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NASA’s OSIRIS-REx spacecraft, pictured here at asteroid Bennu, could have an extended mission by visiting another near-Earth asteroid, Apophis, when it flies over Earth in 2029. (Credit: NASA / GSFC ) |
by Jeff Foust
Monday, November 30, 2020
On April 13, 2029 – a Friday the 13th – the asteroid Apophis will pass remarkably close to Earth, less than 31,000 kilometers from the Earth’s surface, or closer than satellites in geostationary orbit. In late 2004, shortly after its discovery, astronomers projected a one in 37 chance of a collision at one point in 2029, but further observations quickly ruled out any impact. A small risk of impact in April 2036 persisted for a few years, particularly if the asteroid passed through a narrow “keyhole” in space near Earth during its 2029 flyby (see “Sound an alarm , cautiously, ”The Space Review, May 31, 2005), but that too has since been dismissed.
With the risk of short-term impact eliminated, Apophis has gone from a threat to an opportunity. This close flyby of 2029 makes the asteroid, several hundred meters in diameter, an ideal target for studies by ground-based telescopes and radars. This also puts it within reach of spacecraft missions, including relatively small and inexpensive missions.
“It’s a really rare natural experience,” 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 put online due to the pandemic.)
He likened the flyby to comet Shoemaker-Levy 9, fragments of which collided with Jupiter in 1994, providing both a scientific boon and greater awareness of impact threats. “We are all struggling with what we can experience.”
| “It’s a really rare natural experience,” Binzel said of the 2029 flyby. |
Scientists discussed the study of Apophis both as a member of the near-Earth asteroid population and to support planetary defense. The close flyby, for example, will allow scientists to study the effects of tidal forces on the asteroid, changing its rotation or disrupting its shape, if the asteroid is a “rubble heap” of smaller objects. like 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 close flyover. The last day of the meeting was devoted to presentations of various mission concepts to visit Apophis before, during and after the closing approach. The concepts, by teams in the United States, Europe and Asia, included a cubesat-class spacecraft, a small lander based on the MASCOT spacecraft dropped to the surface of asteroid Ryugu during the Hayabusa2 mission, and even A Larger Spaceship Launched On A Space Launch System Rocket To Collect Samples For Return To Earth.
The prospect of multiple missions from different agencies all going to Apophis at the same time has raised concerns that they might interfere or even clash. Some participants suggested that the equivalent of an air traffic control system might be needed to avoid conflict.
One of the more intriguing Apophis mission options doesn’t require a new spaceship 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, it’s you”, The Space Review, October 19, 2020). As the sample return container lands in the Utah Desert, the main spacecraft will fly through 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 allow the spacecraft to perform a series of flyovers that bring it back to Earth in April 2029, around the same time. where Apophis goes. This would allow OSIRIS-REx to encounter the asteroid later in the month. Once the spacecraft reaches Apophis, he said, “we can stay there as long as we want.”
The spacecraft is in good condition and its instruments, developed for close studies of Bennu, could be trained on Apophis. “We have this great payload which looks very healthy,” he said. “Having OSIRIS-REx there means we can provide extensive support for further characterization efforts on the ground and in space.”
While the flyby is still over eight years away, planning for spacecraft missions begins now. Some of this simply reflects spacecraft development timelines, especially those looking to study the asteroid before 2029 approaches. Even OSIRIS-Rex needs to think ahead: Lauretta said the project will need to prepare an extended mission proposal for NASA, likely in 2022, which would detail its plans for an Apophis approach or other mission.
There are also programmatic reasons, such as the influence of the recently started Decadal Survey of Planetary Science, the scope of which includes planetary defense. NASA’s planetary defense program has grown dramatically over the past decade, from $ 4 million per year to $ 150 million, spurred at least in part by the asteroid redirection mission at the start of the decade. and maintained even after the disappearance of ARM. Much of this funding supports 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. to deflect the asteroids.
| “Apophis’ close pass is a good conversation starter, but it’s not enough on its own to initiate a mission,” Rivkin said. |
The DART will be followed by the Near-Earth Object Monitoring Mission (NEOSM, pronounced “nee-awesome” by those involved in the project), a consequence of the NEOCam mission which was a finalist in the previous phase of the Science Discovery program. planetary smaller missions. NEOSM will pilot a small space telescope to discover near-Earth objects at infrared wavelengths (see “A Scientist’s 15-Year Quest (and Counting) to Save Earth from Asteroid Impacts,” The Space Review, October 28, 2019).
If NEOSM launches around 2025, as currently proposed, it will open funding into the Planetary Defense Program just in time for one or more Apophis 2029 flyby support missions. Some members of the planetary defense community have seen a 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 not be carried out. should be finished. “The resources for space exploration are much more limited than we would like,” said Andy Rivkin, planetary scientist at the Applied Physics Laboratory at Johns Hopkins University. “We need to determine as a community what to really stand up for in terms of Apophis.”
A mission to Apophis, he noted, does not meet a number of scientific goals set by the community of scientists who study asteroids and other small bodies in the solar system. A mission, however, could be useful for detailed dynamic studies in the event that a deviation of the asteroid is required in the future, as well as for comparative studies with other near-Earth asteroids.
“Apophis’s close pass is a good conversation starter, but on its own isn’t enough to start 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 was 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 sizes that we can consider rendezvous targets for this spacecraft?” he said, noting that the mission team was considering different options, not just Apophis.
| “This overview is the best opportunity to date to raise awareness of all aspects of planetary defense,” Betts said. |
Rivkin also advocated a “do no harm” approach to any missions that might steal Apophis. Although an impact in 2029 or 2036 has been ruled out, other scientists have noted that there is still a small possibility of 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 path by several Earth rays by 2068, said Steve Chesley of JPL.
Observations of Apophis over the next few months will likely rule out any impact in 2068. “If that’s excluded, I think interactions with spacecraft in 2029 can be perfectly safe,” Chesley said. “But as long as 2068 remains in the crosshairs, you might want to be very thoughtful and wary about interacting with the asteroid and changing its orbit, especially ahead of the 2029 encounter.”
The flyby and potential missions to Apophis at this time present an opportunity for planetary defense advocates. “This overview is the best opportunity yet to raise awareness of all aspects of planetary defense,” said Bruce Betts of the Planetary Society, educating both the general public and policymakers about this. “If we can take advantage of this, the public will become more aware, interested and supportive of NEO science and threat preparedness, and we hope to be able to make positive changes in NEO and planetary defense policy through the world.
But, Rivkin noted, that interest could have a downside. “People might get scared and try to stop an Apophis mission rather than trying to support it,” he said.
Lauretta, in a discussion at the end of the workshop, noted that OSIRIS-REx has had to deal with these issues before: Bennu has a higher – but still very low – chance of hitting Earth, in his case at the end of the next century. “We have already solved this problem from a public relations perspective,” he said.
“I think Apophis is going to have a greater risk perception than Bennu,” Binzel retorted.
Lauretta recognized him, referring to 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 it. It’s more threatening.
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