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The Space Review: The Apophis Case


NASA’s OSIRIS-REx spacecraft, depicted here on the asteroid Bennu, may have a long-term mission by visiting another near the terrestrial asteroid Apophis when it flies from Earth in 2029 (credit: NASA / GSFC)

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On Friday, April 1

3, 2029 – the 13th – the asteroid Apophis will pass remarkably close to Earth, approaching 31,000 kilometers from the Earth’s surface or closer to the satellites in geostationary orbit. In late 2004, shortly after its discovery, astronomers predicted 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 impact in April 2036 persists for several years, especially if the asteroid passes through a narrow “lock” of space near Earth during its flight in 2029 (see “Alarm, be careful”, The Space Review, 31 May, 2005), but this has been ruled out ever since.

With the short-term risk of impact eliminated, Apophis moved from threat to opportunity. This close flight in 2029 makes the asteroid several hundred meters in diameter an ideal target for ground-based telescope and radar research. It also puts it within the reach of spacecraft missions, including relatively small, inexpensive ones.

“This is a really rare natural experiment,” said Richard Binzel, a professor of planetary science at the Massachusetts Institute of Technology, one of the organizers of the Apophis T-9 Years seminar earlier this month. (The event was originally scheduled for April in France, but was postponed and moved online due to the pandemic.)

He compared the flyby to Comet Shoemaker-Levy 9, fragments of which collided with Jupiter in 1994, providing both scientific prosperity and greater awareness of threats to impact. “We’re all struggling with what we can do with the experiment.”

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

Scientists have discussed the study of Apophis both as a member of the population of asteroids near Earth and to support planetary protection. The close fly, for example, will allow scientists to study any effects that tidal forces have on an asteroid, changing its rotation or disrupting its shape if the asteroid is a “pile of debris” from smaller objects like some other small asteroids.

While some of this work could be done with ground-based observatories, there was a clear interest in developing spacecraft missions to take advantage of the close flight. The last day of the meeting was dedicated to presentations of various concepts of the mission to visit Apophis before, during and after a close approach. Concepts from teams in the United States, Europe and Asia include a cubesat-class spacecraft, a small lander based on the MASCOT spacecraft that landed on the surface of the Ryugu asteroid during the Hayabusa2 mission, and an even larger spacecraft launched into space. System racket for collecting samples for return to Earth.

The prospect of several missions from different agencies going to Apophis at the same time raised fears that they might interfere or even collide with each other. Some attendees suggest that an equivalent air traffic control system may be needed to avoid conflicts.

One of the most intriguing options for the Apophis mission does not require a new spaceship at all. NASA’s OSIRIS-REx spacecraft will return to Earth in September 2023, carrying samples collected from the surface of the asteroid Bennu last month (see “TAG, Bennu, you are,” The Space Review, October 19, 2020). . While the sample return container lands in the Utah desert, the main spacecraft will fly from Earth and can be used for a long-term mission.

Dante Laureta, chief researcher at OSIRIS-REx at the University of Arizona, said an extended mission option would cause the spacecraft to make a series of flights to bring it back to Earth in April 2029, while Apophis flew. This will allow OSIRIS-REx to meet the asteroid later that month. After the spaceship reached Apophis, he said, “We can stay there as long as we like.”

The spacecraft is in good condition and its instruments, developed for close-up studies of Benu, can be trained on Apophis. “We have this great load that looks very strong,” he said. “The presence of OSIRIS-REx there means that we can provide broad support for other attempts at terrestrial and space characterization.”

While the flight is still more than eight years old, planning for spacecraft missions begins now. Part of this simply reflects spacecraft development schedules, especially those that want to explore the asteroid before the upcoming approach in 2029. Even OSIRIS-Rex needs to think ahead: Loretta said the project will have to prepare an extended mission proposal for NASA, probably in 2022, to specify in detail its plans to approach Apophis or some alternative mission.

There are also programmatic reasons, such as influencing the study of the Decade of Planetary Science, which began recently, which covers planetary protection. NASA’s planetary defense program has grown dramatically over the past decade, from $ 4 million a year to $ 150 million, spurred at least in part by an asteroid diversion mission at the beginning of the decade and maintained even after ARM disappeared. Much of this funding supports the development of a special planetary defense mission, the Dual Asteroid Redirection Test (DART), which will launch in July to fly to the asteroid Didymos and collide with its small moon, Dimorphos, to demonstrate the approach. of the kinetic impact element for deflecting asteroids.

“The close submission of Apophis is a good start to a conversation, but it is not enough in itself to start a mission,” Rivkin said.

The DART will be followed by the Near Earth Observation Mission (NEOSM, declared “great” by the project participants), an outgrowth of the NEOCam mission, which was a finalist in the previous round of the smaller planetary science missions program. NEOSM will fly a small space telescope to detect near-Earth objects with infrared wavelengths (see “15-year (and reporting) search by a scientist to save Earth from asteroid impact,” The Space Review, October 28 2019).

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

But some scholars have warned that a mission to Apophis does not mean that Must to be made. “Space exploration resources are much more limited than we would prefer,” said Andy Rivkin, a planetary scientist in the Johns Hopkins University Laboratory of Applied Physics. “We need to define as a community what we should actually stand for in relation to Apophis.”

The mission at Apophis, he noted, did not coincide with a number of scientific goals set by the community of scientists studying asteroids and other small bodies in the solar system. However, a mission can be useful for detailed dynamic research in the event that a deviation of the asteroid is needed in the future, as well as for comparative research with other asteroids near Earth.

“The imminent submission of Apophis is a good start to a conversation, but it is not enough in itself 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.”

Loretta said there was no guarantee that an expanded OSIRIS-REx mission, if approved by NASA, would go to Apophis. “Are there other higher-value targets with similar orbits and sizes that we can consider as targets for this spacecraft?” He said, noting that the mission team is considering different options, not just Apophis.

“This overflight is the best opportunity so far to raise awareness of all aspects of planetary defense,” Bates said.

Rivkin also advocated a “no harm” approach to any missions that could fly to Apophis. Although the impact in 2029 or 2036 has been ruled out, other scientists note that there is still little chance of impact in 2068. A small change in the speed of Apophis before the flight in April 2029 caused by the collision of a small an asteroid satellite could shift the asteroid’s trajectory by several Earth radii in 2068, said JPL’s Steve Chesley.

Apophis’ observations over the next few months are likely to rule out any impact in 2068. “If excluded, I think interactions with spacecraft in 2029 could be completely safe,” Chesley said. “But as long as 2068 stays in sight, you may want to be very careful and cautious about interacting with the asteroid and changing its orbit, especially before the 2029 encounter.”

The overflight and potential missions to Apophis at that time offered an opportunity for the defenders of the planetary defense. “This overflight is the best opportunity so far to raise awareness of all aspects of planetary defense,” said Bruce Bates of the Planetary Society, educating both the general public and politicians. “If we can take advantage of this, society will become more informed, interested and supportive of NEO science and threat preparedness, and we hope that we can make a positive difference in NEO and planetary defense policy around the world.”

But Rivkin noted that interest could have a downside. “People could go crazy and try to stop Apophis’ mission instead of trying to support it,” he said.

Lauretta, in a discussion at the end of the seminar, noted that OSIRIS-REx already had to deal with these issues: Bennu has a greater – but still very small – chance of hitting Earth, in this case at the end of the next century. “We have already solved this problem from a PR perspective,” he said.

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

Loretta acknowledged this, citing the origin of the name: an evil snake in Egyptian mythology (and a villain in the Stargate SG-1 series). “I think Apophis just has a better name for it. This is more threatening. ”

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