes roughly two Earth hours to complete one full orbit. 

The orbit takes LRO over the lunar poles, where interest in human habitation is highest. The orbit slowly processes around the moon such that LRO flies over a specific location about twice an Earth month.

You can see where the LRO is with NASA's live LRO tracker. Arizona State University's website also updates LRO's position every five minutes.

What is the LRO's purpose?

LRO came to the moon with two chief purposes: Studying the moon for scientists and surveying the moon for future missions. LRO carries a toolbox of several instruments that it can use to map the moon's surface, probe the layers just underneath, search for volatiles like water, and test how humans will fare in the moon's surrounding space. 

Related: Amazing moon photos from NASA's Lunar Reconnaissance Orbiter

Scientists can use LRO's data to better understand the moon's surface environment in far greater detail than they could with any mission before. They want to use it to understand the moon's history and composition: everything from the water ice that might lie under the surface to the volcanic activity that might have shaped the moon in its past, according to NASA. 

Meanwhile, mission planners on Earth can rely on LRO's data. They can use LRO's maps to select landing sites or find promising spots for longer-term habitation, such as points with access to subsurface water. LRO data was used to help choose a landing site for NASA's Volatiles Investigating Polar Exploration Rover, or VIPER, mission. VIPER is part of NASA's Artemis Program and is currently scheduled to launch late 2024. The rover needs to be able to stay in contact with Earth while also exploring craters to study water ice, the mission team members were able to use extensive mapping data from LRO and decided to land the rover near the western edge of Nobile crater.

LRO scientists can also use LRO's measurements of the moon's tenuous atmosphere or the radiation around the moon to infer how human space travelers might fare under prolonged exposure.

LRO's purpose has evolved over the course of its mission. In the 2020s, scientists are confident that the moon is home to volatiles. So, LRO now wants to estimate how many volatiles there are, precisely, how they're changing, and why they're there in the first place. Additionally, LRO has been around the moon long enough that scientists can watch changes in its history: for instance, new craters forming as meteors impact the lunar surface. 

Related: Rogue rocket's moon crash site spotted by NASA probe (photos)

The site of the March 4, 2022 rocket crash on the moon is shown in this before-and-after pair of photos taken by NASA’s Lunar Reconnaissance Orbiter on Feb. 28, 2022 and May 21, 2022, respectively. (Image credit: NASA/GSFC/Arizona State University)

How did the LRO image Apollo landing sites?

Perhaps LRO's most well-known instrument is the Lunar Reconnaissance Orbiter Camera (LROC). This instrument consists of three cameras: two Narrow Angle Cameras, which provide 1,640-ft (500-meter) resolution n images over a 3-mile (5-km) wide area; and one Wide Angle Camera, which provide 328-ft (100-m) resolution images over a 37-mile (60-km) wide area.

Not long after LRO arrived at the moon, LROC captured the landing sites of several Apollo missions. The instrument's camera viewed the remaining lunar modules, astronauts' footprints through the moondust, and even scientific instruments that the astronauts left behind. The Apollo landing sites can further be explored on LROC's official website.

LROC has imaged other spacecraft, too: for instance, the Chang'e 3 and its Yutu rover, which landed on and traversed the lunar surface, respectively, in 2013; and the Vikram lander, which lost contact with its parent Chandrayaan 2 orbiter and crashed into the lunar surface in 2019.

Related: Apollo landing sites: An observer's guide on how to spot them on the moon

LRO FAQs answered by an expert

 We asked Noah Petro, project scientist for LRO a few commonly asked questions about the Lunar Reconnaissance Orbiter. 

Noah Petro

Noah Petro is the project scientist for LRO at NASA's Goddard Space Flight Center in Greenbelt, Maryland. 

What can the LRO do to support missions?

First and foremost, we can collect more data. So, "Mission X wants to go to this particular landing site." Well, what data do we have? What data can we collect to really improve our understanding of a particular site.

Turning around and then making more data products: whether it's higher-resolution topographic models, refined understanding of the thermal environment, specific maps of volatile abundance and distribution and what have you, we can turn around specific data products to support these missions and support the specific places.

Hasn't the LRO mapped everything?

We've had a map of the moon for a long time, but the moon's a big place. There are locations on the moon, that we're interested in going to, that we may not have as much data as we would have for other locations. So, part of what we need to do is go around and say: Here's additional datasets that we have to collect. And that takes time. We can't drive the spacecraft: "Okay, go over this place!"

Twice a month, we fly over particular locations, and we have that opportunity to collect data. Some of those datasets require very specific illumination geometries, so we have to wait.

How can the LRO support Artemis?

We've done this now for the Artemis candidate landing sites, for Artemis 3. Where do we want to go? Let's not only pull together all the data that we have, but what additional datasets can we collect? How can that feed into the safe places to go, the scientifically interesting places to go, and what we do when we get there?

And once the missions start flying — so, once Artemis 3 lands — what happens while they're on the surface? Can we image that landing site to see the lander, to characterize the changes to the surface that occur?

Once the mission leaves, we can go back and either fly over that site or try to collect as much data of that site as possible to understand: "Hey, what happened while we were on the surface?"

How long can the LRO's data last?

We recognize that the science team, the LRO team, can't answer every single question. What we're also trying to do is build up the data that future generations can use to try to constrain these problems. So, as much as our science teams are set about answering specific questions, we also know we need to collect as much data as we can. 

What instruments does the LRO carry?

In addition to LROC, the probe carries six other instruments.

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) studies the radiation that rains down upon the moon's environs. Coming in the form of solar radiation and cosmic rays from far outside the solar system, that radiation can pose an existential threat to humans by damaging their DNA. Therefore, the instrument is fashioned from a plastic that mimics human tissue, allowing it to measure the amount of radiation that a human space traveler would experience.

Diviner observes the moon in infrared, mapping its surface temperature. These measurements help scientists understand how the temperature fluctuates through the weeks-long lunar days and nights. Without an atmosphere thick enough to stabilize the climate, lunar surface temperatures can spike to 400K (260.6 ºF) in the day and plunge to 100K (-279.4 ºF) at night. Whereas the Apollo missions all landed on equatorial regions during the lunar day, future missions will land on a much broader swath of the world. Additionally, the moon's infrared emissions and temperature shifts indicate what minerals — including ice — might lie underneath.

The Lyman-Alpha Mapping Project (LAMP) is an ultraviolet sensor that detects Lyman α radiation: ultraviolet emitted from hydrogen atoms. Both distant stars and clouds of hydrogen within the solar system generate this ultraviolet radiation. As it glances off the moon, LAMP can use it to "see in the dark" into places like the poles and the interiors of large craters, shadowed by the sun's visible light. LAMP can measure it to search for ice and other minerals.

The Lunar Exploration Neutron Detector (LEND) measures neutrons streaming from the lunar surface. Cosmic rays striking the moon create high-energy neutrons a few feet beneath its surface, which stream back up. Scientists can scan these neutrons to learn about the minerals they pass through en route. In particular, if neutrons collide with hydrogen nuclei, they quickly lose their energy. Therefore, LEND can easily see hydrogen-bearing compounds, such as water.

The Lunar Orbiter Laser Altimeter (LOLA) scans the lunar surface with a quintet of laser beams, measuring the moon's topography. Scientists have used LOLA to map the moon's elevation down to less than a kilometer. LOLA's high-detail maps can help planners on Earth pinpoint optimal landing sites.

Mini-RF is an experimental radar instrument that scans the lunar poles in search of water ice. In 2011, Mini-RF's transmitter failed, ending its initial experimentation. Mini-RF's receiver, however, is still functioning. Scientists can still use it: for instance, by bouncing radar transmissions off the lunar surface and using Mini-RF to pick them up.

How long can the LRO last?

LRO will last so long as its instrumentation can remain functioning and NASA continues renewing its extended mission. Its current orbit is reportedly very fuel-efficient, and at LRO's present rate of propellant consumption, NASA officials expect the probe to last into the late 2020s. 

Additional resources

Pan around the moon on LROC’s website, and see many more LRO images thanks to NASA. Watch the launch of LRO — and sister satellite LCROSShere on NASA's YouTube channel.


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Rahul Rao
Contributing Writer

Rahul Rao is a graduate of New York University's SHERP and a freelance science writer, regularly covering physics, space, and infrastructure. His work has appeared in Gizmodo, Popular Science, Inverse, IEEE Spectrum, and Continuum. He enjoys riding trains for fun, and he has seen every surviving episode of Doctor Who. He holds a masters degree in science writing from New York University's Science, Health and Environmental Reporting Program (SHERP) and earned a bachelors degree from Vanderbilt University, where he studied English and physics.