* The smaller bodies of the solar system, particularly the asteroids, were long a lower priority for early planetary exploration efforts, and it has only been the last few decades that greater efforts have been made to learn more about them. This chapter provides a survey of what is known about the asteroids and current efforts to explore them.
* The first asteroid to ever be imaged in any detail was an NEA named 4769 Castalia. Radar observations of asteroids, even some main belt asteroids, were performed with increasing detail through the 1980s, and when Castalia passed by Earth in August 1989, Stephen J. Ostro and his colleagues at NASA JPL managed to obtain a radar image of it. Castalia proved to have a "peanut" shape, consistent with the idea that it was composed of two 800-meter chunks more or less glued together.
That led to further radar observations of asteroids. The NEA 1620 Geographos, originally spotted in 1951, was imaged by radar during an Earth flyby in 1994, and turned out to be shaped something like a sweet potato with dimensions of 5.1 by 1.8 kilometers. The NEA 4179 Toutatis was imaged during an Earth flyby in 1992 and proved to be two masses stuck together, one about 4 kilometers across and the other about 2.5 kilometers across. Astronomers are beginning to suspect that such "contact binary" or "Siamese twin" configurations are common among asteroids.
This suspicion was reinforced by radar observations performed by Ostro and his colleagues of the main-belt asteroid 216 Kleopatra in November 1999. Analysis of the radar data showed Kleopatra to have a shape like a dumbbell or a dog's bone, with two large knobby ends connected by a narrow neck. Kleopatra, which was originally discovered in 1880, is estimated to have dimensions of 217 by 94 by 81 kilometers, and appears to be made largely of metal. It has an eccentric and highly inclined orbit by the standards of main-belt asteroids. Other asteroids have been since observed by radar, and it turns out that the contact binary configuration is common. Some of the asteroids observed by radar also had tiny moonlets.
* The first asteroid to be photographed close-up was the main belt asteroid 951 Gaspra, which was imaged in 1991 by the NASA Galileo spacecraft, discussed later, while on its way to Jupiter. 951 Gaspra has the appearance of a lumpy potato with dimensions of 19 by 12 by 11 kilometers.
In 1993, Galileo imaged the main belt asteroid 243 Ida, which looked like a lumpy rock with dimensions of 56 by 24 by 21 kilometers. The images were taken from 3,500 kilometers away, and had minimum feature resolutions of 35 meters, twice as good as those obtained from the Gaspra flyby. The images revealed a surprise as well: Ida proved to have a moonlet about 1.4 kilometers in diameter, which was named Dactyl. Analysis of Dactyl's orbit around Ida was used to obtain a value for the mass of Ida, which was so low that it strongly hinted there were voids inside the larger asteroid.
* The first successful space probe intended specifically to explore asteroids was the NASA JPL "Near Earth Asteroid Rendezvous (NEAR)" spacecraft, a Discovery-series spacecraft that was launched by a Delta 7925 booster on 17 February 1996. Its objective was to go into orbit around the well-known NEA 433 Eros, the first NEA to be discovered, back in 1898. The NEAR spacecraft was equipped with cameras, spectrometers, and other instruments to obtain images and perform chemical studies of the composition of Eros.
In 1997, while on the way to Eros, NEAR performed a flyby of the main belt asteroid 253 Mathilde. Mathilde is a primitive carbonaceous asteroid, blacker than coal, and has an eccentric orbit that takes to the outer reaches of the asteroid belt. Mathilde is the largest asteroid visited by a spacecraft to date, with dimensions of 66 by 48 by 46 kilometers. Mathilde's surface is gouged by huge craters that make it look as though large bites had been taken out of it. The largest of the craters is about 30 kilometers across.
A good estimate of Mathilde's mass was obtained by its influence on NEAR's trajectory, and once again the density was surprisingly low, less than half that of the similar "carbonaceous chondrite" meteorites found on Earth. An astronaut trying to walk on Mathilde would only weigh about half a kilogram. Walking on Mathilde would be tricky at best, and the astronaut would likely get thoroughly black and dirty as well.
On Valentine's Day, 14 February 2000, NEAR reached its objective, Eros, and went into orbit around it. The probe was then given the name "NEAR Shoemaker", after the late great planetary astronomer Eugene Shoemaker. NEAR was to have gone into orbit around Eros over a year earlier, but when the probe was being prepared for orbital insertion in December 1998, its communications system decided to go on vacation for about 27 hours. However, mission controllers were able to react and save the mission, rescheduling it for a new try.
Eros has dimensions of 33 by 13 by 13 kilometers, and looks like a long potato. It has two major gouges in it, almost certainly due to impacts. Eros is a particularly interesting NEA, since its orbit does cross that of the Earth, and some astronomers have calculated that it has a 1 in 20 chance of striking the Earth in the next billion years. The impact would be substantially more destructive than that which is believed to have helped exterminate the dinosaurs. By the late summer of 2000, NEAR's observations had made it clear that Eros was not a "flying rubble pile", instead consisting of a single piece of undifferentiated rock that seemed to date back to the origins of the solar system.
NEAR continued to orbit Eros until 12 February 2001, when mission controllers managed to ease the spacecraft down to a "soft landing" on the asteroid. Although the mission was supposed to end after this stunt, very much against predictions the probe was still working, and the mission was extended for ten days to permit close-up measurements with the probe's X-ray and gamma-ray spectrometer instruments. No other instruments were used during this final mission phase, since the "beached" space probe could not direct its high-gain antenna at Earth, meaning that all data had to be funneled back through the low-rate low-gain antenna. The data that could be returned by other instruments with the spacecraft in that position would have hardly been worth the bandwidth.BACK_TO_TOP
* In the meantime, other probes had obtained images of asteroids. In early 2000 the NASA Cassini probe, on its way to Saturn via a Jupiter flyby and also discussed later, imaged the asteroid 2685 Masursky. The asteroid was observed from a distance of 1,545,000 kilometers (960,000 miles). The observations were performed with the spacecraft's wide angle and narrow angle imagers, using various spectral and polarizing filters. The observations showed the asteroid to be from 14 to 19 kilometers across.
Similarly, the NASA Stardust comet probe, launched in 1999, imaged the 4-kilometer-wide asteroid AnneFrank on 2 November 2002; and the ESA Rosetta comet probe imaged the 4.6-kilometer-wide asteroid 2867 Steins on 5 September 2008. These two probes are also discussed in more detail in a later chapter.
* While NASA and the ESA sent probes to asteroids, Earth-based astronomers performed remote inspections of asteroids using new "adaptive optics" telescopes, with mirrors that are deformed under computer control to adjust to optical variations in the Earth's atmosphere and obtain a clearer picture.
In late 2000, a group of astronomers under Dr. William Merline of the Boulder office of SWRI released images of a large double asteroid, and an asteroid with a small moonlet. The double asteroid was the outer belt asteroid Antiope, which consists of two bodies about 80 kilometers in diameter with a separation of 160 kilometers. The images were obtained by the Keck telescope on Mauna Kea on the island of Hawaii.
Images obtained by the SWRI team from the Canada-France-Hawaii Telescope (CFHT), a neighbor of the Keck on Mauna Kea, showed that the 145-kilometer-wide asteroid Pulcova has a moonlet about a tenth its size, orbiting once every four days at a distance of about 800 kilometers. The SWRI group discovered a similar moonlet orbiting the 220 kilometer wide asteroid Eugenia in 1999. The SWRI teams cautions that they inspected 200 asteroids and only found two with moonlets. CFHT images were also obtained of 216 Kleopatra, and had an excellent correspondence to the radar images taken of the asteroid.
The new European Southern Observatory Very Large Telescope (ESO VLT), an array of four big telescopes in Chile, also got into the asteroid observation act, with astronomers announcing in 2005 that VLT imagery had revealed an asteroid with two moonlets. 87 Sylvia, a main belt asteroid, was discovered in 1866 and has a diameter of about 280 kilometers. The two moonlets were a few kilometers across; since Sylvia in Roman legend was the mother of the founders of Rome, Romulus and Remus, the two moonlets were given the names of her sons.
* New missions to the asteroids followed these observations. The Japanese ISAS MUSES-C probe was launched on 9 May 2003 on a mission to the asteroid 1998 SF36, in an orbit between Earth and Mars. The spacecraft was put into space on an ISAS M-5 solid-fuel booster from the ISAS Kagoshima launch center. The probe's name loosely stood for "M-5 Space Engineering Spacecraft Mark C". Once in space, it was given the much more elegant name of "Hayabusa (Peregrine Falcon)".
Hayabusa had a launch mass of 530 kilograms and featured a high-efficiency ion propulsion system, featuring three ion thrusters, as well as a highly autonomous flight control system. After performing an Earth flyby in May 2004 to get a gravity assist, it then performed a rendezvous in mid-September 2005 with the asteroid 1998 SF36, which is about 700 by 300 meters in size; the asteroid was named "Itokawa", after Dr. Hideo Itokawa, one of the founders of the Japanese space program.
At the time of the rendezvous, asteroid Itokawa was about 1.3 times as far away from the Sun as is the Earth. The rendezvous was originally scheduled for June 2005, but an intense solar flare in 2003 damaged the spacecraft's solar panels, reducing the power for the ion thrusters and forcing a modification of the flight trajectory. One of the spacecraft's three reaction wheels had also failed before reaching the asteroid, but the other two were able to keep the probe stable.
The probe moved to a stationary position relative to the asteroid at an altitude of about 20 kilometers. It performed observations for about two months using its instrument suite, which included:
Following this surveillance period, the probe was to then descend to the asteroid to "touch down" momentarily and take samples by firing a metal ball into the surface and then using a retractable funnel-shaped collector to scoop up debris. To perform preliminary scouting, Hayabusa was to release a small hopping landing rover microprobe named MINERVA (Micro-nano Experimental Vehicle for Asteroid), with a mass of 590 grams, carrying three cameras and a set of thermometers. NASA had planned to contribute a larger rover, but it didn't survive budget cuts.
Mission actions following the surveillance period did not go well. MINERVA was released as planned on 12 November 2005, and then disappeared. Two attempts were made to touch down for samples, on 19 and 25 November, the first being a clear failure and the second being judged likely a failure. It did have the distinction of being the first spacecraft to land on an asteroid -- or for that matter, any other celestial body other than the Moon -- and take off again.
After the encounter, Hayabusa was to cruise back toward Earth, arriving in June 2007 and dropping a 20 kilogram sample-return capsule that would parachute onto Australia. However, communications were temporarily lost at a critical time and the probe missed the Earth-return window. Mission planners arranged a retry, with Hayabusa dropping its capsule on 13 June 2010, with the capsule recovered from the outback. Although few believed the capsule contained much of interest, JAXA still had reason for satisfaction in carrying out a highly sophisticated mission, providing useful experience for future missions.
Even without significant samples, Hayabusa still returned high-quality observations of Itokawa. It appears to be a contact binary with a "flying rubble pile" composition, consisting of about 40% voids. The surface was not heavily cratered, suggesting that the asteroid was not very old, far younger than other asteroids observed in close-up so far, though some contrarians believe that impacts may have shaken the object enough to fill in craters and so Itokawa is older than it looks.BACK_TO_TOP
* In late 2001, NASA approved another Discovery-series asteroid probe named "Dawn", which is to orbit Vesta and Ceres and perform detailed observations. The probe was named "Dawn" because Vesta and Ceres are seen as survivors of the first days of the solar system. After a near-death experience in 2005, when NASA considered canceling the program due to cost overruns, Dawn was launched from Cape Canaveral on 27 September 2007 by a Delta II Heavy vehicle, featuring nine solid-rocket boosters.
The Dawn probe was built by Orbital Sciences Corporation, and had a launch mass of 1,218 kilograms (2,685 pounds). It featured a solar-powered xenon-ion electric propulsion system with three thrusters, the technology being derived from the solar-electric propulsion system used on the NASA-JPL Deep Space 1 probe, once again discussed later. Electric power was provided by Dutch-built solar arrays spanning 19.8 meters (65 feet). The instrument suite included:
The probe performed a gravity slingshot around Mars on 18 February 2009, and entered orbit around the asteroid Vesta in August 2011 for observations, the closest approach being 195 kilometers.
Following Dawn's rendezvous with Vesta, as mentioned earlier the next asteroid encounter was by the Chinese Chang'e 2 lunar probe, which flew by the NEA 4179 Toutatis on 13 December 2012. Dawn was not finished with its mission at Vesta, however, having departed Vesta orbit on 26 August 2012 to travel to Ceres, entering orbit in April 2015 -- it would have arrived a month earlier, but its ion propulsion system was temporarily shut down by a cosmic radiation upset that put it into SAFE mode. The probe will continue observations of Ceres until its mission ends, possibly in 2019.
* The Japanese did not give up after the poor showing of their Hayabusa probe. In 2012, JAXA committed to building an improved "Hayabusa 2", which was launched on 3 December 2014. Hayabusa 2 had a launch mass of 590 kilograms, with four ion thrusters for propulsion. Hayabusa 2's sampling system was to fire small projectiles at the asteroid, knocking particles back into a collector, to be placed in a re-entry capsule. The spacecraft also carried three payloads of its own:
Hayabusa 2 performed an Earth flyby in December 2015, and will arrive at the target, asteroid (162173) 1999 JU3 in July 2018. It will depart in December 2019, and return to Earth in December 2020, dropping its sample capsule on Australia.
The flight also included three other, separate payloads, one being an asteroid flyby spacecraft named "Proximate Object Close Flyby with Optical Navigation (PROCYON)". It was a technology demonstrator with a launch mass of 59 kilograms (130 pounds), to show how small spacecraft could be used for planetary missions. After an Earth flyby in 2015, it was to perform a flyby of at least one asteroid from 2016, the primary purpose being to demonstrate optical navigation techniques. However, PROCYON's ion propulsion system failed in March 2015, with the flyby then necessarily cancelled.
* NASA is now readying to fly an asteroid sample-return mission, named "Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx)". The 1,529-kilogram probe will be launched by an Atlas V booster in the fall of 2016.
Along with the asteroid sampling system and sample-return capsule, the probe's instrument suite includes:
After launch in 2016, OSIRIS-REx will rendezvous with the asteroid 101955 Bennu -- a carbonaceous asteroid -- in 2018, to begin an extended mapping phase. After selection of a landing site from the mapping data, the probe will collect samples in September 2019, to then begin the return journey to Earth. The sample capsule will re-enter the Earth's atmosphere in September 2023, with the capsule being recovered at the Utah Test & Training Range. OSIRIS-REx was the third in the NASA "New Frontiers" series of deep-space probes, along with the "Juno" Jupiter orbiter and the "New Horizons" Pluto flyby probe, both discussed later.BACK_TO_TOP
* There's been interest in a crewed mission to an asteroid, using NASA's Orion space capsule, as a precursor to more ambitious deep-space missions. However, it turns out that it's not as straightforward as it sounds. A list of 44 candidate asteroids was examined by NASA, being screened for size -- they had to be over 50 meters across -- and for orbits that would permit a round-trip time of 180 days or less, in the 2020 to 2050 timeframe. Only three NEAs made the cut:
That proved disappointing, so NASA's focus moved on to evaluating schemes to protect the Earth from errant asteroids, the exercise being designated the "Asteroid Redirect Mission (ARM)". The program is being conducted in three phases:
The Asteroid Identification Segment is leveraging off existing asteroid searches, with a candidate to be selected no earlier than 2020. NASA has already identified four preliminary candidates:
In the second phase Asteroid Redirect Robotic Mission, NASA will send an automated spacecraft to fly to, rendezvous with, and characterize an NEA. It will will then retrieve a boulder from the asteroid's surface using a robot arm with a claw grip, and place it in a "distant retrograde orbit (DRO)" of the Moon. NASA envisions the robotic mission as using solar electric propulsion, and leveraging off commercial space systems.
In the Asteroid Redirect Crewed mission, an Orion capsule with two crew and kitted for asteroid investigation will visit the robot and boulder placed in lunar orbit. The mission would last a bit over 24 days.
* ARM is seen as a precursor to more ambitious asteroid redirection missions, either to nudge them away from an Earth impact, or place them in lunar orbit for exploitation. Studies have been performed in which a SEP robot tug brought a larger asteroid into lunar orbit. There may be very good reason to do this: Jeffrey Kargel, an astrogeologist with the US Geological Survey in Flagstaff, Arizona, has suggested that asteroids may be a source of massive quantities of valuable metals. Kargel focused on metals that are relatively rare and precious on Earth, but which might be available in quantity in a metallic asteroid: platinum, and its relatives iridium, osmium, palladium, rhenium, and rubidium. He estimated that a metallic asteroid a kilometer in diameter should contain 400,000 tonnes of these metals.
Kargel performed his analysis using 1990 US dollar prices for these metals, and concluded that their total value would be $5 trillion USD. Of course, introduction of large quantities of such rare metals into the world economy would cause their price to fall dramatically, but even taking that into consideration the value would still be over $300 billion USD.
NEAs orbit the Sun at velocities similar to that of the Earth, and so sending a spacecraft to one is relatively easy. A team of asteroid miners could fly to a suitable NEA, set up a solar power system and a smelter, and use an electromagnetic gun to shoot the ingots produced by the smelter back to Earth. The ingots could be carried in aerobraking shells to enter the Earth's atmosphere, where the aeroshells would then deploy parachutes to drop their payload into a desert or other remote location.
Transferring all 400,000 tonnes of metal would require three electromagnetic gun shots a day for 20 years. Given revenue of $15 billion USD a year, the profits would very likely justify the expense. Kargel's study estimates that there are likely to be about six candidate NEAs that fit the bill.BACK_TO_TOP