Nasa launched the Osiris-Rex spacecraft from the Kennedy Space Center in early September on a seven-year mission to collect samples from the surface of the asteroid Bennu and bring them back to Earth. It is a sixty million metric ton rock with an Earth-crossing orbit places it on the list of more than 1,700 potentially hazardous asteroids. Bennu is also a remnant of the Solar System’s earliest days and could contain the kinds of organic molecules and amino acids that allowed life to evolve on Earth.
“The asteroids that we selected [for Target Asteroids!] will help us to understand the observations that we’ve already made of Bennu,” Hill explained. “We have a list of asteroids that… are analogues to Bennu. Some of them are near-Earth asteroids some are Main Belters. We generally select asteroids where not very much is known about them or we need more data. The amateur observations can perhaps help us untangle some of that information.”
More than three hundred and fifty people in forty-seven countries have sign up with Target Asteroids! “It’s a great distribution,” Hill said. “Our most active observers are not nearly that many of course. We have people who have been extraordinarily dedicated in their observations. Probably a third of them have provided regular observations. We have one fellow who was an AAVSO variable star observer and happened to see our website and thought well, the skill set is the same for asteroids the only difference is they’re moving and got hooked. He is now one of our best observers.”
Collecting these data requires skill and equipment beyond the casual stargazer. “It is a pretty high level program,” Hill admits. “It’s not just looking at something.” The minimum setup can easily cost several thousand dollars. The telescope itself must be at least eight inches in diameter to collect the asteroids’ faint light. A motorized mount must hold the telescope on target by compensating for the Earth’s rotation. Astronomical cameras and filters convert that light into the kinds of data that will help scientists study asteroids.
“You don’t necessarily need a large telescope,” Hill explained. “Larger aperture is wonderful but we’ve found that the people who tend to submit the most observations are the ones with the smaller telescopes not the giant ones. So just get out there and observe what you can when you can.”
Even without a telescope, amateurs have several ways to contribute. “We encourage people to get together with their local astronomy club that might have a club observatory,” Hill explained. "In addition there are telescope services [like] our fantastic partners iTelescope [and] Sierra Stars Observing Network. They have telescopes all over the world. Anyone can contact them and say ‘I would like to observe these objects’. You select the telescope that would be best suited for them. And they will arrange the images.”
Target Asteroids! offers another program that lets amateurs work with data from professional observatories through the International Astronomical Search Collaboration. “Fifteen teams of three individuals actually receive images from world-class observatories such as the Pan-Starrs observatory in Hawai’i and the Catalina Sky Survey here in Tucson. They receive those images and both scan them for new asteroids and measure the astrometry and photometry of known asteroids.”
The economic constraints that limit professional scientists help drive amateur space exploration. Making giant leaps is so expensive that the professionals cannot afford to do everything. That frees amateurs to make the lower-priority scientific observations. Searching for asteroids was a low priority until the world watched a comet slam into Jupiter. Now multi-million dollar sky surveys like Pan-Starrs and the Catalina Sky Survey that make almost all of the asteroid discoveries.
“Although it is much more difficult to compete in the arena of discovery of new asteroids,” Hill explained, “there are two important ways amateurs can contribute. When surveys discover a new asteroid, that’s all they do. They discover it, they confirm that it was a real object, and then they rely on the follow up observers. But most of the asteroids are just discovered and then nothing else is done with them after that. They are either too faint or professionals don’t have enough observing time to look at each and every one. [Amateurs] can make huge contributions in the follow up work that is so desperately needed when a new asteroid, a new near Earth object in particular, is discovered.”
These observations, called astrometry, map an asteroid’s position against a backdrop of stars. Over time the asteroid moves in its orbit and changes position relative to the stars. As tens and then hundreds of observations come in, scientists reconstruct the asteroid’s orbit around the Sun. Amateurs can make two other kinds of observations. Photometry measures the way sunlight reflected off the tumbling asteroid’s surface varies. Scientists use those data to learn about the asteroid’s size, shape, and other physical features. Spectroscopy measures the spectrum of reflected light to give scientists an idea of the asteroid’s surface composition.
Hill explained how photometric and spectroscopic observations comprise the second important contribution amateurs make to asteroid science. “They can assist with characterization of the asteroids. Even though there are more than 710,000 asteroids known, we actually know the details about very very few. So the more continuous observations amateurs [make], if it is done on a long enough time period, astronomers can determine the size of the object, a little bit about the taxonomy of the object, a little bit about the inherent albedo of the object. All of these things together can help us characterize that asteroid and if it might belong to an asteroid family.”
Hill’s day job as a meteoriticist at the Lunar & Planetary Laboratory lets her hold pieces of the entire Solar System. Collisions between asteroids and even between asteroids and planets scatter fragments into space, some of which survive the fiery path through Earth’s atmosphere to land on the surface - sort of nature’s own sample return missions. Scientists like Hill analyze meteorites collected from the arid deserts of Australia, Africa, and Antarctica. They then try to match the meteorites’ structures and compositions with telescopic asteroid observations to reconstruct the Solar System’s formation and evolution.
“As a meteoriticist it has been very frustrating to be able to analyze the meteorite and tell you so many details about it but - with very few exceptions - I can’t tell you exactly where it came from,” Hill explained. “An asteroid in the sky from the Earth is just a dot of light. The spectra are necessarily integrated over that entire spot. So we have a difficult time discerning whether that asteroid is homogeneous or not. There are many different mineral combinations that can produce that spectrum. But we don’t know which one is the correct one. In addition, the spectra are a little bit different between asteroids and meteorites. We think we understand why but we don’t know for sure. There are effects of space weathering that can affect the spectra of asteroids. Do impact melts look different from fresh broken surfaces? We just don’t have enough data points.”
A few of those data points arrived when the first asteroid sample return mission, Japan’s Hayabusa, brought back 1,500 particles from the asteroid Itokawa. The follow-on mission, Hayabusa 2, will bring back several times as many particles from Ryugu, an asteroid similar to Bennu. “We don’t know if Ryugu and Bennu were ever part of the same parent body or not,” Hill said.
Osiris-Rex will collect as much as two kilograms of surface material from Bennu. That giant leap will revolutionize the study of asteroids and meteorites. “All of the Target Asteroids! observations and the analyses of the Bennu samples when they return will provide a tremendous ground truth check on astronomical observations,” Hill explained. “Once we get the sample in hand we will be able to make those kinds of calibrations. So this mission is a real pathfinder that will help us understand those connections much much better.”
The amateurs’ observations also will help scientists conduct research well into the future. The Minor Planet Center is the world’s archive for asteroid observations which it uses to keep orbital calculations current. Scientists use the MPC data to conduct their own research, and space agencies use the data to develop future asteroid missions.
Amateur astronomers who contribute to Target Asteroids! also send their data to the MPC which has a long history of working with the amateur community. “When people submit these images,” Hill explained, “I liken it to Tycho Brahe and [Johannes] Kepler. Tycho Brahe beautifully made very precise careful observations and he didn’t know what they would ever be used for in the future. Kepler was able to rely on the care that he took [to derive the laws of planetary motion]. So we’re doing a similar thing. We’re compiling an archive of data that will be used in the future. People can really leave a legacy of observations behind that will live on. Your observations can take [an asteroid] from just a dot of light [in the sky] to a world that we can imagine and think about and travel to someday.”
pdate 10/18 - Corrected Tyco to Tycho. Changed geographical distribution from 33 countries to 47 countries.