On Deflecting Near Earth Objects

The B612 Foundation continues to examine the danger of near-Earth objects (NEOs). As noted earlier in these pages, B612 points to the continuing evidence for asteroid and comet impacts and their role in shaping the planet’s history; the much discussed demise of the dinosaurs, due to a likely asteroid strike in the Yucatan, is but one of the instances where the planetary ecology has been altered. We know that the Earth orbits in a swarm of near-Earth asteroids, with a probability of collision in this century that the Foundation pegs at an unacceptably high 2 percent.

Given these concerns, and the possible dangers posed by the object called NEO 99942 Apophis, the Foundation has engaged in a dialogue with NASA about possible missions to this asteroid. Apophis (also known as 2004 MN4) is on course for a near-miss in 2029 , with the 400-meter asteroid approaching to within 32,000 kilometers. What happens afterwards as the near-miss itself disrupts the orbit of this object remains a concern; we may be looking at an uncomfortably close 2036 encounter.

Asteroid impactThe B612 Foundation has suggested placing a radio transponder on the asteroid to plot its orbit more accurately. The complete communication between the Foundation and NASA on this subject is now available on its Web site. NASA concluded that a transponder mission and a follow-up deflection mission, if required, could be performed after the 2013 radar acquisition of Apophis, and that no transponder mission was therefore required at this time. A deflection mission decision would have to be made by 2021. By that time, Arecibo radar may well have determined whether deflection is necessary without the need for the earlier mission, although the agency does not completely rule out earlier missions given the growing interest in near-Earth objects and the opportunity this one presents for study.

Image: A scenario to avoid, and a major reason why deep space technologies to prevent such events should be a priority. Credit: European Space Agency.

These documents make for fascinating reading, especially the Foundation’s analysis of Apophis as a highly unusual NEO — its Earth-like orbit means that a relatively inexpensive deflection technique should suffice to protect Earth from a 2036 impact if such a mission is ever needed. Apophis is not typical of the NEO impact threat, however, and a typical NEO impact scenario would require a more sophisticated mission. Out of all this come three recommendations:

  • That NEO radar capability be assured to support early warning and rational deflection planning.
  • That power and propulsion technologies appropriate to such missions be developed for use on the general population of NEOs.
  • That the responsibility of protecting the Earth from future impacts be assigned to a single US government agency.
  • Centauri Dreams has long advocated an awareness of the danger of near-Earth objects and their devastating potential in the event of an impact. Asteroid and cometary impacts are a key driver for our push into the outer Solar System, for the sooner we can get to an object, the more time we will have to deflect it from a potential encounter with Earth. Not all space science occurs because of curiosity; some occurs out of self-defense, and the development of superior propulsion techniques cannot be looked upon as a luxury but a matter of planetary survival.

    Robert Goddard on Interstellar Migration

    “A manuscript I wrote on January 14, 1918 … and deposited in a friend’s safe … speculated as to the last migration of the human race, as consisting of a number of expeditions sent out into the regions of thickly distributed stars, taking in a condensed form all the knowledge of the race, using either atomic energy or hydrogen, oxygen and solar energy… [It] was contained in an inner envelope which suggested that the writing inside should be read only by an optimist.”

    — Robert Goddard, “Material for an Autobiography,” (1927), available in Pendray, G. E. and Goddard, E. C. (Eds.), The Papers of Robert H. Goddard (New York: McGraw Hill, 1970).

    Photon Pressure Affects Japanese Spacecraft

    The Japanese spacecraft Hayabusa, now in a ‘parking orbit’ above the asteroid Itokawa, is providing good evidence of just how useful the pressure of solar photons can be. Japan’s Institute of Space and Aeronautical Science (ISAS) reports that the force being experienced by the spacecraft is 1/100th of the thrust produced by its ion engines, but fully ten times larger than the gravity of Itokawa itself. The effect is consequential enough that it must be factored into Hayabusa’s descent close to Itokawa’s surface; the spacecraft will deploy a small surface ‘hopper’ called MINERVA to take measurements on the asteroid.

    Hayabusa (once known as MUSES-C and renamed for a Japanese rocket pioneer) thus becomes both a testbed for current technologies and a reminder of a future one. Its electric propulsion or ion drive engines have met the challenge of asteroid rendezvous, although their performance was degraded by solar panel damage from solar flares in 2003. The spacecraft also carries an autonomous navigation system, a sample collection system and a sample capsule for return of asteroid material to Earth. As to that intriguing photon pressure, ISAS has this to say in a news release:

    The solar radiation pressure ideally amounts to the gravity force acting on one yen coin per 1,000 square meters. But reflection is not perfect and actual light force is reduced. Hayabusa uses this light force for descending and station keeping purposes. However, contemporary research has studied the application to revolutionary new spaceships, Solar Sails. JAXA also has investigated the new generation spaceships utilizing hybrid propulsion combining electric propulsion with this light force.

    Despite the failure of the Planetary Society’s Cosmos 1 mission, solar sail research continues in a number of venues because the cumulative acceleration offered by photons gives us the best understood method to reach the outer Solar System while leaving bulky chemical fuel systems behind. Solar sailing offers up a number of problems, including especially the difficulty of deployment of the large, thin surfaces needed for this work, but the theory behind it is absolutely sound and understood by any engineer who has studied the orbital behavior of large objects like communications satellites in space.

    Ahead for Itokawa: surface samples are due to be collected in November with return to Earth in June of 2007. A useful backgrounder on the mission can be found here.

    A Gravitational Lens at Work

    Gravitational lenses of the sort discussed in yesterday’s post are now widely discussed. The idea that gravity can bend light may seem counterintuitive but we’ve seen numerous demonstrations of the effect, starting with the famous eclipse studied by Arthur Eddington in 1919. Hoping to test Einstein’s general theory of relativity, Eddington traveled to the island of Principe, off the coast of West Africa. There, despite initially cloudy skies, he was able to take the crucial photograph that verified Einstein. Stars in the Hyades Cluster that should have been blocked by the Sun were revealed in the image, offset by an amount close to that predicted by Einstein.

    Some have questioned whether Eddington’s equipment was sufficiently precise to make accurate readings, but whatever the case, the bending of light as a result of gravity has stood up. Among the various images that show this effect in deep space, none is as dramatic as the one below. Here we’re looking at multiple bluish images of a galaxy which actually is found behind the foreground galactic cluster seen in yellow roughly in the center of the image. Look for images of this galaxy at 4, 8, 9 and 10 o’clock around the center of the cluster. There may be still other artifacts of the background galaxy elsewhere in the image. The cluster and the dark matter surrounding it are acting as a gravitational lens, one captured dramatically in this Hubble photograph.

    A Hubble view of a gravitational lens

    Image: A background galaxy viewed as multiple images, the effect of gravitational lensing caused by the foreground cluster of galaxies. Credit: W.N. Colley and E. Turner (Princeton University), J.A. Tyson (AT&T Bell Labs, Lucent Technologies), Hubble Space Telescope, and NASA.

    Now ponder the possibilities for a spacecraft using the Sun’s gravity as a lens. At 550 AU, the probe could be positioned to examine a multitude of objects in various wavelengths, with unprecedented magnification provided by the lensing effect. FOCAL, the mission to the gravity focus championed by the Italian space scientist Claudio Maccone, could become one of the most significant space missions ever flown if we can overcome key technical and cultural challenges to make it happen. We’ll be examining those challenges in coming weeks.

    A Mission to the Gravity Focus

    Voyager 1 is, in a sense, our first interstellar spacecraft, with evidence mounting that it has reached the heliopause, that area marking the boundary between the Sun’s outward-flowing particles and the true interstellar medium. The New Horizons mission, scheduled for launch in January, will go on to explore at least part of the Kuiper Belt. But what will our first true interstellar mission be; i.e., when will we launch a spacecraft designed from top down to studying nearby interstellar space?

    The answer may well be a mission to the Sun’s gravity focus. Located at 550 AU (3.17 light days), some 14 times farther from the Sun than Pluto, the focus is that point to which the Sun’s gravity bends the light from objects on the other side of it. The effect is to magnify distant images in ways that could be observed using the proper equipment. The effect of gravitational focus, first studied by Einstein in 1936, had already borne observational fruit by 1978 in the discovery of a ‘twin quasar’ image caused by the gravitational field of a galaxy between us and the distant object. Stanford’s Von Eshleman applied the same theory to the Sun in 1979, and suggested sending a spacecraft to 550 AU to take advantage of this magnifying effect at microwave wavelengths like that of the hydrogen line at 1420 MHz.

    The SETI potential is obvious, for 1420 MHz (21 cm) is the famous ‘waterhole’ frequency thought by some to be likely for interstellar transmissions. Magnifying such signals holds the clear potential for aiding the search for other civilizations, as Frank Drake pointed out in 1987 at the Second International Bioastronomy Conference. But it was not until 1992 that space scientists turned serious attention to studying the gravity focus in terms of a mission. That took place at the Conference on Space Missions and Astrodynamics in Turin, Italy, led by Claudio Maccone. The proceedings were published in the Journal of the British Interplanetary Society. Maccone would go on to submit a formal proposal to the European Space Agency to fund design for a potential mission.

    In the years since, it is Claudio Maccone, a space scientist based in Turin, who has emerged as the champion of the mission he originally called SETISAIL, and now calls FOCAL. His book The Sun as a Gravitational Lens: Proposed Space Missions (Aurora, CO: IPI Press, 2002) contains the key papers that define the mission and suggest its ramifications. Given the length and complexity of FOCAL, it became clear to Maccone early on that the mission could not retain a purely SETI-based focus. Instead, it should broaden its study to consider factors such as the computation of the parallaxes of distant stars and the detection of gravity waves, among other experiments.

    On the subject of the gravitational lens itself, Maccone has this to say in his book:

    As each civilization becomes more knowledgeable they will recognize, as we now have recognized, that each civilization has been given a single great gift: a lens of such power that no reasonable technology could ever duplicate or surpass its power. This lens is the civilization’s star, in our case, our sun. The gravity of each star acts to bend space, and thus the paths of any wave or particle, in the end creating an image just as familiar lenses do.

    The potential of this lens for observation is staggering. Maccone again:

    This lens can produce images which could take perhaps thousands of conventional telescopes to produce. It can produce images of the finest detail of distant stars and galaxies. Every civilization will discover this eventually, and surely will make the exploitation of such a lens a very high priority enterprise. One wonders how many such lenses are being used at this moment in time to scan the universe, capturing a flood of information about both the physical and the biological realities of our time.

    Given a possible flight time of 50 years or more, will FOCAL ever be flown? Maccone relates the response of French professor Roger Bonnet, who at that time was Director of Scientific Programs for ESA. Bonnet told the physicist that had FOCAL been approved by the agency, it would have provided work not just for ESA’s current generation of employees, but for both their children and their grandchildren. Some would say this disqualifies the mission from serious consideration. Centauri Dreams begs to disagree. FOCAL and other interstellar concepts are a classic case of the need to rearrange our cultural priorities to think and plan for the long-term, to strive for accomplishments begun by one generation and maintained by its descendants. To think, in other words, beyond the limitations of a single human lifetime for the good of the species.

    FOCAL thus becomes, in many respects, a way for Centauri Dreams to focus anew on these key issues. We will be examining a number of aspects of the gravitational lens mission in coming weeks in both a scientific and cultural context. Those interested in background material might want to begin with Eshleman’s “Gravitational Lens of the Sun: Its Potential for Observations and Communications over Interstellar Distances,” Science Vol. 205, (1979), pp. 1133-35.