In most scientific disciplines, if we want to examine an object closely and in-depth, we can collect some kind of sample specimen of that object, take it to our laboratories, and perform any number of direct analysis examinations of that specimen. For the most part, in astronomy we can’t do that; we are usually restricted to examining objects from far away, usually by examining the light we receive from them. The one exception to this comes from meteorites, where, in essence, the objects have come to us, and indeed, as covered in previous “Special Topics” presentations, we have learned quite a bit about various “small bodies” in our solar system, and even a little bit about the moon and Mars, as a result of laboratory analysis of meteorites that have been collected.
With meteorites, of course, we are restricted to whatever nature decides to send our way; if we want samples from anywhere else, we have to go out and collect them somehow. The one object where we have done so is the moon: the moon-landing Apollo missions in the late 1960s and early 1970s returned over 382 kg of lunar samples – most of which is still in storage and remaining to be examined – and three unmanned Luna probes (Luna 16, 20, and 24) from the then-Soviet Union during the early- to mid-1970s returned an additional 300 grams of lunar samples. Just last week China launched its Chang’e 5 mission for the collection of lunar samples and, if successful, it is expected to return those shortly after mid-December. Meanwhile, the Perseverance rover launched this past July is planned as the first step in a multi-mission joint NASA/ESA endeavor to collect Martian samples for return to Earth towards the end of this decade.
As of now, there have been four spacecraft missions that have been flown with the intent of collecting and returning samples from the solar system’s “small bodies.” The first of these was NASA’s Stardust mission, which was launched on February 7, 1999, and which flew through the coma of Comet 81P/Wild 2 – the Week 1 “Comet of the Week” – on January 2, 2004. By means of an extremely lightweight and porous substance called “aerogel” Stardust collected over one million samples of dust particles from Comet 81P’s coma, and then returned these to Earth on January 15, 2006. The Stardust mission as a whole is discussed in a previous “Special Topics” presentation, and specific results from the analyses of the returned samples are discussed in Comet 81P’s “Comet of the Week” presentation.
The second mission was JAXA’s Hayabusa mission, which was launched on May 9, 2003, and which arrived at the Apollo-type asteroid (25143) Itokawa in September 2005. Hayabusa was beset by various difficulties but during one of its brief landing attempts managed to collect approximately 1500 grains (with a total mass of slightly less than one gram) of surface material, which it eventually returned to Earth on June 13, 2010. The Hayabusa mission as a whole is discussed in a previous “Special Topics” presentation.
The remaining two sample-collection missions are going on at this time. The first of these is JAXA’s Hayabusa2 mission, which was launched from the Tanegashima Space Center in southern Japan on December 3, 2014. Hayabusa2’s destination was the Apollo-type asteroid (162173) Ryugu, which was discovered on May 10, 1999, by the LINEAR program in New Mexico (and later named for the Dragon Palace from Japanese folk tales); it is a roughly spherical object approximately 1 km in diameter and is a dark-colored primitive object somewhat similar to the carbonaceous chondrite meteorites (discussed in a previous “Special Topics” presentation). Hayabusa2 arrived at Ryugu on June 27, 2018, and went into orbit around it.
Although not every aspect of Hayabusa2’s planned mission profile has gone as planned, overall it has been distinctly more successful than its predecessor. On September 21, 2018, Hayabusa2 successfully deployed two small MIcro-Nano Experimental Robot Vehicle for Asteroid (MINERVA) rovers onto Ryugu’s surface, and two weeks later deployed the larger German-built Mobile Asteroid Surface Scout (MASCOT) rover as well. All three of these performed as expected, with several scientific instruments aboard MASCOT performing detailed scientific investigations. Meanwhile, Hayabusa2 itself briefly touched down twice onto Ryugu’s surface, once on February 21, 2019, to collect surface samples, and again on July 11, 2019, to collect sub-surface material that had been exposed by a previously-deployed impactor. Both of these operations were successful and a third touchdown and sample collection maneuver was deemed not necessary and thus was cancelled.
Hayabusa2, with its collected material samples, left Ryugu on November 13, 2019, and is due to arrive back at Earth next week on December 6. Upon doing so it is expected to release the capsule containing the sample materials, which will then land at the Woomera Test Range in South Australia where it can then be retrieved and the samples subsequently sent out for analysis. There is enough propellant left in the main spacecraft such that after it passes by Earth it will be sent towards a high-speed flyby of the Apollo-type asteroid (98943) 2001 CC21 in July 2026 and a rendezvous with another Apollo-type asteroid, 1998 KY26, in July 2031.
The other current mission is NASA’s Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) mission, which was launched from Cape Canaveral, Florida on September 8, 2016. OSIRIS-REx’s destination was the Apollo-type asteroid (101955) Bennu, which was discovered by LINEAR on September 11, 1999; following a worldwide naming contest, it received the name of the Egyptian bird-shaped deity that was suggested by a 9-year-old student from North Carolina, Michael Puzio. Like Ryugu, Bennu is also roughly spherical in shape, but is somewhat smaller, being approximately 500 meters in diameter; it, too, is a dark-colored and primitive object.
OSIRIS-REx arrived at Bennu on December 3, 2018, although it did not go into orbit around it until the very end of that year. OSIRIS-REx contains a suite of scientific instruments that have been carrying out a wide range of investigations of Bennu, and among one of the earliest findings was the presence of water-related substances on Bennu’s surface in the form of hydrated clays, a result consistent with the results of some recent ground-based studies. In early 2019, when Bennu was near perihelion, OSIRIS-REx detected several comet-like eruptions of dust grains and larger particles off Bennu’s surface, indicating that Bennu can be considered one of the “active asteroids” that are the subject of next week’s “Special Topics” presentation.
One of the primary elements of the OSIRIS-REx mission is the collection of samples from Bennu’s surface, a task which turned out to be somewhat more difficult than expected due to the fact that the surface contains a large number of boulders and not much in the way of “smooth” areas. Four potential sites were identified in August 2019, with the final selection of the site dubbed “Nightingale” being made in December. By means of a Touch-and-Go Sample Acquisition Mechanism (TAGSAM) device, wherein short bursts of nitrogen gas were fired at the surface, OSIRIS-REx successfully collected well over 200 grams of surface material this past October 20.
OSIRIS-REx is expected to remain in the vicinity of Bennu for a few more months, with a current planned departure date of March 3, 2021. Arrival at Earth is expected to occur on September 24, 2023, at which time the capsule containing the acquired samples will be deployed into the atmosphere, with touchdown expected at the Utah Test and Training Range west of Salt Lake City. From there, the retrieved samples will be disbursed to various laboratory facilities for analysis.
At this time there is one additional “small bodies” sample return mission in development, JAXA’s Martian Moons Exploration (MMX) mission. As currently planned, MMX would be launching in September 2024, and after arrival at Mars orbit the following year would attempt one or two sample-collection landing maneuvers on Phobos, with the goal of collecting up to 10 grams of material. Afterward, MMX would perform several flybys of Deimos before leaving Mars orbit in August 2028 for arrival back at Earth in July 2029.
Incidentally, (162173) Ryugu will be passing 0.061 AU from Earth this coming December 29, although it will remain a relatively faint object of 17th magnitude. Meanwhile, (101955) Bennu, which passed 0.015 AU from Earth a week and a half after its discovery, has been identified as a Potentially Hazardous Asteroid (in the full sense of that term), and can make very close approaches to Earth on occasion, including one of 0.005 AU on September 23, 2060. The possibility of an Earth impact at some point in the future after that date remains a possibility, although numerical simulations indicate a more likely fate of impacting the sun someday. If an Earth impact does happen to become a realistic scenario someday, perhaps the information gleaned from OSIRIS-REx will help in determining the appropriate actions humanity will need to take.