Why the PACE team is nocturnal this week

A tall building is centered in the image. It has an American flag facing downwards on the left side of the building and the NASA meatball logo on the right side. The building sits right on the horizon. Behind the building is a blue sky that is darkening after the sunset. At the horizon the sky is still slightly orange. Below the building in the foreground of the image is a lake which is reflecting the building and sky off of it.

Editor’s note, Feb. 6, 2024: NASA and SpaceX stood down from PACE’s Feb. 6 launch attempt due to unfavorable weather conditions. Launch is now targeted for 1:33 a.m. EST Wednesday, Feb. 7.

Editor’s note, Feb. 7, 2024: NASA and SpaceX stood down from PACE’s Feb. 7 launch attempt due to unfavorable weather conditions. Launch is now targeted for 1:33 a.m. EST Wednesday, Feb. 8.

There’s a good reason why NASA’s PACE satellite is launching in the early morning hours. Late tonight, I’ll venture out in the chilly Merritt Island air to catch a glimpse of a historic sight. At 1:33 a.m., February 6, NASA is slated to launch the Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) satellite atop SpaceX’s Falcon 9 rocket.

Why so early in the morning?  The launch is timed to accommodate the satellite’s orbit around Earth.

PACE will be in a Sun-synchronous orbit, meaning it’s synced to always maintain the same position relative to the Sun. This also means it will cross Earth’s equator at the same local time for each orbit, and the angle at which the sun illuminates Earth will be consistent for each image that it takes. This allows scientists to collect consistent data.

“An Earth-observing satellite generally wants the Sun well overhead during observations,” said Scott Patano, flight dynamics system development lead for PACE at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

To get this level of lighting during its orbit, PACE wants the Sun to be almost behind it as it observes the Earth. Though if the Sun happens to be directly behind the satellite, there may be glare or reflections off the ocean, which isn’t ideal, especially considering one of its main purposes is to collect ocean data. To prevent glare, PACE will be slightly offset – not directly in front of the Sun. If you imagine the Sun is at the 12:00 angle, PACE will orbit at 1:00.

So why the 1:33 a.m. launch? The best answer to that question is… math. Really cool math. By launching south out of Florida on the dark side of the Earth, the math works out perfectly to get the satellite right into place on the approaching India as it crosses the equator for the first time on the daylight side of the Earth by 1:00 p.m. local time.

Centered in the image is a rocket, mid-launch. The launch is taking place at night, so all around the rocket and it's stand is black. The only light is coming from the fire emerging from the bottom of the rocket in an orange glowing color, illuminating a cloud of smoke coming from the rocket as well.
A previous night launch at NASA’s Kennedy Space Center in Florida. A SpaceX Falcon 9 rocket launches with NASA’s Imaging X-ray Polarimetry Explorer (IXPE) spacecraft onboard from Launch Complex 39A, Thursday, Dec. 9, 2021. Launch occurred at 1:00 a.m. EST. Photo Credit: (NASA/Joel Kowsky)

While some satellites launch first into a temporary orbit, before moving into their permanent position, PACE will be directly injected into its final orbit, “an effectively instantaneous launch,” said Joel Parker, flight dynamics lead for PACE at Goddard.

This leaves little wiggle room for the launch time: a mere 90-second window for the launch to proceed. A tense minute and a half for years of striking data.

So while I’ll be prescribing a late afternoon nap for myself, I know that when I wake up, I’ll be getting ready to see PACE rocketed up to its new home – where it will provide a stunning new view of ours.

Header image caption: The dark water of the turn basin at NASA’s Kennedy Space Center mirrors the night lights and the Vehicle Assembly Building and Launch Control Center, silhouetted against the post-sunset sky. Photo credit: NASA/Kim Shiflett

By Erica McNamee, Science Writer at NASA’s Goddard Space Flight Center