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99942 Apophis: The Value of a Close Approach

The approach of the asteroid 99942 Apophis in April of 2029 offers an opportunity to study a sizeable asteroid through both radar and optical telescopes. Marina Brozović, a radar scientist at the Jet Propulsion Laboratory, points out that radar studies of the object might resolve surface details that are no more than a few meters in size. No surprise, then, that Apophis is the subject of much discussion at the 2019 Planetary Defense Conference in College Park, Maryland.

This is the same conference at which NASA Administrator Jim Bridenstine warned about the critical nature of planetary defense, noting the Chelyabinsk event in 2013 that delivered some 30 times the energy of the Hiroshima bomb. NASA has contracted with SpaceX to provide launch services for its Double Asteroid Redirection Test (DART), which is expected to launch in 2021 via a SpaceX Falcon 9 and test asteroid deflection through high-speed collision.

DART’s target will be the tiny moon of an asteroid called Didymos, which it will reach by solar electric propulsion in October of 2022, when the asteroid closes to within 11 million kilometers of Earth. Bridenstine pointed out that the NASA plan to detect and characterize 90 percent of near-Earth objects measuring 140 meters in diameter and above is “only about a third of the way there,” adding that events like Chelyabinsk are expected roughly every 60 years.

So it’s heartening to see missions like the Japanese Hayabusa2 and NASA’s OSIRIS-REx probing the nature of these objects, even as we look toward the Apophis opportunity in 2029. Numerous small objects on the order of 5-10 meters have been found passing as close to the Earth as Apophis, but the latter is substantial, a 340-meter asteroid that will be widely studied as it approaches to within 31,000 kilometers of the surface. The asteroid will become a naked eye object in the night sky over the southern hemisphere on April 13, 2029.

Image: This animation shows the distance between the Apophis asteroid and Earth at the time of the asteroid’s closest approach. The blue dots are the many man-made satellites that orbit our planet, and the pink represents the International Space Station. Credit: NASA/JPL-Caltech.

You may recall that following its discovery in 2004, early calculations showed a 2.7% chance that Apophis might impact the Earth in 2029, but follow-up observations have ruled that out. Now we can take advantage of the close passage to study Apophis’ size, shape and composition. A key question: Can we use the flyby to learn more about the asteroid’s interior?

“We already know that the close encounter with Earth will change Apophis’ orbit, but our models also show the close approach could change the way this asteroid spins, and it is possible that there will be some surface changes, like small avalanches,” said Davide Farnocchia, an astronomer at JPL’s Center for Near Earth Objects Studies (CNEOS), who co-chaired the April 30 session on Apophis with Marina Brozović.

Apophis’ passage in 2029 will take it within the distance some spacecraft orbit the Earth, and there remains the possibility of a mission to the object. As to future collision risks, the trajectory of Apophis is well established, but gravitational interactions between asteroid and Earth make it necessary to continue to recalculate the orbit. As we assemble the catalog of potentially hazardous objects, the need for missions like DART — and others testing a range of mitigation strategies — is clear. Because if we ever do find an asteroid of this size that presents a danger to our planet, we need to know what we’re going to do.

You can watch video from the 2019 Planetary Defense Conference here.


{ 14 comments… add one }
  • Michael C. Fidler May 2, 2019, 10:56

    Looks like we may have some fireworks this summer, in fact starting next month!

    “Our poster is up at the #PlanetaryDefense Conference. The Taurid Complex hypothesis predicts a concentration of NEOs that will pass close enough to Earth to be observable this summer. If it does not exist, it can be falsified by observation. If it exists, it can be quantified.”


    The meteor shower that brought Tunguska is due in June!

    That disastrous rock may now looks to have been a Beta Taurid passenger.
    Analysis of the Tunguska tree-fall patterns suggests a familiar source for the asteroid that caused it.
    Its timing also fits perfectly with a late June annual meteor shower.
    Nonetheless, it’s more interesting than dangerous. Put down that helmet.


    This is a daytime meteor shower meaning fireballs and bolides will be visible. The stream is entering the earth’s atmosphere on the sunlit side of earth. We could be in for some surprises!

    • Michael C. Fidler May 6, 2019, 4:22

      Preprint 6 May 2019

      The 2019 Taurid resonant swarm: prospects for ground
      detection of small NEOs.

      David Clark, Paul Wiegert, Peter G. Brown

      In June 2019 the Earth will approach within 5 ◦ mean anomaly of the centre of the Taurid resonant swarm, its closest post-perihelion encounter with Earth since 1975. This will be the best viewing geometry to detect and place limits on the number of NEOs proposed to reside at the swarm centre until the early 2030s. We present an analysis of the optimal times and pointing locations to image NEOs associated with
      the swarm.

      The June-August 2019 encounter of the TSC provides a
      unique opportunity to identify additional NEOs of the
      swarm, helping to either substantiate or refute the giant
      comet hypothesis of Clube & Napier (1984) and the Tau-
      rid Complex hypothesis of Asher (1992) and Asher (1992).
      Dedicated surveys will at the very least be able to place
      limits on the NEO density near the swarm centre. The
      encounter will yield two opportunity windows of observa-
      tion: A) 2019/07/05 -2019/07/11 (JD 2458669.5 – 2458675.5)
      where southern observatories may detect m 22 (all appar-
      ent magnitudes assume H=24) objects with in-sky motion
      less than 2.25 ◦ /day, and B)2019/07/21 – 2019/08/10 (JD
      2458685.5 – 2458705.5) where both northern and southern
      observatories may view dimmer objects (m > 22.5 in late
      July, m > 25 in early August) with in-sky motion ranging
      from near 0 to 2 ◦ /day. The recommended target sky posi-
      tion for TS object detection is in the southern sky just east
      of the galactic plane, α = 263 ◦ , δ = −48 ◦ for opportunity A,
      α = 272 ◦ , δ = −38 ◦ for opportunity B.


  • Supernaut May 2, 2019, 15:29

    I’ve seen the animation several times now. However, what’s confusing is that Apophis is to come withing 31,000 km from the surface of Earth at closest approach; the geostationary ring, also seen in the animation, is ~35,800 km from the surface of Earth so why is Apophis’ path showing it passing outside the ring?

    • John walker May 2, 2019, 20:45

      Good question. The satellites illustrated are in geostationary orbit above the equator. It appears that Apophis has it’s closest approach north of that. ie it will be over 35786km away when directly above the equator and at closest approach above Europe by the looks of that gif.

  • David Evans May 3, 2019, 10:10

    Is it too late to engineer a collision between Apophis and a small, inert object? It seems to me that at that distance we could make some interesting observations of the debris cloud, and the probe wouldn’t have to go far!

    • Nic May 4, 2019, 20:46

      I think that ten years is plenty of time to construct and launch a relatively small probe to impact the asteroid. This could easily be done at it’s closest approach with a probe launched specifically for the purpose of the experiment, or even sing a ‘dead’ satellite redirected if it’s mission is completed before 2029.
      But there would also be the risk of the creation of a large amount of debris in the area of other satellites in or close to GEO. This could be averted by having the impact further from Earth, but this would come at a cost.

      An interesting thought though… it would be entirely possible in the time we have

  • Charley May 4, 2019, 23:57

    “Killer asteroid flattens New York in simulation exercise”

  • Charley May 6, 2019, 18:03

    Does anyone here think that it might be an extremely wise thing to rendezvous with this asteroid when it is at its closest approach to Earth with the intent of placing on said body a radio transmitter/receiver? This I think would be a unique opportunity to perform detailed studies of the orbital path using ranging techniques as well as Doppler shifts to track the position and velocity of this object. Such information could be absolutely invaluable in giving a snapshot over time of the gravitational effects that the body would be subjected to as it moves through the solar system. Here I’m thinking about getting gravitational mapping due to small as well as large bodies affecting the object as it orbits the sun. The more data you collect upon such objects which at the present time is limited in terms of its accuracy. I think could be absolutely invaluable for general studies of these objects as classes of near earth passerby’s.

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