The case for Apophis


OSIRIS-REx

NASA’s OSIRIS-REx spacecraft, depicted here at the asteroid Bennu, could have an extended mission visiting another near Earth asteroid, Apophis, when it flies by Earth in 2029. (credit: NASA/GSFC)


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On April 13, 2029—a Friday the 13th—the asteroid Apophis will pass remarkably close to the Earth, coming within 31,000 kilometers of the Earth’s surface, or closer than satellites in geostationary orbit. In late 2004, shortly after its discovery, astronomers projected at one point a 1-in-37 chance of a collision in 2029, but additional observations soon ruled out any impact. A small risk of an impact in April 2036 lingered for a few years, particularly if the asteroid passed through a narrow “keyhole” of space near Earth during its 2029 flyby (see “Sounding an alarm, cautiously”, The Space Review, May 31, 2005), but that, too, has since been ruled out.

With the near-term risk of an impact eliminated, Apophis has shifted from a threat to an opportunity. That 2029 close flyby makes the asteroid, several hundred meters across, an ideal target for studies by ground-based telescopes and radars. It also puts it in reach of spacecraft missions, including relatively small, low-cost ones.

“This is a really rare natural experiment,” said MIT planetary science professor Richard Binzel, one of the organizers of the “Apophis T-9 Years” workshop held earlier this month. (The event was originally scheduled for April in France, but delayed and moved online because of the pandemic.)

He compared the flyby to comet Shoemaker-Levy 9, fragments of which collided with Jupiter in 1994, providing both a scientific bonanza as well as greater awareness of impact threats. “We’re all grappling with what we can make out the experiment.”

“This is a really rare natural experiment,” Binzel said of the 2029 flyby.

Scientists discussed studying Apophis both as one member of the population of near Earth asteroids, and to support planetary defense. The close flyby, for example, will allow scientists to study any effects tidal forces have on the asteroid, altering its spin or disrupting its shape, if the asteroid is a “rubble pile” of smaller objects like some other small asteroids.

While some of that work could be done with ground-based observatories, there was a clear interest in developing spacecraft missions to take advantage of the close flyby. The meeting’s final day was devoted to presentations of various mission concepts to visit Apophis before, during, and after the close approach. The concepts, by teams in the US, Europe, and Asia, included cubesat-class spacecraft, a small lander based on the MASCOT spacecraft dropped onto the surface of the asteroid Ryugu during the Hayabusa2 mission, and even a larger spacecraft launched on a Space Launch System rocket to collect samples for return to Earth.

The prospect of several missions from different agencies all going to Apophis at the same time raised concerns that they might interfere or even collide with one another. Some attendees suggested there may be the need for the equivalent of an air traffic control system to avoid any conflicts.

One of the more intriguing Apophis mission options doesn’t require a new spacecraft at all. NASA’s OSIRIS-REx spacecraft will return to Earth in September 2023, carrying samples it collected from the surface of the asteroid Bennu last month (see “TAG, Bennu, you’re it”, The Space Review, October 19, 2020). While the sample return container lands in the Utah desert, the main spacecraft will fly by Earth and could be used for an extended mission.

Dante Lauretta, principal investigator for OSIRIS-REx at the University of Arizona, said one extended mission option would have the spacecraft perform a series of flybys that bring it back to Earth in April 2029, at the same time Apophis flies by. That would allow OSIRIS-REx to rendezvous with the asteroid later that month. Once the spacecraft reaches Apophis, he said, “we can stay there as long as we like.”

The spacecraft is in good condition and its instruments, developed for closeup studies of Bennu, could be trained on Apophis. “We’ve got this great payload that seems to be very healthy,” he said. “Having OSIRIS-REx there means we can provide extensive support for other ground- and space-based characterization efforts.”

While the flyby is still more than eight years away, planning for spacecraft missions is starting now. Part of that simply reflects the development schedules for spacecraft, particularly those seeking to study the asteroid ahead of the 2029 close approach. Even OSIRIS-Rex needs to think ahead: Lauretta said the project will need to prepare an extended mission proposal for NASA, likely in 2022, that would detail its plans for either an approach to Apophis or some alternative mission.

There are programmatic reasons as well, such as influencing the planetary science decadal survey that recent started, one whose scope includes planetary defense. NASA’s planetary defense program has grown dramatically over the last decade, from $4 million a year to $150 million, stimulated at least in part by the Asteroid Redirect Mission early in the decade and sustained even after ARM faded away. Much of that funding is supporting the development of a dedicated planetary defense mission, the Double Asteroid Redirection Test (DART), which will launch next July to fly to the asteroid Didymos and collide with its small moon, Dimorphos, to demonstrate the kinetic impactor approach for deflecting asteroids.

“Apophis’ close pass is a good conversation starter, but by itself is not enough to launch a mission,” Rivkin said.

DART will be followed by the Near Earth Object Surveillance Mission (NEOSM, pronounced “nee-awesome” by those involved with the project), an outgrowth of the NEOCam mission that was a finalist in the previous round of the Discovery program of smaller planetary science missions. NEOSM will fly a small space telescope to discover near Earth objects at infrared wavelengths (see “One scientist’s 15-year (and counting) quest to save Earth from asteroid impacts”, The Space Review, October 28, 2019).

If NEOSM launches around 2025, as currently proposed, it will open up funding in the planetary defense program just in time for a mission, or missions, to support the 2029 Apophis flyby. Some in the planetary defense community have seen an Apophis mission as a natural next step for a line of planetary defense missions.

But, some scientists cautioned, just because a mission to Apophis could be done doesn’t mean it should be done. “Resources for space exploration are much tighter than we would prefer,” said Andy Rivkin, a planetary scientist at the Johns Hopkins University Applied Physics Lab. “We need to determine as a community what to actually advocate for in terms of Apophis.”

A mission to Apophis, he noted, doesn’t align with a number of the science goals set by the community of scientists who study asteroids and other small solar system bodies. A mission, though, could be useful for detailed dynamics studies in case deflection of the asteroid is needed in the future, as well as comparative studies with other near Earth asteroids.

“Apophis’ close pass is a good conversation starter, but by itself is not enough to launch a mission,” he said. “We need to focus on the investigations that can only be done at Apophis and really figure out what can actually be done there.”

Lauretta said there’s no guarantee that an OSIRIS-REx extended mission, if approved by NASA, would go to Apophis. “Are there other, higher value targets with similar orbits and sizes that we can look at as rendezvous targets for this spacecraft?” he said, noting that the mission team was looking at various options, not just Apophis.

“This flyby is the best opportunity to date to raise awareness of all aspects of planetary defense,” said Betts.

Rivkin also advocated for a “do no harm” approach for any missions that might fly to Apophis. While an impact in either 2029 or 2036 has been ruled out, other scientists noted that there is still a small possibility of an impact in 2068. A tiny change in Apophis’ velocity before the April 2029 flyby, caused by the collision of a small satellite with the asteroid, could shift the asteroid’s trajectory by several Earth radii in 2068, said Steve Chesley of JPL.

Observations of Apophis in the next few months will most likely rule out any 2068 impact. “If it is ruled out, I think interactions with spacecraft in 2029 may be perfectly safe,” Chesley said. “But as long as 2068 remains in the crosshairs, you might want to be very thoughtful and circumspect about interacting with the asteroid and changing its orbit, especially before the encounter in 2029.”

The flyby, and potential missions to Apophis at that time, presents an opportunity for planetary defense advocates. “This flyby is the best opportunity to date to raise awareness of all aspects of planetary defense,” said Bruce Betts of The Planetary Society, educating both the general public and policymakers about the topic. “If we can leverage this, the public becomes more aware, interested, and supportive of NEO science and threat preparedness, and hopefully we can get positive changes to NEO and planetary defense policy across the world.”

But, Rivkin noted, that interest could have a downside. “People could get freaked out and try to stop an Apophis mission rather than try to support it,” he said.

Lauretta, in a discussion at the end of the workshop, noted that OSIRIS-REx had already had to deal with those issues: Bennu has a higher—but still very small—chance of hitting the Earth, in its case in late in the next century. “We already solved that problem from a PR perspective,” he said.

“I think Apophis is going to have a bigger perception of risk than Bennu,” countered Binzel.

Lauretta acknowledged that, referencing the origins of the name: an evil serpent in Egyptian mythology (and a villain from the “Stargate SG-1” series). “I think Apophis just has a better name for that. It’s more menacing.”

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