The Space Review: the case of Apophis

NASA’s OSIRIS-REx spacecraft, pictured here on asteroid Bennu, may have an extended mission by visiting another near-Earth asteroid, Apophis, when it flies close to Earth in 2029. (credit: NASA / GSFC)

by Jeff Foust
Monday 30 November 2020

On April 13, 2029, Friday the 13th, the asteroid Apophis will pass considerably close to 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 predicted at one point a one in 37 chance of a collision in 2029, but further observations soon ruled out any impact. A small risk of impact in April 2036 remained for a few years, particularly if the asteroid passed through a narrow “keyhole” of near-Earth space during its 2029 flyby (see “Sounding an alarm, cautiously “, The Space Review, May 31, 2005), but that too was ruled out.

By eliminating the short-term risk of an impact, Apophis went from a threat to an opportunity. That close flyby of 2029 makes the asteroid, several hundred meters in diameter, an ideal target for ground-based telescopes and radar studies. It also puts it within reach of spacecraft missions, including relatively small and low-cost ones.

“This is a very rare natural experiment,” said MIT Professor of Planetary Sciences Richard Binzel, one of the organizers of the “Apophis T-9 Years” seminar held earlier this month. (The event was originally scheduled for April in France, but has been delayed and moved online due to the pandemic.)

It compared the flyby to comet Shoemaker-Levy 9, whose fragments collided with Jupiter in 1994, providing both a scientific mine and greater awareness of impact threats. “We are all struggling with what we can understand in the experiment.”

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

While some of that work could be done with ground-based observers, there was a clear interest in developing spacecraft missions to take advantage of close flyby. The last day of the meeting was dedicated to the presentation of various mission concepts to visit Apophis before, during and after the close approach. Concepts, by teams in the United States, Europe and Asia, included a cubesat-class spacecraft, a small lander based on the MASCOT spacecraft that crashed to the surface of asteroid Ryugu during the Hayabusa2 mission, and even a spacecraft largest launched into space Launch the 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 each other. Some participants suggested that the equivalent of an air traffic control system may be needed to avoid conflicts.

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

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

The spacecraft is in good condition and its tools, developed for Bennu’s close-up studies, could be trained on Apophis. “We have this large payload that appears to be very healthy,” he said. “Having OSIRIS-REx there means we can provide broad support for other terrestrial and space characterization efforts.”

While the flyby is still over eight years away, spacecraft mission planning begins now. Part of that simply reflects development programs for spacecraft, particularly those seeking to study the asteroid before the 2029 close approach. OSIRIS-Rex also needs to think ahead: Lauretta said the project will have to prepare a extended mission proposal for NASA, likely in 2022, detailing its plans for an approach to Apophis or some alternative mission.

There are also programmatic reasons, such as the influence of the recently initiated ten-year survey of planetary science, whose scope includes planetary defense. NASA’s planetary defense program has grown substantially over the past decade, from $ 4 million annually to $ 150 million, spurred at least in part by the asteroid redirection mission early in the decade and sustained even after the disappearance. by ARM. Much of this 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 impact approach. to deflect asteroids.

DART will be followed by the Near Earth Object Surveillance Mission (NEOSM, pronounced “nee-awesome” by those involved in the project), a consequence of the NEOCam mission which 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 “Scientist’s 15-Year (and Current) Research to Save Earth from Asteroid Impacts,” The Space Review, October 28 2019).

If NEOSM is launched around 2025, as currently proposed, it will open funding in the planetary defense program just in time for one or more missions, to support the Apophis 2029 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 have warned, just because a mission to Apophis could be done doesn’t mean that they should be made. “The resources for space exploration are much more limited than we would prefer,” said Andy Rivkin, a planetary scientist at Johns Hopkins University’s Applied Physics Lab. “We need to determine as a community what to actually support in terms of Apophis.”

A mission to Apophis, he noted, is not in line with a series of scientific goals set by the community of scientists studying asteroids and other small bodies in the solar system. A mission, however, could be useful for detailed dynamic studies if deflection of the asteroid is needed in the future, as well as for comparative studies with other near-Earth asteroids.

“Apophis’ close passing is a good conversation starter, but in itself it isn’t enough to start a mission,” he said. “We need to focus on investigations that can only be done in Apophis and really understand what can actually be done there.”

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

Rivkin also advocated a “do no harm” approach to all missions that could fly to Apophis. Although an impact in 2029 or 2036 has been ruled out, other scientists have noted that there is still a small chance of an impact in 2068. A small change in Apophis’ speed 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 JPL’s Steve Chesley.

Apophis observations over the next few months will most likely rule out any impact in 2068. “If that’s ruled out, I think interactions with spacecraft in 2029 could be perfectly safe,” Chesley said. “But as long as 2068 remains in the sights, you may want to be very careful 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 present 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 on the subject. “If we can leverage this, the public becomes more aware, interested and supportive of NEO science and threat preparedness, and hopefully we can achieve positive changes to NEO and planetary defense policy around the world.”

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

Lauretta, in a discussion at the end of the seminar, noted that OSIRIS-REx had already faced these problems: Bennu has a greater, but still very small, chance of hitting Earth, in her case at the end of the next century. “We have already solved the problem from a public relations point of view,” he said.

“I think Apophis will have a greater perception of risk than Bennu,” Binzel retorted.

Lauretta recognized this, referring to the origins of the name: an evil snake in Egyptian mythology (and a villain from the “Stargate SG-1″ series). “I think Apophis simply has a better name for it. It is more threatening. ”

Jeff Foust ([email protected]) is the editor and publisher of The Space Review and a senior writer of SpaceNews. He also manages the Spacetoday.net website. The views and opinions expressed in this article are those of the author only.

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