Last month, President Donald Trump signed a bill into law giving NASA $19.5 billion to spend on a variety of projects in the 2018 budget year, including planning further exploration of the planet Mars.

From the Oval Office, the president said:

For almost six decades, NASA’s work has inspired millions and millions of Americans to imagine distant worlds and a better future right here on Earth. I’m delighted to sign this bill. It’s been a long time since a bill like this has been signed, reaffirming our national commitment to the core mission of NASA: human space exploration, space science and technology.

But while NASA’s budget will aid the agency’s long-term plans for a manned exploration of Mars, it also halts the space agency’s Asteroid Redirect Mission, Scientific American reported last month.

Walter Myers/Stocktrek Images via Getty Images In this illustration, a manned asteroid lander is docked to a combination Deep Space Vehicle and Extended Stay Module on a mission to a deep space asteroid. In the background is the asteroid destined for exploration.

That project would have included a robotic attempt to rendezvous with an asteroid, then collect and haul a giant boulder from it for future study by a manned crew. Part of this plan was also to redirect the asteroid’s trajectory away from any path that would bring it close to Earth.

But Trump’s budget, which emphasizes Mars exploration, doesn’t leave enough funding for this project. HuffPost reached out to NASA to ask how the space agency feels about not having enough funds for the Asteroid Redirect Mission.

“We remain committed to the next human missions to deep space, but we are not pursuing the Asteroid Redirect Mission with this budget proposal,” a NASA spokesperson told HuffPost. “However, we will continue to work on the needed technologies, such as solar electric propulsion, which will advance future in-space transportation needs.”

graphics.vp via Getty Images Artist rendering of asteroids in space.

Sometimes referred to as “minor planets,” asteroids are, for the most part, irregularly shaped ― some nearly spherical ― rocky space rubble that originated from the early creation of our solar system more than 4 billion years ago.

Most known asteroids orbit the sun in an area of space known as the asteroid belt, a doughnut-shaped mass between Mars and Jupiter. These cosmic pieces of rock run the gamut between 33 feet and more than 600 miles in diameter. They don’t just travel around the sun in circular orbits ― they also rotate and tumble as they move through space.

Those attributes would present interesting challenges to any manned mission with the goal of actually landing on one of these constantly moving minor planets.

There have already been several unmanned missions and flybys to asteroids.

The Japanese Hayabusa spacecraft landed on a near-Earth asteroid, Itokawa, in 2005, to collect small samples and return them to Earth for study by scientists.

NASA’s OSIRIS-REx spacecraft was launched last September en route to the asteroid Bennu to scoop up some rock samples and bring them home in 2023, with the hope that the material will provide important clues of how our solar system formed and evolved.

But what about a manned mission to an asteroid? In a 2013 Science Direct technical paper, Purdue University researchers explained how this could possibly come to pass for a landing on the largest known asteroid, Ceres, using “a nuclear electric propulsion mission.”

“It’s so large, it has enough of a gravitational field to pull itself into a round shape, unlike most other asteroids, which just look like potatoes and funny-shaped rocks,” Purdue University aerospace engineer James Longuski, told National Geographic in 2013.

Longuski’s Science Direct aerospace engineer co-author Frank Laipert agreed that Ceres is an important target for a manned mission. “There’s a lot we could learn about the birth of the solar system from Ceres, since it’s essentially a large leftover from the solar system’s formation,” he told National Geographic. “And a human could be a lot more effective as a scientist on Ceres than a robotic probe.”

Still, there’s another intriguing reason for humans to check out Ceres, according to National Geographic:

Ceres may have vast amounts of frozen water beneath its crust. Some researchers even think Ceres may have an ocean of liquid water under its surface, potentially making it of interest to scientists looking for signs of extraterrestrial life, since there is life virtually wherever there is liquid water on Earth.

But it comes down to no money, no mission. Maybe 2019’s budget will be different.