Earlier this month, the White House released a report detailing the current status of planetary defense efforts in the United States. The report discusses the ways that US government agencies would respond to an anticipated or unanticipated impact event, and outlines the steps that they should take to mitigate the danger. Unfortunately, this is just a report—any policy changes or future missions will require further action from the President and Congress.
This was a heartening but also concerning admission ahead of Asteroid Day. I’m encouraged that America is taking planetary defense seriously. Protecting against asteroid and comet impacts is arguably the single most important thing that we can be doing in space right now. However, it has not been the primary or even secondary focus of any major space agency. Much of the work has been limited to university researchers and private-sector organizations.
On this day, the 100th anniversary of the Tunguska Event, I would like to discuss our progress in planetary defense. While NASA and other organizations have done an excellent job tracking near-Earth asteroids and assessing their threat to humanity, we are not dramatically closer to impact prevention capabilities. Nevertheless, there is reason to be cautiously optimistic. Let’s discuss the major ongoing and future missions to explore and detect asteroids that could potentially threaten Earth.
To begin: just this week, the Japanese Hayabusa2 spacecraft arrived at the asteroid 162173 Ryugu. For the next eighteen months, Hayabusa2 will orbit the 900-meter-wide space rock. In addition to orbital observations, Hayabusa2 will release several surface probes and retrieve samples for return to Earth in late 2020. These samples will include material taken from the interior of the asteroid, extracted by detonating a small explosive device on the asteroid’s surface.
Hayabusa2 performs surface sample extraction at Ryugu.
Hayabusa2 is primarily focusing on planetary science, with consideration for eventual resource extraction. However, understanding the structure and dynamics of near-Earth asteroids is essential for predicting their impact odds and planning redirection maneuvers if need be.
Asteroid 162173 Ryugu as imaged on June 26.
Planetary defense is a major motivation of the OSIRIS-REx mission, which is continuing on its way towards asteroid 101955 Bennu. Astronomers estimate that there is a small chance that Bennu may impact Earth in about 200 years, so studying the asteroid up-close will considerably refine their calculations.
(Before someone asks: OSIRIS-REx Principal Investigator Dante Lauretta wrote a blog post specifically outlining the damage that Bennu would cause if it were to impact Earth. Note that all recent estimates place the impact odds far, far below one percent.)
OSIRIS-REx launched in 2016 and will arrive at Bennu in August of this year. The spacecraft performed a burn last January to put it on a trajectory for an Earth flyby, which occurred on September 22, 2017. This maneuver put the spacecraft on an appropriate orbit to rendezvous with the small asteroid.
Like Hayabusa2, OSIRIS-REx is also a sample return mission. Allowing scientists to study asteroidial material in laboratories on the ground massively increases the information we can glean from them, because many of the most interesting measurements and experiments are simply too heavy, bulky, or complex to cram within an interplanetary spacecraft.
OSIRIS-REx performs surface sample extraction at Bennu.
Sample return is just one aspect of the OSIRIS-REx mission. In addition to the standard planetary science objectives, OSIRIS-REx will study the impact of non-gravitational forces on the orbit of a celestial body. Many of these forces are well-understood for conventional satellites, but the data for asteroids and comets is hazy when it exists. Characterizing these effects for objects like Bennu will go a long way towards predicting their orbits and thus the impact odds long-term.
Once we understand these risks, though, we need the capability to deflect any objects we find headed towards Earth. Here the progress has been less inspiring.
First, a mission is currently under development called the Double Asteroid Redirection Test, or DART. Planned to launch in December of 2020 or early 2021, DART will rendezvous with the asteroid 65803 Didymos in October of 2022. Didymos is actually a double asteroid, and DART with collide with the smaller moon when the asteroid is near its closest approach to Earth. Ground-based observers will measure the effect that the collision has on the moonlet, giving researchers some hard data on just how well the kinetic impact technique would work for asteroid deflection.
This is an interesting mission, both for its planetary defense implications and for its technology. DART is expected to be the first spacecraft to test the NASA Evolutionary Xenon Thruster (NEXT), and will sport the Roll-Out Solar Array (ROSA) design tested on the International Space Station in 2017. Unfortunately, there has been some uncertainty over whether this mission would actually fly. Though required by law, it was not included in the 2018 NASA Budget. The John Hopkins University Applied Physics Laboratory, however, has continued working on the project and as of April was discussing final design and fabrication later this year.
On a related note, the Asteroid Redirect Mission (ARM), which the mainstream science media so eloquently described as “giving the Moon a moon”, was recently cancelled in favor of pursuing manned Lunar missions. This is a questionable decision. ARM was a logical development conduit for key technologies in robotics, propulsion, and astrodynamics. All of these would have contributed greatly to manned exploration of the Solar System. Thankfully, much of this technological investment will be preserved, and may eventually pay dividends in planetary science and defense.
ARM grapples an asteroidial boulder to return to the Earth-Luna system.
Nevertheless, having a six-ton asteroid in Lunar orbit would have provided a compelling intermediate target for human spaceflight. (The Deep Space Gateway would naturally fill the role of an intermediate target, but I’ll save the discussion of its advantages and disadvantages for another day.) Landing is a much bigger challenge than just returning to cislunar space, and the scientific knowledge from returned samples would also have been tremendous.
Another important project has also faltered: the Sentinel Space Telescope. This mission was designed by the B612 Foundation to detect asteroids that Earth-based telescopes miss by putting a dedicated spacecraft in solar orbit near Venus. Sentinel would have launched on a Falcon 9 rocket in 2018 or 2019 and cost $450 million to develop.
This proved way too expensive for a niche non-profit. NASA provided some funding, but this ended in late 2015, and B612 was unable to raise the remainder. Retired astronaut Ed Lu confirmed that the project was dead last year.
Rendering of the Sentinel Space Telescope on-orbit.
Source: B612 Foundation
B612 is instead focusing on telescopes mounted to significantly smaller satellites. They will use a new technique called synthetic tracking to spot smaller asteroids than previously possible. Few and fewer potential “killer” asteroids remain to be discovered as better and better telescopes come online, so the overall emphasis is turning towards the smaller rocks. These can still be deadly: the Chelyabinsk meteor was only about 20 meters across but caused thousands of injuries and millions of dollars in damages.
If mounting space telescopes on small satellites sounds ambitious, consider that the Canadian Space Agency launched a comparable spacecraft called NEOSSat with similar goals in 2013. Small satellites are much more affordable for non-governmental organizations, in part because launch vehicle cost can be distributed over multiple missions—NEOSSat rode to orbit with six other spacecraft. An audit in 2013 estimated the program was significantly over-budget. Total cost: CDN$25 million.
NASA and the Jet Propulsion Laboratory are also pursuing a Near Earth Object Camera, which would join the fleet at the Earth-Sun L1 point. NEOCam was passed over for the latest Discovery Mission opening, though was still under development as of April under the aegis of NASA’s Planetary Defense office. Without a clear funding commitment, though, mission planners are severely limited in developing the program.
This is a bit concerning, because NASA has a Congressional mandate to detect 90% of near-Earth objects 140-meters and larger by 2020. Much of this work was completed by the Wide-field Infrared Survey Explorer during its extended mission. WISE discovered over 3000 asteroids and comets, including 262 near-Earth asteroids. 47 of these are considered potentially hazardous. Detecting the remainder will probably require new hardware, so I’m doubtful that NASA will meet the Congressional mandate. If current legislators are serious abut this mission, they need to allocate funds—and quickly.
Artistic rendering of the WISE spacecraft.
The probability that an undetected asteroid will hit Earth before that objective is met—on time or not—is low, but every day that passes risks another Chelyabinsk. Only once we know the threats can we take actions to mitigate them. Having the necessary technology ready-to-go would vastly improve our odds.
On the whole, I’m glad to see that the United States is taking planetary defense seriously, but there’s a lot of work left to do. Check back again next Asteroid Day to review the advances we make in the coming months.