The moon is a pretty inhospitable place. So it’s been a long-standing head scratcher as to why rock samples from the Apollo missions contain amino acids, important precursors for the development of life. Scientists never really thought the amino acids were native—they almost certainly represent contamination—but had they been introduced during handling on Earth or had they rained down to the lunar surface from micrometeorites and space?
Last year, chemist Jaime Elsila of NASA’s Goddard Spaceflight Center was finally able to answer the mystery. By looking at different isotopes of elements in the amino acids, she saw telltale signs that the organic chemicals likely came from terrestrial contamination. The discovery speaks to the amazing longevity of scientific output from sample return missions.
“[Elsila] wasn’t born when the samples were collected,” said Jason Dworkin, Chief of Astrochemistry at Goddard. “And she answered questions beyond the capabilities of the Apollo scientists and engineers, using tools that had not yet been invented.”
Before sample return missions, the only way for researchers to get their hands on extraterrestrial real estate was to wait for a meteorite to fall to Earth, likely altering it and introducing all sorts of contamination. But since the 1960s, astronauts and robotic explorers have brought back pieces of the moon, sun, asteroids, and comets, providing important ground truth information that couldn’t be obtained in any other way.
Next month, NASA plans to launch its newest sample return mission—the Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx)—the agency’s first in more than a decade. The spacecraft will visit a near-Earth asteroid called 101955 Bennu, map it, and return as much as 2 kilograms of material. The rocks and regolith will answer important questions about the origins of our solar system and life on our planet. And they will establish a legacy; when new questions arise, future researchers will be to turn to these resources. “A sample is the gift that keeps on giving,” said Dworkin, who is also the Project Scientist for OSIRIS-REx. “For science now and generations to come.”
Compared to flyby missions and orbiters, there have been very few sample return missions in history. The earliest happened when Apollo astronauts cumulatively hauled back 382 kilograms of lunar rocks, soil, and drill cores over the course of six missions. In the 1970s, the Soviet Union added to this using robotic landers to obtain around 300 grams of lunar material. These treasures have been scrutinized for decades, resulting in thousands of journal articles. Scientists have performed radiometric dating on the samples, making the moon the only other body in the solar system where the geologic age of different rocks is known with precision.
Sample return missions provide exceptional and unique information.
In 2004, NASA brought back its first extraterrestrial samples since Apollo using the Genesis spacecraft. Genesis collected particles from the solar wind, a stream of charged particles blown off the sun’s surface, that have helped researchers determine how the solar system formed. Two years later, NASA’s Stardust spacecraft (which had actually launched before Genesis) returned captured grains from the tail of comet Wild 2 that proved the small body contained liquid water and a wide range of organic compounds.
A 1.5 mm long impact track of a meteoroid captured in aerogel exposed to space by the EURECA spacecraft. Credit: NASA
Japan has already sampled a few particles from an asteroid, 25143 Itokawa, with its Hayabusa mission and the follow-up, Hayabusa-2 is currently on route to its asteroid target. China intends to get in the sample-return game with its next lunar rover, Chang’e 5, launching next year. And someday in the not-too-distant future, NASA hopes to pull off a sample return mission on Mars. The successor to the Curiosity rover, set to launch in 2020, will cache rocks and soil for a subsequent mission to fetch.
Sample return missions provide exceptional and unique information. Orbital satellites can give researchers a rough idea of a planet’s surface composition but the details left out can be frustratingly huge. “If you look at the infrared spectrum of an apple and the orange, they’re basically the same,” said Dworkin. Actually getting a sample in hand and being able to run it through a bevy of testing gives scientists the chance to ask much more precise questions about its age and chemistry.
Even when sending a highly capable robot like Curiosity, which is essentially a mobile laboratory, there are many trade-offs. Instrument designs have to be frozen years before launch so that they can be ready in time, meaning that researchers can’t have state-of-the-art tools on another planet. Those selected must also be robust enough to withstand the rigors of space travel, and light and small enough to fit on a robot.
The newest sample return mission, OSIRIS-REx, is scheduled to reach asteroid Bennu in 2018 and spend two years extensively studying the surface. It will create three-dimensional topographic maps, take temperature readings, and photograph details of the asteroid at a microscopic level. All this will allow mission scientists to determine the most interesting place to pick up a sample. When it’s ready, OSIRIS-REx will swoop in and use a system that Dworkin described as “kind of like a reverse vacuum cleaner”—blowing a jet of nitrogen gas to stir up regolith from Bennu that will then be collected through an air filter.
Bennu is about half a kilometer across, roughly the size of the Empire State Building. Scientists think it is a small chunk broken from a larger body that formed 4.6 billion years ago, preserving a mineral record of the solar system’s earliest days. This information has been erased on Earth, whose oldest rocks have mostly been subducted into the planet’s interior. It’s therefore hard to know whether the early Earth was rich in organic compounds—a good environment for life to form in—or whether these chemicals rained down with comets and asteroids. If they came from space, then perhaps the same processes delivered organics to places like Mars or Europa.
Almost coincidentally, Bennu will provide researchers with another opportunity—to learn how to deflect an asteroid. There is a one in 2,700 chance that Bennu could crash into our planet in the late 2100s, making it one of the most hazardous known objects. The details of whether or not it will depend on its surface composition. As it flies through the solar system, the asteroid absorbs photons from the sun and then later remits them, creating what is essentially a low-intensity thruster that can push the object in one direction or another. Data from OSIRIS-REx will help arm future scientists and engineers with the information they need to predict its orbit and, if needs be, nudge it away from a collision course with Earth.
Three-quarters of the material OSIRIS-REx brings back will be archived for future use. Everything will be carefully tagged and labeled, and great care will be taken to future proof the archive. The spacecraft has been cleaned to modern requirements but it’s likely that future techniques will be much more precise and detailed. Therefore, samples of spacecraft materials, lubricants, adhesives, and other potential sources of contamination will be included so that researchers will hopefully be able to tell what part of the asteroid materials are pristine.
This documentation will greatly increase the scientific value of the mission, said Dworkin, because it’s unknown at this point what ideas future researchers will come up with. When you send a robot with instruments to another world, “you can only answer the questions that the mission was designed to answer,” he said. “But if you bring it back, you can answer questions the mission team hadn’t even thought of asking.”