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Compensatory Time Off

Fact sheet: compensatory time off, description.

Compensatory time off is

• Time off with pay in lieu of overtime pay for irregular or occasional overtime work, or
• When permitted under agency flexible work schedule programs, time off with pay in lieu of overtime pay for regularly scheduled or irregular or occasional overtime work.

Employee Coverage

Compensatory time off may be approved in lieu of overtime pay for irregular or occasional overtime work for both FLSA exempt and nonexempt employees who are covered by the definition of "employee" at 5 U.S.C. 5541(2).

Compensatory time off can also be approved for a "prevailing rate employee," as defined at 5 U.S.C. 5342(2), but there is no authority to require that any prevailing rate (wage) employee be compensated for irregular or occasional overtime work by granting compensatory time off.

Compensatory time off may be approved (not required) in lieu of regularly scheduled overtime work only for employees, including wage employees, who are ordered to work overtime hours under flexible work schedules. See 5 U.S.C. 6123(a)(1).

Agencies may require that an FLSA exempt employee (as defined at 5 U.S.C. 5541(2)) receive compensatory time off in lieu of overtime pay for irregular or occasional overtime work, but only for an FLSA exempt employee whose rate of basic pay is above the rate for GS-10, step 10. No mandatory compensatory time off is permitted for wage employees or in lieu of FLSA overtime pay.

Time Limits

Flsa-exempt employees.

An FLSA-exempt employee must use accrued compensatory time off by the end of the 26th pay period after the pay period during which it was earned.

An agency may provide that an FLSA-exempt employee who (1) fails to take earned compensatory time off within 26 pay periods or (2) transfers to another agency or separates from Federal service before the expiration of the 26 pay period time limit-

• Receive payment for the unused compensatory time off at the overtime rate in effect when earned or
• Forfeit the unused compensatory time off, unless failure to use the compensatory time off is due to an exigency of the service beyond the employee's control. (An FLSA-exempt employee whose earned compensatory time off would otherwise be forfeited due to an exigency of service beyond the employee's control must receive payment for the unused compensatory time off at the overtime rate in effect when earned.)

FLSA-nonexempt employees

An FLSA-nonexempt employee must use accrued compensatory time off by the end of the 26th pay period after the pay period during which it was earned.

If accrued compensatory time off is not used by an FLSA-nonexempt employee within 26 pay periods or if the FLSA-nonexempt employee transfers to another agency or separates from Federal service before the expiration of the 26 pay period time limit, the employee must be paid for the earned compensatory time off at the overtime rate in effect when earned.

Separation or leave without pay status due to service in the uniform service or on-the-job injury

An FLSA-exempt or nonexempt employee must be paid for compensatory time off not used by the end of the 26th pay period after the pay period during which it was earned at the overtime rate in effect when earned if the employee is unable to use the compensatory time off because of separation or placement in a leave without pay status (1) to perform service in the uniformed services or (2) because of an on-the-job injury with entitlement to injury compensation under 5 U.S.C. chapter 81.

Compensatory time off to an employee's credit as of May 14, 2007

See 5 CFR 550.114(e) and 551.531(e) for special rules regarding the administration of compensatory time off to an employee's credit as of May 14, 2007.

1 hour of compensatory time off is granted for each hour of overtime work.

Questions and Answers on Compensatory Time Off in Lieu of Overtime Pay

FLSA-exempt employees earn compensatory time off in lieu of title 5 overtime pay under 5 U.S.C. 5542 and 5 CFR 550.113, and are subject to OPM's compensatory time off regulations at 5 CFR 550.114. FLSA-nonexempt employees earn compensatory time off in lieu of overtime pay under section 7 of the FLSA (29 U.S.C. 207) and 5 CFR 551.501, and are subject to OPM's compensatory time off regulations at 5 CFR 551.531. Both 5 CFR 550.114 and 5 CFR 551.531 are derived from the statutory authority governing compensatory time off in 5 U.S.C. 5543 and, for employees under flexible work schedules, 5 U.S.C. 6123(a)(1). An employee's unused compensatory time off is subject to the regulations under which it was earned, regardless of the employee's current FLSA exemption status.

Example: An employee earns 16 hours of compensatory time off under 5 CFR 551.531 in lieu of FLSA overtime pay while employed in an FLSA-nonexempt position.

The employee is promoted to an FLSA-exempt position 6 months later, but does not use the 16 hours of compensatory time off within 26 pay periods after the pay period during which it was earned. As provided by 5 CFR 551.531(d), the employee must be paid for the 16 hours of unused compensatory time off at the overtime rate in effect when earned. The employee is entitled to receive payment for the compensatory time off even if the employing agency's policy under 5 CFR 550.114(d) is to require forfeiture of compensatory time off earned in lieu of title 5 overtime pay if the compensatory time off is not taken within 26 pay periods.

Agencies must provide payment for, or require forfeiture of, compensatory time off under the conditions set forth in 5 CFR 550.114(d)-(f) and 5 CFR 551.531(d)-(f), as applicable. The general rule is that accrued compensatory time off must be liquidated (i.e., paid) or forfeited (as applicable under agency policies) if not used by the end of the 26th pay period after the pay period during which it was earned. Exceptions to the general rule relate to the following circumstances: (1) the 3-year grandfathering period for any compensatory time off to an employee's credit as of May 14, 2007, which must be used by the end of the pay period ending 3 years after May 14, 2007; (2) transfer to another agency; (3) separation from Federal service; and (4) separation or placement in a leave without pay status in connection with service in the uniformed services or entitlement to workers' compensation based on an on-the-job injury. (When exception #4 applies, the employee must be paid for the unused compensatory time off; forfeiture is not an option.)

• What entities are considered an "agency" for the purpose of triggering the payment/forfeiture of unused compensatory time off at the time of transfer to another agency? View more For the purposes of the compensatory time off regulations, the term "agency" is defined under 5 CFR 550.103 to mean a department (as defined in this section) and a legislative or judicial branch agency which has positions subject to subchapter V (Premium Pay) of chapter 55 of title 5, U.S. Code. The term "department" is defined under 5 CFR 550.103 to mean an executive agency and a military department. (See 5 U.S.C. 101-105 and 5541(1).) Since the military departments (Army, Navy, and Air Force) have standing as separate agencies, the rest of the Department of Defense as a group is treated as a separate agency.

Yes. Agencies are required to provide payment for accrued compensatory time off under the conditions set forth in 5 CFR 550.114 and 5 CFR 551.531. However, an agency has discretionary authority to provide payment for accrued compensatory time off in other circumstances. This discretionary authority should be exercised by issuing formal policies so that employees are treated consistently.

Additional situations in which an agency may choose to provide for payment of compensatory time off include, but are not limited to, the following:

Example 1: A Federal employee moves to a position with a different FLSA exemption status (i.e., from an FLSA-nonexempt position to an FLSA-exempt position or vice versa).

An employee moving to a position within the same agency that has a different FLSA exemption status may have his or her compensatory time off balance paid out according to that agency's policy at the time of his or her change in exemption status. Alternatively, an agency may choose to maintain that employee's compensatory time off after a change in FLSA exemption status, subject to the regulations under which the compensatory time off was earned. (Under this latter alternative, the agency must maintain separate balances for compensatory time off earned under 5 CFR 551.531 in lieu of FLSA overtime pay versus compensatory time off earned under 5 CFR 550.114 in lieu of title 5 overtime pay.)

Example 2: An employee transfers to another component within the same agency.

The agency may choose to provide payment for compensatory time off when a Federal employee transfers to another component within the same agency. Conversely, the agency may allow the employee to maintain his or her compensatory time off balance in the new position, subject to the regulations under which it was earned. (See A3 for information on the definition of "agency" for this purpose.)

Example 3: An employee is placed in a Senior Executive Service (SES) position.

Each agency should establish policies governing the handling of accrued compensatory time off for an employee who is placed in an SES position and thus no longer covered by 5 U.S.C. 5543. The agency may choose to (1) provide payment for any balance of compensatory time off accrued before placement in the SES position at the rate at which it was earned; or (2) allow the employee to use the accrued compensatory time off while in the SES position, subject to the normal time limits established in OPM regulations.

• May an agency require an employee to forfeit unused compensatory time off under circumstances in addition to those identified in OPM regulations? View more An agency may require an employee to forfeit legitimately earned compensatory time off only under the circumstances identified in OPM regulations. An agency may not establish a policy requiring an employee to forfeit unused compensatory time off earned under 5 CFR 550.114 in lieu of title 5 overtime pay in circumstances not identified in that section. An agency may not require an employee to forfeit compensatory time off earned under 5 CFR 551.531 in lieu of FLSA overtime pay under any circumstances.
• 5 U.S.C. 5543 and 5 U.S.C. 6123(a)(1)
• 5 CFR 550.114 and 551.531
• Comptroller General opinions: B-183751, October 3, 1975, and
• October 19, 1976; 58 Comp. Gen. 1 (1978)
• Section 1610 of Public Law 104-201, the National Defense Authorization Act, 1997

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Gateway by the numbers.

Gateway's Power and Propulsion Element (PPE) will generate 60 kW, making it the most powerful solar electric spacecraft ever flown.

5 Space Agencies

Five space agencies, NASA, CSA, ESA, JAXA, and MBRSC, are contributing to Gateway.

4 Crew Members

Early Gateway configurations can accommodate up to four astronauts.

Gateway will have a minimum 15-year lifespan, with the potential for extension well beyond that timeframe.

Gateway will complete one full orbit around the Moon approximately every 6.5 days.

Frequently Asked Questions

Welcome to the FAQ page for NASA’s Gateway space station. From its purpose and design to its role in future space exploration, discover everything you need to know about Gateway, humanity’s first space station in lunar orbit.

What is Gateway?

Gateway is a next-generation space station that will orbit the Moon. It will enable a wide range of research activities and serve as a vital staging point for deep space exploration. Gateway will feature habitation and research facilities, crew and payload airlocks, and docking ports for visiting spacecraft, including Orion, lunar landers, and logistics resupply craft. As the first permanent infrastructure at the Moon, Gateway is a crucial component of NASA’s Artemis missions , which aim to return humans to the lunar surface in preparation for the first crewed missions to Mars .

What’s the purpose of Gateway?

Gateway will accomplish several key objectives crucial to NASA’s Artemis campaign:

• Enable a sustained presence around the Moon.
• Support human crewed missions to deep space and the lunar surface.
• Facilitate scientific research and discovery in a deep space environment.
• Enable, demonstrate, and validate technologies critical for lunar missions, with applications for Mars and other deep space destinations.
• Cultivate industry and international partnerships to lead an unprecedented global coalition to the Moon.

Why Gateway? Why don’t we just go straight to the Moon like we did with Apollo?

Gateway will enable longer stays on and around the Moon, and help ensure the safety of crew in the harsh deep space environment, while significantly expanding research and exploration opportunities beyond what was envisioned or achievable during the Apollo missions . Additionally, Gateway is part of a broader strategy for sustained and permanent lunar exploration. It plays a vital role in NASA’s preparations for future missions to Mars by offering essential knowledge and experience that will prove invaluable for those endeavors.

When will Gateway be ready and how long until it’s finished?

Gateway will be an incrementally built and deployed spacecraft, similar to the International Space Station (ISS). Assembling Gateway in lunar orbit will involve four rocket launches and will take approximately six years, in contrast to the ISS’s 42 assembly flights over about 13 years. This streamlined assembly process for Gateway is made possible by its smaller size and advancements in technology. Gateway’s elements are taking shape on the ground, with hardware fabrication actively underway in facilities worldwide. Gateway’s first two modules—(PPE) Power and Propulsion Element and HALO ( Habitation and Logistics Outpost )—will be integrated and launched together on a SpaceX Falcon Heavy rocket no earlier than 2025. Following launch, these components will spend about a year traveling to lunar orbit. During this phase, Gateway will begin its scientific investigations, enabled by autonomous systems and remote operations. Gateway’s next module, the Lunar International Habitation module (Lunar I-Hab), is expected to reach the Moon no earlier than 2028, as part of the Artemis IV mission . During this mission, a crewed Orion spacecraft will transport the Lunar I-Hab to Gateway and dock it with HALO. This will mark the first time astronauts enter the space station.

How long will Gateway operate?

Gateway will have a minimum 15-year lifespan, with the potential for extension well beyond that initial timeframe.

How big is Gateway going to be?

Gateway will be approximately 1/5 the size of the ISS by volume and 1/6 by mass. When fully assembled, Gateway will weigh around 63 metric tons, or 63,000 kg, and measure around 20 meters in length. By comparison, the ISS is 109 meters long.

How many astronauts will live and work aboard Gateway?

Early configurations of Gateway can accommodate up to four astronauts.

What will astronauts do on Gateway?

Astronauts aboard Gateway will conduct scientific research, prepare for lunar surface missions, and perform maintenance tasks to ensure the smooth operation of the station.

How will astronauts get to Gateway?

Artemis astronauts will travel to Gateway using NASA’s Orion spacecraft .

Will Gateway be visible from Earth?

No, Gateway will not be visible from Earth like the ISS. While the ISS orbits at a distance of approximately 250 miles from Earth in low-Earth orbit, Gateway will be located much farther away, in lunar orbit, about 1000 times farther from Earth than the ISS.

How is Gateway different from the ISS?

• While the ISS has been a cornerstone of space research in low-Earth orbit for over two decades, Gateway expands this legacy into the deep space environment.
• Unlike the ISS, which is shielded from radiation by Earth’s magnetosphere in its low-Earth orbit, Gateway will operate in orbit around the Moon, where radiation is a greater concern due to the lack of this protective shield.
• While the ISS is permanently inhabited, Gateway will not be. Instead, Gateway will focus on pushing the boundaries of solar electric propulsion, robotics, and autonomous operations. This will enable Gateway to conduct science investigations and support missions even when crew are not present.
• When fully assembled, Gateway will be much smaller than the ISS. Gateway will be approximately 1/5 the size of the ISS by volume and 1/6 by mass. When fully assembled, Gateway will weigh around 63 metric tons, or 63,000 kg, and measure around 20 meters in length. By comparison, the ISS is 109 meters long.

How will Gateway be resupplied?

NASA has selected SpaceX as the initial U.S. commercial provider for Gateway’s logistics services . SpaceX will deliver critical pressurized and unpressurized cargo, science experiments and supplies to Gateway. One logistics services delivery is anticipated for each crewed Artemis mission to Gateway.

Gateway’s Orbit

Where will Gateway be located?

Gateway will be located in a polar orbit around the Moon. This highly elliptical Near Rectilinear Halo Orbit (NRHO) will bring Gateway within about 1,000 miles (1,500 km) of the Moon at its closest approach, and about 43,500 miles (70,000 km) at its farthest.

What key advantages does Gateway’s orbit provide?

Gateway’s unique polar orbit will provide Artemis astronauts and their spacecraft access to the entire lunar surface, including the critical lunar South Pole region which is the focus of the Artemis missions. It will also ensure uninterrupted communications between the Moon and Earth, provide unique scientific opportunities within the deep space environment, and its high stability will enhance operational efficiency and reduce costs.

How long will it take Gateway to complete one orbit around the Moon?

How fast will Gateway travel?

At its fastest, Gateway will travel at nearly 3.2 kilometers per second.

International and Industry Partnerships

Who is building and operating Gateway?

Gateway signifies NASA’s expansion of human spaceflight partnerships into deep space, with contributions from international and commercial partners. Space agencies making significant contributions to Gateway include Canadian Space Agency (CSA), European Space Agency (ESA), Japan Aerospace Exploration Agency (JAXA), and the Mohammed Bin Rashid Space Centre (MBRSC).

What contributions are each of NASA’s partners providing for Gateway?

Partner contributions are as follows:

• Gateway’s NASA-led components, HALO and PPE, are being supplied by commercial partners Northrop Grumman and Maxar Technologies , respectively.
• CSA is providing the Gateway External Robotics System (GERS), which features the next-generation robotic arm, Canadarm3. CSA will also supply Gateway External Robotic Interfaces (GERI) across the Gateway modules.
• ESA is contributing three elements to Gateway: the Lunar I-Hab, Lunar View, and Lunar Link.
• JAXA will supply the Lunar I-Hab’s Environmental Control and Life Support System (ECLSS), thermal control system functions and cameras, provide batteries to power HALO, Lunar I-Hab, and Lunar View, and offer a HTV-XG spacecraft for launch and delivery of a logistics resupply mission for Gateway.
• MBRSC will supply Gateway’s Crew and Science Airlock.

Astronauts from Canada, Europe, Japan, and the United Arab Emirates will work aboard Gateway on Artemis missions.

Gateway Science

What scientific research will be conducted on Gateway?

Gateway will serve as a year-round deep space laboratory , offering both internal and external utilization capabilities, and enable a diverse array of scientific research. This unique platform will support scientific investigations that are not feasible on Earth’s surface or in low-Earth orbit, as well as technology demonstrations crucial for long-duration missions to deep space destinations. Scientific areas of focus will include heliophysics, human health research, space biology and life sciences, astrophysics, and fundamental physics investigations. The knowledge gained from operations in the lunar vicinity through Gateway will be used to send the first astronauts to Mars.

What are some examples of specific science experiments or instruments planned for Gateway?

Three science payloads are set to accompany PPE and HALO on their launch into lunar orbit: the European Radiation Sensors Array (ERSA), the Heliophysics Environmental and Radiation Measurement Experiment Suite (HERMES), and the Internal Dosimeter Array (IDA). These instruments will facilitate high-priority research on space weather and solar radiation in deep space, aiding scientists in understanding how to mitigate their effects on future missions to the Moon and Mars.

• HERMES – This space weather instrument suite will operate on the exterior of Gateway, studying the nearby magnetic field, protons, electrons, ions, and lower-energy electrons of solar wind. Managed by NASA’s Goddard Space Flight Center, HERMES will be the first space weather monitoring platform on a crewed space station.
• ERSA – Located externally on Gateway, ERSA will play a critical role in ensuring astronaut safety by monitoring radiation exposure in Gateway’s orbit. Beyond its role in studying crew health, ERSA’s ionizing radiation instrument is essential for safeguarding electronics in space, as ionizing radiation can induce voltage spikes that may cause short-circuits and potential damage. ESA is developing and managing the ERSA instrument.
• IDA – Led by ESA with contributions from JAXA, this is an internal radiation instrument will assess how effectively Gateway shields its habitable volume from radiation. IDA’s data will be compared to external data from instruments including ERSA and HERMES to help evaluate crew risks related to cancer and the effects of radiation on the cardiovascular and nervous systems.

Learn More about Gateway

Discover the wonders of Gateway with our collection of educational resources.

Gateway Capabilities

Explore a snapshot of Gateway's extensive capabilities for sustained exploration and research in deep space.

Gateway News

Stay up-to-date with the latest Gateway news from NASA.

NASA is giving the moon its own time zone. Why the clock is ticking.

It’s going to start getting crowded on the moon, and we’ll soon need a precise lunar timekeeping system, a shared clock , to coordinate missions and prevent disasters.

Time moves a bit faster on the moon than on Earth because the moon's mass is smaller than that of Earth and its gravity is weaker. That means the moon is going to get its own time zone .

NASA has been asked to establish a unified standard time for the moon and other celestial bodies by the end of 2026. That request came from the White House – specifically the Office of Science and Technology Policy – on April 2.

"The same clock that we have on Earth would move at a different rate on the moon ," said Kevin Coggins, NASA's space communications and navigation chief, in an interview with Reuters.

The difference is slight: 58.7 microseconds per Earth day. (A microsecond is one-millionth of a second.) Microseconds are often used as measurements in electronics, computing and telecommunication applications.

On Earth, Coordinated Universal Time, or UTC , is the standard used to set all time zones around the world. Eastern Time is four hours behind UTC.

The new lunar time zone will be known as Coordinated Lunar Time, or LTC . It's not yet determined whether the moon will have one or multiple time zones.

Why do we need precise time on the moon?

That 58.7 microseconds may not sound like much to worry about. An eye blink, for example, takes 0.1 to 0.4 seconds , or 100,000 to 400,000 microseconds.

But the time discrepancy becomes crucial in space missions, which require perfect precision for communication, tracking and navigation.

Unable to view our graphics? Click here to see them.  

LTC is also needed for secure data transfers between spacecraft and for matched communication among Earth, lunar satellites, bases and astronauts, Reuters reported.

And because future missions will be multinational, all participants will need to be synchronized.

Nations have been using their own standard time in space missions. NASA, for example, used Mission Elapsed Time, or the amount of time from the moment of liftoff, to coordinate space missions.

Where will Coordinated Lunar Time be used?

LTC will be used in operations in cislunar space , the region around the Earth that extends to the moon and past its orbit. It includes:

• The lunar surface.
• Lunar orbits.
• Earth-moon Lagrange points.

Lagrange points are gravity balanced positions in space between two bodies – the Earth and moon, in this example – in which objects such as spacecraft can stay in position. Lagrange points are easy to reach, and many spacecraft use them to perform observations , Space.com says.

Will the moon's time system be similar to that of Earth?

It could be. Earth's UTC uses a group of 450 highly precise atomic clocks in 85 time laboratories around the world. The Office of Science and Technology Policy directive suggests "an ensemble of clocks" on the moon could be used to set up the moon's LTC.

The White House directive also says LTC must have:

• Traceability or a calculated connection to Earth's Coordinated Universal Time.
• Specificity and accuracy to measure very short spans of time, necessary for precise scientific study and spacecraft landings.
• Self-sufficiency , in case connection with Earth is lost.
• Scalability , so other celestial objects or space environments could use it.

NASA will work with the departments of Commerce, Defense, State and Transportation to create and implement the lunar time zone.

NASA also will need to consult the 36 nations that have signed the Artemis Accords , an agreement that spells out countries' activities in space and on the moon.

China and Russia, the two main U.S. rivals in space, have not signed the agreement.

SOURCE USA TODAY Network reporting and research; NASA; Office of Science and Technology Policy; European Space Agency; space.com ; itu.int; Florida Today; Center for Strategic and International Studies

Why does NASA want a time zone on the moon?

NASA has been tasked with determining a standard time zone for the moon, but it’s more complicated than you might think.

The United States government has tasked its space agency, NASA, with establishing a standard time zone for the moon, which will be known as Coordinated Lunar Time (CLT).

In a memo issued on April 2, the US Office of Science and Technology Policy (OSTP) stated: “Federal agencies will develop celestial time standardisation with an initial focus on the lunar surface and missions operating in Cislunar space [the area within the moon’s orbit], with sufficient traceability to support missions to other celestial bodies.” “Traceability” means that CLT can be kept in sync with time zones on Earth.

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The memo outlined the following features for the new CLT:

• Traceability to Coordinated Universal Time (UTC – a compromise for both English and French speakers);
• Accuracy sufficient to support precision navigation and science;
• Resilience to loss of contact with Earth (meaning CLT can operate independently of Earth); and
• Scalability to space environments beyond the Earth-moon system (meaning other space stations beyond the moon would be able to use CLT as well).

Don’t expect your favourite time zone and calendar apps to have CLT as an option yet; NASA has until the end of 2026 to establish CLT.

Why does the moon need its own time zone?

In layman’s terms, we need a reliable “lunar time” earth-syncing system because lower gravity on the moon causes time to move slightly faster there than on Earth – by just 58.7 microseconds (there are 1 million microseconds in a single second) faster within every 24 Earth hours.

This is not science fiction, even though it is a main feature of Hollywood blockbusters such as Interstellar. Known as “gravitational time dilation”, the passage of time is impacted by gravity.

Although small, these time discrepancies can cause issues with syncing satellites and space stations in lunar orbit.

An unnamed OSTP official told Reuters: “Imagine if the world wasn’t syncing their clocks to the same time – how disruptive that might be and how challenging everyday things become.”

How would we tell time on the moon?

Earth uses UTC or Coordinated Universal Time to sync time zones around the world. UTC is determined by more than 400 atomic clocks that are maintained in national “time laboratories” in about 30 countries around the world. An atomic clock uses the vibrations of atoms to achieve extreme precision in keeping track of time.

Similar atomic clocks would be placed on the moon to get an accurate time reading.

Known as Positioning, Navigation and Timing (PNT), this precision-timing system allows communications systems to measure and keep accurate timing. The Ordnance Survey, the British organisation that has been producing maps since 1791, explains that PNT has three core elements:

• Positioning – the ability to precisely determine one’s location and orientation, predominantly two dimensionally on a printed map, although three-dimensional orientation can be determined when required.
• Navigation – the ability to determine both the current and desired position (either relative or absolute), and apply corrections to course, orientation and speed to reach a desired position from anywhere in the world, from sub-surface (below the Earth’s surface) to surface, and from surface to space.
• Timing – the ability to maintain accurate and precise time from anywhere in the world.

Does NASA have plans for time zones in other parts of outer space?

Although there has been no mention of time zones on other planets, in 2019, NASA’s Deep Space Atomic Clock (DSAC) mission tested an atomic clock to improve spacecraft navigation in deep space. The DSAC mission, on SpaceX’s Falcon Heavy rocket, was launched on June 22, 2019. The rocket tested the atomic clock in Earth’s orbit for one year.

Typically, spacecraft keep accurate time by bouncing signals to atomic clocks on Earth and then the signal is sent back to the spacecraft. In this mission, the on-board atomic clock was tested to keep precise time without relying on this two-way communication between the spacecraft and the atomic clocks on Earth. The accuracy of the timing is tied to getting accurate positioning, while helping the spacecraft reach the intended location in space successfully.

As NASA’s Jet Propulsion Laboratory, the centre for robotic exploration of the solar system, explains: “A two-way system that sends a signal from Earth to a spacecraft, back to Earth and then to the spacecraft again would take an average of 40 minutes. Imagine if the GPS on your phone took 40 minutes to calculate your position. You might miss your turn or be several exits down the highway before it caught up with you. If humans travel to the Red Planet [Mars], it would be better if the system was one-way, allowing the explorers to immediately determine their current position rather than waiting for that information to come back from Earth.”

The mission successfully ended in 2021, with the on-board atomic clock maintaining the correct timing and navigational positioning.