WATCH: SpaceX Falcon Heavy rocket blasts off from Kennedy Space Center

CAPE CANAVERAL, Fla. – A NASA spacecraft with “green propellant” and a space atomic clock launched early Tuesday on the third flight for SpaceX’s Falcon Heavy rocket. 

The liftoff happened at 2:30 a.m, three hours after the launch window opened at the Kennedy Space Center’s Launch Complex 39A. 

 The middle-of-the-night rideshare also features a solar sail and human ashes, including an astronaut's.

The two side boosters on the Falcon Heavy successfully touched down back on the ground several minutes later. The center booster was to land on a barge ship in the Atlantic, but mission mangers say the landing failed. 

The 24 payloads include missions with the Department of Defense’s Space Test Program-2 (STP-2), for NASA, NOAA, the U.S. Air Force and multiple research institutions.

What time is it? Space time

Atomic clocks, which use mercury ions to measure time, are not a new concept, but ground-based atomic clocks are bulky and not easily transportable. For the last 20 years, scientists at NASA's Jet Propulsion Laboratory have been developing a deep space atomic clock to utilize this technology for space exploration.

A test version of the Deep Space Atomic Clock will launch with STP-2 to try out this concept for one year.

The spacecraft is about the size of a loaf of bread and uses less than two cans of tuna worth or mercury ions, according to NASA.

NASA says the atomic clock can eliminate spacecraft time errors down to 1 microsecond per decade. 

The mission will benefit future spacecraft navigation accuracy and radio science.

"This new capability could forever change the way we conduct deep-space navigation -- by eliminating the need to 'turn signals around' for tracking," according to NASA.gov.

The space fuel of the future

NASA and Ball Aerospace & Technologies Corp. have developed a fuel for spacecraft that could be more efficient-- resulting in longer missions -- and environmentally friendly.

The Green Propellant Infusion Mission, or GPIM, will also reduce hazards with fueling spacecraft and cut down the amount of ground processing time to launch, according to NASA.

"We are increasingly reliant on satellites for communications, for monitoring weather and conditions on Earth and for exploration of the universe," NASA's Space Technology Mission Directorate associate administrator Steve Jurczyk said in a news release. "It's important that we develop technology that increases protections for launch personnel and the environment, and that has the potential to reduce costs."

When NASA announces the end of a planetary science mission -- think Cassini at Saturn or Messenger at Mercury -- it's typically because the spacecraft has run out of fuel.

The SmallSat launching this week will test the new "green" propellant, which will provide a safer alternative to toxic chemical propellants currently used, and which, in turn, could mean longer duration missions for spacecraft in deep space.

[LISTEN BELOW: WKMG's Emilee Speck discusses the upcoming SpaceX launch on Supercluster's podcast]

More launch facts

Bill Nye's nonprofit The Planetary Society is launching its second mission with Falcon Heavy.

LightSail 2 is a small spacecraft, known as a CubeSat, which will further test a propulsion system using a large sunlight-powered sail. The second mission follows LightSail 1. The Planetary Society raised $7 million through donations and crowdfunding to make LightSail-2 possible.

The concept of solar sailing dates back to Carl Sagan, the late science communicator who co-founded The Planetary Society. Sagan even showed off a light sail model on "The Tonight Show with Johnny Carson" in the 1970s.

The landing and recovery

This is the first time SpaceX's heavy lift rocket will launch on boosters previously used on another Falcon Heavy.

SpaceX also may attempt to recover the rocket's nose cone, or fairings, in which the payloads are enclosed. Another boat called GO Ms. Tree has a giant net to catch the $6 million rocket nose cone.