German astronomer Gustav Witt discovered the asteroid Eros (image at top of post) on 13 August 1898. Eros was both the first asteroid found to orbit entirely outside of the Main Belt of asteroids between Mars and Jupiter and the first known planet-crosser; its path crosses the orbit of Mars. Eros orbits the Sun in a little more than 643 days. Eros and Earth pass nearest each other - about 14 million miles apart - every 81 years.
In March 1966, Eugene Smith, an engineer with Northrop Space Laboratories in Hawthorne, California, presented a paper outlining a piloted Eros flyby mission at the Third Space Congress in Cocoa, Florida. In it, he wrote that Eros exploration might help scientists to understand Main Belt asteroids and small planetary moons (for example, the martian satellites Deimos and Phobos). He noted that Eros - which he described as "brick-shaped" - would pass within 14 million miles of Earth on 23 January 1975, its closest approach of the 20th century.
At the time Smith presented his paper, NASA and its contractors devoted considerable effort to studies of piloted free-return Mars and Venus flyby missions based on Apollo and Apollo Applications Program technology. The first of these was expected to depart Earth for Mars in late 1975. Among other expected benefits, a Mars flyby would provide interplanetary flight experience ahead of 1980s piloted Mars landings. Smith noted, however, that a Mars flyby mission would likely be so heavy that placing all of its components and propellants into space would need either a Saturn V rocket with a nuclear-thermal upper stage or multiple all-chemical Saturn Vs followed by assembly through multiple dockings in Earth orbit. He called instead for a 1975 piloted Eros flyby that would provide experience applicable to Mars landings, yet could depart Earth on a single uprated Saturn V rocket.
Smith argued that "the value of the Eros mission to subsequent manned planetary flights having a higher level of difficulty and complexity is of no small consequence." He added that "interplanetary experience comes only from interplanetary missions: less difficult flights, such as that to Eros, could significantly enhance experience acquired in Earth orbital and lunar activities, and could thereby increase the probability of success for the missions to follow." Smith provided no estimate of the cost of his mission.
Smith's 527-day Eros flyby mission would begin with launch from Cape Canaveral on 3 May 1974, at the opening of a 30-day launch window. The Eros mission Saturn V and its payload, the Eros Flyby Spacecraft Vehicle (EFSV), would have have stood 21 feet taller than the 363-foot-tall Apollo Saturn V. From top to bottom, the EFSV would have comprised the Eros Launch Escape System, outwardly a duplicate of the Apollo Launch Escape System tower; the conical 10.5-foot-tall, 12.7-foot-diameter Eros Command Module (ECM), outwardly a duplicate of the Apollo Command Module (CM); the Eros Service Module (ESM), a 21.5-foot-diameter, 34.3-foot-long replacement for the 12.8-foot-diameter, 25.7-foot-long Apollo Service Module; the 21.5-foot-diameter, 30-foot-long Eros Mission Module (EMM); and a 21.7-foot-diameter Instrument Unit (IU) bolted atop a 21.7-foot-diameter, 58.4-foot-long S-IVB stage, both of which would be outwardly identical to those used on the Apollo Saturn V.
The Saturn V IU and S-IVB together measured 61.4 feet long; combined with a streamlined adapter that covered the S-IVB's single J-2 engine, they were 61.6 feet long. The adapter flared to a diameter of 33 feet to join the bottom of the S-IVB stage with the top of the 33-foot-diameter, 81.5-foot-long S-II stage, the Saturn V second stage. The S-II in turn sat atop the 33-foot-diameter, 138-foot-long Saturn V first stage, the S-IC.
Upon arrival in 100-nautical-mile-high low-Earth parking orbit, the EFSV would have comprised (from fore to aft) the 33.6-ton Eros Command Module/Eros Service Module (ECM/ESM), the 33.2-ton Eros Mission Module (EMM), and the S-IVB stage with a mass of 98.6 tons, bringing total payload mass to 165.4 tons. For comparison, the Apollo Block II Command and Service Module (CSM) weighed 33.4 tons and the Apollo Lunar Module (LM) weighed about 16.2 tons, for a total Apollo payload mass of about 49.6 tons. During Apollo missions, the Saturn V IU and S-IVB were not generally counted as payload; rather, they were treated as part of the Saturn V rocket.
When Smith presented his paper, the Apollo Saturn V was still more than a year from its maiden flight. NASA, however, expected that it would be able to launch about 130 tons into 100-nautical-mile parking orbit. Smith cited studies that proposed boosting Saturn V launch capacity to 165 tons by uprating the five J-2 engines in its S-II second stage. Alternately, the rocket's S-IC first stage could be fitted with twin 260-inch-diameter solid-propellant strap-on boosters, increasing its launch capacity to a whopping 215 tons. The latter, Smith wrote, would provide ample margin for EFSV weight growth during development. It would, of course, also constitute a more dramatic (and thus costly) change to the basic Saturn V design than would S-II engine uprating.
During Apollo missions, the S-IVB would ignite following S-II stage burn-out and separation and would burn for 2.5 minutes to place itself, the IU, a shroud housing the LM that also served to link the top of the IU to the bottom of the Apollo CSM, and the CSM into parking orbit about the Earth, about 115 nautical miles high, 11 minutes and 40 seconds after launch. About two hours and 44 minutes after launch, the S-IVB would ignite again and burn for six minutes to put the CSM and LM on course for the moon.
When used as part of Smith's EFSV, the S-IVB's first burn would occur as in Apollo missions, but the second would be different. Upon arrival in parking orbit, the six-man crew in the ECM/ESM would check out the EFSV's systems. Assuming that all appeared normal, they would then ignite the S-IVB engine at perigee to raise the ESFV's apogee and gain over 90% of the velocity needed to depart Earth orbit for Eros. S-IVB burnout would see them still orbiting Earth, but in a highly elliptical "Intermediate Departure Orbit" with a period of two days.
The astronauts would next separate the ECM/ESM from the EMM/spent S-IVB, then turn it end for end and dock the ECM's nose-mounted probe docking unit with a drogue unit mounted at the bottom of a conical recess atop the EMM. After detaching it from the spent S-IVB stage, the crew would transfer to the EMM, their main living and working space throughout the Eros flyby mission. They would deploy the EMM's eight solar panels, a steerable "sensor turret," a large dish antenna, and a "support structure" which, along with the conical recess in the top of the EMM, would shield the ECM from harsh sunlight and micrometeroids. After linking the EMM and ECM/ESM electrical and control systems, they would check out all EFSV systems a second time.
If the EFSV failed its second checkout, the astronauts could abort their mission by separating from the EMM in the ECM/ESM and firing the ESM's engines at perigee to reduce speed so that they would reenter Earth's atmosphere in the ECM. The ESM would include twin RL-10A-3 main engines that would burn high-performance liquid hydrogen (LH2) and liquid oxygen (LOX) cryogenic propellants in place of the Apollo CSM's single large Service Propulsion System engine, which would burn hypergolic (ignite-on-contact) propellants. The twin engines would provide redundancy, for the ESM would need only one engine to perform most maneuvers. If the EFSV checked out, the astronauts would fire the ESM engines at next perigee on 5 May 1974 to add enough velocity to place the EFSV on course for Eros.
Smith envisioned that his cylindrical EMM would contain near its center a spherical pressurized habitat module not very different from the one in NASA Marshall Space Flight Center's February 1965 Apollo-based Mars/Venus piloted flyby study report. In the event of solar flares, the crew would retreat to a "storm cellar" with hatches at both ends. The forward hatch would lead to the ECM and the aft hatch to the habitat sphere. A centrifuge would divide the sphere into forward (crew quarters) and aft ("mission task area") halves. Smith hoped that periodic "centrifugation" in the small centrifuge would be sufficient to maintain crew health during the 17.5-month Eros voyage, since spinning the entire EFSV to generate acceleration the crew would feel as gravity would also generate engineering problems. Smith wrote that most of these problems - for example, designing solar arrays so that they would track continuously on the Sun as the spacecraft rotated - could be solved only by increasing the spacecraft's mass so that it could no longer depart from Earth on a single uprated Saturn V.
A hatch in the aft end of the habitat sphere would lead to a pressurized equipment room, which would in turn lead to a round probe hatch. The EFSV's disk-shaped solar panels would ride to Earth orbit folded and stacked under the round aft end of the EMM. They would deploy as two arrays of four panels each. The arrays and aft end of the ELSV would point toward toward the Sun during most of the flight. This would place the ECM/ESM in shadow, which, along with heavy insulation, would prevent the liquid hydrogen fuel in the ESM from boiling away during the long voyage.
On 18 January 1975, the astronauts would begin to track Eros using radar, a five-foot-long reflecting telescope with a 30-inch primary mirror, and other instruments mounted in the EMM's sensor turret. On 23 January 1975, they would fire the ESM engines to ensure an Eros close-approach distance of about 50 miles and would begin gathering Eros science data. About eight hours before closest approach, the astronauts would "catapult" a 200-pound automated probe out the probe hatch toward the asteroid. The probe would function much as had the Block III Rangers; that is, it would image Eros until it smashed into its surface and was destroyed, yielding detailed close-up images in its final seconds. The EMM's dish antenna would relay to Earth data from the probe's TV camera and other instruments.
Closest approach to Eros would occur about 14 million miles from Earth on 28 January. The piloted spacecraft would spend about 90 seconds within 200 miles of the asteroid's sunlit side and about 30 seconds within 100 miles. On 30 January 1975, the crew would end Eros tracking and fire the ESM engines to correct course deviations imparted by the Jan. 23 maneuver, the automated probe launch, and the weak tug of Eros's gravity.
The astronauts would load the ECM with scientific data - film, mainly - and check out its systems beginning on 10 October 1975. On 12 October, they would abandon the EMM and use the ESM engines to place the ECM on course for Earth atmosphere reentry. They would then jettison the ESM, reenter Earth's atmosphere at about 40,000 feet per second - that is, about 3500 feet per second faster than Apollo lunar-return speed - and descend to the surface on parachutes.
Congress killed NASA's plans for piloted Mars and Venus flyby missions in August 1967, in the aftermath of the Apollo 1 fire, which shook confidence in NASA's capabilities. Smith's piloted Eros flyby proposal received little attention. The only U.S. piloted mission of 1975 was the Apollo-Soyuz Test Project, which saw the final Apollo CSM spacecraft dock with the Soviet Soyuz 19 spacecraft in low-Earth orbit.
When NASA at last explored a near-Earth asteroid, it explored Eros. The $112-million Near-Earth Asteroid Rendezvous (NEAR) mission - the first mission in NASA's low-cost Discovery Program - left Earth on 17 February 1996, 22 years after the planned launch date of Smith's piloted Eros flyby.
On 20 December 1998, NEAR failed to enter Eros orbit because its computer aborted a crucial engine burn. Three days later, after some quick reprogramming, NEAR flew past the 22-mile-long, 13-mile-wide asteroid at a distance of 2375 miles, returning 222 images. They revealed that Eros is shaped like a ballet slipper or, as some would have it, a banana.
On 14 February 2000, after another revolution about the Sun, NEAR at last orbited its target. NASA renamed the spacecraft NEAR Shoemaker in March 2000 to commemorate renowned planetary geologist and asteroid and comet discoverer Eugene Shoemaker, who had died in a car crash in Australia in 1997 while looking for asteroid impact craters. During the year that followed, the spacecraft radioed to Earth more than 160,000 close-up images of Eros. New images revealed many odd smooth "ponds" of dust.
Though designed as an orbiter, NEAR Shoemaker succeeded in landing on Eros on 12 February 2001. It may have landed in a dust pond, helping to cushion its impact. It returned gamma-ray spectrometer data from the asteroid's surface until 28 February 2001.
Eros flew past Earth on 31 January 2012, at a distance of 16.6 million miles, its closest approach since 1975. The asteroid will pass slightly closer to Earth than it did in 1975 on 24 January 2056.
Reference:
"A Manned Flyby Mission to Eros," Eugene A. Smith, Proceedings of the Third Space Congress, "The Challenge of Space," pp. 137-155; paper presented at the Third Space Congress in Cocoa Beach, Florida, 7-10 March 1966.
