Picture: An artist’s interpretation of the OSIRIS-REx spacecraft rendezvousing with the asteroid Bennu. Image credit: NASA
One of the challenges we face in astronomy is the fact that we can’t touch the things we study. In chemistry, you can run experiments by mixing chemicals together. In biology, if you want to study mice, you catch (or order) a bunch. If you’re a sociologist, you can go poll thousands of people.
But when studying astronomy, we’re limited by the vast distances between us and what we study. Most of the time, the only way we’re able to get information about objects in space is by studying the light they emit, absorb, or reflect. Within the Solar System, we’re at least able to send the occasional rover to Mars or orbiter to Saturn (RIP Cassini). And sometimes, when the funding aligns, we can send a sample-return mission.
Sample-return missions are missions which collect samples from something in space, then return those samples to the Earth. Samples are very useful to scientists because they allow us to examine extraterrestrial material more closely. While probes we send to other objects are able to accomplish a large amount of science, they are limited by the instruments we are able to pack onto them. By bringing samples back to the Earth, we can run more experiments on them, using more powerful instruments.
The most famous sample-return missions are the Apollo missions. The six manned moon missions returned 382 kilograms of moon rock and dust to the Earth. Although we would like to send astronauts all over the Solar System, such missions are complicated and costly. So we mostly rely on robotic lackeys to explore for us. Over the past few decades, several space probes have been sent to bring samples back to Earth, with more planned in the future. Here are some of the few sample-return missions we’ve conducted.
The Luna Missions
During the Space Race, the USSR sent multiple probes to the moon as a part of the Luna program. While any of these probes failed, they did accomplish some firsts, including the first man-made object to land on the moon, first pictures of the far side of the moon, and the first robotic sample return. Luna 16, Luna 20, and Luna 24 returned a total of 326 grams of moon dust to the Earth. These three probes were each equipped with a drill, a hermetically-sealed container, and a rocket to return the samples to Earth.
Stardust was a probe sent to examine the comet Wild 2. As a part of this project, it collected material from the comet’s coma, a large cloud of gas and dust which surrounds its nucleus. This material was captured using aerogel. Aerogel is an ultralight material which was used to slow down the particles without damaging them. This aerogel also collected about 45 grains of interstellar dust. Some of the particles returned by Stardust indicate the presence of liquid water on the comet. Researchers have also found an amino acid called glycine in the sample. Amino acids make up the basis of life, so this discovery supports the idea that the makings of life are relatively common in space.
Run by the Japanese Aerospace Exploration Agency (JAXA), the Hayabusa probe (“Peregrine Falcon” in English) was the first spacecraft to make contact with an asteroid. Flying to the asteroid Itokawa on experimental ion engines, Hayabusa made contact in November 2005. Despite a few setbacks, Hayabusa returned about 1,500 grains of material from Itokawa.
Astronomers like to study asteroids, because they provide us with a snapshot of what the Solar System was like during its formation. The rocks in asteroids have not been subject to the same weathering processes that rocks on planets experience. This means that they have remained largely unchanged since they were formed.
Hayabusa has been succeeded by Hayabusa2, which is scheduled to reach the asteroid Ryugu in 2018. Once it arrives, it will orbit and survey the asteroid for a while. Then it will release a small shaped charge to create an artificial impact crater. The probe will collect a sample from the crater, to get material which has not been weathered.
The Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (astronomers are bad at acronyms), was launched in September 2016. Recently, it completed a gravitational assist Earth flyby to change its direction and increase its velocity. During this flyby, it took an image of the Earth with its primary camera. This flyby also allowed operators to test the functionality of its instruments. It is scheduled to arrive at the asteroid Bennu in 2018. Bennu is an asteroid known as a carbonaceous chondrite, a remnant from the formation of the Solar System. Its orbit also intersects with the Earth’s, and Bennu will possibly impact the Earth sometime in the 22nd century. By sending OSIRIS-REx to study this scientists hope to learn several important pieces of information:
- Bennu could provide clues as to how the Solar System formed, and how life began on the Earth.
- Understanding Bennu’s makeup could help us prepare a deflection mission to redirect the asteroid away from the Earth.
- Asteroids like Bennu could be sources of useful resources like water, iron, and precious metals.
So far, we’ve collected samples from the Moon, some asteroids, a comet, and a the solar wind. But one of our large goals, which is more complex than any previous sample-return missions, is to get samples from Mars. Asteroids and comets are relatively easy to get samples from, since a probe can collect the samples from orbit. But Mars has a much higher gravity, so to get samples back from Mars, they have to be launched back to the Earth.
Currently, NASA is hoping to collect samples of Martian materials using the Mars 2020 rover (it doesn’t have a cool name yet). The rover will identify locations of interest, collect samples, then store those samples to be picked up by a later return mission. If we succeed in this, we will finally have a chance to examine another planet in the lab.
In order to get the samples back to Earth, usually the probe has to be put on an orbit to intercept our planet. The samples are loaded into a special return capsule (shown below). This capsule is designed with a very hardy heat shell, to survive the reentry process. When the probe gets close enough to the Earth, it releases the capsule, which plummets to the ground. Airbraking and parachutes are used to ensure that the samples survive the landing.
But the mission doesn’t stop when the samples hit the ground. Once the capsule has been recovered, the samples need to be handled very carefully. Since these samples have been brought from space, extreme care is used to make sure that they are not contaminated by terrestrial materials. If Earth dust were to accidentally get into the sample, this would result in inaccurate measurements. Contamination protection goes both ways. Scientists aren’t sure what exactly will be coming back, so precautions are taken to prevent the possibility of releasing extraterrestrial life (if it exists) into Earth’s biosphere. This is the same reason the Apollo astronauts were quarantined for several weeks upon their return from the moon. This policy is known as planetary protection, and warrants its own discussion later.
The samples are transported to special storage facilities, which have been prepared to keep them in extremely clean environments. This way, scientists can study them without worrying about contamination. Most of NASA’s samples are stored at Johnson Space Center, in Houston, Texas. Once the samples have been examined, they are stored in as pristine a condition as possible. This includes keeping them at near-vacuum pressures and specific temperatures. Scientists across the world can request to have samples to conduct research. Some samples are used for education and outreach, and are loaned to schools and colleges.
Sample-return missions offer scientists a way to get their hands on that which we can usually only look at. Since we don’t always know what we’ll find when we send a probe to some object, we can only guess at what instruments we’ll need on it. But bringing samples back allows us to run tests in the comfort of our own labs, with whatever equipment we need. Hopefully one day we’ll be able to walk around on the surfaces of other planets, but for now, our robots will have to do it for us.