NASA’s Lucy flies past thousands of objects to help Earth’s gravity

Mission engineers will track NASA’s Lucy spacecraft nonstop as it prepares to fly near Earth on October 16 to use that planet’s gravity to position itself on a course toward the Jupiter Trojan bring asteroids.

But they’ll also be closely tracking something else: more than 47,000 satellites, debris, and other objects orbiting our planet. With a probability greater than 1-10,000 that Lucy will collide with one of these objects, the mission engineers will have to adjust the spacecraft’s trajectory slightly.

Although adaptation is unlikely and collisions are rare, the odds increase as the number of objects in Earth orbit grows, NASA experts say.

The International Space Station, for example, has avoided space debris 31 times since 1999, including three times since 2020.

“Low Earth orbit is becoming increasingly crowded, that has to be part of the consideration these days, especially for missions that fly low like Lucy,” said Dr. Dolan Highsmith, chief engineer of the risk analysis group for conjunction assessments at Goddard, NASA Space Center in Greenbelt, Maryland. The group determines the probabilities of collisions between NASA’s robotic spacecraft and Earth-orbiting objects. NASA’s Johnson Space Center in Houston does the same for manned spacecraft as the space station.

Launched on October 16, 2021, Lucy is on a 12-year journey to study several Trojan asteroids up close. It will be the first spacecraft to visit these remnants from the early Solar System, helping scientists refine their theories about how the planets formed 4.5 billion years ago and why they landed in their current configuration.

But Lucy still has a long way to go before she arrives with the Trojans in 2027. The forthcoming gravity assist is one of three the starship will rely on to propel itself to its goals in space.

When Lucy comes closest to Earth for her first gravity assist, she will be hovering 220 miles (350 km) above the surface. That’s lower than the station’s altitude and low enough that the spacecraft will be visible to the naked eye from Western Australia for a few minutes starting at 18:55 local time (10:55 UTC). On her way down, Lucy will fly through the crowded layer of Earth’s orbit overseen by the US Space Force’s 18th Space Control Squadron. The squadron helps NASA identify close approaches.

Engineers began collision analysis for Lucy a week before the spacecraft approached Earth. Starting the process earlier would render collision predictions useless, Highsmith said: “The further you forecast into the future, the more uncertain you are about where an object will be.”

Determining the positions of spacecraft as well as orbiting satellites and debris is challenging, especially when trying to predict the future. This is mainly because the sun plays an important role in pulling or pushing objects around, and future solar activity is difficult to predict. For example, the Sun’s activity – how much plasma and radiation it emits – affects the density of the atmosphere, and thus how much friction tugs at a spacecraft and slows it down.

So, the closer the collision rating is to the Earth flyby time, the better. NASA broadcasts Lucy’s whereabouts daily to the Space Force squadron. When the squadron determines that Lucy may be interfering with something, Highsmith’s group calculates the likelihood of a collision and works with the mission team to move the spacecraft if necessary.

With a mission this valuable, you really have to make sure you’re able to get out of the way in case it’s a bad day,” Highsmith said.

Lucy’s navigation engineers have two maneuver options ready in case the spacecraft needs to avoid an object. Both maneuvers require engine firings to accelerate the spacecraft, which is traveling at about 12 km per second. Each maneuver can increase Lucy’s next approach to Earth by 2 or 4 seconds respectively.

“That’s enough to avoid anything that might get in the way,” said Kevin E. Berry, Lucy’s flight dynamics team leader at NASA Goddard.